Residual stress relaxation of railway gradient S38C steel during fatigue crack growth by neutron imaging and diffraction

Hu, F. F.*; Qin, T. Y.*; Su, Y. H.  ; He, L. H.*; Ao, N.*; Parker, J. D.*; 篠原 武尚   ; Wu, S. C.*

Hu, F. F.*; Qin, T. Y.*; Su, Y. H.; He, L. H.*; Ao, N.*; Parker, J. D.*; Shinohara, Takenao; Wu, S. C.*

The relaxation behaviors of residual stress and the influence on the fatigue resistance of S38C axles with induction hardening were investigated by using advanced Bragg-edge transmission imaging and neutron diffraction as well as XFEM-based numerical modeling. The Bragg-edge imaging results indicate that lattice parameters change as the crack develops, which then leads to a decrease of residual strain. In addition, it was observed from neutron diffraction results that all three residual stress components decrease with the crack growth. By contrast, the compressive residual stress in the axle surface was almost fully released when the crack propagated to the matrix material zone. Furthermore, residual stress relaxation was validated by the extended finite element method. These new insights confirm stress relaxation in railway S38C axles during crack propagation, which must be considered while carrying out the structural integrity assessment.