Increased Interlaminar Fracture Toughening Through Distinct Fiber Bridging Effect of rCF Staple Fiber Yarn Composite

This study investigates the influence of fiber bridging on the interlaminar strength of carbon fiber-reinforced polymer (CFRP) made from recycled carbon staple fiber yarn (rCF), compared to CFRP made from new fibers (vCF). Double-cantilever beam (DCB) tests measure the resistance of both materials against crack formation and the corresponding energy release rate (ERR). Several microscopic tools (SEM, CT) were then used to analyze the fracture surfaces and characterize the underlying failure mechanisms of the fiber bridges. The resulting ERR of rCFRP is four times (2140 J/m2 compared to 587 J/m2) higher than that of vCFRP. SEM images of the fracture surface reveal that the fracture mechanism is fiber debonding followed by fiber pull-out with constant friction. This finding is confirmed by calculating the fiber bridging stress using the mathematical formulation of this effect resulting in a fiber bridge tension of approximately 70 N/mm2. The main reason for the increased ERR of rCFRP compared to vCFRP is the extensive occurrence of fiber bridges in rCFRP due to the inhomogeneity of the rCF roving. This results in a pronounced nesting effect between adjacent rCF layers. The influence of the nesting effect on the ERR was investigated by testing samples with an increased layer orientation difference of 3° and 5°. This results in an ERR decrease of 26% in rCF and 30% in vCF. The nesting effect can be eliminated in vCFRP, but in rCFRP higher layer orientation, nesting is still visible. This finding suggests that the coarse, inhomogeneous structure of the rCFRP roving causes nesting regardless of the layer orientation and leads to a pronounced tendency to form fiber bridges.