c
CAD See computer-aided design.
CAD/CAM See computer-aided design; computer-aided manufacturing.
CAE See computer-aided engineering.
CAES See compressed-air energy storage.
cage The frame that holds and separates the balls in a ball bearing or the rollers in a roller bearing.
caisson A large chamber in the form of a vertical cylinder or a rectangular box, which may have an open end, used for example to house water turbines or allow sub-surface work in a river or sea. See also Pendulor wave-energy converter.
calandria 1. A term sometimes used for a tube or bundle of tubes in a shell-and-tube heat exchanger, particularly one used for distillation or evaporation. 2. The core of the CANDU (Canada deuterium uranium) nuclear reactor, a pressurized heavy-water reactor.
calibration 1. Establishing constants in empirical formulae from experimental data or exact numerical solutions. 2. Choice of disposable parameters or constants in FEM or CFD simulations, to make the calculations agree as closely as possible with experimental data or analytical solutions. 3. Comparing the output of an instrument against an instrument of known high accuracy or a standard when the same input is applied to both instruments. A calibration curve is a plot of calibration data indicating the correct value for every reading indicated by an instrument, often incorporating a fitted curve such as a polynomial. A calibrated instrument is one that has undergone calibration.
Callendar and Barnes continuous-flow calorimeter An apparatus for measuring the mechanical equivalent of heat 
(unit J/cal). It consists of two long concentric glass tubes with an electrical heating coil along the length of the inner tube through which there is a constant flow of water with mass flow rate ṁ. The space between the two tubes is evacuated and so insulates the inner tube from the surroundings. If the temperature rise of the water is Δθ, = Ė/ṁΔθ where Ė is the rate at which electrical energy is supplied to the coil.
Callendar’s thermometer The first resistance thermometer to use platinum wire as the resistance element. See also platinum-resistance thermometer.
calliper (caliper) 1. A device (often with electronic readout) like dividers but with curved feet for measuring inside and outside dimensions; when the feet point in the same direction, the device is used to measure the pitch of gearing. 2. See disc brake.
calorically-imperfect gas See perfect gas.
calorically-perfect gas See perfect gas.
caloric equation of state An equation for the dependence of the specific internal energy of a substance on its temperature and specific volume.
calorie (cal, gram calorie, small calorie) An obsolete (i.e. non-SI) unit of energy equal to 4.186 8 J. It is the amount of energy needed to raise the temperature of 1 gram of pure air-free water from 14.5°C to 15.5°C at standard atmospheric pressure. See also British thermal unit; mechanical equivalent of heat.
Calorie See kilocalorie.
calorific value of a fuel (heating value of a fuel, heat value) (Unit kJ/kg for solid and liquid fuels, but also used is kJ/kmol and, for gaseous fuels, kJ/m3) The energy released per unit mass of fuel in complete combustion with oxygen, i.e. the heat of reaction. Values are determined by burning fuel in a bomb calorimeter under specified conditions. The gross or higher calorific value, GCV or HCV, corresponds to the water in the combustion products being in the liquid phase and the net or lower calorific value, NCV or LCV, to the vapour phase. The two are related by
where hfg is the latent heat of vaporization of water, mH2O is the mass of water produced for a mass of fuel mFUEL. The values given are normally determined at constant pressure, but constant-volume values are also found. See also Boys gas calorimeter.
calorifier See non-storage calorifier; storage calorifier.
calorimeter An apparatus used to measure such quantities as the calorific value of a fuel, the enthalpy of reaction of a chemical reaction, and the thermal capacities of materials. The apparatus typically consists of an insulated vessel, which may be open or closed, surrounded by stirred water. Temperature changes in the water are analysed to determine the quantity of interest. See also bomb calorimeter; Boys gas calorimeter; Callendar and Barnes continuous-flow calorimeter.
CAM See computer-aided manufacturing.
cam A component of a mechanism that imparts a prescribed reciprocating motion to a cam follower, the output element of a cam mechanism which is in contact with the cam profile. It is typically a rod which slides in a guide with a roller or shaped end (translating follower) or a pivoted arm (oscillating or rotating follower). The motion is determined by the shape of the cam surface (cam profile) and the geometry of contact with the follower. Rotating cams (profiled discs) that open and close valves in engines are examples, but there are linear (wedge) cams, and cylindrical cams in which the end of the follower runs in a groove profiled to give the desired motion. The profiled part of a cam by which movement is imparted to the follower is the cam nose. Cam dwell is that part of a cam profile where no motion of the follower takes place. For a rotating cam it is an arc of a circle with its centre at the centre of cam rotation. An eccentric cam is a cam with circular profile rotating off centre. Rotating cams are mounted on a camshaft, the rotation being effected by the camshaft drive, a toothed belt (as in the diagram), gear train, or chain through which power is transmitted from the crankshaft of an engine. The drive may also power peripherals such as the coolant pump. The tensioner transmits no power.
camshaft drive
camber line See aerofoil.
CANDU reactor See calandria (2).
canopy 1. The transparent cover over the cockpit of some types of aircraft, especially high-performance military aircraft and helicopters. Most modern canopies are vacuum-formed acrylic. Where appropriate, the canopy is an integral part of the ejection seat system. 2. A term sometimes used for a car door which lifts up to allow access. 3. The term used to describe the principal components of a parachute.
cantilever A beam that is built in at one end and unsupported at the other. See also fixity.
caoutchouc See rubber.
cap A cover, often in the form of a short cylinder, one end of which is closed. Typically used to close an orifice or pipe end, on to which it can be pushed, welded, screwed, or attached with fasteners.
capacity factor (load factor, plant factor, energy utilization factor, utilization factor) The ratio, for a power plant, of its annual output divided by its maximum possible output.
capacity rate See number of transfer units.
capillarity correction (Unit mmHg) A part of the correction which must be applied to the reading of a mercury-in-glass barometer to account for the convex shape of the meniscus.
capillary See capillary tube.
capillary constant See surface tension.
capillary effect (capillary action, capillarity) 1. The tendency of liquids to move along a capillary tube as a consequence of the surface-tension force which arises at the tube wall–liquid interface. If the liquid wets the surface, as is the case for water, a concave meniscus is formed in a vertical tube of radius R with its lower end submerged below the liquid surface. The liquid rises by an amount 2σcosθ/(ρgR) where σ is the liquid–air surface tension, θ is the contact angle, ρ is the liquid density, and g is the acceleration due to gravity. If the liquid is non-wetting, as is the case for mercury, a convex meniscus is formed and the liquid level in the tube is depressed. 2. Similar forces to those described in (1) arise in liquid flow through porous media and microchannels.
capillary filling The application of capillary action to introduce liquid into a microchannel (capillary flow) such as in a lab-on-a-chip.
capillary fitting (capillary joint) A pipe joint where a male end having soldering flux over its surface fits into a female end, and application of heat, typically using an LPG blowtorch, melts solder fed to the joint, which then is drawn into the annular gap by capillary action.
capillary-force valve (capillary valve) A valve that uses surface tension to control the flow of a liquid into a microchannel filled with a second immiscible fluid, usually a gas.
capillary number (Ca) A non-dimensional number that characterizes a liquid flow having an interface with an immiscible fluid (gas or liquid) affected by both viscous and surface-tension forces. It can be defined as Ca = μV/σ where μ is the dynamic viscosity of the liquid, σ is the surface tension between the two fluids and V is a characteristic velocity of the flow. An important parameter for flow in a microchannel. See also Bond number; Eötvös number.
capillary reactor A capillary tube, typically in the shape of a U, through which there is flow within which a chemical reaction occurs.
capillary tube (capillary) A circular tube, often of glasss, of sufficiently small internal diameter (typically 1 mm) that the interface between two immiscible fluids (liquid–liquid or gas–liquid) within the tube will be significantly affected by surface-tension forces. See also conduit.
capillary viscometer 1. (suspended-level viscometer) An apparatus used to determine the kinematic viscosity ν of a Newtonian liquid of density ρ by measuring the time t for a known volume 𝓥 (typically 5.6 ml) of liquid to flow through a vertical capillary tube of diameter d (typically in the range 0.64 to 6.76 mm) and length l (typically 90 mm). The apparatus is held at a fixed temperature, usually 20°C. The principle of the device is that the flow through the capillary tube is Poiseuille so that, in principle, ν can be calculated directly from the measurements. In practice, the viscometer is calibrated against a liquid of known kinematic viscosity νCAL and ν is determined from ν = νCALt/tCAL where tCAL is the time required for volume 𝓥 of the calibration liquid to flow through the capillary tube. See also Ostwald viscometer; Ubblehode viscometer. 2. An apparatus used to determine the dynamic viscosity μ of a Newtonian fluid by measuring the pressure drop Δp and volumetric flow rate 
for flow through a capillary tube of length l and internal radius r. Assuming Poiseuille flow through the tube, μ is determined from the equation 
. See also Saybolt Universal viscometer.
capillary wave (ripple) A small-amplitude wave at the interface of two immiscible liquids or a liquid–gas interface in which the restoring force is due to surface tension. For capillary waves on a liquid–liquid interface, the dispersion relation is given by
where ω is the angular frequency, σ is the surface tension, ρH and ρL are the densities of the heavier and lighter liquids, respectively, and λ is the wavelength. See also gravity–capillary wave.
cap nut A nut with a blind threaded hole, for example a dome nut to cover the end of a bolt.
cap screw A bolt where the thread runs right up to the head and engages in a threaded hole or captive nut in an adjoining member. See also machine screw.
capstan A winding drum having a vertical axis.
capstan lathe A lathe having a number of tools already set up in a moveable tool holder (the capstan), often hexagonal in shape, to perform a specified sequence of operations in mass production of components. The tools are brought into action by rotation of the capstan about a vertical axis. See also turret lathe.
captive nut A nut attached loosely or rigidly to a sheet member that is too thin to thread and which engages with a cap screw.
capture efficiency The ratio of the net rate of heat transfer into a solar collector to the solar radiation incident on the collector (i.e. the product of the surface area and the irradiance).
carbide A compound of carbon and another element, usually a metal. Important examples include titanium carbide and tungsten carbide, used in cutting tools and abrasives, and iron carbide (cementite), an important microconstituent in steels and cast irons. See also cemented carbides; steel microstructures.
carbide cutting tool A cutting tool made of sintered or hot-pressed ‘hard-metal’ carbides such as boron carbide, titanium carbide, tungsten carbide, etc., bonded with mixtures of cobalt, chromium and nickel.
carbon capture and storage (CCS) To reduce its contribution to global warming, a system for removing CO2 from combustion exhaust gases at source, concentrating it and removing to a remote storage location, such as an underground cavern.
carbon fibre A filament reinforcement material, consisting of fibres 5 to 10 µm in diameter and composed primarily of carbon atoms, used in composites. In addition to having a much greater fracture strength than glass fibre (~21 GPa vs ~1 GPa), it is also much stiffer (Young’s modulus = 750 GPa vs 70 GPa). In consequence working strains (deflexions) in a design using CFR composites can be kept at similar levels to those when using metals, unlike designs using GFRP, where strains and deflexions can be appreciable.
carbon-fibre-reinforced plastic (carbon-fibre-reinforced polymer, CFRP) A composite material consisting of two principal components: an underlying polymeric resin and reinforcing carbon fibres.
carbon-fibre tow Several thousands (1k to 48k) of twisted filaments of pure carbon, typically 5–8 µm diameter, which can be woven to produce a carbon-fibre cloth.
carbon nanotubes (buckytubes) Tubes of single or multiple layers of graphene a few nm in diameter and from less than 1 μm to several mm in length. They can have closed or open ends and are produced by chemical vapour deposition, laser ablation, or electric arc discharge. The unique molecular structure results in a Young’s modulus of up to 1 000 GPa, high tensile strength (~10–100 GPa), high electrical and thermal conductivities, and, with few or no unbonded atoms, low chemical activity. Applications to exploit these properties include micro-composites, tissue engineering, energy storage, sporting goods, electromagnetic shields, thin-film electronics, and black coatings. The last of these is demonstrated by Vantablack®, a super-black coating that absorbs electromagnetic radiation in nanotube cavities over the wavelength range from ultraviolet to the far infrared. It absorbs 99.965% of light at a wavelength of 750 nm. The name is derived from an abbreviation of vertically aligned nanotube arrays.
carbon steels (plain carbon steels) Steels in which carbon is the principal alloying element, and the amount of manganese does not exceed 1.65% and the copper and silicon contents are less than 0.60%. There are three main types: low-carbon steels (0.08–0.35% carbon); medium-carbon steels (0.35–0.50% carbon); and high-carbon steels (0.50–2.0% carbon). See also steel microstructures.
carburettor (carburetor) A device that supplies the air–fuel mixture to a spark-ignition engine. The air is drawn in by the downward movement of a piston and passes through a Venturi. The resulting low pressure at the Venturi throat sucks fuel through an atomizing jet and into the air flow, where it mixes. The intake air can flow either upwards through the main body into the engine intake manifold (updraught carburettor) or downwards (downdraught carburettor). Fuel-injection systems have replaced the carburettor in modern engines.
carburettor icing Ice formation on the surfaces of a carburettor, particularly the butterfly valve, owing to expansive cooling and evaporation of petrol which causes water vapour in the airflow to condense and freeze. Icing is particularly a problem with very humid air such as is encountered by an aircraft flying through clouds, but is preventable by carburettor heating.
carburizing See case hardening.
carcass (casing) The structural body of a tyre which withstands the internal pressure and to which the tread and sidewall are bonded. It is made up of layers of textile cord (termed plies).
Cardan joint See Hooke’s joint.
Cardan shaft See drive shaft; propeller shaft.
Cardan’s suspension A system that uses three gimbals with orthogonal axes so as to support a component in a fixed orientation despite rotation of the mounting of the gimbals. It is typically used to support gyroscopes for navigational use and for experiments on bodies rotating about a fixed point (O in the diagram) as opposed to a fixed axis.
Cardan’s suspension
Carnot cycle An ideal four-stage reversible cycle consisting of isothermal expansion (1–2) with the reception of heat from a heat source at high absolute temperature TH; isentropic expansion (2–3); isothermal compression (3–4) with the rejection of heat to a heat sink at low absolute temperature TL; and isentropic compression (4–1). The cycle efficiency (Carnot efficiency) η is given by η = (TH − TL)/TH. The diagram shows the cycle as a plot of pressure (p) ʋs specific volume (ʋ).
Carnot cycle
Carnot engine The diagram shows a possible arrangement for an engine operating on a Carnot steam cycle. Saturated water is evaporated in a boiler at constant pressure to produce saturated steam (steps 1–2); the steam is then expanded in a turbine or steam engine to produce power (2–3) and partially condensed at constant pressure (3–4). The cycle is completed by compression by a rotary or reciprocating compressor (4–1).
