Failure mode, fracture and fatigue analyses of resistance spot welds and spot friction welds.

Abstract

Failure modes and fracture of resistance spot welds are investigated based on experimental observations and a mechanics approach. An elastic analysis indicates that the initial yielding starts on the two side edges of the nugget in the sheet. An elastic plastic finite element analysis suggests that the initial necking failure occurs near the middle of the nugget circumference in the sheet as observed in experiments and as indicated by the forming limit diagram (FLD) for ductile sheet metals. Next, closed-form solutions of the mode I stress intensity factor for spot welds in lap-shear specimens are derived based on the classical Kirchhoff plate theory. Closed-form solutions for mode I, II, and III stress intensity factor solutions of spot welds in lap-shear specimens are suggested. Failure modes of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets under quasi-static and cyclic loading conditions are also investigated. Optical and scanning electron micrographs of the welds before and after failure show that the microstructure, geometry, and the failure modes of the welds made by different tools are different. For the welds made by the concave tool under quasi-static loading conditions, the failure starts from the necking of the indented sheet outside the weld. Under cyclic loading conditions, the failure comes from the kinked fatigue crack growing in the non-indented sheet outside the stir zone. For the welds made by the flat tool under quasi-static loading conditions, the failure starts from cracking near the boundary of the stir zone close to the indented sheet surface inside the weld. Under low-cycle loading conditions, the failure comes from the kinked fatigue crack growing near the boundary of the stir zone inside the weld. Under high-cycle loading conditions, the failure comes from the kinked fatigue crack growing in the non-indented sheet outside the stir zone. Based on the experimental observations of the dominant fatigue cracks, a fatigue crack growth model is then adopted to estimate the fatigue lives of the spot friction welds.