DISCONTINUOUS CRACK-GROWTH IN POLYETHYLENE UNDER A CONSTANT LOAD

The kinetics of slow crack growth were measured in a polyethylene copolymer in a notched tensile specimen under constant load. The microscopic changes in the crack morphology were linked to the crack opening displacement and to the crack advance. The jump distance during the discontinuous crack growth decreased as the applied stress decreased. The jump distance decreased as the temperature decreased because the yield point was increasing. These observations are explainable by the Dugdale theory. The initiation time for fracture depends on the rate of disentanglement of the fibrils in the craze and occurs in the fibrils adjacent to a tough skin at the base of the craze. The crack grows until it meets fibrils whose strength matches the value of the stress intensity at which point the crack is arrested. Re-initiation of fracture occurs when the fibrils at the root of the crack have been sufficiently weakened by the process of disentanglement. Thus, the overall kinetics depend on (1) the rate of disentanglement of the fibrils, (2) the gradient in the fibrilla strength between the base and the tip of the craze, (3) the value of the stress intensity, and (4) the yield point of the matrix.