Figure 1. Bending fatigue results for 304 stainless at a calculated maximum stress of 189 MPa. The data preliminarily indicates that supersaturation hydrogen exposure conditions may reduce bending fatigue life.
Samples of 304 stainless steel were subjected to a range of hydrogen exposure conditions including 1 week at 1 atm, and up to 3 weeks at 138 MPa. Samples were tested in bending fatigue and tensile testing. Increased hydrogen exposure correlated with loss of fatigue life for bending fatigue. Tensile test data appears to show increased strain rate sensitivity after exposure to gaseous hydrogen at one atm pressure for one week.
One of the objectives of the current investigations is to contribute to the design database for components that will be subjected to hydrogen exposure and cyclic stresses. Previous investigations at Gonzaga have shown that the bending fatigue life of 304 stainless decreased after supersaturation levels of hydrogen exposure (e.g. up to 138 MPa hydrogen at 300C for 21 days) [1–3]. Supersaturation hydrogen exposure appears to reduce the fatigue life of 304 stainless bending fatigue specimens from 75,070 cycles (sx=27840, n=15) cycles to 9,460 cycles (sx=1960, n=5) at an estimated maximum stress level of 183 MPa. Statistically significant changes in fatigue life for specimens subjected to 1 atmosphere of hydrogen pressure for one day have not been found. Intermittent hydrogen exposure (e.g. re-exposing after each subsequent 6000 bending fatigue cycles) does not appear to correlate with reduced fatigue life for preliminary tests that have been run to date [3].
Proposed mechanisms to explain the reduced fatigue life may be based on surface adsorption, decohesion or on a dislocation interaction. For example, if surface adsorption is the mechanism, hydrogen ahead of the propagating crack tip in fatigue possibly reduces the energy for creating new surfaces, thereby lowering the threshold requirements for crack tip advancement [4].
The preliminary results reported here are for fatigue and tensile experimentation on flat samples of a nominal grade of 304 stainless steel exposed to a range of hydrogen conditions.
The bending fatigue tests were performed using a VSS-40H bending fatigue testing machine (Fatigue Dynamics, Walled Lake MI). The cycling frequency was between 300 and 350 cycles per minute. After the specimen failed, it was removed and the location of the failure was identified by measuring the distance from the start of the specimen radius to the middle of the failure location on specimen. Bending fatigue specimens were plasma cut from 0.9 mm thick sheets of nominally type 304 stainless steel. The plasma-cut specimens were belt-ground along the periphery to remove slag. The specimens were hand sanded and polished, with a final sand paper application of 600 grit. Hydrogen exposure conditions included no exposure, one week at 1 atm hydrogen and 138 MPa at temperatures as high as 375°C for 15 days (‘supersaturation conditions’). The typical hydrogen concentration in 300 series stainless steels after similar conditions is approximately 140 ppm, by weight [5]. Other hydrogen charging protocols are described by Mine et al. [6] The bending fatigue experimental equipment and procedures may be found in previous papers [1–3].
Tensile tests were performed on a MTS tensile testing machine using a 2.5 kN load cell. Strain rates of 3.81 mm min−1 and 0.41 mm min−1 were used to test specimens that were exposed to hydrogen ranging from no exposure to supersaturation levels. The specimens were plasma cut from 304 stainless sheet, and sanded and polished with a final application of 600 grit sand paper. The hydrogen exposure conditions were the same as that described for the bending fatigue specimens.
Fig. 1 shows bending fatigue results at a maximum bending stress of 189 MPa for 304 stainless samples of 0.9 mm thickness. The data appears to indicate that hydrogen supersaturation reduces the bending fatigue life. No apparent effect is seen for samples that are exposed to hydrogen for one week at 1 atm. The average for the one week exposed samples (avg = 114 kilocycles) is nearly identical to that of the unexposed samples (avg = 109 kilocycles).
Figure 1. Bending fatigue results for 304 stainless at a calculated maximum stress of 189 MPa. The data preliminarily indicates that supersaturation hydrogen exposure conditions may reduce bending fatigue life.
Fig. 2 shows a bar chart of the ultimate force to failure of 304 samples at two different strain rates and two different hydrogen exposure conditions. Each bar represents the mean and plus or minus one standard deviation for a data population of twenty five samples. The data in fig. 2 appears to preliminarily indicate that lower strain rates reduce the ultimate force to failure. The data also shows that exposure to one week of hydrogen accentuates the strain rate effect. T tests that were run on the four populations for the data bars shown in fig. 2 only indicate a statistically significant difference between the two populations that were exposed to hydrogen, each pulled at different strain rates. In other words, the strain rate effect on ultimate force to failure appears to be increased by one week exposure to hydrogen.
Figure. 2. Bar graph of tensile test results for 304 stainless steel. Each bar represents tensile test results for 25 samples. The data is for ultimate force to failure for combinations of strain rate and hydrogen. The populations that were exposed to one atmosphere hydrogen for one week (‘low hydrogen’) are significantly different, indicating an enhanced strain rate effect from low hydrogen exposure.
