2. Ductile Mode Cutting Mechanism

In this chapter, ductile mode cutting mechanism of brittle material is analysed theoretically and systematically. The coexisting crack propagation and dislocation extension in the chip formation zone are examined based on an analysis of cutting geometry and forces in the cutting zone, both on Taylor’s dislocation hardening theory and strain gradient plasticity theory. Ductile chip formation is a result of large compressive stress and shear stress in cutting zone, of which shields the growth of pre-existing flaws by enhancing material’s yield strength and suppressing its stress intensity factor K_I. Large compressive stress in cutting zone is obtained by satisfying two conditions: (a) very small undeformed chip thickness, and (b) undeformed chip thickness being smaller than tool cutting edge radius. Experimental verification shows that thrust force F_t is much larger than cutting force F_c in cutting of brittle material, which indicates that a large compressive stress is generated in cutting zone to enhance material’s yield strength by dislocation hardening and strain gradient, and shields the growth of pre-existing flaws by suppressing its stress intensity factor K_I. Thereafter, ductile mode cutting of brittle material is achieved when two conditions are satisfied, such that work material is able to undertake a large cutting stress in cutting zone without fracturing.