Abstract
Fatigue failure of structural components due to cyclic loading is a major concern for engineers. Although metal fatigue is a relatively old subject, current methods for the evaluation of fatigue crack growth and fatigue lifetime have several limitations. In general, these methods largely disregard the actual shape of the crack front by introducing various simplifications, namely shape constraints. Therefore, more research is required to develop new approaches to correctly understand the underlying mechanisms associated with the fatigue crack growth. This paper presents new tools to evaluate the crack front shape of through-the-thickness cracks propagating in plates under quasi-steady-state conditions. A numerical approach incorporating simplified phenomenological models of plasticity-induced crack closure was developed and validated against experimental results. The predicted crack front shapes and crack closure values were, in general, in agreement with those found in the experimental observations.
Highlights
Significant research effort has been directed to the study of the fatigue crack closure phenomenon, which was first introduced by Elber [1] to explain the experimentally observed features of fatigue crack growth in aluminium alloys
It is commonly accepted that the contributions of various crack closure mechanisms, plasticity-induced crack closure, roughness-induced crack closure, and oxideinduced closure, are significant, and these mechanisms are capable of explaining many fatigue crack growth phenomena, e.g., the influence of thickness on crack growth rates, retardation effects associated with overloads, or higher propagation rates of small cracks in comparison with long cracks [2]
It is well-established that for relatively long cracks propagating in a non-aggressive environment, the plasticity-induced crack closure dominates over the roughness-induced crack and oxide-induced closures
Summary
The main idea behind the evaluation of the steady-state shape of a fatigue crack front proposed in this paper is to select a curve from a parametric family that minimises the deviation of the fatigue driving force along the crack front. The local crack growth rate is a function of the effective stress sity factor range, intensity factor range, i.e., ΔKeff(z) = Kmax(z) − Kop(z) = U(z)ΔK(z). A number of sophisticated finite-element (FE) models were developed to evaluate minimum load at which the crack faces, at point z, which are fully separated. U(z) for different geometries and as discussed above, Bethese low, we consider two simplified methods for the evaluation of the normalised ratio, models have many limitations, and are quite difficult to apply in fatigue load calculations. These methodsthe will be further into the 3D linearThis elastic finite element besimulations performed via the corner singularity method [17], which is briefly presented in Section to evaluate the shape of the through-the-thickness cracks.
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