Evaluation of the Fatigue Behavior of Discontinuous and Continuous Fiber Thermoplastic Composite Laminates

The present investigation summarizes an evaluation of the effect of fatigue loading on continuous and long discontinuous fiber (LDF™) composites manufactured using the AS4/PEKK (poly-ether-ketone-ketone) system. Approximately 100 static and fatigue specimens were tested in a two phase experimental investigation. In the first phase LDF AS4/PEKK thermoplastic specimens of 2 layups [0/90/±45]s and [0/±45/90]s were tested in static tension to determine their respective failure strengths. Constant amplitude tensile fatigue tests were then conducted to evaluate the S-N behavior of these two LDF composite laminates. Specimens surviving one million cycles were statically tested to failure. In the second phase of the investigation, continuous fiber AS4/PEKK thermoplastic laminates were tested in static tension and in fatigue to evaluate the effect of the discontinuous fiber on composite performance. The effect of a known stress concentration on both material systems was also investigated using specimens having a .25 inch center hole. Static tests confirmed a reduction (13%) in unnotched LDF strength compared with continuous fiber thermoplastics. The LDF laminates were significantly less notch sensitive, i.e. with a notch the reduction in strength was only 4%. The endurance limit for both systems was between 60 and 70% of their respective static ultimate strengths. One additional objective was to determine whether the presence of discontinuous fibers, and the resulting stress concentrations, would cause a severe reduction in LDF thermoplastic composite fatigue properties. This was not found to be the case. The static strength and S-N type data generated here has shown that the fatigue behavior of continuous fiber and LDF thermoplastic laminates can be characterized using a methodology developed for thermoset laminates based on stiffness reduction.