Damage trends in cryogenically cycled carbon/polymer composites

Six carbon/polymer composites (three materials, two lay-ups each) were cryogenically cycled by repeated submersion of 5 cm × 5 cm flat plates in liquid nitrogen (LN2). The materials and lay-ups were chosen to study the density of ply-level micro-cracks that formed as a function of number of cycles, prepreg thickness, residual stresses, and the orientation of adjacent plies. IM7/977-2 carbon/epoxy, IM7/5250-4 carbon/bismaleimide, and IM7/3K carbon/epoxy cross-ply ([0/90]2S) and quasi-isotropic ([0/45/45/90]S) laminates were tested for up to 1000 cycles. Several trends in damage development were observed that should be considered when formulating a model for predicting the formation of micro-cracks due to cryogenic cycling. For example, reducing the ply thickness by 30% in the IM7/5250-4 delayed surface ply micro-cracking by up to 200 cycles. However, by 1000 cycles the surface ply micro-crack densities were nearly equal regardless of the ply thickness. Also, micro-crack densities 2.5 cm away from the sample edges were greater than at the edges in plies with large amounts of damage (>0.2 micro-cracks/cm) and interior micro-cracks were often not extensions of micro-cracks observed on the edges of the samples.