Residual stress relaxation during fatigue crack growth in surface-hardened S38C axles studied by time-of-flight neutron imaging and diffraction

Hu, F. F.*; Qin, T. Y.*; Zhang, R.*; Ao, N.*; He, L. H.*; Su, Y. H.  ; Xu, P. G.   ; Wu, S. C.*

The S38C railway axles demonstrate excellent fatigue resistance owing to the large-layer depth compressive residual stress in the hardened surface. However, during fatigue crack propagation, the residual stress may occur the stress relaxation problem, which results in a reduction of the damage tolerance capacity and the service lifetime. To tackle this concern, the time-of-flight neutron scatter methods, including the Bragg-edge transmission imaging and neutron diffraction, were employed to quasi-in-situ study the residual strain and RS relaxation during the fatigue crack advance with single-edge notch bending samples. The BET experimental results show that lattice parameters will change as the crack grows, which then leads to a decrease in residual strain. Moreover, it was discovered from the neutron diffraction test that all three RS components decrease as the crack propagation. The CRS in the axle surface layer is almost fully released when the crack propagates to the matrix material zone.