Fig. 3 shows a bar chart of the elongation of the 304 tensile samples for four data populations, representing two different strain rates and two different hydrogen exposure conditions. Each bar represents the mean and plus or minus one standard deviation for a data population of twenty five samples.
Figure. 3. Bar graph of tensile test results for 304 stainless steel. Each bar represents tensile test results for 25 samples. The data is for elongation for combinations of strain rate and hydrogen.
The axes shown in figs. 2 and 3 indicate the strain rate and hydrogen exposure conditions for the different sample populations, where ‘HSR’ and ‘LSR’ indicate high strain rate and low strain rate respectively. ‘LH’ and ‘NH’ indicate one week of one atm hydrogen exposure compared to no hydrogen exposure, respectively.
Table 1 shows t-tests results for the tensile test sample populations. Each population represents twenty five samples. The t-tests that were run were paired homoscedastic. Statistical significance was determined if the populations were different at a 95% level of confidence. The t-tests indicate that the high strain rate ultimate force to failure population is statistically significanlty different than the low strain rate ultimate force to failure population, for both populations’ samples exposed to one atmosphere hydrogen for one week.
Table 1. T-test results for ultimate force to failure data
| Compared Populations | T-test Result | Different? |
| high strain rate (no H2) v. low strain rate (1 atm H2) | 0.1536 | no |
| low strain rate (no H2) v. low strain rate (1 atm H2) | 0.1971 | no |
| low strain rate (no H2) v. high strain rate (no H2) | 0.3096 | no |
| low strain rate (1 atm H2) v. high strain rate (1 atm H2) | 0.0016 | YES |
The overall objective of the hydrogen embrittlement testing at Gonzaga is to contribute to the design database for future hydrogen designs. With widespread and increasing development of hydrogen fuel cell powered devices, mechanical designers will benefit from greater knowledge of hydrogen’s possible medium- to long-term effects on mechanical properties. The work at Gonzaga focuses on mechanical testing of materials that have been exposed to hydrogen, and has an added benefit of providing a hands-on educational experience for undergraduate engineering students.
The bending fatigue results presented here agree with the predicted trend of higher hydrogen levels leads to worsening mechanical properties, in this case lower fatigue life. Supersaturation levels of hydrogen appeared to have the lowest fatigue life, possibly because dissolved hydrogen reduces the stress required for crack growth ahead of the advancing crack tip. The population of samples that were exposed to one atmosphere of hydrogen for one week did not appear to have reduced fatigue life, compared to unexposed samples. However, continued testing may reveal a slight difference if a sufficient number of samples are tested.
Future bending fatigue work at Gonzaga will focus on higher levels of bending stress for the same exposure conditions as that reported here. Additionally, an investigation of the failed surfaces may show different fractographic appearances based on hydrogen exposure level.
The tensile data presented shows that the strain rate effect appears to be accentuated by low hydrogen exposure levels. The possible accentuated effect was only observed, at a statistically significant level, for ultimate force to failure. The literature suggests that strain rate effects are more likely to be seen for elongation rather than for ultimate force [4]. Future testing includes tensile tests on supersaturated samples, at the high and low strain rates used in the present work. Future tensile testing may also include other strain rates.
The conclusion reached that the strain rate appears to be accentuated by one week hydrogen exposure appears to be valid for the two strain rates tested, which are approximately an order of magnitude different. Future testing may include a more detailed study of varying strain rates to see if there is a threshold strain rate difference below which no effect is significantly observed.
Another noteworthy observation from the tensile test data is that the standard deviations of the populations for all of the low strain rate data appear to be larger than that of the high strain rate data. This observation includes the ultimate force as well as the elongation data.
Supersaturation of hydrogen in 304 stainless appears to reduce the fatigue life of bending fatigue samples. After one week of exposure to one atmosphere of hydrogen gas, the data does not show a fatigue life difference although testing is continuing to increase the sample size, and to test other stress levels.
Tensile data for 304 stainless exposed to one atmosphere hydrogen for one week appears to have an accentuated strain rate effect on ultimate force to failure. Samples that were tested at two different strain rates after two different hydrogen exposure conditions preliminarily indicate the low levels of hydrogen exposure may acccentuate the strain rate effect. Higher strain rates appear to correlate with higher force to failure. After one week of hydrogen exposure, the difference in force to failure was significantly different for the two strain rates tested.
The authors acknowledge the support provided by Gonzaga University, and by the faculty, staff and students of the School of Engineering and Applied Sciences (SEAS). The authors particularly acknowledge the help of Floyd Grillo, Steve Klemp, James Moody, Jackie Davis, Cameron Davis, Jason Ross and others in the shop for helping generate test samples.
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2. P. Ferro et al., “Fatigue Testing of Hydrogen-exposed Austenitic Stainless Steel”, Advances in Materials Science for Environmental and Energy Technologies, Ceramic Volumes (2012).
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