Effects of environment on the mechanical properties of plastics under high pressure

Polystyrene (PS), high‐impact polystyrene (HIPS), and polyethylene (PE) have been investigated studying the pressure dependence of stress‐elongation behavior in tension over the range from atmospheric pressure to four kilobars at room temperature. The effect of strain rate was also observed for PS specimens. Tensile deformation of PS and HIPS has shown that the pressure‐transmitting fluid (silicon oil) acts as a stress crazing and cracking agent. Non‐sealed specimens of PS showed a brittle‐to‐ductile transition at 2.95 kbar while specimens sealed from the environment showed the same transition at only 0.35 kbar. Scales HIPS and PE specimens exhibited ductile behavior at all pressures. The extent of plastic deformation for PE was affected when specimens where exposed to the silicon oil environment. Surprisingly, HIPS exposed to the oil exhibited two transitions as the applied hydrostatic pressure was raised: a ductile‐to‐brittle followed by a brittle‐to‐ductile transition. Analysis of the stress‐elongation curves for sealed PS and HIPS specimens indicated that the pressure dependency of craze‐initiation stress differs from that of shear band initiation stress. The brittle‐to‐ductile transition occurred when the initiation stresses of both processes became equal. The principal stress for craze initiation showed almost no pressure dependency, suggesting that crazes initiate when the principal stress level of the tensile specimen reaches a critical value irrespective of the applied hydrostatic pressure.