Effect of yield strength reduction in surface-modified layer on compressive residual stress stability under strain-controlled axial tension–compression fatigue

To clarify the stability of compressive residual stress (CRS) under cyclic axial loading, strain-controlled in situ X-ray stress measurements were performed on AISI 316L steel treated with fine particle peening (FPP) and AISI 4140 steel treated with FPP or surface quenching, exhibiting different stress–strain responses in their surface-modified layers or core regions. The changes in CRS on the surface and yield strength (YS) of the surface-modified layer were analyzed via X-ray diffraction under cyclic axial tension–compression loading with total strain amplitudes of 0.2 % and 0.4 %. Surface CRS in the FPP-treated AISI 316L steel remained stable under cyclic axial loading, even when plastic deformation occurred at a macroscopic scale, because the surface-modified layer exhibited an elastic response in the X-ray stress measurements. CRS induced in AISI 4140 steel via surface quenching exhibited greater stability under cyclic axial loading than that induced via FPP, because of plastic deformation prevention within the surface-modified layer. It was experimentally demonstrated that the YS change of the surface-modified layer differed from that of the entire surface-modified material by considering the cyclic stress–strain responses of the surface-modified layer and core region separately. Surface CRS relaxation occurs because of a gradual reduction in the compressive YS of the surface-modified layer during strain-controlled axial tension–compression fatigue, causing stress acting on the surface-modified layer under compressive loading to repeatedly exceed its YS. Surface CRS stability under cyclic axial loading and fatigue lives were effectively improved by forming a surface-modified layer with a high YS that did not decrease during fatigue.