Spot welds produced via resistance welding have been widely used in the joining of sheet metal for autobodies since the 1950s. Every modern car contains over 2000 spot welds. Failure of the spot weld is therefore an important concern in relation to autobody durability and safty design. Spot welds can fail in two completely distinct modes, namly, nugget pullout failure and interfacial failure. In the present paper, it is first shown that the nugget pullout failure is caused by plastic collapse and the interfacial failure is governed by crack or fracture mechanics. These two failure mechanisms compete with each other and failure of a spot weld occurs when the fracture criterion for one of the mechanisims is satisfied first. A mathematical analysis combining the fracture mechanics and plastic collapse is presented to predict the failure mode, either nugget pullout or interfacial, of resistance spot welds. The analysis yields a critical weld nugget diameter, which distinguishes the two failure modes. The analytical results are validated through comparison with available test data. Furthermore, it is shown that the failure mode can be predicted using the derived formula for spot welds under either tensile (cross tension samples) or shear (lap shear samples) loads, and besides of that the analytical results may be applied to many different grades of steel varying from low carbon mild steels to high strength steels as well as under either quasistatic or impact loading conditions. By comparison with test data and industry standards for sizing weld nuggets, the results obtained in the present paper suggest a simple and improved formula for predicting the transition of failure mode from pullout to interfacial. With further validation, e.g. more test data for thin (t< 10 mm) gauge steels the mathematical modelling could be recommended for sizing of spot welds in steel sheets for automotive industry applications.
Failure mode of spot welds: interfacial versus pullout
Science and Technology of Welding and Joining ; 8 , 2 ; 133-142
2003
10 Seiten, 10 Bilder, 12 Quellen
Article (Journal)
English
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