13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- All simulations were performed using the finite element code ABAQUS/Explicit version 6.9. The calculated force vs. displacement curves for the six different loading situations of the spot welded joints are shown together with the corresponding experimental data in Fig. (12). The measured force vs. displacement curves of all specimens are reproduced well by the numerical results. Also the calculated and experimental load bearing capacities (see Table (2)) agree well. Figure 12. Comparison of measured and calculated force vs. displacement curves for tensile-shear- , KS2-0°, -30°, -60° and -90° and coach-peel specimens Failure of the joints in shear tests, i.e. TS- and KS2-0° tests, is calculated as pull-out fracture inside the softened HAZ of the hot-stamped 22MnB5. During loading of both of the shear-specimens deformation localizes inside the SHAZ of 22MnB5 leading to pull-out fracture of the joint. Failure of the SHAZ occurs due to the Thomason criterion. The calculated fracture location was confirmed in observations of polished and etched cross-sections of the tested TS-specimens (see Fig. (13)). Figure 13. Calculated pull-out fracture of TS-specimen inside the softened HAZ of 22MnB5 (left and middle) Polished and etched cross-section of a tested TS-specimen showing pull-out fracture inside the SHAZ (right) In pure axial loading situations in KS2-90° tests, pull-out fracture of the joints does not initiate inside the softened HAZ, but occurs inside the coarse grain HAZ of 22MnB5 directly adjacent to the weld nugget. The same fracture behavior is observed under combined loading conditions in KS2-30° and KS2-60° tests as well as in CP-tests. Fracture occurs in the ultra-high strength steel due to the lower sheet thickness and the lower ductility of its HAZ compared to that of the microalloyed steel. In all cases the critical criterion for failure initiation of the joints in simulations was found to be the Thomason criterion. As an example Fig. (14) shows the comparison between calculated and experimentally observed fracture appearance and locations for KS2-90° specimens.
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