The effect of orientation on the mechanism of deformation of polymers

The mechanical behavior of HDPE, medium-density PE, and amorphous and amorphous-crystalline PET after their preliminary orientation is studied. The polymers are oriented by rolling at room temperature on lab-scale rolls, tensile drawing at temperatures somewhat higher than their glass-transition temperatures, and extrusion at room temperature. At low degrees of rolling (below 1.5), the tensile yield stress does not actually increase. (In amorphous-crystalline PET, this parameter even decreases.) It seems that the absence of strain hardening at low draw ratios is a common feature of the behavior of polymers below their glass-transition temperatures. In contrast to the tensile yield stress, the engineering strength increases in proportion to the degree of rolling. A new procedure for construction of the dependence of true tensile yield stress on tensile strain is advanced. At low strains, the true tensile yield stress shows practically no increase. This conclusion is verified by theoretical calculations.