Micromechanisms of slow crack growth in polyethylene under constant tensile loading

Circumferentially notched specimens of a first generation and a third gener ation pipe-grade of high density polyethylene with similar weight average m olar masses have been subjected to constant tensile loads at 80 degreesC. A transition from full ligament yielding to failure by stable sub-critical c rack growth was observed as the applied load was decreased. The specimen li fetimes in this latter regime were dependent on the initial stress intensit y factor, Ki, and failure was associated with slow crack propagation preced ed by formation of a wedge-shaped cavitational deformation zone at the notc h tip. The fibril diameters in the deformation zones decreased with stress intensity factor near the transition, the limiting behaviour of a relativel y slow crack growth resistant third generation grade at the lowest Ki being inferred from testing in Igepal (TM) to be the breakdown of diffuse zones of interlamellar voiding. This regime was not directly accessible to testin g in air within the allotted experimental times. However, comparison with t he results of accelerated testing in cyclic fatigue has indicated stable in terlamellar voiding in the third generation grade not to necessitate the pr esence of Igepal. Moreover, in both grades, very similar modes of deformati on were observed in air and in Igepal at relatively high Ki. Igepal was the refore inferred not to lead to qualitative changes in the range of mechanis ms that are characteristic of slow crack growth in polyethylene. (C) 2001 E lsevier Science Ltd. All rights reserved.