A Fracture-Mechanics and System-Stiffness Approach to Fatigue Performance of Spot-Welded Sheet Steels

To improve fuel economy, the automotive manufacturers have been gradually incorporating lighter gage, higher strength sheet steels. For the same fatigue loads, the operating stresses of these spot-welded lighter-gage sheets will be greater causing a concomitant reduction in fatigue life. This paper describes the results of a program using fracture-mechanics concepts to determine the fatigue life of spot-welded connections of various stiffness. A model is developed which predicts the fatigue life, and shows that life increases with increasing joint stiffness. Two types of fatigue cracking were observed and found to be competing throughout the fatigue life. The type of cracking dominating the final failure was also related to the joint stiffness. Above a critical stiffness, shear failure could occur. Below this critical stiffness, sheet failure resulted, and the fatigue life was related to the square root of the change in rotation.