Experimental determination of velocity and strain rate fields in metal cutting of OFHC copper

Abstract

Metal cutting subjects the material being cut to strains greater than 100% at high strain rates between 103/s and 106/s and may be a good test for mechanical properties in this strain rate regime. This thesis presents an experimental study of the velocity field in the primary shear zone (PSZ) while cutting OFHC copper under six different cutting conditions. A new type of ultra high speed camera is used to acquire sequences of high resolution stereoscopic microphotographs of the PSZ at frame rates from 50 kHz to 1 MHz. The velocity field is obtained by 3D stereoscopic correlation of the images. The gradient of the velocity field yields the strain rate field. These fields are averaged over multiple experiments for each of the six test conditions to obtain the average fields for each test condition. It is found that the strain rate field scales up with increase in cutting speed and decrease in depth of cut. Under conditions that result in low strain rates, the PSZ resembles a triangular shear zone, narrower at the cutting edge and wider at the free surface. Under conditions that produce higher strain rates, the PSZ is parallel sided. In both cases, the strain rate is found to decrease monotonically from the cutting edge to the free surface. It is found that the decrease in strain rate causes a corresponding decrease in strain in the chip, from higher strain values in the chip near the tool to lower stains in the chip near the free surface. This occurs in spite of the fact that tools used are very sharp, with cutting edge radius < 1μm and a high rake angle of 30°, in an effort to minimize the indentation component. Slip line fields derived from the velocity fields show that the higher strains closer to the cutting edge are caused by a decrease in the shear angle of the lower boundary of the PSZ. Hencky’s equations indicate an increase in hydrostatic pressure near the cutting edge and this suggests that the increased strain and hydrostatic pressure may be attributable to the friction at the chip-tool interface. While the strain rate distribution usually shows a broad zone of distributed shear deformation, the strain rate distributions in some tests (more often under conditions that result in low strain rates) show fine structural features characteristic of persistent shear bands.