The effects of combined pressure–temperature on mechanical behavior of polypropylene

The effects of combined pressure and temperature on the mechanical behavior of polypropylene have been studied. Tests were conducted in tension and compression superimposed on various hydrostatic pressures up to 7 kbar at temperatures of 20, 50, and 75°C. The experimental data have been analyzed in view of molecular and continuum approaches. It has been observed that the Young's modulus and the yield strength in both tension and compression increased significantly with increasing pressure at all temperatures studied. However, the rate of increase of the Young's modulus undergoes abrupt change about the glass‐transition pressure ( P g ). The P g is linearly dependent on the test temperature and the pressure coefficient of the T g is estimated, from P g versus temperature relations, to be about 18°C/kbar for the polypropylene samples of this study. Pressure dependence of the yield stress is described by a generalized Eyring theory incorporating pressure effects and two flow mechanisms, the α‐ and the β‐relaxation processes. The theory predicts a bilinear dependence of the yield stress of polypropylene on hydrostatic pressure as observed in the tests. The paper also described a method of healing stress whitening in polypropylene by a combination of shear stress and hydrostatic pressure.