Burrs and fractures are two types of shapes formed on exit edges in pricision machining. They affect the quality of workpiece and the efficiency of prodcution seriously. A finite element model of fracture formation process to simulate the machining process of 2024-T3 Aluminum alloy is proposed. According to simulation results, five stages and variation of shear shain in fracture formation process are analyzed. It is found that shear strain plays an important role in fracture forming process. The method of determination of critical shear strain for fracture forming is proposed from simulation results.