The effect of hydrostatic pressure on the tensile properties of plastics

The effect of hydrostatic pressure, up to 112,000 lb/in.2, on the tensile properties of four polymers is reported. The pressure soaking of polystyrene in castor oil had no significant effect on the material's residual properties when tested under ambient conditions. When tested under pressure polystyrene necked, like a metal, and exhibited a brittle ductile transition at 40,000 lb/in2. Between 40,000 to 112,000 lb/in.2 the tensile strength increased by about 30%. Young's modulus and yield strength were only slightly affected by pressure. Similar results were obtained for specimens sheathed to prevent possible plasticization of the polystyrene. Polymethylmethacrylate tested at 112,000 lb/in.2 failed just short of its instability point and with only a slight increase in Young's modulus. These amorphous polymers thus behaved under pressure in a generally similar manner to metals. Pressure had a marked effect on the stress-strain curves of two crystalline polymers polyethylene and nylon. Young's modulus and tensile strength were considerably increased and elongation decreased. Pressure inhibited ‘drawing’ of the materials. Deformation was restricted to a small necked region.