Tensile strength of notched carbon/glass/epoxy hybrid composite laminates before and after fatigue loading

Hybrid laminates consisting of C-glass woven fabric/epoxy composite plies and 3k-carbon woven fabric/epoxy composite plies are studied for fatigue damage and residual strength. Tension–tension fatigue tests were conducted on notched composite laminates at two load ratios of 0.1 and 0.25. The laminates were fabricated with the hand lay-up process for a symmetrical stacking sequence [0/90] _3s made of three 3k-carbon/epoxy composite plies at both top and bottom sections and six C-glass/epoxy composite plies in the middle. Fatigue damage was generated on notched specimens with 4 × 10 ⁴ load cycles to damage for residual strength tests. The hybridization was found to be beneficial for relative damage sensitivity under one of four different fatigue conditions although its effect was marginal while three other conditions were not in favor. A relative damage sensitivity factor expression (or a criterion) was developed for quantitative comparisons between non-hybrid and hybrid composites and was theoretically demonstrated to be valid for any possible cases where various combinations are possible due to differences in strength reduction rate between two different composite systems. A theoretical framework with the relative damage sensitivity factor is proposed as a guide to deal with the complexity involving uncertainties and a large number of variables in the hybrid composite system. New damage mechanisms of the hybrid system due to dissimilarity between two sub-composite systems (i.e. glass/epoxy and carbon/epoxy) were hypothesized and tested to be valid with evidence based on microscopic and macroscopic examinations. The difference between static and fatigue damage is discussed.