Comparing the Long Term Behavior of Tough Polyethylenes by Craze Testing

Abstract

This chapter presents a new method of craze analysis using high constraint circumferentially deep notched tensile (CDNT) specimens. The notion behind the test is that when a specimen is loaded in tension, the deep symmetrical notches develop a highly constrained region within the confined ligament which cavitates and fails thus replicating the worst case damage mechanisms seen in polyethylene pipe in the field. The method involves measuring the local separation properties of a craze under both constant load and constant speed conditions and quantifying the long term behavior of different grades of tough polyethylene on this basis. Circumferentially deep notched tensile specimens were used successfully to assess the behavior of tough polyethylenes on the basis of their craze mechanics under constant speed and constant load conditions. Experimentally measured traction–separation curves under constant speed conditions were shown to provide good discrimination between the grades and highlighted the superior toughness of a particular grade of PE100. Results under constant load conditions provided a useful stress–time characterization of the grades and also highlighted in general the inferior performance of homopolymers compared to copolymers.