Carnot engine
Carnot’s law For an ideal gas, the difference between the specific heats at constant pressure CP and constant volume CV is equal to the specific gas constant R, i.e. CP − CV = R.
Carnot’s theorem (Carnot’s principles) 1. Any reversible heat engine operating between two constant-temperature reservoirs has the same thermal efficiency as any other. This is self-evident from the efficiency of the Carnot cycle. 2. The efficiency of an irreversible (i.e. actual) heat engine is always less than that of a reversible heat engine operating between the same temperatures. The two statements are proved by demonstrating that the violation of either one is a violation of the second law of thermodynamics.
Carreau model See non-Newtonian fluid.
Carreau–Yasuda model See non-Newtonian fluid.
carriage spring See semi-elliptic spring.
Cartesian-coordinate robot (rectangular-coordinate robot) A robot where the first three joints are translational, have axes normal to each other, and move in the three Cartesian coordinate directions of the base frame. The volume that the robot can reach is thus a hollow cube. The diagram shows an idealized Cartesian coordinate robot with the joint offsets d1, d2, and d3 showing the movements of the first three joints. See also cylindrical-coordinate robot.
Cartesian-coordinate robot
cascade See cascade tunnel.
cascade compensation Where a compensator is placed in series with a controller so as to improve the performance of the controller.
cascade control A control system in which the output of a primary controller determines the set point of a secondary controller that provides the input to the controlled plant. For such a system to operate correctly, the secondary controller must have a much shorter settling time than the primary controller so as to allow it to track changes in the set point output from the primary controller. For example, in a rolling mill the overall speed of the strip being rolled is controlled by a primary controller, but each drive motor will have a secondary controller to maintain the motor speed at that set by the primary controller.
cascade refrigeration cycle A cycle in which the refrigeration process involves two or more refrigeration cycles operating in series. A typical application is where a larger temperature range is required than is practical for a single vapour-compression refrigeration cycle.
cascade system In control systems, a system that has been divided into two or more parts so as to allow cascade control to be applied.
cascade tunnel A wind tunnel in which a straight, staggered row of equally spaced turbine or compressor blades (cascade) is used to investigate their aerodynamic characteristics.
case hardening (carburizing) Hardening of the surfaces of low-carbon steel components by heating in an atmosphere of carbon so as to increase the carbon content of the surfaces by diffusion up to, say, 0.6–0.8%, after which quenching transforms this outer layer to martensite that is then stress-relieved. See also electron beam hardening; laser hardening; nitriding.
casing 1. See carcass. 2. A large-diameter pipe, usually of steel, inserted into an oil or gas well and cemented in place. The casing is designed to prevent the borehole from collapsing, or the movement of fluids from one rock formation to another, and to aid in well control. See also perforating.
Casson model See non-Newtonian fluid.
castellated nut A nut having slots across the hexagonal faces, used with a bolt having a drilled hole so that a split (cotter) pin may be inserted through both nut and bolt to prevent unscrewing.
caster (castor) A small-diameter wheel or ball that swivels on an axis perpendicular to the axis of the wheel.
caster angle (Unit °) The inclination of the kingpin (swivel pin) in a steering mechanism, as viewed along the axis of the steered wheels, that gives self-centring steering.
Castigliano’s theorems The deflexion of an elastic body at the point of application of an external load is given by the partial derivative of the strain energy with respect to the component of the applied force in that direction. Strictly, there are two theorems, the one employing differentiation of the complementary strain energy for displacement under a load, and the other using differentiation of the strain energy to give the load causing a displacement, these energies being different from one another in non-linear elasticity but identical in linear elasticity.
casting 1. The process of pouring molten metal into a mould so as to obtain, after cooling, a component having the shape of the mould. 2. A component produced by the process of casting.
casting strains The strains that result from different shrinkage displacements in different parts of a casting upon cooling. They are accompanied by casting stresses.
cast iron A term for alloys of iron containing between 2% and 5% carbon. There are two principal types, called white and grey (gray) from the appearance of fracture surfaces. White iron is an extremely hard form of cast iron, usually having less than 4.3% carbon and containing only very small amounts of silicon. The microstructure consists of pearlite and cementite (but martensitic forms are possible) and, depending on the pearlite spacing, the tensile strength is between 250 and 500 MPa, with a Young’s modulus of 170 GPa. Grey iron contains silicon, which promotes the formation of free graphite flakes that act as sharp cracks. Depending on whether the microstructure is pearlitic or ferritic, the tensile strength is between 140 and 420 MPa, with Young’s modulus varying inversely with flake size between 80 and 140 GPa. Iron phosphide in the microstructure increases the fluidity and makes casting straightforward. Cast iron is brittle unless specially treated to give improved toughness and ductility. Among such modified types are malleable white iron (a traditional form of cast iron with average ductility, initially cast as white iron and then heat treated to produce graphite clusters called temper carbon; both pearlitic (whiteheart) and ferritic (blackheart) forms are made, the two names arising from shiny or dull fracture surfaces); ductile modular grey iron and spheroidal grey cast iron, the ductility of which has been considerably improved by the addition of magnesium and/or cerium to molten grey cast iron; and meehanite iron, which would otherwise be a white iron of low silicon and carbon content, but to which is added in the melt calcium silicide, resulting in finely-dispersed carbon flakes. Alloying elements (nickel, etc.) may also be added to all types of cast iron to improve properties such as corrosion and wear resistance. See also chilled castings; steel microstructures.
cast steel Steel that is cast into shapes. It has superior properties to most cast irons, but is more expensive to produce.
catalytic converter A device installed in an exhaust system to reduce the toxicity of exhaust gases, typically using a catalyst, such as platinum, palladium, or rhodium, to oxidize carbon monoxide and unburned hydrocarbons and reduce oxides of nitrogen.
catastrophic failure A sudden and total failure of a large engineering structure such as an aeroengine, aircraft, space vehicle, bridge, or dam.
catenary The shape taken up by a uniform chain or cable having negligible bending stiffness when suspended between two points. For a cable with each end at the same height, and with the origin of axes at the lowest point, the equation is 
where y and x are vertical and horizontal distances, respectively, w is the weight per unit length of the chain, and H is the (constant) horizontal component of the tension T in the cable, given by T = H + wy. Problems are solved from known conditions, such as the values of the central dip and span, central dip and length of cable, or span and length of cable.
caterpillar (crawler vehicle) A vehicle that ‘lays its own road’ by running on endless belts, driven by toothed wheels, on each side. Used on soft ground where spreading of the load reduces contact stress.
cathodic protection See corrosion.
cation See interatomic bonding.
Cauchy number (Cn) 1. The non-dimensional number that has to be satisfied for equality of inertia and elastic forces, given by 
where V is velocity, ρ is density, and E is Young’s modulus. If Poisson’s-ratio effects can be neglected, the compressional wave speed in a solid is given by 
so that Cn = V/c which is analogous to the Mach number in a gas. See also Atkins number. 2. A non-dimensional number sometimes used to characterize compressible flow and defined as 
(in fluid mechanics the symbol Ca is more usual) where K is the isentropic bulk modulus of the fluid. For an isentropic flow process, Cn = M2 where M is the Mach number of the flow.
Cauchy’s equations The set of general partial differential equations governing the flow of any fluid satisfying the continuum hypothesis. They relate the temporal and spatial derivatives of the velocity components with the spatial derivatives of the normal and shear stresses together with any specific (i.e. per unit mass) body forces and are derived directly from Newton’s second law of motion. See also continuum mechanics.
causal system A system where the output at a particular time T depends only on the inputs over the time t in the range 0 ≤ t ≤ T. This is the case for normal physical systems which cannot anticipate future events. However, in the digital filtering of previously captured data, a non-causal filter is frequently used where the filter output at time T depends on inputs both before and after T.
cavitation The formation of vapour- or gas-filled cavities in a liquid due to reduction of the local pressure, often due to acceleration of the fluid, such as in flow through a convergent nozzle. If there is no dissolved gas in the liquid, vaporous cavitation occurs when the pressure falls below the saturated vapour pressure. If the bubbles are formed due to high temperature, the process is termed boiling. If there is dissolved gas, gaseous cavitation occurs due to pressure reduction, temperature increase, or diffusion (degassing). At the tips of marine propellers and in hydraulic machinery, the collapse of cavitation bubbles can cause noise and vibration and lead to surface damage in the form of pitting. Cavitation in a fluid flow is characterized by the non-dimensional cavitation number (σ) defined by 
where p∞ is the ambient static pressure, pV is the vapour pressure, ρ is the fluid density, and V is the flow speed. See also Euler number.
cavitation-resistance inducer An axial-flow pump used upstream of a main pump in order to prevent cavitation in the latter by increasing the inlet head.
cavitation tunnel A closed-circuit recirculating water tunnel in which the static pressure can be reduced to sufficiently low levels for cavitation studies to be performed.
cavity radiator A heated chamber having a small hole through which radiation, approximating blackbody radiation, passes out.
cavity resonator See Helmholtz resonator.
CCR See steel microstructures.
CDD See degree day.
cellular materials There are three broad classes of materials that have a sponge-like structure containing many small closed or open (interlinked) pores or cells. Natural cellular materials include bone, cork, sponge, and wood, and have numerous well-known uses. Wide ranges of cellular plastics are produced using blowing or foaming agents, such as air, ammonium carbonate, sodium bicarbonate, etc., to create pores during the processing of thermoplastic or thermosetting polymers. Rigid foams have moderate compressive strength and can be moulded and machined. Ceramic foams produced by introducing gaseous cells during processing have excellent thermal-shock resistance and insulation properties. Their uses include catalyst supports, filters, and components for gas diffusion. It is also possible to produce cellular glasses and metals for niche applications. See also foam (2); polyurethane; sponge.
Celsius temperature scale (centigrade temperature scale) A relative, non-SI, temperature scale now defined in terms of the Kelvin absolute temperature scale as °C = K − 273.15, where °C is the symbol for degrees Celsius. The scale was previously called the centigrade scale, with two fixed points: the melting point of ice (the ice point) as 0°C, and the boiling point of water (the steam point) as 100°C.
cemented carbides Sintered mixtures of refractory metal carbides (e.g. tungsten carbide) in a metal matrix binder such as cobalt, nickel, or iron. They have high melting point, toughness, compressive strength, and wear resistance. Applications include use in grinding wheels and papers, cutting tools, drill bits, wire-drawing dies, and ball-point pen tips. Sometimes known as hard metals when the application is to machine tools.
cementite See steel microstructures.
centi (c) An SI unit prefix indicating a multiplier of 0.01; thus centimetre is a unit of length equal to one one-hundredth of a metre or 10 mm.
centigrade heat unit (Celsius heat unit, CHU) An obsolete (i.e. non-SI) unit equal to the energy required to increase the temperature of one pound of pure, air-free water from 14.5°C to 15.5°C at a pressure of one standard atmosphere.
central gear See toothed gearing.
centreless grinding Grinding of a cylinder not supported along its axis, but by an underneath knife edge and a feeding wheel or pressure roller.
centre line 1. In an engineering drawing, a line of symmetry. 2. An imaginary line along a pipe, duct, or shaft that defines an axis of symmetry. 3. An imaginary straight line parallel to the intended direction of a surface located such that the areas above and below the line and the real wavy (rough) surface cancel out.
centre-line average (CLA, Ra) The arithmetic mean height of the peaks and valleys determined by a profilometer or optical interferometer in surface-roughness measurements.
where y is the height of the surface above the mean line at distance x from the origin and L is the overall length of the profile under examination.
centre of buoyancy The point in a floating or submerged body, located at the centroid of the displaced volume, through which the upthrust acts. See also Archimedes principle; metacentre.
centre of flexure See shear centre.
centre of gyration The point in a rotating body about which its angular momentum can be regarded as being concentrated. See also radius of gyration.
centre of mass The point within a system or body at which all the mass can be imagined to be concentrated. If the system or body is of uniform density, its centre of mass coincides with its centroid. If the system is in a uniform gravitational field, it coincides with its centre of gravity, the fixed point through which the resultant gravitational body force (i.e. its weight) acts. When a rigid body is accelerated, the resultant force caused by its inertia acts through its centre of mass.
centre-of-mass coordinate system (centre-of-momentum coordinate system) A reference frame that moves at the velocity of the centre of mass of a body or system so that the latter is always at rest with respect to the reference frame and the total momentum of the system is zero. See also accelerating frame of reference.
centre of oscillation For a rigid body so suspended that it can rotate freely about a horizontal axis through a point O, the length L of a simple pendulum with the same period of oscillation is given by 
where c is the distance of the centre of gravity G of the body from O and kG is the radius of gyration of the body about G. If the point of suspension is shifted to another point O′ on the line OG extended, and the periodic time remains unaltered, it follows that 
, i.e. 
. The point O′, related in this way to the point of suspension O, is called the centre of oscillation.
centre of percussion The location along the handle of a hammer, bat, etc. where, when struck at one end, there is zero impulsive reaction, i.e. no ‘sting’ to the hand of the person holding the shaft. For a uniform rod of length 2L struck at one end, the centre of percussion is at a distance 2L/3 from the other end.
centre of pressure 1. The location, on an aerofoil or other body that develops lift, of the resultant lift force. 2. The location, on a surface submerged in a liquid, of the resultant force due to the pressure acting on the surface. Because hydrostatic pressure increases with depth, the centre of pressure is generally below the centroid of the surface.
centre of twist See shear centre.
centre punch A conical-ended punch used to ‘dot’ the surface of a component where a hole is to be drilled in order to prevent chisel-ended drills from wandering. See also pilot bit.
centrifugal Acting or moving in a direction away from the axis of rotation of a rotating body.
centrifugal acceleration See accelerating frame of reference.
centrifugal clutch A clutch that engages and disengages at a defined speed of rotation of the driving shaft, as when expanding friction shoes act against the inside of a drum.
centrifugal compressor A compressor in which kinetic energy is added to a fluid by radial acceleration in an impeller and then converted into a pressure increase by flow though a diffuser.
centrifugal fan A machine with a rotor consisting of a number of blades mounted around a hub and used for moving air or other gases. The gas enters the rotor axially and is discharged radially at increased pressure.
centrifugal force (Unit N) The inertial reaction force to the centripetal force. It is equal in magnitude but opposite in direction.
centrifugal pendulum A small pivoted mass, attached at a distance r from the axis of a disc rotating at angular velocity ω, acts as a pendulum in the centrifugal acceleration field given by 
and has frequency 
where L is the distance from the pivot to the centre of mass of the pivoted mass. Used to advantage in dynamic dampers since vibration frequency is proportional to speed and therefore damps out-of-balance engine vibrations at all speeds. See also pendulum damper.
centrifugal pump A pump into which liquid enters axially through the eye of the casing and is then accelerated through an impeller, thereby increasing both its kinetic energy and pressure before being delivered to a ring diffuser (the volute) that further increases the liquid pressure and from which it leaves.
centrifugal pump
centrifuge A machine incorporating a rapidly spinning drum in which liquids and suspended particles of different densities are separated by centrifugal action.
centripetal Acting or moving in a direction towards the axis of rotation.
centripetal acceleration (Unit m/s2) The radially-inward component of acceleration of a particle moving along a curved path. If the local radius of curvature is r and the instantaneous velocity of the particle is V, the centripetal acceleration is V2/r = rω2 where ω is the instantaneous angular velocity. It arises because the velocity vector, even if constant in magnitude, continually changes direction. For a particle of mass m moving along a curved path, the centripetal force is mV2/r or mrω2 directed towards the centre of rotation. See also angular acceleration; radial MOTION; tangential acceleration.
centrode The locus of the instantaneous centre of rotation of a moving body.
centroid See centre of mass.
ceramic foams See cellular materials.
ceramics Materials in one of the following classes: abrasives, cements, clay products, glasses, refractories, and advanced ceramics.
cermets Composite materials consisting of ceramics such as metal borides, carbides, nitrides, or oxides, and metal binders such as aluminium, cobalt, chromium, iron, nickel, tantalum, or titanium. They are made at high temperature by powder metallurgy techniques involving pressing and sintering. They have excellent high-temperature strength and wear resistance, and high oxidation resistance. Used for high-temperature applications such as high-speed machining tools, components in turbines, rockets and pumps, and high-temperature coatings. See also metal-matrix composite.
cetane number (CN) A number that defines the ignition quality of a diesel fuel. The scale is defined by blends of two pure hydrocarbon reference fuels, n-cetane (or hexadecane) with CN = 100 and heptamethylnonane (HMN), a fuel with very low ignition quality (CN = 15). It is given by CN = percent n-cetane + 0.15 percent HMN. See also octane number.
