Effects of Processing Variables on Fatigue in Molded PEEK and Its Short Fiber Composites

The effects of various material and processing parameters on the fatigue behavior of PEEK and its short glass and carbon fiber reinforced composites have been investigated. In particular, the significance of molecular weight, fiber type and increased matrix crystallinity, induced by post-forming annealing, are considered in this paper. Standard S-N (peak stress against log (cycles to failure(( fatigue data curves have been generated and crack propagation rates have been monitored as a function of the alternating stress intensity factor. Scanning electron microscopy (SEM), optical microscopy, X-ray diffraction and differential scanning calorimetry (DSC) have all been used to characterize the microstructural parameters and failure mechanisms. Results show that the increased crystallinity, induced by annealing, significantly improves fatigue crack resistance for both neat PEEK and its short fiber composites. Generally, superior fatigue behavior is associated with the higher molecular weight matrix and with carbon rather than glass fibers. However, it proved difficult to unravel the complex interdependence of various matrix factors such as molecular weight and crystallinity, and fiber type, length, aspect ratio and preferred orientation. Nevertheless, a combination of the tie molecule density concept and a reinforcing efficiency model, developed to take into account these various parameters, has helped to explain the complexity of the results.