Prestraining Effects

For stainless steel the influence of prestraining was investigated by prestraining specimens in strain control at fully-reversed 2% strain amplitude for 10 cycles, and then fatigue testing under either load control or strain control. To avoid a large mean stress resulting from prestraining, the strain amplitude was gradually reduced after prestraining [65], as was explained in Chapter 3 (see Figure 3.3). For aluminum prestraining consisted in 10 fully-reversed cycles at 1.4% strain amplitude. Tests were conducted under four different conditions with the smaller cycles at four locations of the prestrain cycle hysteresis loop, as described in Chapter 3 (see Figure 3.5).

Based on the linear damage rule, prestraining cycles consumed only about 3% of life for SS304L and 6% of life for Al 7075-T6. Stress amplitudes at the 10^th cycle of prestraining were 501, 501, and 562 MPa for SS304L CLI, SS304L THY, and aluminum, respectively. Results for prestrained tests listed in Tables 5.1 to 5.3 are from midlife constant amplitude cycles following prestraining.

Midlife stress amplitudes from fully reversed tests on prestrained specimens are presented in Figure 4.9, superimposed with the monotonic and cyclic stress-strain curves.

Considerable hardening is induced in SS304L CLI (about 35%), as prestrained test data are well above the cyclic stress-strain curve. For SS304L THY, due to secondary hardening for virgin material, when taking midlife values, only about 10% of hardening

90 is observed at 0.3% strain amplitude for the prestrained material. For both SS304L CLI and SS304L THY grades, prestraining led to failure of the specimen for tests conducted at the virgin material runout level (0.2% and 0.25% strain amplitude for SS304L CLI and SS304L THY, respectively) and no secondary hardening was observed. A runout specimen for the prestrained SS304L CLI material was obtained at the lower level of 0.175% strain amplitude and no significant secondary hardening was observed in this test either.

Comparison of the stress responses obtained for SS304L CLI from prestrained tests and from mean strain tests with Rε = 0.75, where the maximum strain was greater than 3% shows more hardening due to prestraining (see Table 5.1). Thus, the hardening in prestrained tests mostly results from cycling at high strain amplitude, rather than from high maximum strain.

In contrast, the effect of prestraining on the deformation behavior of aluminum does not exist, since the prestrained data are similar to those for the virgin material (see Figure 4.9(c)). Prestraining neither affected the deformation nor the fatigue behavior of aluminum. Figures 5.17(c) and 5.18(c) show prestrained and virgin material fully-reversed (R = -1) data overlap on both strain-life and stress-life curves. Mean stress tests conducted on prestrained specimens for aluminum (i.e. tests with stress-strain paths C and D in Figure 3.5) showed no effect of prestraining with respect to deformation or fatigue behaviors either, as data points for mean stress tests for virgin and prestrained specimens also overlap in Figures 5.17(c) and 5.18(c).

The effects of prestraining for SS304L were dependent on the test control modes, however. Prestraining followed by strain-controlled tests led to about the same lives as

91 those for the virgin material for the SS304L CLI grade. This is in spite of the stress amplitudes for prestrained tests being about 30% higher than the corresponding virgin material tests, due to the initial hardening (see Table 5.1). For tests at 0.175% strain amplitude, hardening slope in stress amplitude response versus cycles was steeper for the virgin material, as compared to the prestrained material (which was nearly flat), as can be seen from Figure 5.19(a). This is consistent with the fact that secondary hardening was observed to be related to plastic strain amplitude. The prestrained material exhibited lower ductility and consequently slower hardening than the virgin material.

In strain-controlled tests, prestraining was found to have more effect on the fatigue behavior of SS304L THY than SS304L CLI. At 0.25% strain amplitude, prestraining led to 30% increase in stress response in SS304L CLI. At identical strain amplitude of 0.25%, for SS304L THY, the virgin material presented secondary hardening, leading to a stress response at midlife greater than the one for the equivalent prestrained test. When comparing stress responses at a given number of cycles, however, prestraining in SS304L THY led to a level of hardening similar to the one observed in SS304L CLI (about 40%). Reduction in fatigue lives by a factor of more than five were observed due to prestraining in SS304L THY, whereas fatigue lives for prestrained and virgin SS304L CLI materials were very similar. Therefore, the behavior of SS304L THY grade is more sensitive to prestraining than the behavior of the SS304L CLI grade.

In load-controlled tests, however, SS304L THY and SS304L CLI presented similar behaviors, as considerable increase in fatigue lives were observed due to prestraining. For load-controlled tests of SS304L, prestrained specimens had strain amplitudes which were smaller than those for the virgin specimens at identical stress

92 levels (see Table 5.1 for SS304L CLI and Table 5.2 for SS304L THY). In LCF, under load control, fatigue life of prestrained SS304L CLI specimens was slightly longer than the virgin material (LCF tests were conducted on SS304L CLI only). In HCF, prestrained materials exhibited much longer life than virgin materials, resulting in runout tests.

Similar behavior was observed for SS304L CLI and SS304L THY in HCF, since prestraining resulted in a reduction of strain amplitude of about 53% on average, leading to runout tests for both materials at the lower stress levels.

When considering strain-life or stress-life approach, prestraining appears to have little effect from strain-life point of view (Figures 5.17(a) and 5.17(b)), while it has significant beneficial effect based on the stress-life curve (Figure 5.18(a) and 5.18(b)) where prestrained data are well above the curve. Therefore, using a damage parameter that considers both stress and strain is a more appropriate approach for fatigue life prediction and correlation of load-controlled and strain-controlled prestrained fatigue data. Such data correlations are presented and discussed in Section 5.6.