Research On Mesoscopic Fracture Criterion Of Dual Phase Steel

Automobile Lightweight makes people are increasing looking for new materials, in order to accommodate the requirements of safety aspect after lightweight. The dual phase steel is gradually applied to various beam structure of car body, because of its good mechanical properties and formability. However, dual phase steel easily occur shear fracture during forming, it’s limitation research of dual phase steel shear fracture failure criterion from the macroscopic phenomenological. In recent years, mesoscopic damage has becomed a hot research topic, due to it can understand the process and physical nature of material damage. In this paper, GTN(GursonTvergaard-Needlman) and modified GTN are used to research different fracture of dual phase steel, in order to built micromechanical criterion to predict fracture of dual phase steel.Firstly, in the article, the Hill ’48- GTN mesoscopic damage model is builded and the corresponding constitutive equation is deduced. Combined with the explicit finite element method, Fotran language is used to write the VUMAT subroutine of Hill ’48 – GTN.Secondly, because of it is difficult to obtain accurate GTN damage model parameters, a new method is proposed based on power index material hardening model of damage parameters. By using the method of experimental design to select different damage parameters combination, establish the relationship between damage parameter and hardening parameter, using the inherent basic parameter of material when establishing evaluation function, it can obtain more accurate damage parameters, and the damage parameters of DP780 is determined through this method.Thirdly, due to steel phase is easy to produce the shear fracture, shear damage is introduced to the original void volume fraction, the contribution of shear deformation is equivalent to the void volume growth. Two different damage growth is used to simulation the tensile and tensile- shear specimens, the void volume considering shear damage evolution is proved better to predict the failure in pure, but the damage value is high in the higher stress triaxiality.Finally, shear damage is defined by the shear ultimate plastic fracture strain, the damage is introduced to GTN constitutive equation. Through the tensile shear test, the relation is received between shear damage and stress triaxial, using the least squares fitting out the function. Mesoscopic fracture criterion of dual phase steel was established according to the shear damage ultimate value and void volume fraction at different stress triaxial. The simulation result of the stretch bending forming under the different radius is proved the correctness of the criterion. The floor beam fracture process is simulated.