THE EFFECTS OF A SINGLE-MODE-III LOADING CYCLE ON MODE-I CRACK INITIATION AND GROWTH TOUGHNESSES IN A CROSS-PLY [90-DEGREES 0-DEGREES](2S) TI-6AL-4V/SICF COMPOSITE/

The effects of a single Mode III loading cycle on the subsequent Mode I crack initiation and growth toughnesses in a cross-ply [90 degrees/0 degrees](2sS) Ti-6Al-4V/SiCf composite have been investigated. A Mode III cycle with Delta J(III) = 8 kJ/m(2) was found to have no measurab le effect on either the initiation or growth toughnesses. In contrast, a Mode III cycle with Delta J(III) = 14 kJ/m(2) resulted in a signifi cant decrease in the subsequent Mode I crack initiation toughness. It also caused a reduction in the crack growth toughness until the crack had extended beyond the damage zone produced during the Mode III excur sion. The results are rationalized on the basis of the fracture mechan isms as well as the deformation field produced by Mode III loading. Th e Mode III damage effect has implications for failure criteria for com posites under multiaxial loading.