4.3 Cyclic Softening and Fatigue Life
The cyclic softening behavior is also essential for the isotropic
parameter identification (c and d in Eq.
(8))1-3,49 in Chaboche UVP model and even damage
parameters62. Thus, the measured peak stress
evolutions versus fatigue lives (N ) for both SSFS and MTP tests
subjected to saw-tooth and dwell-type loadings are compared in Fig.12.
Similar to SSFS fatigue test, significant cyclic softening is observed
in MTP specimens with different magnitude levels and rates, as the
primary mechanism of cyclic softening in the investigated material is
low angle boundary dislocation annihilation63,64. In
general, the trends of the measured peak stress evolutions in every
cycle in the MTP specimens under saw-tooth loading are similar to that
of in SSFS specimen. However, cyclic softening and fatigue lives of MTP
specimens exhibit a strong geometry dependency. For example, MTP-3 with
the weakest geometry constraint effect gives the closest fatigue life
comparing with the SSFS test subjected to saw-tooth loading, while MTP-2
is the intermediate case. The shortest fatigue life can be observed in
MTP-1 due to the strongest geometry constraint effect. The measured peak
stress evolution subjected to saw-tooth waveform in Fig.12 is consistent
with the peak stress evolution as predicted in Fig.8c, which further
confirm the feasibility of the testing methodology and FE analysis used
in this work. The fatigue life of MTP-3 subjected to dwell-type loading
is significantly lower than the test under saw-tooth loading because the
hold dwell effect can aggravate the damage accumulation, which is
consistent with our previous experimental observations on the
investigated material1,3,63,64. Table 4 tabulates the
comparison of fatigue lives (Nf ) for all the
tests within this test program.