Tension-tension fatigue of hybrid composite rods

The tension-tension fatigue behavior was investigated for a hybrid composite rod comprised of a unidirectional carbon fiber core and a glass fiber shell. Fatigue tests were performed at three R-ratios and four maximum applied stress levels (MAS) while recording the secant modulus at each cycle, and acoustic emission (AE) sensors were employed to monitor the activation of fatigue mechanisms. Fatigue failure occurred when the composite rod was no longer able to support the applied cyclic load. For a MAS level of 70% of the ultimate tensile stress (UTS), composite rods tested at higher R-ratios showed AE activity through a larger percentage of fatigue life, but exhibited a greater resistance to fatigue failure, whereas samples cycled at lower R-ratios displayed AE activity only near the end of fatigue life, and showed a lower resistance to fatigue failure. The hybrid composite showed modes of progressive fatigue damage at high R-ratios and low strain amplitudes in the form of longitudinal splitting of the GF shell. In contrast, failure of the CF core was catastrophic and non-progressive. The fatigue resistance and damage mechanisms of the composite rod were dependent on the MAS level and R-ratio. Fatigue cracks initiated because of fretting between the CF shell and grip surface, which led to the observed longitudinal splitting of the CF shell. Fatigue damage occurred along the GF/CF interface where non-uniform strains developed because of the clamping force of the grip on the CF surface. At an R-ratio of 0.85, a fatigue stress of 70% UTS caused catastrophic fatigue failure, while at lower stresses, composite rods did not fail and withstood cyclic loads up to 1 million cycles. The research conducted is the first to investigate the degradation in fatigue performance arising from grip/composite rod interactions and suggests that the results from the study provide new information for composite materials in industries that utilize unidirectional composites in cylindrical form. (C) 2012 Elsevier Ltd. All rights reserved.