A material sensitive modified wheeler model for predicting the retardation in fatigue response of AM60B due to an overload

摘要

Application of an overload within an otherwise constant-amplitude loading scenario causes retardation in crack propagation. Several models have been proposed for predicting retardation in crack propagation due to an overload cycle. Among them, the widely used Wheeler model, assumes the "affected zone dimension" to be a function of the current and overloaded plastic zone radii. When one considers the actual shape of the plastic zone, however, one realizes that the affected zone dimension does not agree with that assumed by Wheeler. In this paper, the influence of a single overload (but by considering three different overload ratios) on the fatigue crack growth retardation of center-cracked AM60B magnesium alloy plates is experimentally investigated. The retardation effect on crack growth due to an applied overload within a random-amplitude loading scenario, using various "clipping levels", is also investigated. The sensitivity of this material to overload is compared with the response of some other materials. The actual radius of the plastic zone is evaluated for various stress intensity factors, using the finite element method. The results indicate that depending on the material, the affected zone would be sometimes larger or smaller than that produced by Wheeler's model. Subsequently, a new parameter, hereafter referred to as the "sensitivity parameter" (β), is introduced that enables one to evaluate the affected zone dimension more accurately. It is shown that the proposed modified model is more effective than the original one in predicting the retardation response of the alloy. The integrity of the modified model is also investigated by evaluating the retardation in some other materials.