Abstract
When the constraint of the fill yarns was removed from a woven fiber composite, delamination followed an intrayarn pathway. Despite extensive fiber-matrix debonding during crack growth, resin toughness translated directly to composite toughness. This suggests that damage from intrayarn cracking contributes significantly to toughness of woven fiber composites even though the crack is expected to follow an interply pathway. Effects of fiber undulation were isolated from the effects of yarn interlacing by studying specimens a single yarn in width. In the double cantilever beam (DCB) test, intrayarn crack growth caused the crack to follow the undulating pathway of the fibers. The orientation of the crack changed as the crack moved along the undulating fibers; thus, the extent of mode II loading relative to mode I loading increased as the fibers deviated from the plane normal to the loading direction. Because composite toughness increases with the degree of mode II loading, the variation in toughness induced a characteristic stop-start crack growth mechanism. The toughness of the matrix was varied by changing the soft segment content of the urethane-methacrylate resin. Soft segment content affected the crack initiation toughness more than crack arrest toughness. Consequently, longer crack jumps were observed with the tougher resins.