Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements

Abstract

Synchrotron-based, high-energy X-ray diffraction measurements are used to study the local strain fields underlying the transient fatigue crack growth rate retardation produced by a single overload cycle known as the overload effect. Specifically, 4140 steel compact tension specimens fatigued for varying levels of crack growth after an overload cycle have been studied with in-situ diffraction under varying external loads. The load responses of the strain at the overload-position, versus at the crack tip, are focused upon in detail. The large compressive residual strain at the overload-point is observed to remain essentially unchanged even after the overload-point is left in the wake of the propagating crack tip. The differential strain-load response at the crack-tip/overload position before and immediately after the overload is seen to be unchanged. Once the overload point is behind the crack tip, a highly nonlinear behavior is observed in which the load response of the strain field transfers from the overload -point to the crack tip when the load exceeds a critical value. The results are discussed in terms of plasticity-induced crack face contact at the overload point as an important local mechanism contributing to the “overload effect” in this specific system.