Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite

An apparatus was developed to thermally cycle coupon-sized mechanical test specimens to - 196 degreesC. Using this device, IM7/5250-4 carbon/bismaleiraide cross-ply ([0/90](2S) and [90/0/90/0/90/0/90/0/90]) and quasi-isotropic ([0/45/ - 45/90](S)) laminates were submerged in liquid nitrogen (LN2) and returned to room temperature 400 times. Ply-by-ply micro-crack density (transverse cracks), micro-crack span, laminate modulus, and laminate strength were measured as a function of thermal cycles. The composite micro-cracked extensively in the surface plies followed by sparse micro-cracking of the inner plies. The tensile strength of the two blocked lay-ups (lay-ups with adjacent plies of the same orientation) decreased by 8.5% after 400 cycles. Sectioning of the samples revealed that the micro-cracks in the surface plies spanned the full width of the sample while many of the micro-cracks observed on the edge of the inner plies did not extend to the center of the samples, implying that a rectangular specimen with exposed free edges may result in a significantly different micro-crack density than a sample without free edges. (C) 2003 Published by Elsevier Science Ltd.