Dilatometric studies of polymers undergoing high and low rate tensile deformation

Specific volume change and stress-strain data were obtained simultaneously during tensile deformation on several plastics known to be resistant to impact loading. Tensile deformation rates of 20 percent/minute and 106 percent/minute and temperatures of −190° to 55°C were employed. A common sequence of deformation modes was observed in all materials studied (rubber modified acrylics and styrene, ABS materials, polycarbonate, impact grade polypropylenes, and high density polyethylene). In all cases the major mode of deformation to failure at low rates and/or higher temperatures is volume conserving and primarily a shear flow process. At higher rates of deformation or lower temperatures, a transition occurs and the specific volume of the material increases in direct proportionality to the tensile strain above the apparent yield point. Volume increases of 17 to 50% were observed and these were equal to 85 percent or more of the observed tensile strain at failure. These observations indicate that microcavitation may be the major process available for the absorption of mechanical energy at impact rates in plastic materials.