Correlation between the pivot node concept and fatigue crack closure

Abstract

The study of fatigue crack growth has been commonly done by means of bi-dimensional models and assuming a homogeneous behaviour through the thickness. According to the specimen thickness, a state of plane stress or plane strain is presumed. However, recently, it has been shown that thickness effects influence the crack tip behaviour. These works have revealed a series of effects along the thickness with a strong influence on the crack front growth. One of the experimental evidences that can be explained as a direct consequence of these effects is the curvature of the crack. It is observed that when the crack advance, the crack front changes adopting a curved shape, growing faster at the interior than at the exterior. Two mechanisms can explain this effect: the first one is related to the crack closure effect near the surface. The second one, related to the plastic zone size decrement observed in a small region close to the surface, is due to ΔK being smaller near the surface than in the interior. Both mechanisms are difficult to evaluate separately. A series of works were devoted to study these effects. A research line has been focused in the analysis of the stress intensity factor distribution. These works evaluate the finite element model of an Al 2024-T35 compact tension specimen with no plastic wake effect introduced, according to the methodology developed by the authors. The three-dimensional behaviour in the vicinity of the crack front is simulated through numerical analysis with ANSYS code and J-integral method is used to determinate the curves of K evolution along the thickness. The main finding of these studies is that the distribution of K is not homogeneous. The overall values for the whole model accurately agree with the nominal K applied. The K profiles along the thickness are characterized by a series of parameters that allow us to analyze the distribution of K in terms of the expected Knom against variations of geometrical and external factors.