Determination of fatigue mesoscopic mechanical properties of an austenitic stainless steel using depth-sensing indentation (DSI) technique

Abstract

In this paper, the depth-sensing indentation (DSI) technique was used to investigate the mechanical properties at the mesoscopic scale of 18Cr–8Ni austenitic stainless steel subjected to low-cycle fatigue loading. For this purpose, several representative analytical approaches for extracting basic mechanical properties, such as the indentation hardness ( H), Young's modulus ( E), yield strength ( σ ys), strain hardening exponent ( n) and plastic energy ( W p), from the indentation load–depth ( P– h) curve were introduced. A series of experiments including constant amplitude low-cycle fatigue tests, DSI measurements and transmission electron microscopy (TEM) observations were carried out to obtain the change characteristics of the mechanical properties at the mesoscopic scale of the material as well as their micromechanisms. It is shown that, for the polycrystalline metal investigated, the cyclic plastic deformation remarkably influences its subsequent elastic–plastic response of the small volume material to the indentation process. As the strain amplitudes increases, the mesoscopic mechanical properties ( H, E, σ ys, and n) show increase, while the plastic energy of the indentation ( W p) exhibits a decrease process. Microstructure observations using TEM reveal that, with increasing strain amplitudes, the dislocation substructures in the material tend towards cell formation and the size of dislocation cells tend to progressively decrease. The dependence of the microstructures on the applied strain amplitude is thus responsible for the change characteristics of the mechanical properties at the mesoscopic scale of the material subjected to low-cycle fatigue loading.