CF See carbon fibre.
CFCC See continuous fibre ceramic composites.
CFD See computational fluid dynamics.
CFR engine See cooperative fuel research committee engine.
CFRP See carbon-fibre-reinforced plastic.
cgs system (cgs units) An obsolete (non-SI) system of units based on the centimetre, gram, and second.
chain A series of connected links, typically of steel. For lifting, pulling, securing, etc., each link is a closed loop, often in the form of a ring. For power transmission, the links are designed to mesh with the teeth of a sprocket wheel. In contrast to belt drives, chain drives tend to be employed in high-torque applications. See also kinematic PAIR; roller chain; silent chain.
chain dimensioning On an engineering drawing, where the end point of one dimension is the starting point for the next. Parallel dimensioning is preferred as chain dimensioning can lead to the accumulation of tolerances. See also combined dimensioning.
chain hoist (differential chain hoist) A lifting device using a chain running over a sprocket to lift the load. The sprocket may be driven by a motor, or by force applied manually to another chain and sprocket with a gearbox joining the two sprockets to provide the mechanical advantage.
chain lines Thin lines on an engineering drawing with long sections separated by short dashes or dots. Principally used to denote centrelines and lines of symmetry.
chalcogen See periodic table.
chamfer An edge machined at an angle, typically 45°. See also bevel.
change of phase 1. A term used in thermodynamics for the change from one state (solid, liquid, vapour, or gas) to another. See also p–v–T diagram. 2. A change within a given state, such as solid-state transformation in metals at different temperatures (e.g. austenite, ferrite, cementite, martensite in steels). See also steel microstructures.
Chapman–Jouguet point A point on the detonation adiabat corresponding to sonic conditions downstream of a detonation wave. According to the Chapman–Jouguet rule (Chapman–Jouguet condition), the Mach number behind a detonation wave is unity, i.e. it propagates at the sonic velocity of the reacting gases. A Chapman–Jouguet detonation is a detonation wave with the Chapman–Jouguet point as the end state.
Chapman viscosity law A proportional approximation for the dependence of dynamic viscosity μ on absolute temperature T widely used in aerodynamics and given by μ = μREFT/TREF where μREF is the viscosity at a reference temperature TREF.
characteristic equation See equation of state.
characteristic length See scaling parameter.
characteristics 1. In the propagation of acoustic waves through a gas with sound speed c, lines given by t ± x/c that trace the progress of the waves through the gas, where x represents distance and t time. See also compatibility conditions. 2. See slip line.
Charles law (Gay–Lussac law) The volume of a fixed mass of gas at constant pressure is proportional to its absolute temperature. See also Boyle’s law; perfect gas.
Charpy test (Charpy impact test, Charpy V-notch test) A notched-bar impact test in which the specimen, unrestrained at both ends and with the notch vertical, is struck from behind the notch by a pendulum. The subsequent decrease in height of the swing of the pendulum is a measure of the energy absorbed in fracturing the testpiece. See also impact testing; Izod test.
chart recorder An instrument that plots experimental data in the form of a dependent variable against an independent variable. It can be a Cartesian or a polar plot.
chatter 1. The noisy vibration of a cutting tool when the tool fixture is insufficiently stiff. 2. The vibration that can occur in a control valve, sometimes referred to as flutter.
Chebyshev’s mechanism See Watt’s mechanism.
check valve (clack valve, non-return valve) A mechanical device that allows fluid flow in one direction only. The numerous designs include ball, diaphragm, disc, lift, split disc, and swing check valves.
cheese head A cylindrical head on a screw or bolt. For driving, it may be slotted, or hexagonally recessed (Allen® screw).
chemical irreversibility See irreversibility.
chemical reaction A process in which two or more substances are mixed together and interact to form products. An endothermic reaction is accompanied by the absorption of energy. In an exothermic reaction chemical energy is released in the form of heat.
chemical vapour deposition (CVD) A chemical process by which volatile precursors react and decompose on a substrate to produce thin films or substrate surface enhancement. It is widely used in electronics and optoelectronics for depositing materials such as silicon, silicon nitride, silicon dioxide, carbon (diamond and graphene), and some metals.
chilled castings Iron castings cooled at a rate that results in white iron (slow enough to avoid the formation of martensite, yet fast enough to prevent any silicon present from causing decomposition of cementite into iron and graphite). See also cast iron.
chimney stack (chimney) See stack.
choked flow (choking flow, critical flow) The flow of a compressible fluid through a contraction at the maximum possible flow rate ṁ. For a perfect gas,
where γ is the ratio of specific heats for the gas, ρ0 is the stagnation density of the gas (i.e. ρ0 = p0/RT0, R being the specific gas constant, and p0 is the stagnation pressure), c0 is the sound speed corresponding to the absolute stagnation temperature T0 (i.e. 
), and A* is the throat (minimum contraction) area (the superscript * denotes choked flow). The gas speed at the throat is equal to the speed of sound at that location, i.e. the Mach number equals unity. See also Fanno flow; Rayleigh flow.
choked nozzle A convergent or convergent–divergent nozzle for which choked flow occurs at the throat.
choke valve 1. A control valve for compressible fluids operating under choked-flow conditions. 2. A valve, usually a butterfly valve, that limits air flow into the carburettor of a petrol engine to assist starting by providing a richer mixture. 3. A heavy-duty valve, of plug or needle type, typically used in the oil industry to control liquid flow.
chopped-strand mat (CSM) A thin mat formed of random discontinuous fibres (usually of glass) held together by resin, from which three-dimensional composite shapes may be manufactured which have quasi-isotropic in-plane properties. See also prepreg.
chordal thickness See toothed gearing.
CHP plant See combined heat and power plant.
chuck A rotating cylindrical device for gripping drill bits and other rotating tools or work pieces. See also collet; four-jaw chuck; Jacobs chuck; three-jaw chuck.
chuck key A wrench-like tool to tighten or loosen the jaws of a chuck.
churn flow A regime of gas–liquid flow, with relatively high gas velocity, between the slug-flow and annular-flow regimes. See also boiling.
CIM See computer-aided manufacturing.
circlip (snap ring) An external or internal retaining ring that locates parts of circular cross section in an axial direction. It consists of an incomplete ring, with holes on either side of the gap, that may be expanded by a plier-like tool to pass into a groove in a shaft or contracted to pass into a groove in a bore.
circlip
circular frequency See angular frequency.
circular motion The motion of a rigid body or fluid in which all particles move along circular paths about the same axis.
circular pitch (circumferential pitch) See toothed gearing.
circulation (velocity circulation, Γ) (Unit m2/s) Within a fluid flow, the anticlockwise line integral with respect to arc length around a closed curve C of the velocity component tangential to the curve. Thus 
where V is the flow velocity, α is the angle between the velocity vector and the tangent to the curve, and ds is an element of arc. See also Kelvin’s circulation theorem.
circumferential winding The manufacture of a continuous filament-reinforced composite tube by winding on a mandrel, often at angles to the axis of the pipe. See also axial winding; helical winding
CLA See centre-line average.
clamped See boundary conditions; fixity.
Clapeyron‐Clausius equation (Clausius–Clapeyron equation) An equation derived from the Clapeyron equation that can be used to determine the variation of saturation pressure pSAT with saturation temperature TSAT for a liquid-to-vapour phase change of a substance, according to
where hfg is the latent heat of vaporization for the substance and R is the specific gas constant assuming the substance behaves as an ideal gas. For a solid-to-vapour phase change, hfg can be replaced by the latent heat of sublimation.
Clapeyron equation The equation from which the Clapeyron–Clausius equation derives. For changes of phase, either solid to liquid or liquid to vapour, application of the first and second laws of thermodynamics leads to the equation
where p is pressure, TSAT is the saturation temperature, hfg is the latent heat of vaporization, ʋ is specific volume, and the subscripts g and f refer to the saturated vapour and liquid states, respectively. (Sometimes called the Clapeyron–Clausius equation, but not with the same meaning as the previous definition.)
classical control The design and analysis of control systems using frequency-domain methods that operate on the transfer function, i.e. on the Laplace transform of the ratio of the output to the input.
classical mechanics (Newtonian mechanics) A sub-field of mechanics based on Newton’s laws of motion.
Clausius equation An equation of state for real gases that takes into account the fact that molecules cannot move to positions occupied by other molecules. It can be written p(ʋ − b) = RT where p is the gas pressure, ʋ is its specific volume, T is its absolute temperature, R is the specific gas constant, and b is the actual volume of 1 kg of the molecules. See also perfect gas.
Clausius inequality (Clausius theorem) See second law of thermodynamics.
Clausius statement See second law of thermodynamics.
clean room A specially-prepared room in which a controlled atmosphere may be achieved, free of dust particles and other contaminants. Used for the assembly of delicate equipment, semi-conductor devices and the preparation of composite components.
clearance (Unit m or μm) 1. The distance (if any) between mating components in an assembly. 2. The distance between two moving parts, or a moving part and stationary part, in a machine (e.g. the gap between a piston and a cylinder head). 3. With threads, the major clearance is the distance between the design form at the root of an internal thread and the crest of its mating external thread; the minor clearance is the corresponding dimension between the crest of an internal thread and the root of the external thread. See also screw.
clearance angle (relief angle) (Unit °) The angle between the underneath or flank of a cutting tool and the machined surface.
clearance fit A range of clearances ranging from close sliding to loose running, i.e. a fit in which the limits for the mating parts always permit assembly. See also limits and fits.
clearance hole A hole of specified size such that a bolt, stud, etc. of the same nominal size will always pass through.
clearance volume (Unit m3) The ‘dead’ volume above a piston, including the recess in the cylinder head, in a reciprocating compressor or engine when the piston is at top dead centre.
clearness index The ratio of the solar radiation received by a horizontal surface in a prescribed period (usually one day) to the radiation that would have been received by a parallel extra-terrestrial surface in the same period.
cleavage fracture A fracture created by splitting (cleavage), as between layers in materials like slate or mica. In brittle metals, and brittle microconstituents in alloys, cleavage occurs along particular crystal planes. See also ductile–brittle transition.
clevis A U-shaped hook with holes at the ends through which a retaining bolt or pin (clevis pin) passes.
climb See dislocation.
climb milling See down milling.
clinometer See inclinometer.
clip gauge A displacement gauge consisting of two thin strain-gauged cantilever arms attached through knife edges to a testpiece to give the load–line displacement in fracture mechanics test pieces, or used as an extensometer in tensile tests.
clock gauge See dial gauge.
closed cycle A thermodynamic cycle in which the working fluid is returned to the initial state at the end of the cycle and is recirculated. No fluid is exchanged with the surroundings, although there is energy transfer in the form of work and heat. Examples of idealized cycles include binary vapour cycle, Brayton or Joule cycle (closed-cycle gas turbines), Carnot cycle, combined gas-vapour cycle, Diesel cycle, Ericsson cycle, Otto cycle (petrol engines), Rankine cycle (closed-cycle steam turbines), Stirling cycle, and vapour-compression refrigeration cycle. See also open cycle.
closed cycle turbine
closed-die forging The forming of a workpiece by compression within a pair of dies having the female form of the component to be manufactured (closed dies), superfluous metal being expelled as flash where the dies meet. See also open-die forging.
closed feedwater heater See regenerative Rankine cycle.
closed kinematic pair A kinematic pair where continuous contact between members is ensured by the constraints, as in pinned links.
closed loop A closed loop uses a signal from the output of a controlled plant to modify the input so as to attempt to make the output match a specified set point irrespective of disturbances. A closed-loop control system is one using one or more closed loops.
closed-loop gain The gain of a controller when the feedback loop is connected.
closed pair A pair of bodies both constrained so that no relative motion is possible.
closed system A closed thermodynamic system consists of a fixed amount of mass. No mass can cross its boundary although energy can, in the form of work or heat, and its volume can change. See also isolated system; open system.
close-off rating The maximum pressure difference to which a control valve may be subjected when fully closed. It is usually determined by the available actuator power.
close sliding See limits and fits.
clutch A device for connecting and disconnecting rotating shafts, for example between an engine and a gearbox.
CNC See computer numerical control.
cnoidal wave A long shallow-water wave of permanent type and finite amplitude. The formula for the wave profile involves the second Jacobian elliptic function cn u, cn standing for cosine and leading to the term cnoidal. Such waves are characterized by sharp peaks and flat troughs.
coalescence-type separator A steam separator in which wet steam passes through an obstruction, such as a demister pad. Entrapped water droplets coalesce and fall to the bottom of the separator.
Coanda effect The tendency of a fluid jet flowing close to a flat or curved solid surface to attach to it due to entrainment of fluid into the jet.
coarse thread See screw.
coated abrasive An abrasive tool consisting of a flexible backing material, such as a woven cloth, paper or vulcanized fibre, a bond material, such as a glue or synthetic resin, and grit. See also bonded abrasive.
coaxial A term for components having a common axis such as concentric shafts.
Coble creep See creep mechanisms.
COD See crack tip opening displacement.
