Crystalline Phase Transformation of Polytetrafluoroethylene in a Fatigue Test

In this study, we aimed to characterize the mechanical response of polytetrafluoroethylene (PTFE) laminates under a tension-tension load-control fatigue test (frequency = 5 Hz, load ratio = 0) and provided an analysis of the failure patterns of the PTFE material with consideration of crystalline phase transformation. In the final results, the evolution of the cyclic creep strain and stress-number of cycles to failure (S-N) curves presented duplex properties accompanying the fatigue life increasing to high cycles (cycle fatigue > 10(5)). A simple phenomenological damage index was defined in this study to describe the cyclic creep process. Additionally, the scanning electronic machine investigation suggested that local fibrosis caused by crystalline phase transformation to phase I led to the initiation of fatigue crack, and the fiber formation and orientation was found to be beneficial to a higher tensile strength and better resistance to crack propagation. The aspect of cyclic-load-induced crystallization was observed by the microfocus hard X-ray diffraction beam-line from a new insight. The crystalline phase transformation led to a gradient distribution of crystallinity and lateral crystallite size along the crack propagation direction. (C) 2014 Wiley Periodicals, Inc.