FATIGUE CHARACTERIZATION OF AM60B MAGNESIUM ALLOY SUBJECTED TO CONSTANT AND VARIABLE AMPLITUDE LOADING WITH POSITIVE AND NEGATIVE STRESS RATIOS

Abstract

AM60B magnesium alloy is being increasingly used in auto industry in applications that usually involve various formats of cyclic loading scenarios. Therefore, the fatigue response of this alloy is investigated in this thesis. Our investigation is focused on characterization of the influence of compressive stress cycles within a given cyclic loading scenario on alloy’s crack propagation response.

In the first part of this dissertation, fatigue crack growth rate (FCGR) of AM60B alloy subject to cyclic loadings with various stress ratios (both positive and negative) is investigated and a modified model is proposed to predict the FCGR under a wide range of stress ratios. Subsequently, using the modified model, the experimental results of the crack propagation tests are condensed into a single line in a logarithmic scale and the integrity of a proposed FCGR model is investigated. The investigation is continued by studying the influence of compressive stress cycle (CSC) on FCGR. Constant and random amplitude loadings with several magnitudes of CSCs are applied, leading to considerable acceleration in FCGR. The stress distribution ahead of the crack tip is also studied using the finite element method. The tensile residual stress and plastic zone are characterized upon the removal of the CSCs. The acceleration in the crack propagation is shown to be governed by the tensile zone ahead of the crack tip.

Furthermore, application of an overload within an otherwise constant amplitude loading (CAL) has been known to retard the crack propagation, thus increase the fatigue life. This retardation would be a function of the affected zone and retardation magnitude. It is shown in this thesis that the affected zone would be influenced by the “sensitivity” of the material to overload. Moreover, it is also demonstrated that the nature of baseline CAL loading would also affect the retardation response and dimension of the affected zone. Therefore, modification to the Wheeler model is proposed, thereby enabling the model to account for material’s sensitivity and nature of the baseline loading. The integrity of the proposed model is verified by the experimental results obtained in this project, as well as those reported by other investigators for other alloys.