CREEP AND YIELD BEHAVIOR OF POLYETHYLENE

Extensive studies of the creep and recovery behaviour of oriented poly ethylene have been undertaken because of the potential engineering app lications of high modulus polyethylene fibres and large section orient ed materials made by the Leeds die-drawing process. Two primary requir ements ate a valid protocol for the practical assessment of the creep behaviour and an adequate formal representation which can provide the basis for a physical interpretation of the creep processes in terms of the structure of the oriented materials. The practical requirements o f improved creep performance lead to examination of the effects of mol ecular weight and copolymerisation on creep and recovery, together wit h studies of the effects of radiation cross-linking either before or a fter drawing. A particular new feature has been the development of a p rocess for cross-linking in acetylene, where it has been shown that di ene bridge crosslinks are produced with very little chain scission. Th e protocol adopted for assessing creep behaviour is to construct creep strain rate/strain (Sherby-Dorn) plots which, for highly oriented fib res, show constant strain rates after an initial period. These equilib rium strain rates can be modelled by two thermally activated processes acting in parallel. In recent research these studies have been extend ed to fibres of lower draw ratio and to isotropic polyethylene. It has been possible to establish links between creep and yield behaviour, a nd to show that slow crack propagation can be understood in terms of a combination of yield and creep to failure.