Failure mode and fatigue behavior of weld-bonded lap-shear specimens of magnesium and steel sheets

• Examine fatigue of ultrasonic spot welds of Mg and steel with or without adhesive.
• Spot welded specimens fail from kinked crack growth.
• Adhesive-bonded specimens fail from cohesive failure and kinked crack growth.
• Weld-bonded specimens fail in cohesive, interfacial failure and kinked crack growth.
• Estimate fatigue lives of adhesive-bonded and weld-bonded specimens.

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 were investigated. The spot welded specimens failed from the kinked crack growth mode. The adhesive-bonded specimens failed from the cohesive failure through the adhesive and the kinked crack growth through the magnesium sheet. The weld-bonded specimens failed from the cohesive failure through the adhesive, the interfacial failure through the spot weld, and the kinked crack growth through the magnesium sheet. The estimated fatigue lives for the adhesive-bonded and weld-bonded specimens failed from the kinked crack growth mode are lower than the experimental results.