Tham khảo tài liệu 'metal machining - theory and applications episode 2 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Cutting edge design 253 Fig. Initial finite element model used for chip breakability analysis angular velocity Vh Rc-t2 at point h from the ICM analysis is always greater than that of Vg Rc at point g without a force at B the chip path would penetrate the tool. The contact forces at C are assumed to obey Coulomb s laws of friction. While the chip slips on the work surface t mdơt holds. If the relative velocity between the chip and workpiece becomes zero then the chip is regarded as adhering to the surface. The adhered chip does not slide again until t msơt. The static and dynamic friction coefficients ms and md are assumed to be and respectively. As for a fracture criterion in the chip beyond its formation region it is assumed that a crack nucleates and develops from the chip s rough free surface when the maximum principal stress or the maximum shear stress exceeds a critical value s1c or tc. A crack that satisfies s1c propagates in the direction of minimum principal stress whereas one that satisfies tc grows in the direction of maximum shear stress. In this work s1c 880 MPa and tc 440 MPa have been found to give good representations of practice. To follow the crack growth it is necessary to subdivide the elements around the crack tip and this requires reorganization of the node connectivity too. Remeshing around the point B is also required - and a small time step of 10-6 s for the cutting speed of 100 m min is also needed. Simulation results Figure shows the chip shape simulated with changing WG and hB 0. As WG increases from mm to mm the radius of the broken chips becomes larger and at WG mm chip breakage does not occur. 254 Applications of finite element analysis Fig. Predicted chip shape with changing W- rB 0 Increasing hB aids chip breakage. Figure shows the development of chip shape with time for WG mm but hB mm. Plastic deformation with e 10 s 1 takes place at the hatched regions in the figure and the chip .
