SUMMARY

The goal of this research was to investigate the tensile fatigue behavior of a carbon fiber / epoxy composite material. The project sponsors, Boeing, hoped to extend a failure model to the case of fatigue and this investigation provided empirical data to that end. The research primarily consisted of experimental testing of cross-ply laminates. Since composite fatigue damage typically initiates in and propagates from the 90 degree plies in layups which contain them (including the ubiquitous “traditional 0/45/90” layup), this work provides valuable insight into the fatigue behavior of those plies which originate fatigue damage in a composite. Unidirectional 90 and 10 degree specimens were also tested, but the bulk of testing was done on cross-ply laminates and the presentation will focus on these tests: - Experimental results for static and fatigue crack initiation: x-ray images, edge replicates, dye penetrant testing. Plots of crack initiation and damage development. - Expperimental explorations of the effect of R-ratio, loading frequency, and surface roughness on fatigue crack initiation. - Comparisons of experimental results with other investigations of this type of layup. - Comparison of experimental results with model predictions of this layup's behavior.