Fatigue fracture and morphology in injection molded polyacetal

Variations in viscoelastic properties during fatigue of injection-molded polyacetal were evaluated by means of a fatigue test. Fatigue fracture and morphology of the specimen were investigated by optical microscopy, infrared spectroscopy, and differential scanning calorimetry studies; At low stress amplitude and low test temperature, fracture behavior was fundamentally brittle. When the stress amplitude and the test temperature were increased, fracture behavior became more ductile. When the fatigue behavior was changed from brittle to ductile mode, the fatigue life time was greatly reduced. In brittle fracture, the crack propagated along one fracture surface. In ductile fracture, however the specimen was stretched and the crystals were largely deformed. The crystallinity in a local region near the fracture surface of the specimen was decreased by the fatigue. When the ductile fracture took place, the crystallinity all over the specimen was increased by self-heating during the fatigue process.