CODATA The Committee on Data for Science and Technology, which in 2006 provided a self-consistent set of values for the fundamental physical constants (including the Avogadro, Boltzmann, molar gas, Planck, and Stefan–Boltzmann constants) for international use. Minor adjustments to the values of these constants were made effective on 20 May 2019. See also International System of Units
http://www.codata.info/resources/databases/key1.html
• Internationally recommended values of the fundamental physical constants
https://www.bipm.org./utils/common/pdf/si-brochure-draft-2016b.pdf
• Full details of the International System of Units. Draft of ninth brochure.
coefficient of area expansion See thermal expansion.
coefficient of bulk viscosity See bulk viscosity.
coefficient of cubical expansion See thermal expansion.
coefficient of discharge (CD) For flow through a nozzle or orifice plate, the ratio of the actual mass flow rate to the theoretical mass flow rate calculated assuming the flow to be isentropic. The coefficient of velocity (velocity coefficient) is the corresponding ratio of the actual average velocity to the theoretical value. For incompressible flow, the theoretical flow rate and velocity can be calculated using Bernoulli’s equation. See also contraction coefficient.
coefficient of friction (coefficient of kinetic friction, coefficient of sliding friction, friction coefficient, μ) The ratio of the frictional force F to the normal force N between two surfaces in contact, i.e. μ = F/N. Static friction is when there is no relative sliding; kinetic friction when there is. See also Amontons friction.
coefficient of heat transfer See heat transfer.
coefficient of linear expansion See thermal expansion.
coefficient of mass transfer See mass transfer.
coefficient of performance (COP) A measure of the efficiency of a refrigerator or heat pump, defined as the ratio of the desired output to the required input. For a refrigerator, 
where 
is the rate of removal of heat from the refrigerated space and Ẇ is the power input into the compressor. For a refrigeration cycle, Ẇ = H - L where H is the rate at which heat is transferred from the condenser to the surroundings. Thus 
and it can be seen that COPR can exceed unity. For a heat pump, COPH = 
where H is the rate of heat transfer from the heat pump and Ẇ is again the power input into the compressor, and 
.
coefficient of permeability See permeability.
coefficient of restitution (e) The ratio of the relative velocity of two colliding bodies after collision to that before. In perfectly elastic collisions e = 1; when all the impact energy is dissipated, e = 0.
coefficient of rigidity See shear modulus.
coefficient of rolling friction The ratio of force parallel to a surface, on which an object rolls, to the normal force. Unlike sliding friction, rolling friction depends on the size of the contact patch and the radius of the rolling element, and the behaviour depends on whether the contact is elastic, viscoelastic, or plastic and on hysteresis losses.
coefficient of superficial expansion See thermal expansion.
coefficient of thermal expansion See thermal expansion.
coefficient of velocity See coefficient of discharge.
coefficient of volumetric expansion See thermal expansion.
coefficients of expansion See thermal expansion.
coextrusion The simultaneous extrusion through the same die of two or more materials in combination. See also pultrusion.
Coffin–Manson–Tavernelli relation See fatigue.
cog A tooth on the edge of a wheel (cog wheel), a series of which form a gear. Often applied to gears where the teeth are rudimentary and do not have involute or other precise form.
COGAS plant See combined gas and steam plant.
cogeneration plant (COGEN) See combined heat and power plant.
coherent structure A term given to the larger eddies of turbulent shear flow, such as boundary layers, jets, and wakes, that show distinctive correlated patterns of motion.
coherent derived units See International System of Units.
cohesive strength (Unit Pa) A theoretical fracture strength for solids based on interatomic forces, approximately equal to E/10 where E is Young’s modulus.
cohesive zone In fracture-mechanics modelling and simulation, the region at the crack tip over which an assumed traction (load-displacement) relation has to be overcome to permit initiation and propagation of a crack.
coil spring A spiral (‘clockwork’) or helical (cylindrical) spring.
coining A forging operation, employing a closely-fitting punch and die from which no metal is allowed to escape, in which the surface pattern on the punch and die is imprinted on the blank.
Colburn equation For fully-developed turbulent flow in a smooth pipe, an empirical formula for the Nusselt number given by Nu = 0.023Re0.8Pr1/3 for Prandtl numbers in the range 0.7 ≤ Pr ≤ 160 and Reynolds numbers greater than 4 000. Nu here is based on the pipe diameter D and Re on D and the bulk velocity of the flow.
Colburn j-factor (heat-transfer j-factor, jH, jM) A non-dimensional parameter that arises in convective-heat-transfer analysis and is defined as jH = St Pr 2/3 where St is the Stanton number and Pr is the Prandtl number. For mass transfer, the definition is jM = StM Sc 2/3 where StM is the Stanton number for mass transfer and Sc is the Schmidt number.
cold-air standard assumptions Air-standard assumptions for which the air has constant specific heats specified at 25°C.
cold joint See soldering.
cold trap A device used to trap vapour from water and other solvents in a gas flow by passing it through stainless-steel or glass tubing cooled by liquid nitrogen, dry ice, a dry ice–alcohol slurry, or a dry ice–acetone slurry. The vapour condenses to liquid or ice crystals on the tube wall. A common application is to the inlet or outlet of a vacuum pump.
cold working The plastic deformation of a metal, by rolling (cold rolling), drawing, forging (cold forging), etc. at a temperature well below its recrystallization temperature, which results not only in permanent shape change but also increase in strength and loss of ductility owing to work-hardening. See also hot working; strain hardening; warm working.
Colebrook equation An empirical correlation for the Darcy friction factor fD as a function of Reynolds number Re in fully developed incompressible turbulent flow through smooth and rough pipes, and the basis for the Moody chart. It can be written as
where D is the pipe diameter, ε represents the equivalent roughness of the pipe-wall surface, and Re is based upon D and the bulk velocity of the flow. See also Blasius pipe-friction law.
collapse load (Unit N) The applied load at which a structure becomes a mechanism owing to the formation of sufficient plastic hinges for collapse to occur. See also plastic design.
collapse mechanism The pattern and location of plastic hinges in a structure that would permit it to become a mechanism and thereby collapse. Various patterns may be envisaged for a given structure: that requiring least work (least collapse load over the same displacement) is the natural one.
collar A ring secured to, or integral with, a shaft to give axial location. See also circlip.
collar bearing See bearing.
collector See solar collector.
collector efficiency See capture efficiency.
collet A type of chuck for holding small, circular workpieces or tools. It consists of a slotted sleeve with an external cone at the gripping end that holds the workpiece or tool when pulled against an internal cone on the lathe mandrel.
colloidal system A system in which microscopic particles are dispersed uniformly throughout another substance. Both the dispersed and the continuous phases may be solid, liquid or gas (although a gas‐gas combination is not possible). See also aerosol; emulsion; foam; gel; hydrosol; sol.
colour temperature (Unit K) The absolute temperature of an ideal blackbody that radiates light of the same colour as a given light source. The range is from about 1 000 K (candlelight) to about 20 000 K (blue sky). For forging and hardening of steel, the range is from 873 K (brown-red) to 1 473 K (white), and for tempering from 483 K (light yellow) to 603 K (grey-green).
column A vertical member of a structure that withstands axial compressive loads. See also prop; strut.
column ends See fixity.
combined dimensioning The use of chain dimensioning and parallel dimensioning on the same engineering drawing.
combined gas and steam plant (COGAS plant, combined cycle power plant, CCPP, combined cycle gas turbine plant, CCGT plant) A power plant in which the exhaust gases from a gas turbine are used in the production of steam for a steam turbine. COGAS plants are commonly used for marine propulsion. See also combined heat and power plant.
combined gas–vapour cycle The thermodynamic cycle on which a combined gas and steam plant operates, such as that shown in the schematic diagram for an open-cycle gas turbine and a closed-cycle steam turbine. Exhaust gas from the gas turbine is the heat source for the water boiler. The gas and steam cycles are shown superimposed on a temperature (T) vs specific entropy (s) plot.
combined gas–vapour cycle (COGAS)
combined heat and power plant (CHP plant, COGEN, cogeneration plant, total-energy plant) A plant for the simultaneous production of more than one useful form of energy from the same energy source, such as process heat and electric power. In the arrangement shown, partially expanded steam is extracted from the steam turbine and used to produce process heat.
combined heat and power plant (COGEN)
combined stresses The stress state at a point in a component subjected to combination of axial, bending, torsional loadings etc., acting along all reference axes.
combustible (inflammable) A term for substances that can be ignited and burned.
combustion An exothermic chemical reaction in which a fuel and an oxidant, typically air, react together to release a significant quantity of thermal energy in the presence of a flame. In the case of fluid fuels, ignition may be from an electric spark or compression of the fuel–oxidizer mixture. Solid fuels are usually ignited by a small flame. See also combustion efficiency.
combustion chamber 1. In a piston engine, the volume between the head of an individual cylinder and the crown of the piston in which the fuel–air mixture burns during each power stroke of the engine. 2. (combustor) In a gas-turbine engine, a ramjet, an afterburner, or a rocket motor, the component, often cylindrical or annular in shape, in which the fuel–air mixture burns in a continuous-flow process.
combustion deposit Ash, carbon, and other incombustible solids, often due to impurities in the fuel, that build up on any surfaces exposed to products of combustion. They can lead to corrosion, reduced heat transfer and so higher flue-gas temperatures and reduced efficiency.
combustion efficiency (source efficiency, ηCOMB) A performance measure for combustion equipment defined by ηCOMB = amount of heat released during combustion/(calorific value of the fuel burned).
combustion safeguard See thermal flame safeguard.
command The set point input to a controller.
comminution The fracture or pulverization of brittle materials into small fragments by passing the material through rollers, ball mills, or grinders.
common normal See toothed gearing.
community heating See district heating.
comparator 1. In a control system, a two-input device that determines which of the two inputs is of greater magnitude. 2. A device used to compare an object, typically during manufacture, against a standard master example. See also mechanical comparator; optical comparator.
compatibility conditions (compatibility equations) 1. Mathematical relations between strains in solid mechanics that must arise because the six components of strain at a point can be expressed in terms of only three coordinate displacements. May be expressed in Cartesian, cylindrical, etc. coordinate systems. 2. In the solution of the equations for two-dimensional supersonic flow, which is governed by a hyperbolic partial differential equation, the relationships between the Prandtl–Meyer function ν and the local flow direction θ: ν ± θ are both constant along characteristics. Similarly in the theory of plasticity, slip-line fields are orthogonal curvilinear lines (characteristics) along which p ± 2kφ are constant, where p is hydrostatic stress, k is the shear yield stress, and φ is the inclination of the characteristics.
compensation The improvement of the performance of a control system through the use of an additional control element, a compensator, frequently designed to modify the root locus of the system.
complementary strain energy (Unit J) The complementary strain energy is given by 
where δ is displacement and P is load. In contrast, the strain energy is given by 
. In linear systems they have the same value, but in non-linear systems they do not. See also Castigliano’s theorems; fracture mechanics.
complex modulus See non-Newtonian fluid.
complex pendulum A system of two or more simple pendulums connected end-to-end. The resulting oscillations are chaotic.
complex plane The graphical representation of complex quantities, where the real parts are plotted on the abscissa and the imaginary parts on the ordinate. Used, for example, in control-system design to produce the pole-zero plot and in potential-flow theory. See also conformal mapping.
complex viscosity See non-Newtonian fluid.
compliance (Unit m/N) 1. The reciprocal of stiffness, being the displacement resulting from the application of unit load. In the case of beams, flexibility is an alternative term used for compliance. 2. For a spring, the inverse of spring rate.
component See microconstituent.
component pressure See Dalton’s law.
component volume See Amagat’s law.
composite material (composite structure) A general term used of two or more materials or structures acting in combination (e.g. concrete, reinforced concrete, filament-reinforced polymers, laminated materials, particulate-reinforced materials, flitched beams), resulting in values of strength, stiffness, or toughness greater than the base matrix material alone.
http://www.toolingu.com/class-750125-traditional-composites.html
• Composite materials
composite modulus (E) (Unit Pa) The Young’s modulus of two or more materials in combination is given by the weighted average of the E values for the individual components. When loading parallel to the fibres in a filament-reinforced composite (assuming that the strains in every component are identical) E = ʋf1E1 + ʋf2E2 +…. where ʋf1 etc. are the volume fractions and E1 etc. are the Young’s moduli of the individual components (the Voigt upper bound). The composite strength (σ) is obtained in the same way i.e. σ = ʋf1σ1 + ʋf2σ2 +…. where σ1 etc. are the strengths of the individual components. When loading across the fibres in a filament-reinforced composite (assuming that the stresses in every component are identical) 
i.e. the weighted harmonic mean (the Reuss lower bound). The strength in this case is that of the weakest component.
composite property In thermodynamics, a property defined in terms of the properties of a closed system and its surroundings, such as the non-flow exergy function.
composite wall In heat transfer, a wall consisting of two or more layers of material with different thermal conductivities. For a plane composite wall, with convective heat transfer on either side and an overall temperature difference ΔT, the heat flux 
″ is given by the equation 
where hi and ho are the convective heat transfer coefficients on the outer surfaces, Δi is the thickness of layer i, and ki is its thermal conductivity. The term Δi/ki is called the thermal or conduction resistance while ″/ΔT is called the overall heat-transfer coefficient and given the symbol U, values of which are often quoted for double-glazed windows and cavity walls.
composition The proportions of the constituents of any mixture, chemical compound, alloy, etc. Normally expressed in weight percent (wt%) being the mass of the constituent concerned divided by the total mass and multiplied by 100. Sometimes used, particularly in materials science, is atom percent (at%). For example, for a binary alloy with nX and nY atoms (or moles) of constituent X and Y, respectively, the atom percent of X is 
.
compounding In a steam engine (compound steam engine) or impulse turbine, the progressive reduction in pressure (expansion) across two or more stages in series. See also Curtis stage; pressure-compounded steam turbine; velocity-compounded steam turbine.
compound pendulum A rigid body free to swing about an axis. See also simple pendulum.
compound steam-turbine system A steam-power plant where several turbines are connected together. See also cross-compound steam-turbine system; tandem-compound steam-turbine system.
compressed-air energy storage (CAES) Gas (usually air) is compressed to about 100 bar during periods of low energy demand and stored, generally in an underground cavern such as a disused mine. During periods of high demand the pressurised gas is expanded through a peaking plant (gas turbine) driving a generator.
compressed liquid A liquid subjected to a pressure greater than the saturation pressure corresponding to its temperature. See also subcooled liquid.
compressibility (Unit 1/Pa) A measure of the reduction in volume or increase in density when a substance is subjected to an increase of pressure. It is defined as the reciprocal of the bulk modulus. Liquids and solids are normally considered incompressible, whereas gases are highly compressible. See also bulk modulus; incompressible; isentropic BULK MODULUS; isothermal compressibility.
