Periodic Overload Tests

Periodic overload test procedures used were discussed in Chapter 3. For Al 7075-T6, POL tests were conducted with smaller cycles at 0.5% strain amplitude and periodic overloads at 1.4% strain amplitude were applied about every 10% of the expected fatigue life based on the LDR. This resulted in a load block composition of one overload every

142

142

one thousand small cycles, as shown in Table 6.4. Tests were carried out in load control, while measuring strain and ensuring desired levels of strain were reached. This method avoided mean stress during smaller cycles. However, mean stress was present during the overloads, as either fully compressive or fully tensile overloads were applied (see Figure 3.7).

For SS304L CLI, fully-reversed periodic overloads in between fully-reversed small cycles were applied. Duplicate tests were conducted, one with overloads starting in tension, the other one with overloads starting in compression (Figure 3.8). Both methods led to similar fatigue lives. One purpose of the periodic overload tests was to compare the results with H-L step tests. Therefore, similar levels of stress, strain and damage ratios per block were applied. Periodic overloads were conducted at 1% total strain amplitude in strain control, and 370 MPa in load control, while the lower levels were carried out at 0.4% or 0.25% fully-reversed strain amplitude in strain control, and at 240 MPa stress amplitude in load control. For the lower level of 0.25% strain amplitude, different overload to smaller cycle ratios were used, as indicated in the block composition column in Table 6.4.

For SS304L THY, only one test was conducted, with overloads at 0.4% strain amplitude every 5000 cycles at 0.25% strain amplitude. This test was meant to investigate the effect of periodic overload on secondary hardening behavior.

In contrast to Al 7075-T6, where neither prestraining nor periodic overloads affected the deformation behavior, progressive hardening was observed as a consequence of overloading for SS304L. In strain control, the stress response slowly relaxed in

143

143

between overloads, as shown in Figure 6.16 for one test of each grade, where stress responses for overloads and several small cycles at different instants in the block are represented. The larger stress amplitudes for the small cycles in Figure 6.16 correspond to cycles located right after the overload cycle, while the lower response amplitudes for the small cycles correspond to cycles near the end of a block. Stress response for the fully-reversed constant amplitude strain-controlled fatigue test is also presented for comparison. For SS304L, for H-L strain-controlled step tests, the material was softening all throughout the second step, whereas progressive hardening was observed in periodic overload tests with slight softening in between subsequent overloads. The frequent application of periodic overloads prevents any significant softening, resulting in more hardening than in the corresponding H-L step tests.

In SS304L THY, secondary hardening was observed in the periodic overload test.

Therefore, the hardening induced by the periodic overloads was limited, due to the level of the overloads (0.4 % strain amplitude, versus 1% for SS304L CLI), and the stress response was nearly identical to the fully-reversed constant amplitude test (see Figure 6.16(b)). Therefore, periodic overloads at relatively low strain amplitude do not prevent secondary hardening from occurring, and in this case, did not induce failure of the specimen (runout).

As seen in Figure 6.17, mean stress levels remained low in all periodic overload tests in strain control, although the strain amplitude was gradually reduced after the overloads only in the SS304L THY test. Mean stress data are represented in the same fashion as the stress amplitude data in Figure 6.16.

144

144

In load-controlled POL tests, which were only conducted for SS304L CLI, hardening was characterized by lower strain amplitude as compared to the strain amplitude in fully-reversed constant amplitude test at equivalent stress amplitude level, as shown in Figure 6.18. Only one cycle for the smaller cycles is represented in this figure, since very small variations within a load block were observed. Steady behavior was obtained fairly rapidly.

Comparison of periodic overload and H-L step tests at identical strain or stress amplitudes is shown in Table 6.5. In strain control, more hardening results from periodic overloading than from H-L step tests at the same equivalent overload level. This is manifested by higher stress amplitude at the lower strain level in POL tests, as compared to H-L step tests. More fatigue damage was caused by periodic overloads than by the high level in H-L step tests for equivalent cycle ratios (see Table 6.5). This is consistent with the hardening behavior, as higher levels of stress amplitude were observed in periodic overload tests, at identical strain amplitude to that in H-L step tests.

With regard to fatigue lives, results are presented in Figures 6.19 to 6.22. For Al 7075-T6, tests conducted with tensile overloads led to slightly longer fatigue lives (about 20%) as compared to tests with compressive overloads. Prestraining which was applied for 10 cycles at 1.4% strain amplitude in these tests can complete the microcrack nucleation process. Propagation of microcracks can then represent the major part of the subsequent fatigue life [28, 74]. Under periodic overloads, tensile overloads induce beneficial residual compressive stress at the tip of cracks, increasing the threshold stress intensity during subsequent cycles. In contrast, compressive overloads tend to aid

145

145

microcrack opening and facilitate crack growth, reducing fatigue lives. For fatigue life prediction, use of the LDR associated with strain-life, stress-life, SWT or FS curves led to similar and relatively accurate predictions for aluminum (see Table 6.4 and Figures 6.19(c) and 6.22).

For SS304L CLI, similar to step tests, the presence of hardening led to inaccurate life prediction using the LDR in conjunction with strain-life or stress-life curves. Shorter lives than predicted are obtained in strain control because of high stress levels, while conservative predictions are obtained in load control due to lower strain amplitudes (see Table 6.4 and Figure 6.19(a)). The SWT and FS parameter approaches led to more accurate predictions, as shown in Figure 6.20. For SS304L THY, only one test was conducted and was a runout. Due to the use of considerably longer blocks, resulting in a small total number of blocks, this test is not represented in Figures 6.19(b) and 6.21.