Mechanics of fatigue crack growth under large-scale plasticity: A direct physical approach for single-valued correlation of fatigue crack growth data

Abstract

The major challenge in the mechanics of elastic-plastic fatigue crack growth is to find a physically based driving force parameter to correlate the crack growth rates under different loading conditions. Specifically, a parameter that can provide single-valued correlation of crack growth rates, in stress-controlled and strain-controlled fatigue, is needed. Approaches of the past used either cyclic strain (strain intensity factor) or nonlinear-fracture-mechanics based (cyclic J-integral, ΔJ) parameter, to correlate fatigue crack growth. The validity of these approaches, however, has not been established. Further, the J-integral based approach requires experimental load-deflection curves, recorded after every crack length increment, and geometry-correction factors, which are numerically determined and complicated. In the present work, a physically based approach, based on thecumulative change in the cyclic strain energy of the net-section, is used to develop new parameters that can successfully correlate fatigue crack growth behavior in a variety of loading situations. The driving force parameter, capturing the change in the net-section cyclic strain energy, is determined analytically from elastic-plastic behavior of material and from the relative sizes of cracked and uncracked sections in the crack plane. Load-deflection measurements, geometric correction factors, or numerical methods are not needed in the present approach. Remarkably, excellent correlations of fatigue crack growth, in a variety of specimen geometries and for various stress/strain levels, have been found in both stress- or strain-controlled fatigue conditions. This work, extending the author’s earlier works for elastic fatigue crack growth, validates that the change in net-section strain energy is a fundamental quantity in the mechanics of fatigue crack growth.