Fatigue behavior and structural stress analysis of coach-peel and lap-shear friction stir welded joints of AZ31 magnesium alloy
In this work the fatigue behavior of coach-peel and lap-shear friction stir linear welded joints of AZ31 magnesium alloy sheet were evaluated under different loads and the results were compared using structural stress analysis. Lap-shear coupons of 30 mm in width were obtained from a welded overlap configuration. However, since coach-peel configuration coupons were not available, aluminum L-shaped brackets were adhesed weld to create the coach-peel configured specimen. The experimental fatigue life results showed an inverse relationship between applied load and the number of cycles to failure for both types of coupons. However, due to differences in the configuration of the joint leading to higher applied stress, coach-peel coupons failed at comparatively lower loads than lap-shear coupons. In order to correlate the fatigue data with stresses developed at the weld, finite element analysis, using shell/plate elements, was employed to model the joints under cyclic loading. The modeling effort focused on several post-process techniques including equivalent stress and structural stress methods. While the structural stress approach has merits over conventional nominal equivalent stress approach for welded joints including accuracy of the results and mesh insensitivity, it could not correlate the lap-shear and coach-peel fatigue results into a master design curve. As such, a fatigue damage parameter (FDP) was introduced to correlate the fatigue results of both the joints, thus establishing a basic relation to account for differences in stress at the weld for lap-shear and coach-peel welded configurations.