The mechanical and thermal behaviour of polymers under high strain rate compression
Relatively few studies have been carried out on polymers at high rates of deformation compared to more traditional materials such as metals, and it is therefore important to develop constitutive models to help predict how materials will behave under specified conditions. The stress-strain behaviour of polymers shows a very marked dependence on time (or rate) and temperature. Polymers (including polyethylene, polypropylene, nylon 66, polyetherketone, polyetheretherketone, a liquid crystal polymer, polyethersulphone and polycarbonate) have been compressed at strain rates of -10-3 to 1O-1s-1 (using an "Instron" which measures deformation versus time at constant rates of compression) and _103s-1 (using a novel drop-weight impact machine) to strains of up to -100%. This drop-weight system is different from commercially available machines in that it operates in compression rather than fracturing specimens and provides stress-strain data directly. The initial crystallinity and orientation of specimens were examined using x-ray diffraction, and kinetic decomposition parameters were obtained using differential scanning calorimetry. Also thermocouples were inserted into some specimens compressed at high strain rates in order to measure any rises in temperature. Several important results have emerged so far: I) sufficiently high bulk temperature rises occ;ur during high rate deformation to considerably alter the stress-strain curve from isothermal conditions; 2) localised deformation in the form of cracking or shear banding in tough polymers appears to lead to temperature rises sufficiently high for significant thermal decomposition to occur; 3) data obtained at lower rates could be approximately fitted to the Eyring Theory unlike that obtained at the highest rate; 4) initial investigations suggest that Poisson's ratio varies with strain and strain rate and is not a constant of 0.5 as generally assumed.