Step Load Tests

H-L and L-H step tests at different levels and with different damage ratios were conducted for all materials, in either strain control or load control. For the H-L sequence in strain control, different procedures were used to switch from the higher to the lower level, to control the amount of mean stress resulting from the higher level, due to residual plastic strain, as discussed in Chapter 3. For SS304L, one test procedure involved the presence of mean stress at the lower strain amplitude level. Another procedure used consisted of gradually decreasing the higher strain amplitude to zero, before switching to the lower strain amplitude level [65] (similar to prestrained tests, Figures 3.3 and 3.4).

The latter procedure produced negligible mean stress for this material. For Al 7075-T6, H-L step tests were conducted in different conditions, with the smallest cycles at different locations of the hysteresis loops of the highest level, as shown in Figure 3.5. Due to fully elastic behavior at the lower level, this level was conducted in load control so mean stress could be easily controlled.

Experimental test conditions and results for the step tests of all three materials are gathered in Tables 6.1 to 6.3. Tests are grouped by loading sequence used and test control mode. Stress or strain values listed represent the stabilized response, and damage ratios were calculated using the LDR, with different approaches, as specified.

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No change in deformation behavior due to load sequence effect was noticed for Al 7075-T6 in strain-controlled or load-controlled tests, for either H-L or L-H sequence, as expected (see Figures 6.1(c) and 6.2(c)). In contrast, significant hardening was observed for SS304L at the lower level in H-L step tests, as shown in Figures 6.1(a) and 6.1(b) for stress response in strain control and in Figures 6.2(a) and 6.2(b) for strain response in load control. For comparison purposes, constant amplitude fully-reversed responses are also presented in these figures.

In strain-controlled tests, hardening was characterized by an increase of the stress response of up to 40% for SS304L CLI, as compared to midlife stress amplitude for fully-reversed constant amplitude test at equivalent strain amplitude (see Figure 6.1(a)). In load control, hardening led to a reduction of up to 60% in strain amplitude of the second level for SS304L CLI (see Figure 6.2(a)). The amount of hardening increased with more difference between the two levels and increasing the number of cycles applied at the higher level.

For SS304L THY, in load-controlled H-L step tests, hardening led to a 50%

reduction in the second step strain amplitude (see Figure 6.2(b)). In strain-controlled H-L step tests, however, the differences at midlife between second level stress amplitude and constant amplitude test at the same level were less drastic, since significant secondary hardening occurred in the fully-reversed constant amplitude test at 0.3% (see Figure 6.1(b) and Table 6.2). Nonetheless, hardening due to high level of strain was still significant for SS304L THY, in the first few percent of fatigue life at lower level, as

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shown in Figure 6.1(b). Continuous softening occurred during the second step of the H-L step test.

Figures 6.3 and 6.4 present mean stress responses in strain-controlled, and mean strain responses in load-controlled H-L step tests for SS304L CLI and SS304L THY. In strain-controlled tests for SS304L THY, the level of mean stress remained low all throughout the second step, due to gradual reduction of the strain amplitude in the transition from the higher strain level to the lower strain level, as shown in Figure 6.3(b).

For SS304L CLI, however, the procedure of gradually reducing the strain amplitude was not used for all tests, and mean stress was present in some cases (see Figure 6.3(a) and Table 6.1).

In load-controlled tests, due to high maximum stress, some ratcheting was observed in the first step of H-L step tests for both grades of SS304L (see Figure 6.4).

Nonetheless, during the second step, mean strain levels presented a steady behavior, similar to the fully-reversed constant amplitude tests at equivalent load level, responses of which are also represented in Figure 6.4 for comparison.

With regard to fatigue life, load sequence effect was observed for both Al 7075-T6 and SS304L. Cycle ratios were calculated with the LDR based on strain-life curve for strain-controlled tests and stress-life curve for load-controlled tests. Cycle ratio sums were found to be smaller for H-L sequence than for L-H sequences, in strain control, as can be seen in Figures 6.5 and 6.6 and Tables 6.1 to 6.3. For a H-L sequence, microcracks can initiate during the higher level and grow during the second step, as the stress or strain amplitude may be higher than the crack opening stress. In contrast, for a

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L-H sequence, the stress or strain amplitude during the first step may be low enough that crack initiation is very limited. Therefore, the fatigue life at the higher level is only slightly affected, if at all, by the first step, leading to higher calculated cycle ratios.

