Zones of material separation in simulations of cutting

FEM simulations of orthogonal cutting are reported in which both the Johnson Cook (JC) constitutive relation and the Johnson Cook separation (fracture or damage) criterion are used. Results demonstrate that the damaged regions, in which separation of material occurs at the tool tip, form thin boundary layers on the top of the machined surface and on the underside of the chip. Damage was calculated in terms of the parameters of the Johnson Cook fracture criterion appropriate for A2024-T351 aluminium alloy. The size of the damaged layers is some 35 mu m and appears to be independent of the uncut chip thickness to over the range investigated (50 < t(0) < 500 mu m). In most cases, the highly-damaged boundary layers make up only a very small proportion of the uncut chip thickness, so the deformation fields by which chips are formed are essentially the same as if the damage zone were absent. The result explains the success of variables-separable algebraic models of cutting with continuous chips in which the component works of chip plasticity, friction and separation are uncoupled. The FEM simulations predict quasi-linear relations between cutting force and uncut chip thickness, with an intercept on the force axis. This is exactly what is found experimentally and is predicted by algebraic models of continuous chip cutting under the assumption of sharp tool tip, where the slope of the plot relates to the yield stress of the workpiece and the intercept to its fracture toughness. The fracture toughness and the parameters of the JC damage relation, along with the size of the boundary layers of damage, are shown to be related. (C) 2016 Elsevier Ltd. All rights reserved.