compressibility factor (Z-factor) A non-dimensional parameter, dependent upon reduced temperature and reduced pressure, employed to indicate deviations from the ideal-gas equation (Z ≠ 1), and given by Z = pʋ/RT = p/ρRT, where p is the gas pressure, ʋ is the specific volume of the gas and ρ (= 1/ʋ) is its density, R is the specific gas constant, and T is the absolute temperature. See also principle of corresponding states; perfect gas.
compressible flow A gas flow in which the Mach number M is sufficiently high for the gas density to change significantly. For air, this is when M > 0.3.
compression 1. Loading, the principal effect of which is to squeeze and shorten a component or testpiece. 2. The reduction in volume and increase in density of a substance as a consequence of increased pressure.
compressional wave speed (c) (Unit m/s) In a uniform, isotropic, elastic solid, plane compression (and tension) waves travel at speed 
where K is the isentropic bulk modulus of the solid, G is its shear modulus, and ρ is its density. In a long rod, thin enough that Poisson’s-ratio effects do not produce lateral stresses to curve the wave front, 
where E is Young’s modulus.
compression crease A crease formed during the compression of composites having a high volume fraction of filaments. These composites fail in compression by forming a crease at an angle to the loading axis.
compression failure The reduction or removal of a component’s load-bearing capacity in compression, caused by buckling, fracture, crease formation in fibre composites, etc.
compression fitting A screwed joint for pipework made resistant to leakage by permanent deformation of a closely-fitting ring, called a ferrule or olive, on tightening.
compression-ignition engine See Diesel engine.
compression member A structural component, the major loading on which is compressive. See also column; prop; strut.
compression modulus See bulk modulus.
compression moulding The manufacture of components by moulding plastics (polymers) in granular or pellet form using heat and pressure. See also rotational moulding; transfer moulding.
compression pressure (Unit Pa) The pressure produced in a cylinder of a piston engine by compression of air in the absence of fuel.
compression ratio For a piston engine, if the swept volume is 𝓥SW and the clearance volume is 𝓥CL, the compression ratio is given by (𝓥SW + 𝓥CL)/𝓥CL, i.e. it is a volume ratio rather than a pressure ratio.
compression refrigeration See vapour-compression refrigeration cycle.
compression ring See piston.
compression spring A spring that resists compression forces, usually in the form of a helix with separated coils (giving a linear axial stiffness) or a cone with separated coils (giving a non-linear axial stiffness). See also Belleville washer.
compression stroke The stroke in a reciprocating compressor or engine during which the working fluid is compressed.
compression test The determination of the stress–strain curve of a material by axial loading of a specimen in compression. In brittle materials failure is in the elastic range; more ductile materials will yield before fracture; very ductile materials will plastically deform extensively before failure. The compression strength (compressive strength), with unit Pa, is the compressive stress that causes failure in a component or structure. See also crushing strain.
compression wave (dilatation wave) In a fluid or a solid, a progressive wave or wavefront that compresses the medium through which it propagates. See also rarefaction wave; shear wave.
compressive stress (Unit Pa) The compressive load per unit area at a point in a component. See also force.
compressor A turbomachine, of either axial or radial type, that increases the pressure of a gas or vapour.
compressor blades The aerofoil-shaped vanes that form the rotor(s) and stator(s) of an axial-flow compressor. The aerodynamic design is more critical than is the case for turbine blades because there is an increase in pressure across each row of blades. The arrows in the diagram indicate the flow direction relative to the blades. See also velocity triangle.
compressor blades
compressor bleed The removal of air before the final stage of a multistage compressor operating below design speed, to prevent the final stage from choking.
Comprex® A type of supercharger comprising a cylindrical rotor with longitudinal channels which is belt driven from the crankshaft of an internal-combustion engine. The inlet air is compressed by wave motion within the channels.
computational domain In CFD, FEM, and other numerical methods, the area or volume within which calculations are performed and on the periphery of which the boundary conditions are specified.
computational fluid dynamics (CFD) The solution of fluid mechanics problems using direct numerical simulation (DNS) or a system of algebraic equations, based upon finite-difference, finite-volume, or other discretization schemes to approximate the differential equations and the constitutive equations that govern fluid motion. See also turbulent flow.
computed path control In CNC or robotics, the use of a control program to determine the required path for the tool or end effector. This required path forms the set points for the motor controllers moving the machine tool or robot.
computer-aided design (CAD) Generally, design and calculations performed by computer; more specifically, the use of computer graphics and models to communicate design concepts. Computer-aided design and computer-aided manufacturing (CAD/CAM) is where component dimensions resulting from CAD are passed by electronic means directly to machines for manufacture.
computer-aided engineering (CAE) The non-trivial use of computers in all branches of engineering, for example to solve equations, create engineering drawings and schematic diagrams, apply CAD/CAM, CFD, and other software packages.
computer-aided manufacturing (CAM, computer-integrated manufacturing, CIM) The use of computers in all branches of manufacturing, not only to control machines and robots for manufacturing and assembly, but also for process planning, and monitoring progress of materials and components during production, etc.
computer control The control of an engineering device or system by pre-programmed computer or by feedback control.
computer numerical control (CNC) A term relating to machine tools in which the movements of a tool and/or the workpiece are controlled by computer.
computer vision The digitization and processing of optical images/patterns by computer in order to recognize parts, orientation, etc. in manufacturing.
concentrated load A load on a component which is distributed over a very small area, idealized as the line load of a wedge or knife edge, and the point load of a cone.
concentrated solar power plant (CSP plant) A power plant in which solar radiation is concentrated using mirrors or lenses, typically using parabolic trough mirrors which focus solar radiation on to receiver tubes along the trough’s focal line. In an alternative arrangement molten salt, heated by solar receivers mounted on a tower, is circulated through a steam generator.
concentration ratio For a concentrating solar collector, the ratio of the projected area of the concentrator facing the solar beam to the actual area of the receiver.
concentrator (concentrating solar collector) See solar concentrator.
concurrent engineering The integration of the procedures for product design, material selection and manufacturing method to include life-cycle analysis.
condensate strainer A filter in a steam plant used to remove particulate matter from condensate before it is added to feedwater.
condensation 1. The change of vapour into the liquid state when its temperature falls below the saturation temperature TSAT. This usually occurs on a surface having a temperature (condensation point, liquefaction point) below TSAT but can also occur spontaneously throughout the vapour. See also dropwise condensation; film condensation. 2. (condensate) A liquid that has condensed from a vapour. 3. A measure of the strength of a shock wave defined as the increase in gas density across the shock divided by the density of the gas upstream of the shock.
condensation shock For supersonic flow of a moist gas through a divergent nozzle, condensation occurs in the form of spontaneous nucleation at some point downstream of that at which the temperature falls to the saturation temperature. The condensation process proceeds rapidly, and results in a fairly thick discontinuity termed a condensation shock.
condensation trail See vapour trail.
condenser A heat exchanger in which a substance is changed from its vapour phase to its liquid phase by reducing its temperature to below the saturation temperature. See also shell-and-tube condenser; steam condenser; surface condenser.
condenser vacuum The sub-atmospheric pressure imposed on the condenser of a steam-power plant which leads to an appreciable increase in overall efficiency.
condensing boiler A relatively small industrial or domestic boiler that burns sulfur-free natural gas so that the products of combustion do not contain sulfuric acid and can be allowed to condense on heat-transfer surfaces without danger of corrosion.
conductance See heat transfer.
conducting polymer See electrical conductivity.
conduction resistance See heat transfer.
conduction shape factor (S) (Unit m) For a solid body with two surfaces maintained at constant temperatures T and T + ΔT, the rate of conductive heat transfer 
between the surfaces is given by 
where k is the constant thermal conductivity of the solid medium and S is a factor that accounts for the geometry of the body, e.g. for a sphere of diameter D a distance z (> D/2) below the surface of a semi-infinite solid, S = 2πD/(1 − D/4z).
conduction thickness See heat transfer.
conduit A hollow object, of metal, plastic, glass, ceramic, etc., which is relatively long compared with its lateral dimensions and through which there is fluid flow. A tube is a closed conduit assumed, unless otherwise stated (e.g. Dall tube, Venturi tube), to be of constant, circular cross section. The term tube is often used interchangeably with pipe although the latter is normally assumed to be rigid whereas a tube may be flexible. A hose is a highly flexible tube, often made from rubber or polymers, used to carry gases, liquids, or slurries. Hoses are sometimes reinforced to allow high pressure and to protect against wear. A particular application is to convey oil and water from one part of an engine to another, e.g. water from the engine block to the radiator. A connected system of pipes is termed piping or tubing. A pipeline is a pipe, often with an internal diameter up to 2 m, used to transport such fluids as natural gas, oil, and water over long distances (often thousands of kilometres). The more general term duct is used for a closed conduit of any cross section and may include nozzles, diffusers, and confusers. A capillary tube is a circular tube, often of glass, with an internal diameter of about 1 mm. A hole bored through or cast into an object, such as a cylinder block, to permit flow is termed a flow channel (flow passage). An open channel is a conduit which is open to the atmosphere and through which there is flow of a liquid (usually water) with a free surface. A flume is an open water channel, typically rectangular in cross section, used to study the behaviour of marine craft, aircraft, structures, wave motion, etc. in a laboratory. See also capillary tube; hydraulic diameter; Venturi flume.
cone bearing See bearing.
cone clutch A friction clutch in which an internal cone moves axially in or out of engagement with an external cone. One or both surfaces is lined with high-friction material.
cone pulley A stepped pulley having several diameters which, when linked by a laterally-moveable transmission belt to a corresponding pulley, gives a series of speed ratios.
configuration In robotics, the description of the structure of a robot in terms of the type of each joint (i.e. translational or rotational) and the directions of the joint axes. There are five standard robot configurations: articulated (revolute), Cartesian-coordinate, cylindrical-coordinate, SCARA, and spherical-coordinate.
configuration factor See view factor.
confined flow The flow of a fluid through a pipe or duct, or flow within a closed container, for example a short cylinder with one end closed and the other end being a closely-fitting rotating disc.
conformal mapping (conformal transformation) The transformation, in two-dimensional potential-flow theory, of a function in one complex plane to another. This transformation preserves angles between lines in the two planes. See also Joukowski transformation.
confuser A converging duct. The opposite of a diffuser.
congruent melting point (Unit K) The temperature at which a solid substance at a specified pressure changes phase to a liquid of identical composition.
conical pendulum A pendulum in which the bob rotates in a horizontal circle with constant angular velocity about the vertical axis.
conical spring An open-coiled spring in the form of a cone which gives a non-linear axial stiffness. See also coil spring.
conjugate action See toothed gearing.
conjugate teeth profile See toothed gearing.
connecting rod (con rod) A link that transmits power from one system to another, often changing linear to rotary motion, as in the rod connecting the piston to the crankshaft in a reciprocating compressor or pump or to the crankshaft in an internal-combustion engine, as in the diagram. The big end (bottom end) is the larger end that connects to the bearing on one of the crankpins of the crankshaft. The little end (small end) is joined by a gudgeon pin to the piston.
connecting rod
conservation equations The four equations or principles which form the basis of Newtonian mechanics are (a) matter cannot be created or destroyed (conservation of mass); (b) energy cannot be created or destroyed but may change its form (conservation of energy), a statement which derives directly from the first law of thermodynamics; (c) the total vector momentum remains constant in a system where bodies are subjected only to forces exerted by other bodies in the same system (conservation of linear momentum). This statement derives directly from Newton’s second law of motion; (d) the total vector angular momentum remains constant in a system where bodies are subjected only to forces exerted by other bodies in the same system (conservation of angular momentum).
For fluid flow, each of the four equations is expressed in terms of the time rate of change of the conserved property contained in a control volume (CV), the net flow rate of that property across the surfaces of this volume (CS), and the effects of any external forces (linear-momentum conservation) or external moments (angular-momentum conservation) acting on CV or the net rate of heat transfer across CS less the power input into CV (energy conservation). If the vector velocity is V and the density of the fluid is ρ, then the mass-conservation equation (continuity equation) can be written as
where 𝓥 represents the volume and A the surface area of CV and n is the unit normal vector directed out from the control surface.
The momentum-conservation equation can be written as
where ΣF represents the sum of all the external forces acting on CV. The angular-momentum-conservation equation can be written as
where r is the position vector from the axis of rotation to the elemental mass ρd𝓥 and ΣM represents the sum of all the external moments acting on CV.
The energy-conservation equation can be written as
where the heat-transfer rate in,CV is the net rate of energy exchange between the control volume and its surroundings because of temperature differences, and Ẇin,CV is the work transfer rate into the control volume by the surroundings, i.e. the power input. Ẇin,CV includes power transferred across CS by a rotating shaft, by fluid pressure, and by shear stress. If u represents the specific internal energy of the fluid, g the acceleration due to gravity, and z the height above a datum level, then the stored energy 
, i.e. the sum of u, the kinetic energy per unit mass and the potential energy per unit mass.
