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In this paper, failure modes of dissimilar laser welds in lap-shear specimens of low carbon steel and high strength low alloy (HSLA) steel sheets are investigated based on experimental observations. Micro-hardness tests across the weld zones of dissimilar laser welds were conducted. The hardness values of the fusion zones and heat affected zones are significantly higher than those of the base metals. The fatigue lives and the corresponding failure modes of laser welds as functions of the load ranges are then examined. Optical micrographs of the laser welds before and after failure under quasi-static and cyclic loading conditions are then examined. The failure modes and fatigue behaviors of the laser welds under different loading conditions are different. Under quasi-static loading conditions, a necking failure occurred in the upper low carbon steel sheet far away from the laser weld.

In this paper, the analytical stress intensity factor and J integral solutions for welds in lap-shear specimens of two dissimilar sheets based on the beam bending theory are first reviewed. The solutions are then presented in the normalized forms. Next, two-dimensional finite element analyses were selectively conducted to validate the analytical solutions based on the beam bending theory. The interface crack parameters, the stress intensity factor solutions, and the J integral solutions for welds in lap-shear specimens of different combinations of steel, aluminum, and magnesium, and the combination of aluminum and copper sheets of different thickness ratios are then presented for convenient fracture and fatigue analyses. The transition thickness ratios for critical crack locations for different combinations of dissimilar materials are then determined from the analytical solutions.