Modelling Spotweld Fracture Using CrachFEM

In recent years, a number of research institutes have concentrated on trying to develop fracture models that are generally applicable to a wide range of engineering problems. Examples of some of these fracture models are from Gurson and various extensions to Gurson[1], Dell and Gese (CrachFEM)[2], Xue-Wierzbicki[3], Wilkins (EWK model)[4], and du Bois (*MAT_GISSMO)[5]. The key features of these models are a dependency of the fracture strain on the stress triaxiality and a means of accounting for void growth or instability due to necking. Some of these models also incorporate non-linear strain accumulation, kinematic hardening and sophisticated plasticity models, which may be necessary for modelling certain types of materials. The Dell and Gese (CrachFEM) material model is a popular choice in the European automotive industry and has been used in this study. One of the application areas of concern is at or near to spot welds, where the material properties of the weld and Heat Affected Zone (HAZ) are very different to the sheet and fracture predictions can be signicantly affected by this. This work investigates the potential for developing an accurate 3D weld model to describe the lap shear and cross tension plug fracture modes observed in DP600. Obtaining stress strain curves for the weld nugget and HAZ is a challenge. The standard approach is to use heat treated test coupons to perform a range of non-standard material coupon tests, with test coupons having the same micro-structures as the weld nugget and HAZ. To prepare the heat treated test samples requires spot welding simulation to determine the required temperature time curves observed during spot welding followed by Gleeble testing to reproduce the required temperature time cycles on test coupons. This is difficult to achieve in practice and several iterations may be needed to achieve the required micro-structures. This set of tasks is a significant undertaking and has been the subject of a number of university PhD and post-doctorate research studies. Dancette [6] and Sommer [7] are very good examples of this research. In this study, a simplified approach has been adopted using weld micrographs and micro-hardness indentation tests to infer the geometry and stress strain properties and to assume the fracture properties are the same as the sheet. This approach lacks the rigour of material testing coupons with tailored heat treatments but does provide a simpler approach that can be implemented more easily in industry.