Research On Plastic And Damage Behavior Of Sheet Metal By Using Crystalline Plasticity

Sheet metal forming is one of the most important industrial processes. The mechanical properties of a polycrystalline metal depend on its microstructures. The strength and formability of the sheet metal are related to its crystalline structure, especially in terms of the orientation distribution and the feature of dislocation slip. To describe the effect of microstructures on macroscopic properties, the evolution of microstructure property has been the hot topic of numerical simulation on sheet metal forming.The mechanical properties of the microstructure are embodied in the constitutive framework of polycrystalline plasticity theory. Mechanical properties such as plastic anisotropy at the macroscopic level are derived from averaged microscopic responses. Crystalline plasticity has been more and more widely applied for it's advantage on describing the microstructure. By using theoretic analysis, numerical calculation and experiment verification, the influence of crystallographic orientation, heterogeneous plastic deformation at grain level and microstructure evolution on plastic and damage behavior of sheet metals has been investigated. The results are as follows:To analyze the softening effect of surface grain, a 3D FE model for FCC polycrystal is established, in which each grain is given a different orientation, and the grain boundary constraint is simulated by element boundary. The FCC flow stress responses are simulated for various relative grain size, and the results are analyzed statistically; It is showed that the influences of crystal orientation and grain boundary constraint are described effectively by using crystal plasticity constitutive, and the meso-mechanism of the scale effect caused by surface grain softening is well interpreted.The compression of a cylindrical void in aluminum single crystal was analyzed, and the simulation of crystal plasticity model is effective by comparing with experiment. 3D single crystal and bicrystal FE model are established to simulate the evolution of sphere void in FCC single crystal with different orientation, and it shows that the crystallographic orientation,grain boundary orientation,load orientation are the important factors which affect the evolution of void shape, void growth direction, void coalescence and the crack formation. The damage mechanics and its'evolution can be described well, and the relation ship between meso-scale void damage and macroscopic anisotropic fracture behavior can be established by crystal plasticity model, that is: the active slip system of adjacent area near the void is different due to the different crystallographic orientation, and the grain-scale inhomogeneous plastic forming, then the crack is induced. This model can be used to analyze fracture behavior of sheet metal with fewer grains in micro plastic forming.The forming limit of FCC polycrystal is investigated by using crystal plasticity constitutive combined with M-K approch. The effects of initial defect,strain-rate sensitivity,hardening parameter,initial and deformation induced texture on FLD have been revealed. It shows that crystal plasticity theory, by taking the effects of initial texture and its deformation-induced evolution into account, is more effective than phenomenological model on predicting FLD of anisotropic sheet metal. Then the forming limit of BCC polycrystal with latent hardening is investigated. It shows that the FLD is much lower than that without latent hardening, which is especially obvious at biaxial tension stage.The FLD under proportional and non-proportional (pre-tension,pre-biaxial tension,pre-plane strain tension) strain paths have been simulated by crystal plasticity. The effect of texture on forming limits of FCC and BCC sheet metals are analyzed in meso-scale. The wide blank tension and circular blank drawing process are designed, and the simulation results are compared with experiment. It shows that texture evolution and slip system hardening induced by prestrain have an important effect on FLD. This kind of crystal microstructure effect can be described well by crystal plasticity, which is valuable to predicting the failure of sheet metal under complicated strain path in plastic forming process.