Crystallisation effects during high rate deformation of polymers

Abstract
Previous work with PEK, PEEK and nylon has shown a close correlation between the rapid increase in yield stress as a function of strain rate of these semi-crystalline polymers at strain rates > 10³ s^-1 and the crystallinity content of samples recovered after a test. It has been speculated that the 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 extensive investigation into the morphology of tested strain limited PET samples using X-rays and scanning calorimetry has shown that the crystallinity does not develop until the latter part of a test and that crystallinity changes cannot therefore contribute to yield and flow stress increases.