Numerical modeling crack propagation of sheet metal forming based on stress state parameters using XFEM method

A microcosmic view field model containing a single micro-crack with different angles (0°, 45° and 90°) is investigated using the eXtended Finite Element Method (XFEM). Peak principal stress criterion is used in calculations for description of the crack propagation based on the experimental data concerning strength and fracture mechanics parameters. The stress state parameters are applied to explore the crack propagation conditions under various loading modes. The features of crack growth and fracture in different stress states are simulated, and the relations of crack grow directions and modes with the stress state parameters are discussed. The results show that the stress triaxiality is closely related with the crack propagation, and the higher the value, the easier the crack grow, while the crack would tend to close when the value is negative. The soft coefficient can reflect the situation of the failure mode, the crack mainly is mode I if the value is less than one and the crack is mostly mode II if the value is more than one. The Lode parameter reflects the strain states in mechanism and the deformation energy ratio can precisely reflects the influence degree of stress states on crack propagation. The results from these analyses on crack propagation are in good agreement with the experimental ones, and from micro-view point are reasonable for sheet metal forming.