FEM design of cruciform biaxial tensile specimen based on limit stress analysis

The cruciform biaxial tensile test is an effective and practical method to realize the process of complex strain paths for the investigation of subsequent yield behavior and forming limit of sheet metal, and the key questions should be solved including the large deformation, homogeneousness of stress distribution and the method to measure the stress in the center region. Using cruciform specimen with thinning center region, the necking even fracture was realized. Three types of cruciform specimen with different geometrical shape of thinning center region were presented, and the homogeneous distribution of stress, the appearance of largest deformation and the avoidence of stress concentration in the center region were analysed. A mathematical method was discussed to calculate the stress in the center region. The result shows that the cruciform specimen with square thinning center region is the superior device for the investigation of forming limit stress diagram (FLSD) to obtain the more homogeneous stress distribution in the center region, to realize the large deformation and the fracture appears in the center region more easily, and to control the stress concentration preferably.