Strength evaluation of ultra-high strength steels and spot weld of automotive bodies

Cockcroft-Latham fracture criterion was applied to predict the fracture of high strength steels. A Marciniak-type biaxial stretching test of the four classes of high strength steels was carried out to measure the material damage limit of Cockcroft-Latham fracture criterion. Furthermore, in order to improve the simulation accuracy, the local anisotropic parameters depending on the plastic strain (strain dependent model of anisotropy) were measured by digital image correlation method and incorporated into Hill’s anisotropic yield condition by authors. To confirm the validity of Cockcroft-Latham fracture criterion, the uniaxial tensile tests based on JIS No.5 tensile specimen were performed. The force-displacement history and fracture happening strokes were predicted with high accuracy. Then, Cockcroft-Latham fracture criterion was applied to predict the failure of four types of spot welded joints. To simulate the local bending and warping deformations around the heat-affected zone, the discrete Kirchhoff triangle element was adapted. FEM results for four classes of high strength steels and four types of spot welded joints had a good correlation with experimental ones.〖KH-*2〗〖KH-*2〗