An experimental study of the translaminar fracture toughnesses in composites for different crack growth directions, parallel and transverse to the fiber direction

This work studies how the direction of crack propagation affects the translaminar fracture toughness in composites. In particular, this work focuses on propagation mode-I toughness. The two extreme cases are analyzed: crack propagating in the direction either parallel or perpendicular to the fibers (always without breaking fibers). To evaluate the effect of the direction of crack propagation, an experimental campaign was carried out by two new cracked Three Point Bending (TPB) specimens specially developed for this purpose. Using these specimens, the fracture toughness was measured for different directions of the crack propagation. In addition, a set of standard Double Cantilever Beam (DCB) specimens (corresponding to the longitudinal interlaminar fracture toughness) were tested for the sake of comparison. The results show that the translaminar fracture toughness is higher when the crack grows parallel to the fiber direction than when it grows perpendicular. In both cases, the fracture toughness is higher than that measured for the longitudinal interlaminar growth using DCB specimens. The physical reasons for these experimental results are discussed.