| In the field of sheet metal forming, fine-blanking technology has become a significant forming method. In fine-blanking process, the plastic formability of material is greatly improved because of the compressive stress state. However, complicated damage problems still exist in fine-blaking process. The verification and parameter calibration of ductile fracture criterion suitable for fine-blanking process are actualized by shear test. Therefore, it is necessary to investigate the ductile fracture behavior of sheet metal under the low triaxiality state.Based on a new symmetrical compression-shear test device, a series of numerical simulation and experimental work were accomplished in this study. By comparing the simulation results with the experienment results, the forming characteristics and fracture behavior of compression-shear test specimens, which are in different material and different stress states, were analyzed. In addition, the macro and micro fracture morphology are studied by qualitative and quantitative analysis methods to investigate the fracture mechanism. The conclusions are as follows.1) Based on the existing sheet metal shear test method, a new symmetrical compression-shear test is designed in this theis. With the optimization of specimen structure, various low and negative triaxialities could be built in the shear deformation zones.2) The plastic deformation of the compression-shear specimen is better than tensile-shear specimen so that the fracture failure of compression-shear specimen occurred later in the same loading conditions. Besides, the higher the tensile strength is, the larger the forming loads requires and the smaller the fracture displacement is. In addition, Cracks originated in the medial location near the edge of the deformed zones. The angle between crack propagation direction and the main loading direction will decrease when β increases.3) The macro-fracture can be divided into shear plane areas(region I) and dimples concentrated area(region II),which correspond to the typical shear-fracture mode and dimple-fracture mode respectively. In region I, the shear planes are step shaped distributed. In region II, dense parabolic dimples are elongated along the shear direction. The nucleation particles exist at the bottom of the dimples. When β increases, the proportion of region I will increase distinctly and the average dimple diameter will increase slightly.4) The average diameter and the aspect ratio of dimples are little difference in same material specimen. When β increases, the average diameter of dimples increase slightly and the dimple density decreases obviously. The average diameter of dimples is about 0.4~0.5μm. Besides, when the tensile strength increases, the average diameter of dimples will decrease, and the dimple density will change in parabola shape. |
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