A generalized differential form of the conservation equations may be written as
where u is the velocity vector, xi represents the spatial coordinates x (for i=1), y (i=2), and z (i=3), φ is one of many dependent variables, including momentum per unit mass (or velocity), mass of chemical species per unit mass of mixture (or concentration), and specific enthalpy. The symbol Γφ represents the effective (i.e. laminar plus turbulent) diffusional-exchange coefficient, and Sφ is the source of the relevant conserved property per unit volume.
conservative force field A system of forces where the work done on a body depends only upon the initial and final states, and is not dependent on the path taken (i.e. the process is reversible).
conservative property A property of a system whose value is invariant when other parameters vary.
consistency index See non-Newtonian fluid.
consistency number (NLGI consistency number, penetration number) An empirical measure of the softness or consistency of a material, such as a grease, obtained by allowing a weighted cone to penetrate into the material for a specified time at a specified temperature.
consolute temperature (Unit K) The temperature at which two partially miscible liquids become fully miscible. Liquids which are fully miscible at high temperatures but separate into two liquid phases at lower temperatures have an upper consolute temperature (upper critical solution temperature). Liquids which are fully miscible at low temperatures but separate at higher temperatures have a lower consolute temperature.
constant-force spring A spring that has the same restoring force regardless of displacement. The most common type takes the form of a coiled strip that, owing to tight coiling during manufacture, is pre-stressed (a steel measuring tape is an example). The uncoiling force is approximately constant as the change of curvature of the strip is approximately constant. Not to be confused with a clockwork spring from which power can be obtained.
constant-mesh gearbox A gearbox in which the pairs of gears giving different speed ratios are constantly in mesh, different ratios being obtained by connecting or disconnecting the relevant gear to the driving shaft.
constant-pressure gas thermometer An apparatus based on Charles law in which a rigid vessel is filled with a gas, usually hydrogen or helium, at low pressure and its volume measured as its temperature is increased while its pressure is maintained constant. The device must be calibrated at two fixed points, such as the ice and steam points. A constant-volume gas thermometer (gas thermometer) is similar but based on Boyle’s law. The gas pressure is measured as its temperature is increased while its volume is maintained constant.
constant-speed propeller See propeller.
constant-velocity universal joint (CV joint, homokinetic joint) A connexion that transmits constant angular velocity between two shafts that are neither necessarily in line nor whose axial position is necessarily fixed. See also universal joint.
constitutive equation (constitutive relation) 1. In solid mechanics or fluid mechanics, an algebraic or numerical relation for the dependency of stress on deformation, strain, strain rate, temperature, etc. in a material. In solid mechanics such a relation is sometimes called an equation of state. Simple linear examples include Hooke’s law and Newton’s viscosity law. 2. In heat transfer, a relation, such as Fourier’s law of heat conduction, connecting heat flux with temperature gradient. See also Fick’s law.
constraint The restriction of one or more natural degrees of freedom of a system.
constriction A reduction in the cross- sectional area of a pipe or duct which, especially if sudden rather than gradual, can significantly reduce the flow rate for a given pressure difference.
constructive interference See destructive interference.
contact angle (wetting angle, ϕ) (Unit °) At a gas–liquid–solid interface, the angle that the tangent to the liquid surface makes with the solid surface at the point of contact. If ϕ < 90°, the liquid is said to wet the surface. For water–glass–air ϕ is practically zero, for kerosene–glass–air it is about 26° and for mercury–glass–air 130°. The contact line is the interface between a liquid and a gas on a solid surface. See also hydrophilic; hydrophobic; surface tension; Young’s equation.
contact area (contact patch) When two bodies having flat or curved surfaces are pressed together, the region where the surfaces touch is deformed (‘locally flattened’) into, for example, a rectangular contact area (two parallel cylinders in contact), a circular area (two spheres in contact) or generally an ellipse. The term is commonly used for the area of a tyre in contact with the surface of a road. See also contact mechanics; rolling friction.
contact fatigue strength See fatigue.
contact gear ratio (contact ratio) See toothed gearing.
contact mechanics The determination of surface and sub-surface stresses and strains when bodies in contact are loaded. For normal loading within the elastic range, the surface pressure distribution p over a general elliptical contact patch having semi-axes a and b is given by the ordinates of a semi-ellipsoid contained by the surface of contact, viz: p = p0√[1 − (x/a)−2 − (y/b)−2] where p0 is the peak pressure. The total load F is given by the volume of the ellipsoid, i.e. F = 2πabp0/3, so that the peak pressure is 1.5 times the average pressure F/πab. The greatest shear stress, where plastic flow is likely to initiate, occurs beneath the surface at a depth of some 0.79b in the particular case of two cylinders in contact. In the presence of shear loading as well as normal loading (as in gearing), the location of the maximum shear stress moves nearer to the surface and, for coefficients of friction greater than about 0.1, is in the surface.
contact patch See contact area.
contact ratio See toothed gearing.
contact resistance See thermal-contact resistance.
contact strength (Unit N) The maximum allowable load between contacting bodies converted into a stress.
contact stresses See contact mechanics.
contact surface 1. A surface that separates two fluids of different properties, such as water and air, hot air and cold air, or nitrogen and helium. It may be idealized as a surface of discontinuity (slip surface), although for miscible fluids diffusion leads to thickening e.g. for gas–gas contact surfaces. 2. The flattened surface at the contact area between bodies pressed together.
continuity equation See conservation equations.
continuous casting The casting of metal in a continuous stream rather than into ingots.
continuous cooling curve See time–temperature transformation diagram.
continuous fibre ceramic composites (CFCC) Composite materials, such as silicon carbide (SiC) fibres in a matrix of polycrystalline alumina (Al2O3).
continuously-variable transmission (CVT, infinitely-variable transmission) A gearbox that can change steplessly through an infinite number of gear ratios between maximum and minimum values.
continuous path control Control of the position of a robot end effector where the required position is specified at all times, rather than as a series of discrete steps. For example, a robot performing arc welding would require continuous path control so that it could follow the seam being welded.
continuous time A term referring to a system in which information is continuously accessible, rather than being available only at discrete time intervals as occurs with a sampled-data system.
continuum hypothesis (continuum assumption) The assumption that for any substance, a length scale exists which is much larger than the largest scale at which the molecular structure is important, but smaller than the scale at which there are significant spatial property variations, e.g. due to temperature or pressure variations or inhomogeneity of the substance itself or, in polycrystalline solids, anisotropy caused by different orientations of individual grains. In the majority of practical situations such a length scale is found to exist (typically order 1 μm) for most fluids and solids so that mathematical modelling of material behaviour is possible which takes no account of the molecular structure. The continuum hypothesis allows the definition of material properties, velocity, stress, etc. at any point within a substance. Exceptions in fluid mechanics include rarefied gas flow and the flow of fluids in microchannels where the mean-free path of the fluid molecules is of the same order as the flow dimensions. Exceptions in solid mechanics include the deformation of thin sheet material having a grain size comparable with the sheet thickness.
continuum mechanics The subject concerning the deformation of substances obeying the continuum hypothesis when subjected to shear stresses, normal stresses (including pressure), and body stresses (i.e. body forces per unit volume). There are two formulations:
Eulerian description (velocity-field description) Because fluids deform continuously, the preferred method of analysis for fluid flow in which consideration is given to the variations in density, pressure, velocity, etc. at a fixed point in space, i.e. with respect to a fixed frame of reference. The linear momentum equation leads to Cauchy’s equation
where 
represents stress (normal stress if i = j and shear stress if 
), p is static pressure, 
is the Kronecker delta, ρ is fluid density, and fi is the body force per unit mass (that due to the acceleration due to gravity is gi). The term 
represents particle acceleration taking into account changes in velocity vi with time t, 
, and changes in position 
.
The stress components are related to the strain rates induced in a fluid by the constitutive equations. For a Newtonian fluid these are Stokes constitutive equations. Cauchy’s equation for a Newtonian fluid, combined with the equation of mass conservation, then leads to the Navier–Stokes equations. More complex constitutive equations allow the analysis of the flow of non-Newtonian fluids. Introduction of an equation for the conservation of energy is necessary for problems involving heat transfer. See also substantial derivative.
Lagrangian description The analysis of fluid flow and finite-deformation plasticity following the motion of individual fluid particles or material elements as they move along pathlines. Being referred to particles and not to space, accelerations ai are given by simple differentiation of velocity components with respect to time t, i.e. 
. When applied to the dynamic equilibrium equations of solid mechanics, we obtain
where 
. See also Stokes hypothesis.
contraction 1. A reduction in the volume of a solid object, frequently due to a temperature reduction. 2. A reduction in the cross-sectional area of a flow channel, usually gradual (for example, the confuser in a wind tunnel).
contraction coefficient (CC) A factor that accounts for the area reduction of flow through a sharp-edged orifice. It is defined by CC = AV/AO where AO is the orifice area and AV is the minimum jet area. The section of the jet with area AV is termed the vena contracta. See also coefficient of discharge.
contraction crack A crack in a casting formed during cooling.
contraflexure See point of contraflexure.
contrail See vapour trail.
contrarotating propellers Two propellers mounted one behind the other coaxially on the same shaft but rotating in opposite directions. The second propeller counteracts the swirling flow produced by the first.
contrate gear See toothed gearing.
control See control system.
control accuracy See control system.
control element See control system.
control flow See fluidics.
controllability See control system.
controllable-pitch propeller See propeller.
controlled-rolled steel A high-strength, low-alloy (HSLA) steel with a microstructure of bainitic acicular ferrite. See also steel microstructures.
controlled-strain mode See rotational viscometer.
controlled-stress mode See rotational viscometer.
controlled variable 1. (independent variable) In an experiment, a quantity that is kept constant or otherwise controlled. 2. See control system.
controller See control system.
controller–structure interaction The interaction between the incomplete model in a controller and the full plant dynamics. Where a controller uses a mathematical model of the controlled plant, a reduction in the performance of the controller can be caused by feedback of signals from the sensor which represent unmodelled characteristics of the plant behaviour. As a result, the controller can excite these unmodelled characteristics in the plant.
control surface 1. The bounding surface of a control volume. 2. In a control system, where the action of a controller is different for different set points or plant outputs, the control surface determines where switching between different control actions occurs. 3. A moving surface exposed to the airflow over an aircraft, including the ailerons, elevators, flaps, and rudder, used by the pilot to control the aircraft’s flight.
control system A collection of components used to ensure that the output of some system, referred to as the plant, behaves in a required way. The plant being controlled may have a single parameter as the output, such as the temperature of an oven, or numerous parameters, such as the angles of each joint of a robot. The control system takes information on the required plant output, known as the control system reference input or set point, and determines the input to be supplied to the plant to best achieve the required output.
Control systems have one of two different modes of operation: open-loop or closed-loop. In an open-loop control system, the input is determined from the reference input using a mathematical model of the plant behaviour. If the mathematical model is accurate and there are no external disturbances affecting the plant, the actual plant output, also known as the control variable or the controlled variable, will achieve the required value. In practice, models are imperfect and disturbances affect the plant so that the output will not accurately reach the required value. Closed-loop control attempts to overcome the problems caused by an imperfect model and disturbances by measuring the output using one or more sensors and evaluating the error in the output, which is the difference between the reference input and the actual output. This error is then used by the control system to determine the necessary plant input. The use of the error requires a signal to be fed back from the output to the control-system input, and thus closed-loop control is frequently referred to as feedback control, with the signal fed back being the control-system feedback. The control accuracy is the inverse of the error. Hence a low error represents a high control accuracy and vice versa.
The parts of a control system that apply the control equations to attempt to achieve the desired output are known as the control elements. For example, in a PID controller three elements are employed: one providing proportional control, the second integral control, and the third derivative control. The controller is the combination of all of the control elements. In designing the elements of a control system, one of two approaches may be taken: classical or modern control. In the classical method (classical control), the plant and control system are each analysed using the Laplace transform of the ratio of the output to the input. The resulting transform is the transfer function which describes the behaviour in the frequency domain, that is, as a function of frequency. Such an approach is only applicable where both the plant and the control system can be represented by linear differential equations. Where modern control design is applied, the plant is represented by its states and the analysis performed directly on the low-order differential equations relating these states. Non-linear behaviour can also be represented and analysed.
Because a knowledge of the behaviour of the plant is necessary when designing the controller, the performance of the controller will deteriorate if the plant behaviour subsequently changes. Adaptive control allows the controller to automatically track changes in the plant and change the controller to accommodate these.
However the controller is designed, there are three primary aims. Firstly, to ensure that the output is bounded, i.e. that the system is stable. Secondly, to ensure that the behaviour when the reference input changes is within specification, i.e. that the transient behaviour is satisfactory. Finally, to ensure that the error after transient behaviour has decayed is satisfactory, i.e. that the steady state error is small.
The formal design of a control system so as to show stability with the transient and error performance within specification despite external disturbances, measurement noise, and modelling errors is known as robust-control design. Many modern controllers are designed using optimal control methods in which one or more performance measures are selected and the parameters in the control system adjusted through to minimize the cost function or maximize the performance measure.
A formal measure of the ability of a system to respond to a change in the reference input (by changing from a specified initial state to specified final state within a finite time) is the controllability.
Although almost all control systems are now implemented using digital electronics and thus use sampled data, equations in continuous time are often used to design such systems. This is satisfactory provided that the interval between samples is short compared to the time taken for the plant to show a significant change in output. When the response of the plant is very rapid, z-transforms are used instead of Laplace transforms to represent sampled data.
• Website of the International Federation of Automatic Control
• Website of the American Automatic Control Council
• Website of the IMech E Journal of Systems and Control Engineering
http://www.theiet.org/communities/control-auto/
• Website of the Institution of Engineering and Technology Control and Automation Network
• Website of the American Institute of Electrical Engineering and Electronics Control Society
• Website of the IEEE Transactions on Automatic Control
control-system feedback See control system.
control variable See control system.
control volume (open system) In thermodynamics and fluid mechanics, a region in space chosen for study and which may move, deform, and allow mass flow across its boundary, called the control surface.
convected energy See flow energy.
convection See heat transfer.
convection velocity (Unit m/s) The velocity of an identifiable flow structure, such as a turbulent eddy, which is normally lower than that of the bulk flow.
convective derivative In fluid flow, that part of the total change in a quantity associated with movement through regions of spatially different velocity. In Cartesian coordinates, for a quantity B, which could be a scalar such as pressure or temperature or a vector such as velocity (when it defines convective acceleration), it is given by
and in vector notation as (V · ▽)B where V is the vector velocity with components u, ʋ, and w in the x, y, and z directions, respectively. See also local acceleration; substantial derivative; total acceleration.
convective heat-transfer coefficient See heat transfer.
convective mass-transfer coefficient See mass transfer.
conventional drilling Predominantly vertical drilling of oil and gas wells, usually not involving directional (including horizontal) drilling or fracking.
conventional milling See upmilling.
convergent–divergent nozzle (convergent–divergent duct) A flow nozzle with a convergent section (the confuser), in which (for an incompressible fluid) the fluid is accelerated, upstream of a divergent section (the diffuser) in which the fluid is decelerated. If the fluid is compressible, as in the case of a gas or vapour, and if the pressure difference is sufficiently great, the flow at the throat chokes (i.e. reaches sonic conditions) and supersonic velocities are achieved in the divergent section, possibly involving shock waves. See also Venturi.
convergent–divergent nozzle
convergent duct (convergent nozzle) A duct or nozzle for which the cross-sectional area decreases with downstream distance. For subsonic flow, the velocity increases with distance, whereas for supersonic flow it decreases and the nozzle acts as a diffuser.
conversion efficiency See solar-cell efficiency.
convex involute helicoid See toothed gearing.
coolant A fluid used to limit the temperature of any device that is heated. Examples include water circulated through the radiator of a car engine where it is cooled prior to flowing through passages in the cylinder block; water flowing through the tubes of a surface condenser; a film of air that cools the surfaces of gas-turbine blades, a combustion chamber, or a rocket engine exposed to high temperature gas flow; liquid sodium used to cool certain types of nuclear reactor; water-soluble oil used to cool a cutting tool during machining.
cooling coil A simple heat exchanger consisting of a coiled tube, typically of copper or stainless steel, through which is circulated a coolant such as a refrigerant, chilled water, or water mixed with ethylene glycol. Applications include air conditioning, process cooling, and refrigeration.
cooling correction (Unit K or °C) In determining the calorific value of a fuel using a bomb calorimeter, the correction which must be applied to the measured temperature rise, which is slightly lower than the maximum that would be reached if the outer container were perfectly insulated.
cooling curves 1. Plots of temperature vs time as a substance cools. Plateaux of constant temperature (thermal arrest) in the smooth curve of falling temperature indicate freezing points in pure substances or eutectic (or eutectoid) points in mixtures and metal alloys; sharp changes of slope indicate the beginning and end of the temperature ranges over which non-eutectic mixtures transform to a different phase. Employed to construct phase or ‘equilibrium’ diagrams. 2. See lumped-capacity method.
cooling degree day See degree day.
cooling fins (extended surfaces, finned surfaces, fins) Thin metal plates, studs, or pins which project into the cooling fluid from a surface to be cooled in order to increase the effective surface area for heat transfer. Applications include the cylinders of air-cooled piston engines and compressors, boiler superheater tubes, heat exchangers, electrical transformers, and electronic components. Fin performance is defined either as the ratio of the heat transfer rate from a surface with a fin to that without (fin effectiveness, ε) or as the ratio of the heat transfer rate from a fin to the heat transfer rate from a fin if its entire surface were at the base temperature (fin efficiency, η).
cooling tower A large-scale cooler in which ambient air circulates and cools warm water from a power plant, large air-conditioning system, or industrial process. Depending upon how the airflow is driven, cooling towers are classified as natural draft, forced draft, or induced draft. The cooling-tower range is the difference between the temperature of the warm water entering TI and that of the cool water leaving TO. The cooling-tower efficiency (η) is the ratio of the cooling-tower range to the difference between TI and the wet-bulb temperature of the ambient cooling air TWB, i.e. η = (TI − TO)/(TI − TWB). The cooling-tower approach is TO − TWB. See also dry cooling tower; wet cooling tower.
