FATIGUE FRACTURE AND MORPHOLOGY IN INJECT ION-MOLDED POLYPROPYLENE

Mechanical properties and variations in viscoelastic properties during fatigue of injection-molded polypropylene were investigated by means of fatigue and tensile tests. Fatigue fracture and morphology of the s pecimen were investigated by microscope, and infrared spectroscopy stu dies. Fatigue fracture occurred after cracking in spherulitic core reg ion. The fatigue life time decreased with increase in stress amplitude . For high stress amplitude, the growth of cracks and the softening of the specimen occurred locally. The crystallinity of the specimen afte r fatigue test was higher than that before the test because of heating during the fatigue process. The crystallinity increased with increase in the fatigue lifetime. Fracture of tensile test occurred after grow th of a wide crack in the spherulitic core region.