Variable amplitude cyclic deformation and fatigue behaviour of stainless steel 304L including step, periodic, and random loadings

This paper discusses cyclic deformation and fatigue behaviours of stainless steel 304L and aluminium 7075-T6 under variable amplitude loading using strain-controlled as well as load-controlled tests. Load sequence effects were investigated in step tests with high-low and low-high sequences. For stainless steel 304L, strong hardening induced by the first step of the H-L sequence significantly affects the fatigue behaviour, depending on the test control mode used. For periodic overload tests of stainless steel 304L, hardening due to the overloads was progressive throughout life and more significant than in H-L step tests. For aluminium 7075-T6, no effect on deformation behaviour was observed due to periodic overloads. However, the direction of the overloads was found to affect fatigue life, as tensile overloads led to longer lives, while compressive overloads led to shorter lives. Deformation and fatigue behaviours under random loading were also studied for the two materials. To correlate a broad range of fatigue life data for a material with strong deformation history effect, such as stainless steel, it is shown that a damage parameter with both stress and strain is required. The Smith-Watson-Topper parameter as such a parameter is shown to correlate the data reasonably well under different control modes and loading conditions.