Cooperative Fuel Research Committee engine (CFR engine) A single-cylinder piston engine used for the testing of fuels, especially the determination of the Motor Octane and Research Numbers, as well as the testing of lubricants and research into fuels and lubricants generally.
coordinated-axis control The simultaneous control, in CNC and robot control, of all axis or joint drives, so as to achieve smooth movement of the tool or end effector along a profiled path.
coplanar forces A term for forces that act in a single plane.
copying machine Various types of cutting machine, including lathes and milling machines, in which the tool is guided by a template, thus producing identical components.
core 1. The inside material of a sandwich structural component (often foamed or a honeycomb). 2. See gas generator. 3. See reactor core.
coring See segregation.
Coriolis acceleration (Coriolis force) See accelerating frame of reference.
Coriolis-type mass flow meter (Coriolis meter) A mass flow meter in which liquid flows through a vibrating U-tube. The Coriolis force exerted by the fluid distorts the tube, and the phase difference between the tube oscillation and the distortion is proportional to the mass flow rate.
correction time (settling time) The time taken for the output of a controlled plant to fall and remain within a specified percentage (typically 2%) of the final (steady-state) value.
corrective action The action of a control system in varying the plant input so as to minimize the error.
corresponding states See law of corresponding states.
corrosion The deterioration of an exposed metal surface due to electrochemical oxidation with its surroundings. In the oxidation reaction, metal atoms give up electrons which are transferred to form another chemical species by a reduction reaction, usually with hydrogen and/or oxygen. A common example is the formation of the iron oxide rust Fe(OH)3 on iron or steel in moist air or water. Rust breaks away easily, exposing fresh iron to the atmosphere, whereas the corrosion product of some metals forms an adherent layer that prevents further oxidation reaction at fresh metal, an effect known as passivation. Examples include aluminium oxide on aluminium, chromium oxide on chromium, chromium oxide on alloys containing chromium such as stainless steels, and the patina formed on copper and zinc.
Corrosion has a variety of forms that include the following:
). In jet engines and gas turbines, for example, sodium chloride in the air reacts with sulfur in the fuel to form low-melting-point sulfate compounds that are highly corrosive.Corrosion protection takes many forms. Good design to avoid crevices and stress is important. In some situations, substances known as inhibitors may be added to the environment to decrease its corrosiveness. The electrochemical nature of corrosion results in some metals having a greater tendency for oxidation than others. Zinc has a stronger tendency than iron and this is made use of in galvanizing. This is one example of cathodic protection, in which one metal is connected electrically to a more reactive metal, called a sacrificial anode, that oxidizes by giving up electrons and so protects the first metal. Coatings may be used to resist corrosion. Paints, plating, and diffusion coatings are the most common and the electrochemical process of anodizing is used to deliberately form a passive layer of oxide on aluminium alloys. Material selection plays a major role. For example, stainless steels with their high chromium content are widely used in corrosive environments.
corrosion failure 1. The failure of a component or structure after corrosion has reduced the load-bearing area to an unsupportable level. 2. A situation in which a mechanism cannot function owing to corrosion products preventing free movement at joints.
corrosion fatigue See fatigue.
cost function The variable that is minimized in system identification and other optimization methods to achieve the best parameter estimate.
cotter pin (cotter) 1. A tapered wedge or pin passing through a tapered slot or hole in one member and bearing against a second member to fix it in location. 2. A split cotter is commonly called a split pin.
Couette flow The flow in the near-vicinity of a surface in which streamwise gradients of velocity are negligible and the variation of total mean shear stress τ with normal distance from the surface y is given by
where τS is the wall shear stress, ρ is the fluid density, vS is the normal component of velocity at the surface (zero unless there is blowing or suction through the surface), ū is the mean flow velocity parallel to the surface at a distance y, and 
is the streamwise gradient of the mean pressure 
. The equation for τ is valid for both laminar and turbulent flow, but for laminar flow can be further simplified since then 
where μ is the dynamic viscosity of the fluid. Laminar Couette flow is realized practically in the gap between two concentric cylinders of almost equal radii, one rotating relative to the other when 
and u = τSy/μ if vS = 0. See also Taylor–Couette flow.
Couette viscometer See rotational viscometer.
Coulomb damping See friction damping.
Coulomb friction See Amontons friction.
Coulomb–Mohr fracture criterion See Mohr-Coulomb fracture criterion.
Coulomb–Mohr yield criterion See Mohr-Coulomb yield criterion.
counterbalance See counterweight.
counterbore A concentric enlargement of a hole to a limited depth.
counterflow heat exchanger (countercurrent-flow heat exchanger, contraflow heat exchanger) A heat exchanger, typically of shell-and-tube type, in which the overall directions of flow for the two working fluids are opposite.
counterflow heat exchanger
countershaft (counter shaft, jackshaft) An intermediate shaft in gearing.
countersinking The flaring out of the rim of a drilled hole to form a truncated conical depression to receive a screw having a conical head, thus giving a flush fitting.
counterweight 1. A heavy weight close to the fulcrum of a beam, such as the jib of a crane, that balances a load at the other end. 2. A weight, used to partially balance an object such as a lift cage, suspended over a pulley or sheave in order to reduce the torque needed to raise or lower the object.
couple (Unit N.m) If the resultant of two or more force vectors applied to an object is zero, the moment of those forces, tending to rotate the object about an axis, is termed a couple. Two parallel forces of equal magnitude F but with opposite sense, separated by a distance d, give a couple of magnitude Fd. See also torque.
coupling 1. Any mechanical fastening connecting two or more shafts, or parts of a mechanism, in order to transmit power. 2. A device for connecting two vehicles.
coupling agent A compound that provides a chemical bond between two dissimilar materials, often otherwise non-bonding and incompatible. An example is the use of a silane to bond the reinforcement and the resin matrix of glass-fibre reinforced composite materials.
covalent bonding See interatomic bonding.
cowl (cowling) A shroud or rounded panel, usually of sheet metal or plastic, used on a motor vehicle or aircraft to reduce drag or to surround, or direct air into, an engine. See also air scoop; fairing.
crack A thin fissure-like defect in a component or structure across which material continuity is lost and which reduces the strength of the body. See also fracture mechanics.
crack arrest Crack propagation that stops of its own accord when the energy release rate of the loaded component or structure falls below a critical value. If predictable, it can be incorporated into structural-integrity assessments.
crack tip opening displacement (CTOD, crack opening displacement, COD) (Unit m) The amount of stretch at the tip of a crack in a loaded body prior to crack propagation. See also critical crack tip opening displacement.
crank angle (Unit °) The angle between the crank of a slider-crank mechanism and a line from the crankshaft centreline to the piston axis.
crankcase The housing for the crankshaft of an engine. See also sump.
crankcase breather See breather pipe.
crank effort The force acting on an engine’s crank pin.
crank press A press, the stroke of which is driven by a crank mechanism.
crankshaft The main shaft, of which the cranks are a part, of a reciprocating single-or multi-cylinder machine. Crankshafts may be built up in sections or forged as a single component. In an engine, the reciprocating motion of the pistons transmits power to the crankshaft and causes it to rotate, whereas in a pump the crankshaft is driven and its rotation causes the pistons to reciprocate. The crank pin is a short shaft parallel to the axis of the crankshaft but radially offset from it, to which is attached the big end of a connecting rod in a bearing. Sometimes the crank pin is supported at one end only (a wrist pin) but in built-up or one-piece forged crankshafts, the crank pin is supported by thick plates (crank arms, crank webs) at either end. The crank throw is the radial distance from the crank pin to the crankshaft axis and equal to half the stroke.
crankshaft
crawler vehicle See caterpillar.
crazing A characteristic of some polymers below their glass transition temperature, whereby fine crack-like defects that produce bright reflections appear on loading. They are not true cracks, rather regions of highly plastically-deformed material interspersed with voids.
creep Time-dependent and permanent deformation of material under constant load or stress. The strain increases with time at a rate that increases with increasing temperature. Creep can lead to large strain and eventual failure, and so limit the lifetime of components subjected to load at high temperature. Creep in metals becomes significant at homologous temperatures in excess of about 0.4, e.g. 1200°C for tungsten, 504°C for titanium, 450°C for iron, and 100°C for aluminium.
A typical creep curve of strain (ε) vs time (t) (strain-time or creep diagram) shows an immediate elastic and plastic strain when the load is applied, followed by a decreasing rate of strain with time (primary, or transient, creep) in which the increase of stress caused by work-hardening exceeds the decrease in stress caused by thermal softening, resulting in a decrease in creep rate with time. That stabilizes into a steady increase of strain with time (secondary, or steady-state, creep), followed by an increasing rate of increase of strain leading to fracture (tertiary creep). For primary and secondary creep, ε vs t curves (creep-time relations) are often fitted by one of two empirical formulae, viz: 
, where 
is the immediate strain; α, β, and κ are creep coefficients; κt is steady-state creep; and (αlnt and 
are two forms of transient creep, called logarithmic creep and Andrade creep, respectively. See also creep mechanisms; recovery; relaxation.
creep
creep fatigue The effects of combined creep and fatigue at high homologous temperatures in metals, and in other materials such as polymers having time-dependent behaviour.
creeping flow See Stokes flow.
creep limit (Unit Pa) The maximum allowable stress under the action of which creep deformation of a material does not exceed a specified limit.
creep mechanisms Steady-state creep is of concern for high-temperature components because it can accumulate strain over a long time. At constant temperature, strain rate dε/dt varies as 
, where σ is applied stress. At constant stress, dε/dt varies as 
, where Q is the activation energy for creep, 𝓡 is the universal gas constant, and T is the temperature (in kelvins). The creep rate is therefore sensitive to changes in both σ and T.
Creep with n ≈ 4-8 (power-law creep) occurs at σ values just below the yield stress. Dislocations are held up at obstacles in the microstructure and creep occurs when they are released by climb due to the absorption or emission of vacancies. At low stress, 
and creep strain arises from shape change of grains in the material due to diffusion of vacancies from grain boundaries under tension to those under compression, which is equivalent to motion of atoms in the opposite direction. At high T vacancies diffuse through the grains (Herring–Nabarro creep). At low T, they diffuse more readily along grain boundaries (Coble creep). In both cases, dε/dt varies as 1/d2, where d is the mean diameter of grains. Q for power-law creep and Herring–Nabarro creep is found to be equal to the activation energy for self-diffusion and slightly less for Coble creep, thereby demonstrating the role of vacancies.
Good creep resistance requires combinations of properties such as high melting point, since Q is proportional to it; high intrinsic resistance to dislocation glide, e.g. oxides and carbides (up to 
); and alloying to produce stable microstructures of solute atoms, precipitates, and non-metallic particles resistant to dislocation motion, e.g. austenitic stainless steels and low-alloy ferritic steels (up to 
and 650°C, respectively) and nickel-based superalloys (up to 
). Large values of grain size d are also desirable, e.g. directionally solidified alloys.
Creep damage begins in the tertiary creep stage and accumulates in the form of cavities along grain boundaries transverse to the tensile stress. The principal mechanisms are diffusion of vacancies to these boundaries and grain boundary sliding, i.e. relative motion of material on opposite sides of boundaries. The effective reduction of cross-section area results in increasing stress and hence increasing dε/dt until failure (creep rupture) occurs. Creep-resistant components are designed for service to avoid this stage.
Radiation damage can create self-interstitial atoms in materials. Irradiation creep results from the movement of self-interstitial atoms at temperatures below the range at which vacancies are mobile.
