Crystallisation effects during high rate deformation of polymers

Previous work with PEK, PEEK and nylon has shown a close correlation betwee n the rapid increase in yield stress as a function of strain rate of these semi-crystalline polymers at strain rates > 10(3) s(-1) and the crystallini ty content of samples recovered after a test. It has been speculated that t he crystallinity increase may be partially responsible for the flow stress increase. This paper describes work carried out to extend the previous work to PET and has confirmed that the correlation also holds for PET. An exten sive investigation into the morphology of tested strain limited PET samples using X-rays and scanning calorimetry has shown that the crystallinity doe s not develop until the latter part of a test and that crystallinity change s cannot therefore contribute to yield and flow stress increases.