Low Temperature Fracture Strain at High Deformation Rates

Abstract

One undesirable property of most polymers is their brittleness at low temperatures. Only some linear polymers, e.g., PC, PSU, or PE, show some ductility at 4.2 K. On the basis of previous experience, it is expected that brittleness increases with increasing deformation rate. This would be a further limitation on polymeric materials for low temperature applications. Strain rate dependence was tested at low temperatures on some selected polymers. The investigations showed that a decrease of fracture strain, ε_F occurs only up to a certain strain rate specific to each polymer. At higher strain rates, fracture strain increases remarkably. One explanation of this strange behaviour can be found in the relaxation phenomena of polymers. In earlier studies,^1,2 the rate dependence of stress concentration factors (toughness), K_Ic, was investigated on precracked samples. K_Ic is a measure of the tensile uniaxial load just prior to crack propagation. A peak in K_Ic was found on the same frequency-temperature scale as secondary relaxation (damping) maxima. The experimental deformation time was equal to the relaxation time. These experiments were performed at low deformation rates and cause isothermal deformation at the crack tip.