To conclude this lesson, here are the primary rules for stacking composite laminates of aircraft structures, along with their explanations. Use caution, however, as these rules are constantly evolving, and are set to evolve during the coming years (composite materials are still in their youth and evolve every day).
This avoids twisting during cooldown (which would be very inconvenient for building flat pieces and sizing tolerances) and the membrane/bending coupling (the second reason is less important). This rule is very often respected.
We restate that a laminate with as many plies at +45° as at −45° is called a balanced laminate. This rule makes sense due to shear resistance (shear stress produces the same amount of tension at +45° and compression at −45°); it also avoids tension/shear coupling and shearing during cooldown (the two last reasons are less important compared to the 1st one). However, as composite materials are generally more resistant to tension than to compression, engineers will sometimes opt to have less fibers in the direction receiving tensile stress (+45° if shear is positive). This rule remains, nonetheless, widely respected.
Even though this rule is starting to be put into question in order to achieve lighter structures, it is crucial and has different explanations (do note, however, that at the time of writing this, this 10% is tending towards 7–8%). The first reason concerns matrix cracking. When loading is applied in one direction, cracks appear in the matrix in the perpendicular direction to it, and a few plies in that perpendicular direction can protect the laminate. In fact, this is one of the necessary conditions to use a criterion such as the Yamada–Sun which does not account for matrix cracking (or at least only the part that is due to shear). The second reason is not having to account for secondary loads and simply consider maximum loads. If there are no plies in one direction, even a case presenting a low loading in that direction could end up being critical. Another reason is the unexpected variation of external loads. Loads will sometimes vary during the life cycle of a structure in more or less complex ways, and as a way to avoid unexpected load, a minimum of plies are required in each direction to support these unexpected loads. Lastly, having a minimum of plies in each direction provides better support for loads around the rivets and holes. In these areas under high stress level, even if far-field loading is simple, the presence of holes will produce complex local stress fields with all of the components of the resultant force.
Avoid placing too many consecutive plies in the same direction, and try to distribute them throughout the thickness. This limits the interlaminar stress and thus delamination. This rule is generally properly respected.
This increases buckling resistance and protects primary plies that support primary load (often plies at 0°). This rule is generally respected, leading to a great deal of industrial stacking sequences in [+45, −45, …]S.
This limits interlaminar stresses and thus delamination. This rule can be hard to respect and is therefore often overlooked.