Torsional Fatigue Process in Short Carbon-Fiber Reinforced Polyamid 6.6 (Comparison with Fatigue Process of Rotating Bending)

In this paper, torsional fatigue tests and additional rotating–bending fatigue tests were carried out on the injection moulded short carbon–fiber reinforced polyamid 6.6 composites. The fatigue mechanism in the composites was clarified through successive surface observations using the replica method. Moreover, the mechanism of the torsional fatigue was compared with that of the rotating-bending fatigue. The fatigue cracks in both fatigue tests are initiated along fibers aligned in the direction of the principal stress. In the case of the rotating-bending fatigue test the fatigue crack propagates finally at right angles to the direction of the principal stress. However, the cracks initiated along the fibers in the torsional fatigue test can not propagate under high–cycle fatigue. On the other hand, the fatigue cracks initiated from fibers aligned in the direction of the principal shear stress can propagate, and reach to final fracture. From the comparison between bending and torsional fatigue tests the fatigue process of this material under the condition of an arbitrary combined stress can be estimated.