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.