Design of an in situ mechanical test for spot-welded joints

Facing the need of a better understanding of spot-weld strength and failure mechanisms of advanced high strength steels, we have developed a novel test for the in situ failure analysis of spot-welded joints. Providing loading conditions similar to the standard cross tension test, this new test is based upon the use of wedges to peel plates apart in a welded sample that has been cut in half, revealing the weld in the scanning electron microscope. We first perform finite element modeling and show why the wedge test approach is suitable for our purpose. We then perform experimental tests and observe failure in the scanning electron microscope chamber. We are able to both follow crack propagation and control the failure mode. As a corollary, this approach enables the use of local strain mapping, using digital image correlation, to measure local strains in the vicinity of the crack path. The wedge test should be valuable for assessing failure processes in spot-welded joints.

► We designed a mechanical test for spot welds that enables cracking direct observation.
► Wedges are used to peel sheets apart so that the setup fits in a SEM chamber.
► Failure is brittle-like in the fusion zone and ductile in the intercritical zone.
► In the fusion zone, strains around crack path localize on some phases only.
► In the intercritical zone, material deformation spreads further from the crack path.