Effects of an overload event on crack closure in 3-D small-scale yielding: finite element studies

This paper describes the effects of a single overload event, within otherwise constant amplitude cycles, on the plasticity- induced closure process for mode I fatigue crack growth in the small- scale yielding ( SSY) regime. The 3- D finite element ( FE) analyses extend the initially straight, through- thickness crack front by a fixed amount in each load cycle, using a node release procedure. Crack closure during reversed loading occurs when nodes behind the growing crack impinge on a frictionless, rigid plane. A bilinear, purely kinematic hardening model describes the constitutive response of the elastic - plastic material. Extensive crack growth in the analyses, both before and after the overload, allows the crack to grow out of the initial and the post- overload transient phases, respectively. The work presented here shows that the large plastic deformation in the overload cycle reduces the crack driving force through enhanced closure. Further, the residual plastic deformations due to the overload cause a disconnected pattern of closure in the wake long after the crack front passes through the overload plastic zone. The computational studies demonstrate that the 3- D scaling relationship for crack opening loads established in our earlier work for constant amplitude cycling ( with and without a T- stress) also holds before, during and after the overload event. For a specified ratio of overload- to- constant amplitude loading ( R-OL = K-max(OL)/ K-max), the normalized opening load ( K-op/ K-max) at each location along the crack front remains unchanged when the constant amplitude peak load ( K-max), thickness ( B) and material flow stress ( sigma(0)) all vary to maintain a fixed value of K = K-max / sigma(0) root B. The paper concludes with a comparison of the post- overload response predicted by the 3- D analyses and by the conventional Wheeler model.