Fatigue Characterization and Cyclic Plasticity Modeling of Magnesium Spot-Welds

Abstract

The automotive industry is adopting lightweight materials to improve emissions and fuel economy. Magnesium (Mg) alloys are the lightest of engineering metals, but work is required to assess their structural strength, especially for spot-welded applications. In the present research, fatigue behavior of magnesium spot-welds was characterized and compared with steel and aluminum spot-welds. A fatigue model was proposed to predict the failure location and crack initiation life in magnesium structures. The material under investigation, AZ31B-H24 Mg alloy, and its spot-welds were characterized from microstructural and mechanical perspectives. Microstructure and hardness of the base metal (BM) and different regions in the spot-welds were studied. Under cyclic loading, the BM had an asymmetric hysteresis loop. Cyclic behavior of magnesium spot-welds was measured using different specimen configurations, and the effect of geometrical factors on fatigue life was evaluated. A constitutive model was developed to model the asymmetric hardening behavior of wrought magnesium alloys under cyclic loading. An algorithm for numerical implementation of the proposed model was developed. The numerical formulation was programmed into a user material subroutine to run with the commercial finite element software Abaqus/Standard. The proposed model was verified by solving two problems with available solutions. A number of available fatigue models, as well as a new model proposed in this research were assessed by predicting fatigue life of magnesium spot-welds. The new model used a strain energy damage parameter. All models were evaluated by comparing the predicted and experimental fatigue lives for different Mg spot-welded specimens. The effect of considering the asymmetric hardening behavior of wrought magnesium alloys on the accuracy of the fatigue life prediction was not significant for the available experimental data. This was attributed to the limited experimental data on spot-welded specimens. The proposed material model and fatigue damage parameter were verified by simulating a real-life structure manufactured and fatigue tested by the US Automotive Materials Partnership. The results obtained from the proposed asymmetric model were compared with available symmetric simulation results and experimental data. The asymmetric material model along with the proposed damage parameter resulted in more accurate prediction of fatigue failure location and life.