Mean strain effect on low-cycle fatigue of AISI 304 austenitic stainless steel under non-proportional random loading: Experiments and life evaluation methods

Abstract

The mean strain effect on AISI 304 stainless steel was examined through strain-controlled low-cycle fatigue tests under uniaxial and non-proportional random loading. Under uniaxial loading, cyclic stress responses showed initial hardening followed by continuous softening, typical of austenitic stainless steel. In contrast, non-proportional loading resulted in consistent hardening. Mean strain primarily influenced fatigue life by altering the stress range: a reduced stress range generally led to extended life. Mean stress relaxation was observed and analyzed under various mean strains. An improved Itoh–Sakane (IS) method was developed, incorporating the calculation of mean strain coordinates and a new coordinate system. This enhancement allows accurate calculation of principal strain and stress under mean strain conditions, significantly improving life evaluation accuracy, with results typically within a factor of 2. Based on this, a new life evaluation method was established, applicable to strain-based models and their modifications, as well as energy-based models. The advantages and disadvantages of different models were compared.