CYCLIC DEFORMATION-BEHAVIOR AND MORPHOLOGY OF POLYPROPYLENE

The viscoelastic-plastic behaviour and stress relaxation in Polypropyl ene after uniaxial simple tension or cyclic preloading,are studied by an electrohydraulic, servo-controlled testing machine. The stress-stra in curve data of simple tension at different strain rates show that th e magnitude of stress depends strongly on strain rate. The stress rela xation behaviour after simple tension and cyclic preloading indicates complex features and reveals differences in the deformation of molecul ar chains in polypropylene subjected to different cyclic preloadings. The stress-strain curves and the stress relaxation curves under three mean strains are fairly different from each other. The microstructural changes in samples subjected to different cyclic preloadings were det ermined by scanning electron microscopy (SEM). The results of the stre ss-strain curves and the stress relaxation behaviour are discussed in comparison with the observation of SEM fractographs.