The influence of combined gradient structure with residual stress on crack-growth behavior in medium carbon steel

Gradient materials produced by various external surface treatments are widely used in various mechanical structures. In this study, we illustrated how the combined gradient structure and residual stress influences crack-tip plasticity and crack-growth behavior in medium carbon steel S38C. The material used as train axles has a gradient layer of thickness about 8 mm, with fine nanoscale structures in the surface to microstructures of tens of micron in the core. From the surface, the strength in the gradient layer decreases from about 1600 MPa to about 400 MPa, and the corresponding tensile ductility increases from 2% to 13%. Through in-situ fatigue tests, we further revealed slower crack-initiation and faster crack-growth rate in the gradient samples in contrast to their gradient-free coarse-grain counterparts. Using the key material parameters obtained from experiments, we employed finite-element simulations to examine the crack-growth behavior in the gradient structure with compressive residual stress. We showed that both the gradient structure and compressive residual stress reduce the plastic deformation size and hence slow down crack initiation. However, the lower ductility in the high strength gradient layer gives rise to even faster crack growth after the formation of a sharp crack. Based on the definition of the cyclic Delta J-integral for small scale yielding, we saw lower Delta J in the presence of the residual stress distribution, hence smaller driving force for the crack-growth rate as the latter monotonically increases with Delta J.