Residual Stress Relaxation Of Gradient S38c Steel Axles During Fatigue Crack Growth by Neutron Imaging and Diffraction

Abstract

The S38C axles demonstrate excellent fatigue resistance due to large-depth compressive residual stress (CRS). However, RS relaxation may occur, which results in a reduction of damage tolerance capacity and a shortened service lifetime. To tackle this concern, time-of-flight (TOF) neutron methods of Bragg-edge transmission (BET) imaging and neutron diffraction, were employed to quasi-in situ investigate the RS relaxation during fatigue crack propagation in single-edge notch bending samples. The BET experiments show that lattice parameters will change as crack develops, which then leads to a decrease of residual strain ε110. It was found that the maximum ε110 is released by ~31.4% if a crack propagates to 10.0 mm. Moreover, it was observed from neutron diffraction results that all three RS components decrease with the crack growth. The CRS in the axle surface layer is almost fully released when the crack propagates to the matrix material zone. Finally, RS relaxation was validated by the extended finite element method, and the effect of stress relaxation on stress distribution at the crack tip was also evaluated meticulously. These new findings confirm stress relaxation in S38C axles during crack propagation, which should be considered while carrying out the structural integrity assessment of the axle.

Keywords: Residual stress relaxation, Fatigue crack propagation, Neutron scattering characterization, Surface strengthening method, High-speed railway axles.