Influence of crack closure and local near-tip stress on crack growth life estimation

Abstract

Abstract Currently, among other models for crack growth prediction, crack closure models that consider the decrease in stress intensity factor (SIF) range associated with cyclic loading asymmetry are more popular. One of drawbacks of these models is impossibility of considering the loading history sequence. The theory related threshold SIF range and ΔKth and local near-tip stresses, induced by overloads, and postulated that the overload effect in the near-threshold region of growth rates is caused by residual local stresses. The proposed model applies the local stress and strain approach to estimate stress σ* in the stress concentration region for fatigue analysis. This region is characterized by local near-tip stress σ*, whose amplitude is determined by cyclic inelastic reaction at crack tip. Further development of the model is due to varying nature of the threshold SIF range ΔKth. As a basis, the Forman-Mettu formula was adopted, which describes the fatigue crack growth curve in all three regions of fatigue crack growth rate. The range ΔK was determined by the peak load ΔP of the load history. The crack closure was considered by the Schijve equation, considering the asymmetry of the half-cycle U = f(R), and effective SIF was estimated by ΔKeff = ΔK*U. Given known SIF range ΔK, the value of the local stress σ* at distance from the crack tip r* was determined for each half cycle by Neuber and Ramberg-Osgood equations, and threshold SIF was estimated from the analytical formula of Кth=f(σ). Thus, known loading history made it possible to determine ΔKeff, Kmax, and Kth on each cycle for fatigue life estimation. Mathematical modeling of fatigue crack growth life, especially in near-threshold region of its growth, according to the Sunder’s scheme, showed that investigated aluminum alloy 2024-T3 exhibited crack growth sensitivity to various types of force action, including various types of random loading.