Determining fracture toughness from cutting tests on polymers

A cutting test method has been developed for polymers and analysis schemes derived for the determination of the fracture toughness, Gc from the cutting data. The experimental scheme requires the measurement of forces for a cut of width b, in both the cutting direction, Fc/b and the transverse direction, Ft/b. Depths of cut were varied from 0.025 mm to 0.25 mm and the tool rake angle was varied from −20° to 30°. Cutting was performed at a speed of 10 mm s−1 on three polymers (HIPS, PA 4/6 and LLDPE). In addition, values of fracture toughness and yield strength were determined for the polymers using standard tests for comparison with the values obtained from cutting. Three analysis methods were derived to analyse the cutting data with the most favoured scheme based on an energy balance and using Merchant’s force minimisation criterion to determine the shear plane angle. This avoids the need to measure the cut chip thickness. Results for HIPS and PA 4/6 gave values of Gc in good agreement with the values determined via LEFM. However, the cutting method is intended for materials such as LLDPE which has a low yield stress and moderately high toughness, i.e. materials which cannot be tested using standard LEFM fracture mechanics tests. The cutting analysis appeared to give valid values of Gc for LLDPE in that they were independent of rake angle. There were some complications when analysing this polymer due to visco-elastic recovery effects in the chip and these have been considered. Finally, the cutting analyses always determined high values of yield stress which would appear to indicate work hardening.