Large strain bulk deformation and brittle tough transitions in polyethylenes

Some tough, crystalline polymers can fail by fast brittle fracture. This thesis explores the role of ductile 'shear lips', which form at the fracture surface verges, in brittle-tough transitions. A new laboratory method was used to isolate this region, and to test its ability to draw rapidly, in polyethylenes. The test uses a conventional Charpy type specimen that is deeply notched and impact loaded in three-point bending by a single striker. The ligament, rapidly loaded in almost pure tension, first yields, and then necks down until failure. Initial results are encouraging and correlate well with the in-service performance. A fundamental study of large strain deformation, that avoids the complexity associated with impact tests, was then conducted with the aim of isolating the dominating influences that furnish a polymer with the ability to sustain rapid large strain deformation. True stress vs. true strain curves have been interpreted using the one dimensional spring dashpot model of Haward and Thackray (H-T). The analysis led to the computation of load extension curves in both plane strain compression and uniaxial tension and has successfully simulated analogous necking and drawing behaviour at rates spanning both isothermal and adiabatic conditions. The excellent comparison between computed results and those measured experimentally vindicates use of the HT model and corroborates the physical principles upon which it is based. Finally, the relevance of this behaviour to craze fibril drawing, and hence to fracture, is explored