Stress Dependence of Poisson's Ratio and of the Softening Temperature of Plastics

Poisson's ratio was measured on a variety of plastics as a function of load up to 4000 psi by two methods. Poisson's ratio generally remained constant or decreased slightly with load. However, with crazed polyblends the ratio drastically decreased from 0.33 to 0.15 at high loads applied for long times. The low value can be explained by the growth of cracks perpendicular to the direction of the applied tensile stress. It has been shown that the lowering of the heat-distortion temperature with stress can not be explained in terms of the lowering of the glass temperature resulting from the volume increase on application of a stress. Instead, the lowering of the softening temperature can be explained in terms of the shape of the modulus vs. temperature curve. The new theory agrees quite well with the experimental values. Materials with a broad softening range have a heat-distortion temperature which is strongly dependent upon the load.