| Abstract: | Three experimental poly(ethylene terephthalate) fibers have been spun and then drawn at three different temperatures(70°C, 90°C, and 110°C) in such a way that the drawn fibers possess different orientations and crystallinities but retain the same diameters. Wide angle X-ray diffraction and birefringence measurements have been used to characterize orientation and crystallinities of the fibers. The influence of fine structure on the torsional fatigue behavior of the melt spun and drawn PET fibers has been studied by subjecting them to 1.7 Hz torsional cyclic deformation at various amplitudes. Fracture morphology was found to be strongly influenced by the degree of orientation and crystallinity. Highly oriented and crystalline structures tended to separate into a highly fibrillated structure. Fibers of low draw ratio exhibited initial deterioration of the surface structure with the generation of transverse cracks (perpendicular to the fiber axis). Subsequent torsional loading of the structure generated an increase in longitudinal cracks which finally resulted in the catastrophic failure of the fiber. The extent of fibrillation was found to be a function of draw ratio (orientation) and crystallinity. The amplitude of torsional strain was also found to have an effect on the intensity of fibrillation and the number of cycles to fiber failure. |
