DELAMINATION FAILURE OF A SINGLE YARN GLASS-FIBER COMPOSITE

When the constraint of the fill yarns was removed from a woven fiber c omposite, delamination followed an intrayarn pathway. Despite extensiv e fiber-matrix debonding during crack growth, resin toughness translat ed directly to composite toughness. This suggests that damage from int rayarn cracking contributes significantly to toughness of woven fiber composites even though the crack is expected to follow an interply pat hway. Effects of fiber undulation were isolated from the effects of ya m interlacing by studying specimens a single yam in width. In the doub le cantilever beam (DCB) test, intrayarn crack growth caused the crack to follow the undulating pathway of the fibers. The orientation of th e 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. Be cause 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 segm ent content affected the crack initiation toughness more than crack ar rest toughness. Consequently, longer crack jumps were observed with th e tougher resins.