Microcracks mentioned here are small cracks and do not affect the overall mechanical behavior of the material. They are discussed in further details in Chapter 7.

Figure 6.7 presents pictures of such cracks observed in the gage section, above the fracture cracks, in H-L and L-H step tests for SS304L CLI and Al 7075-T6. Cracks appear longer and deeper in the H-L sequence than for the L-H sequence. This indicates that cracks most likely initiated during the higher level in H-L sequences and grew at the lower level.

For SS304L CLI, in H-L step tests, at equivalent damage for the first step (D = 0.1), shorter lives were obtained in strain-controlled tests as compared to load-controlled tests, and when the high level was conducted at higher strain amplitude (1% versus 0.5%

strain amplitude), as can be seen in Table 6.1 and Figures 6.5(a) and 6.6(a). Comparison of H-L step tests conducted with the higher strain amplitude level at 1% with those at 0.5% strain amplitude showed greater cycle ratios in the latter case, based on LDR and the strain-life curve. One explanation of this observation is more hardening, and therefore higher resulting stress induced by the high step, when the high step is at higher strain amplitude. Another explanation is that cycling at 1% strain amplitude produces more crack nucleation sites than at 0.5% strain amplitude, and leads to shorter life in the second step. For SS304L THY, tests were only conducted with a higher level of 1%

strain amplitude (Table 6.2 and Figures 6.5(b) and 6.6(b)).

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Although similar trends in hardening behavior were observed in strain control and load control, the test control mode had a strong influence on the outcome of the step tests, regarding fatigue lives, for SS304L. In strain-controlled H-L step tests, hardening induced by the higher level led to smaller cycle ratios (i.e. shorter lives) due to higher stress amplitudes at the second strain level. Similar to prestraining, high level of loading was found to be more damaging for the SS304L THY grade than for the SS304L CLI grade (comparison between H-L step tests at equivalent cycle ratio of 0.1 in the first step can be made in Tables 6.1 and 6.2). In load-controlled tests, however, the hardening induced by the first load level resulted in smaller strain amplitude at the second load level leading to larger cycle ratios, as seen in Figures 6.5(a) and 6.6(a). Cycle ratios referred to here were calculated using the common approach of utilizing LDR with a strain-life curve in strain control, or with a stress-life curve in load control.

This type of damage accumulation calculation leads to erroneous conclusions, as H-L sequence fatigue life is overpredicted in strain control (non conservative life predictions), while it is severely underpredicted in load control (overly conservative life predictions). As stated previously, to circumvent this shortcoming, parameters including both stress and strain such as Smith-Watson-Topper [21] (Equation 5.4) or Fatemi-Socie [22] (Equation 5.5) parameters can be used for damage accumulation calculations. The LDR was used with these parameters on experimental data obtained for H-L and L-H sequences with results presented in Figures 6.8 to 6.11 for the three materials. For SS304L CLI (Figures 6.8(a) to 6.11(a)) and SS304L THY (Figures 6.8(b) to 6.11(b)) significant improvement is observed using the SWT or the FS parameter, especially for

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H-L step tests in load control. For Al 7075-T6 (Figures 6.8(c) to 6.11(c)) all three approaches (i.e. strain-life or stress-life, SWT, and FS) led to similar results, as expected.

The SWT and FS parameters also account for the mean stress present in some tests, as previously discussed.

Fatigue life predictions for the second step of the step tests were performed based on LDR in conjunction with strain-life or stress-life, SWT, or FS curves, using:

1

Predicted versus observed remaining lives (i.e. n2) are presented in Figures 6.12 to 6.15 for strain-life or stress-life curves, SWT curve, and FS curve for all three materials. The dashed lines represent scatter bands of a factor of two and five. No significant difference was observed for Al 7075-T6 between strain-life or stress-life and SWT or FS curves.

However, significant improvement is obtained using the SWT or FS parameter for SS304L CLI and SS304L THY, especially for H-L step tests. As also seen in Tables 6.1 and 6.2 and Figures 6.5, 6.8 and 6.10, damage ratios for these tests are close to unity using the SWT or FS parameters. Neither the strain-life nor the stress-life approach account for the hardening due to the higher level in H-L step tests.