Effect of hydrostatic pressure on the yield and fracture of polyethylene in torsion

Yield and fracture of polyethylene have been studied in torsion tests under superposed hydrostatic pressures. Two ductile-to-brittle transitions have been observed. At high strain rates and pressures, a conventional ductile-to-brittle transition was found with increasing strain rate and pressure. A second ductile-to-brittle transition was observed at low strain rates with decreasing strain rate. The yield stress showed a region of low, relatively constant, rate dependence at low strain rates, high temperatures and low pressures and a second region of higher strain-rate dependence at high strain rates and pressures. In contrast, the fracture stress was found throughout to have a relatively constant strain-rate dependence of intermediate value between those obtained for the yield stress. These features confirmed that failure can be considered as competition between yield and fracture processes. The fracture stress became lower than the yield stress at both high and low strain rates where brittle fracture was observed, with fully ductile behaviour resulting in intermediate conditions where the fracture stress exceeded the yield stress. The pressure, strain rate and temperature dependence of the yield stress was well described by two Eyring processes acting in parallel, both processes being pressure dependent.