Mixed mode fracture toughness as a separation parameter when cutting polymers

Material separation at the tool edge during the cutting of polymers has been interpreted using fracture mechanics. The different types of chip that can be produced in the same material under different conditions reflect the cube–square scaling inherent in elastoplastic fracture mechanics. In the case of PMMA, it is shown that both globally-elastic brittle spalls and continuous ribbons formed by plastic shear may be produced merely by altering the depth of cut. In the case of LLDPE and Nylon 66, only continuous chips are formed as it was not possible to take the large depths of cut required in the sledge microtome used for experiments. The shape of the discrete brittle spalls arises because the loading of the tool edge on the workpiece is asymmetrical, resulting in both Mode I and Mode II displacements. The same loading applies when ductile chips are formed. Results seem to show that measured fracture toughnesses when chips form in shear can vary with tool rake angle (or equivalently with primary shear plane angle) and a model (based on a rule of mixtures of critical crack tip opening displacements) is presented that may explain the variation.