It was previously found that material removal in orthogonal cutting of ceramics was characterized by a trajectory of a crack: it unstably extended downwards by some distance from the region near the tool tip, after a pause re-extended, and finally turned upwards to emerge out at the free surface. The crack trajectory was revealed to be considerably affected by the position of crack initiation and the magnitude of the material fracture ahead of the tool rake face during cutting as well as property of the material.
This paper deals with the effect of material plasticity on the position of crack initiation and the role of the material fracture to the trajectory of the crack extension. The finite element method was employed to derive the distributions of stress in models corresponding to those at the incipient stage of orthogonal cutting. The main results obtained are as follows.
(1) A macro crack in an elasto-plastic material initiates near the elastic-plastic boundary in a region ahead of the tool tip and extends both backwards and forwards following a stress trajectory of the minimum principal stress. The backward extention terminates at a position above the tool tip, while the forward extention trends downwards so far as the material fracture does not occur. In the case that the material fracture takes place in the whole region above the extended crack, the crack turns upwards to emerge out at the free surface. If the material fracture is restricted in a region smaller than the above, however, the crack extends downwards after it does upwards for a short distance.
(2) A macro crack in an elastic material initiates near a region ahead of and below the tool tip. Before the material fracture occurs, it extends in a manner similar to that in the elastoplastic material, the end of the backward extention being at a position below the tool tip. Even if the material fracture takes place, the crack continues to extend downwards except that it is arrested by removal of the whole region above the extending crack.
