Failure mode and fatigue behavior of ultrasonic spot welds with adhesive in lap-shear specimens of magnesium and steel sheets

Failure modes and fatigue behaviors of ultrasonic spot welds in lap-shear specimens of magnesium AZ31B-H24 and hot-dipped-galvanized mild steel sheets with and without adhesive are investigated. Ultrasonic spot welded, adhesive-bonded, and weld-bonded lap-shear specimens were made. These lap-shear specimens were tested under quasi-static and cyclic loading conditions. The ultrasonic spot weld appears not to provide extra strength to the weld-bonded lap-shear specimen under quasi-static and cyclic loading conditions. The quasi-static and fatigue strengths of adhesive-bonded and weld-bonded lap-shear specimens appear to be the same. For the ultrasonic spot welded lap-shear specimens, the optical micrographs indicate that failure mode changes from the partial nugget pullout mode under quasi-static and low-cycle loading conditions to the kinked crack growth mode under high-cycle loading conditions. For the adhesive-bonded lap-shear specimens, the optical micrographs indicate that failure mode changes from the cohesive failure mode near the interfaces under quasi-static and low-cycle loading conditions to the kinked crack growth mode under high-cycle loading conditions. For the weld-bonded lap-shear specimens, the optical micrographs indicate that failure mode changes from the cohesive failure mode near the interfaces through the adhesive and interfacial failure mode through the spot weld under quasi-static and low-cycle loading conditions to the kinked crack growth mode under high-cycle loading conditions.