A Discussion on the Damage Mechanism and the Apparent Fracture Strength of Notched Fiber-Reinforced Composite Laminates

Apparent fracture strength of fiber-reinforced composite laminates depends on the notch tip radius even if it is evaluated in terms of the local parameters such as the stress at notch tips or stress intensity factors. Although numbers of phenomenological explanations have been made, this phenomenon has not yet been physically clear enough even today. In order to elucidate its key mechanism, our interest is here focused on the interlaminar crack extension from notch tips in cross-ply laminates subjected to mode-I loading. We find a stochastically expected upper bound of interlaminar crack extension due to the probabilistic breakage process of fibers in load-bearing laminae inside the region covered with interlaminar cracks. This upper bound, i.e., the critical length of interlaminar crack extension, is inherent to the laminate and corresponds to its notched strength. The well-known variation in apparent fracture strength of notched composite laminates with respect to the notch tip radius is clearly explained as the scale effect of this constant critical length in the different displacement distributions ahead of notch tips with different radii.