IMPACT FRACTURE OF POLYETHYLENE - A NON-LINEAR-ELASTIC THERMAL DECOHESION MODEL

A model for brittle dynamic and impact fracture of tough polymers has recently been proposed, according to which a crack-tip Dugdale craze f ails by melting of a thin layer at each cohesive surface. A plane-stre ss, linear-elastic formulation accounted for the measured dynamic frac ture resistance to within the accuracy of measurement, and for the var iation of impact fracture resistance with impact speed, in two pipe-gr ade polyethylenes; but the predicted impact fracture toughness G(c) wa s in error by a factor of up to two. It is shown here that a pseudoela stic formulation, which accounts for non-linearity of the impact load/ displacement trace due to craze extension as well as to non-linear ela sticity, corrects these shortcomings. Impact fracture behaviour of a m edium density and modified high-density polyethylene between -20 and 2 3 degrees C, which embraces a transition to notch-ductile behaviour, i s predicted using stress/strain and thermal property data only.