Residual Stress Analysis of Fatigued Austenitic Stainless Steel

Abstract

The problem of converting of the phase lattice parameters determined by the time-of-flight neutron diffraction measurements in a loaded multiphase system, to strain and subsequently to stress is discussed. An analytical formulation relating the phase applied stress–elastic strain responses in the axial (along the load direction) and transverse directions has been performed which accounts for all the sources of stress measured in the phases. The formulation was applied in the analysis of diffraction spectra from austenitic stainless steel with a martensite phase plastically induced during cyclic tensile-compressive loading. The spectra were obtained during in situ neutron diffraction stress rig experiments on the ENGIN instrument at the ISIS pulsed neutron facility, subsequent to the cyclic loading. The subsequent applied stress–elastic strain responses of the austenitic matrix and martensitic inclusions were obtained by Rietveld refinement of the spectra, and used to determine the elastic constants and residual stresses of the phases as a function of fatigue level. The data shows that the elastic properties of both phases are similar, which allows the simple determination of residual stresses. Only deviatoric components of the residual stress tensor were obtained due to the lack of an unstressed reference for both individual phases. The formation of martensite is connected with volume dilation; since the specific volume of martensite is larger than that of austenite, the martensite phase is generally in hydrostatic compression, where as the austenite is in tension. These plasticity induced phase transformation stresses are superimposed upon the deformation stresses caused by the plastic deformation occurring during low cycle fatigue, resulting in residual stresses which have a non-hydrostatic nature. We have observed that in the axial direction the deviatoric phase stress of the austenitic phase was compressive while it was tensile for the martensite phase. The axial phase stresses are mainly those due to the deformation, while the phase transformation stresses dominate in the transverse direction.