Tham khảo tài liệu 'metal machining - theory and applications episode 2 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Simulation of BUE formation 233 Fig. Experimental distorted grid pattern from a quick stop test at a cutting speed of 25 m min mm d 4 mm 0 10 and without coolant Fig. Relation between flow stress and temperature of the C steel 234 Applications of finite element analysis above 600 C is so steep that deformation occurs easily. The secondary flow zone grid lines in Figure a compared with those in Figure a indicate the collapse of the BUE-range stagnant flow. The almost uniform secondary shear flow stress in Figure c can be attributed to compensation between work hardening and thermal softening. It indicates why despite varying strain strain rate and temperature along the rake face split-tool tests show a plateau friction stress almost independent of distance from the cutting edge although this does of course depend on the constitutive law chosen for the simulation as has been discussed in Chapter . In summary the BUE formation process in steels has successfully been simulated using the finite element method. Under practical cutting conditions where a BUE appears the chip flow property characterized by blue brittleness assists in developing the secondary shear flow into a stagnant zone. At the boundary between the developed stagnant flow and the main body of the chip conditions of high strain concentration low hydrostatic pressure and material brittleness are favourable for the separation of flow to form the nucleus of a BUE. The stagnant flow degenerates at higher cutting speeds because thermal softening prevails over work hardening. Simulation of unsteady chip formation Three examples of unsteady chip formation are described 1 chip flow force and residual stress variations in the low speed 13 mm min machining of a d-brass 60 Cu-40 Zn in conditions that lead to discontinuous chip formation Obikawa et al. 1997 2 changes in chip formation and resulting changes in tool fracture probability during transient chip flow at the end of a .
