Multiscale analysis of the pre-hardening effect on the cyclic behavior and fatigue life of 304L stainless steel

This study investigates the effects of axial and shear cyclic pre-hardening on cyclic behavior and fatigue life of an austenitic stainless steel 304L at room temperature. First of all, the effect of cyclic sequential loading with increasing and decreasing strain amplitude on the cyclic stress-strain curve has been confirmed. Then, the fatigue life of the specimens previously subjected to four increasing-decreasing sequences of pre-straining was investigated. The application of the sequential loading as a pre-hardening made the investigation of the pre-hardening effect without any influence of the mean stress and mean strain possible. Compared to virgin specimens, pre-hardened samples have shown larger flow stress and significantly reduced lifetimes. However, an increase in the strain amplitude during the fatigue test reduced the effect of the pre-hardening. The dislocations observing has shown that pre-strained samples inherit high density of dislocations, structured from the pre-hardening sequence, thus increasing both flow stress and damage. These results were then discussed in terms of long-range backstress and effective stress evolution.