Material removal in orthogonal cutting of ceramics was previously found to be characterized by a trajectory of crack which unstably extends downwards from the region near the tool tip. It was also shown that the crack extended to some distance, re-extended after a pause, and finally turned upwards to emerge out at the free surface. The unstable extension of crack was satisfactorily elucidated in terms of the stress field in the region near the tool tip, but its subsequent extension, especially that to the free surface, still remains unsolved.
This paper confines attention to the stress field which prescribes the trajectory of a crack just after its pause and its upward extension. Photoelastic model plates of diallyl phthalate having a machined pre-crack at the region near the tool tip were orthogonally cut at a relatively low speed (0.1mm/min). The main results obtained are as follows.
(1) The feature that a newly initiated crack extends more downwards from the tip of the precrack and turns upwards immediately after some turbulence in the region near the tool rake face is typical and quite similar to that of ceramics.
(2) A new crack initially extends in the direction which is determined by the stress singularity detectable by isochromatic fringes near the tip of the pre-crack. The trajectory of its subsequent extension is also determined by the isoclinic fringe loops which are formed in such an elliptical vortex generated at the tip of the pre-crack so as to follow a line connecting the farthest ends of the ellipses. In the region far from the tip of the pre-crack, the crack extension is scarcely affected by the pre-crack, and therefore the direction of crack extension is perpendicular to that of the maximum tensile principal stress in the model without any pre-crack.
(3) The upward extension is caused under the condition that the stress intensity factor KII of the crack becomes negative. This condition is realized when some fracture takes place in the region above the extended crack.
