Fatigue behavior of dissimilar ultrasonic welds in lap-shear specimens of AZ31 and steel sheets

Abstract

Fatigue behavior of dissimilar ultrasonic welds in lap-shear specimens in magnesium and steel sheets is investigated. The welds were produced using a Sonobond ultrasonic spot welder to join magnesium AZ31B-H24 to two types of steel. The lap-shear specimens have been machined into a dog-bone profile to approximate a linear weld. The lap-shear linear welds were studied based on experimental observations, closed-form stress intensity factor solutions, and a fatigue life estimation model. Optical micrographs of the welds after testing were examined to understand the failure modes of the welds. The micrographs show that the welds tested under cyclic loading failed from kinked fatigue cracks growing through the magnesium sheets. The closed-form stress intensity factor solutions for each side of the weld are used to explain the location of fatigue crack initiation and growth. Two-dimensional finite element analyses of the lap-shear specimens with the welds were carried out to obtain the global and local stress intensity factor solutions for the main cracks and kinked cracks, respectively. A kinked fatigue crack growth model based on the global and local stress intensity factor solutions for finite kinked cracks obtained from the finite element analyses is adopted to estimate the fatigue life of the welds. The fatigue life estimations based on the kinked fatigue crack growth model generally underestimate the experimental results. The kinked crack growth model shows little sensitivity to the welding indentation or the thickness of the steel sheet.