See also deformation-mechanism map; radiation damage.
creep modulus (Unit Pa) The ratio of stress/strain at a chosen strain level obtained from creep tests, often plotted against time to show changes in stiffness.
creep rupture strength (Unit Pa) The fracture stress at the end of a creep test in metals and polymers, often plotted against time to give a stress-rupture curve.
creep–time relations Algebraic or numerical relations between creep strain and time at constant stress.
crest The highest point of a wave or of a screw thread. See also trough.
crest clearance (Unit m) For screw threads and gearing, the radial clearance between the crest of a thread (or gear) and the root of the engaging thread (or gear).
crippling load See buckling load.
criterion of performance 1. The coefficient of performance for a refrigerator or heat pump. 2. A measure of efficiency for an internal-combustion engine or power plant, such as the overall efficiency, the brake thermal efficiency, the specific fuel consumption, the indicated thermal efficiency, or the mechanical efficiency.
critical angle of attack See stall.
critical compression ratio 1. The compression ratio for incipient knock of hydrocarbon fuels as determined in a variable-compression single-cylinder piston engine. 2. The compression ratio at which an air–fuel mixture will spontaneously ignite due to the temperature increase produced.
critical crack length (Unit m) The crack length at which a given stress applied to a body will result in crack propagation and fracture. See also fracture mechanics.
critical crack tip opening displacement (critical crack opening displacement) (Unit m) The crack tip opening displacement at which crack propagation occurs. See also fracture mechanics.
critical damping The level of viscous damping in a system undergoing free vibrations that brings the system back to equilibrium in the shortest possible time without overshoot or oscillation. See also damping ratio.
critical density See critical state.
critical energy release rate (GC) (Unit J/m2) In linear elastic fracture mechanics, the critical rate of release of energy at which a crack will propagate. See fracture mechanics.
critical exponent (α) A power-law exponent used to approximate the behaviour of a thermodynamic quantity in the immediate vicinity of the critical point. For example, the specific heat C = k|T − TC|α where T is the absolute temperature, TC is the absolute critical temperature, and k is a constant of proportionality.
critical fibre length (Unit m) In fibre-reinforced composites, the load in the matrix is transferred into the filaments (and vice versa) by the interfacial bond strength τ over a distance Lc given by Lc = σmaxd/2τ where σmax is the stress in the fibre, and d its diameter. The critical fibre length is 2Lc.
critical flow 1. See choked flow. 2. Open-channel flow for which the speed of propagation of small-amplitude surface waves is equal to the flow velocity, i.e. the Froude number equals unity (the critical Froude number).
critical frequency See resonance.
critical heat flux See boiling.
critical insulation thickness (Unit m) For some geometries, including cylinders and spheres, there is a certain thickness of insulation that corresponds to a minimum heat transfer rate. Any further increase in the thickness leads to an increase in heat loss. For a circular cylinder, the critical radius of insulation is k/h where k is the thermal conductivity of the insulating material and h is the surface heat-transfer coefficient.
critical isobar The isobar on a T–ʋ diagram, where T is the temperature and ʋ is the specific volume, that passes through the critical point for a given substance.
critical isotherm The isotherm on a p–ʋ diagram, where p is the pressure and ʋ is the specific volume, that passes through the critical point for a given substance.
critical load (Unit N) 1. The applied load that causes propagation of an existing crack of known length, and hence fracture of a component or structure. 2. The applied load that results in buckling of a column of given end fixity.
critically-damped motion See critical damping.
critical Mach number See choked flow.
critical pressure (p*) (Unit Pa) The static pressure at the throat of a choked nozzle. For one-dimensional isentropic flow of a perfect gas
where p0 is the stagnation pressure and γ is the ratio of specific heats for the gas.
critical pressure ratio The ratio of the back pressure to the stagnation pressure that leads to choked flow in a convergent or convergent–divergent nozzle.
critical cooling rate See steel microstructures.
critical Rayleigh number The Rayleigh number for which laminar-to-turbulent transition occurs in natural convection.
critical Reynolds number The Reynolds number for which a flow undergoes transition from laminar to turbulent. The value depends upon the flow type and which characteristic length and velocity are chosen: for fully-developed pipe flow, the accepted value is 2 300; for a flat-plate boundary layer it is about 5 × 105; for flow around a body it depends primarily upon the body shape.
critical speed 1. (Unit m/s) In compressible flow through a convergent or convergent–divergent nozzle, the flow speed at the throat when the flow is choked. It is equal to the sound speed at the throat. 2. See whirling.
critical state (critical point) 1. For a fluid, the thermodynamic state at which the saturated liquid and saturated vapour of a fluid have identical densities. The fluid is then at its critical temperature, critical pressure, and critical density (or critical specific volume). Above the critical temperature, the fluid cannot be liquefied by increasing the pressure. 2. Conditions in gas flow corresponding to unity Mach number, critical properties are often designated with an asterisk superscript, e.g. c*, p*, T*.
critical stress-intensity factor See fracture mechanics.
critical Taylor number The Taylor number for Couette flow in the annulus between rotating cylinders at which Taylor cells first appear.
cross-compound steam-turbine system A multi-cylinder steam-turbine arrangement in which several machines are connected in parallel to separate generators. The diagram shows a high- (HPT) and an intermediate-pressure turbine (IPT) cross-compounded with two low-pressure turbines (LPT). See also tandem-compound steam-turbine system.
cross-compound steam-turbine system
cross-drum boiler A variant of the longitudinal-drum boiler in which the drum is transverse to the heat source.
crossed gears Gears that mesh together on non-parallel axes. See also toothed gearing.
crossed threads When the axis of a nut offered up to a bolt is not aligned with the axis of a bolt (or a screw to a threaded member), it may be possible for the threads to engage incorrectly and even for the nut or screw to advance a turn or more, but ultimately the misaligned threads become locked together. Forcing the nut in such circumstances may irretrievably damage the threads on both.
crossflow See secondary flow.
cross-flow baffle In a shell-and-tube heat exchanger, a metal plate that forces the shell-side fluid to flow back and forth across the tubes, to enhance heat transfer, and also holds the tubes in position.
cross-flow heat exchanger A heat exchanger, typically of compact design, in which, on average, the two working fluids flow transverse to each other. See also plate-fin heat exchanger.
cross-flow heat exchanger
cross-flow turbine 1. See Banki turbine. 2. A vertical-axis wind turbine similar in design to the Banki water turbine that can generate power whatever the wind direction. See also Savonius rotor.
crosshead 1. A reciprocating member, sliding between guides, to which the piston rod is firmly attached on one side and to which the connecting rod is pinned on the other for the conversion of reciprocating into rotary motion. 2. The moveable beam in early designs of mechanical testing machines. 3. A screwhead having slots in a + shape that takes a mating screwdriver.
Crossley meter (liquid-sealed meter, wet gas meter) A drum-type gas flow meter in which gas flows through a partially-submerged four-vane rotor, causing it to rotate. Flow rate is determined from the number of rotations in a given period of time.
Cross model See non-Newtonian fluid.
cross-ply laminate A filament-reinforced composite formed of laminae that are laid up at fixed angles in successive layers.
cross section The shape corresponding to the intersection of an object with a cutting plane.
cross slide That part of a lathe attached to the saddle, over which it traverses at right angles to the lathe bed. The tool is fixed to the cross slide and may be moved to any position by the combined motion of the saddle along, and the cross slide across, the lathe bed.
crown 1. The sharp corner at the crest of some gearing. 2. See piston.
crown gear (contrate gear, crown wheel) See toothed gearing.
crushing strain The supposed single strain that results in comminution of a brittle material. The corresponding stress is termed the crushing strength. Neither quantity can be a material constant as they must obey the laws of fracture mechanics and will depend on the type of testpiece, method of loading etc. See also fracture strength; Weibull function.
cryogenics The study of processes and material behaviour at temperatures below about 123K (i.e. −150°C).
cryoscopic effect The reduction in the freezing point of solvents due to the addition of solutes e.g. salt added to water. See also ebullioscopic effect.
cryostat A vessel, similar to a vacuum flask, used to maintain low temperature levels, often using liquid helium.
crystalline fracture A fracture surface characterized by the shiny facets of trans-granular cleavages as in steels below the ductile–brittle transition temperature.
crystallographic texture See texture.
crystal structure The geometrical arrangement of atoms repeated periodically in three dimensions to form a crystal. Important classes of crystalline materials include metals and alloys, ceramics, semiconductors, and some polymers.
The structure of a perfect crystal is composed of a motif unit of one or more atoms at every site of a lattice. Consider the three-dimensional net formed by three sets of straight, equispaced parallel lines in the figure. The points at the intersection of the lines define a space lattice in which every point has identical surroundings. The lines divide space into equal-sized parallelepipeds (known as a unit cells). The lattice may be constructed by stacking identical unit cells face to face in perfect alignment in three dimensions. Space can be filled by 14 different lattice structures (Bravais lattices). The ones by which the crystal structure of many important materials is defined are based on either cubic (
) or hexagonal (
) unit cells.
lattice and unit cell
The body-centred cubic (BCC) unit cell is shown with one atom (represented by a black sphere) at each BCC lattice site. This is the structure of metals such as alpha-iron, molybdenum, niobium, tantalum, and tungsten. The face-centred cubic (FCC) unit cell shown with one atom at every FCC lattice site is the structure of metals such as aluminium, copper, gold, gamma-iron, lead, nickel, and silver. The hexagonal close-packed (HCP) structure of cobalt, magnesium, alpha-titanium, zinc, and alpha-zirconium is depicted with more than one hexagonal lattice cell in order to make the hexagonal nature clearer. In this structure there is a motif of two atoms per lattice site, corresponding to one at a corner site and one midway between the top and bottom faces of the cell.
common unit cells
These crystal structures for metals allow the atoms to pack closely together, a feature of the metallic bond. This is illustrated for the FCC unit cell with atoms depicted as hard spheres in contact. They occupy 74% of space in the cell. Hard sphere packing density is similar in the HCP structure and 68% in the BCC case. The lattice parameter a in metals is typically 0.25–0.4 nm, and c/a in the HCP metals falls in the range 1.5–1.9.
The cubic and hexagonal structures are also the most common for ceramic materials, with a motif unit of two or more atoms, usually of metallic elements and non-metallic elements such as carbon and oxygen.
CSM See chopped-strand mat.
CSP plant See concentrated solar power plant.
CTOD See crack tip opening displacement.
cubical expansion coefficient See thermal expansion.
cumulative damage Physical or microstructural damage in a component or structure accrued as the result of a number of separate events, leading to weakening.
Cunningham slip correction A correction factor applied to Stokes drag law for small particles in a gas flow. It becomes significant when the particle diameter d is comparable with the mean-free path of the gas λ. It can be written
where a, b, and c are empirical constants for the particular gas.
cup The outer cylinder of a rotational viscometer.
cup anemometer A device for measuring wind speed comprising a number of hemispherical cups, typically four, attached by horizontal radial arms to a vertical shaft which is free to rotate. The rate of rotation of the shaft is a measure of the windspeed. See also anemometer.
cupping See deep drawing.
Curie temperature (Unit K) The temperature above which a ferromagnetic or ferrimagnetic material becomes paramagnetic. By analogy, it is also used to describe the temperature above which ferroelectric materials cease to be permanently polarized. See also ferroelectricity; magnetic materials.
curing time (Unit s) The time required in such processes as thermoplastic moulding, the manufacture of fibre-reinforced composites, and the setting of epoxies, for the completion of the chemical reactions that bring properties to their design levels.
Curtis stage A stage of a velocity-compounded impulse steam turbine consisting of one or more stationary nozzles in which practically all the pressure drop occurs followed by a pair of rotors separated by a fixed ring of guide vanes (stator).
Curtis stage
curved-wall effect See Coanda effect.
curvilinear motion Motion along a curved path.
cusped diffuser A diffuser, used in fluidic devices, with cusped recesses on either side close to the inlet in which stable vortices are created.
cut-in speed The wind speed at which a wind turbine begins to generate usable power; typically about 3 to 4 m/s (10 to 15 kph).
cut-off frequency (Unit Hz) The frequency at which the output of a device is reduced by 3dB with respect to the output within the passband. For example, in a low-pass filter, the frequency at which the output is reduced by 3dB with respect to that at lower frequencies. The cut-off frequency thus defines the passband, i.e. the range of frequencies passed by the filter with less than 3dB attenuation.
cut-off tool A parting tool to separate a finished part from barstock, etc.
cut-off wheel A thin abrasive wheel used to cut through, or cut slots in, a component.
cut-out speed (furling speed) The wind speed, typically about 20 to 35 m/s (70 to 130 kph) at which a wind turbine is either shut down for safety reasons or continues to operate at low efficiency, imposed by stall regulation.
cutter A blade or tool employed to cut or machine materials.
cutting fluid A liquid used in machining operations to cool and lubricate the cutting tool and wash away chips, swarf, and other debris. Mineral and synthetic oils, oil-water emulsions, and water are examples. See also drilling fluid; minimum-quantity lubrication.
cutting plane In an engineering drawing, a plane that passes through a component or assembly and is used to show the corresponding cross section.
cuttings See drill cuttings.
cuttings bed The accumulation of cuttings on the lower part of the wall of a deviated wellbore during drilling of an oil or gas well. See also drilling fluid.
cuttings transport See drilling fluid.
CVD See chemical vapour deposition.
CV joint See constant-velocity universal joint.
CVT See continuously-variable transmission.
cycle 1. See thermodynamic cycle. 2. The sequence of values of a periodically oscillating quantity over a complete period. 3. A mechanical cycle, such as the four-stroke cycle of a piston engine.
cyclic Moving in or following a regularly repeated sequence of events. See also periodic oscillation.
cyclically-pivoting sail windmills A vertical-axis wind machine for which the pitch angle of each sail is cyclically adjusted to optimize its orientation with respect to the wind direction.
cyclic hardening See mechanical hysteresis.
cyclic softening See mechanical hysteresis.
cyclic stress–strain curve A curve generated by cycling loading between fixed limits of stress or strain. The range between the limits may be tension only, compression only, or tension–compression. The deformation may be plastic or purely elastic. See also mechanical hysteresis.
cyclic testing Determination of the mechanical properties of a material, component, or structure under variable loading conditions. See also fatigue.
cyclic train A set of gears in which one or more of the gear shafts rotates around a fixed axis. See also toothed gearing.
cycloidal gear teeth See toothed gearing.
cyclone (cyclone separator) A swirling-flow device with a conical body that flow enters tangentially and leaves axially. It is used to separate particles from a gas, vapour, or liquid, or liquid droplets from a gas or vapour. See also hydrocyclone.
cyclonic-type separator A steam separator in which helical baffles cause the wet-steam flow to swirl. Water droplets are centrifuged to the outer wall and drain down to a steam trap.
cylinder 1. See engine cylinder. 2. The casing of a steam turbine, usually cast for high-and intermediate-pressure turbines, but low-pressure casings include some fabrication. There is often an inner and an outer casing.
cylinder block (block, engine block) The casting, typically of aluminium or magnesium alloy or cast iron, in which are machined the cylinders of a piston engine or a reciprocating pump. Channels are also either bored or cast into the casting to allow the circulation of cooling and lubricating fluids.
cylinder bore (Unit m) The internal diameter of a cylinder in which a piston operates in a reciprocating engine or pump.
cylinder head The machined casting, typically of aluminium alloy or cast iron, that fits above the cylinder block of a piston engine and closes off the cylinders. It normally includes part of the combustion chambers and holes for the valves and spark plugs.
cylinder liner (liner) A replaceable thin metal cylinder or sleeve fitted within the cylinder of an engine or pump which may be replaced when worn, thus allowing continued use of the cylinder block. A dry liner in a piston engine has no contact with the coolant, whereas the outer surface of a wet liner is in direct contact with the coolant.
cylindrical A cylindrical body or surface is one for which the defining generators are parallel straight lines. The cross section is the same at every longitudinal position but need not be circular. In solid mechanics, the term prismatic is more usual.
cylindrical-coordinate robot (turret robot) A robot having a rotational joint, joint angle θ1, with a vertical axis above the base frame; a translational joint, joint offset d2, with a vertical axis above the base frame; and a translational joint, joint offset d3, with a horizontal axis attached to the second joint. The volume that the robot can reach is thus a hollow cylinder centred on the base frame. The diagram shows an idealized cylindrical-coordinate robot. See also Cartesian-coordinate robot.
cylindrical-coordinate robot
cylindrical grinding Accurate finishing of cylindrical components using a high-speed abrasive grinding wheel.