Effect of microcracks on the thermal expansion of composite laminates

A finite element analysis was used to quantitatively predict the effect of matrix microcracks in the 90 deg plies of graphite/epoxy composites on the coefficient of thermal expansion in the 0 deg direction, alpha (y) (perpendicular to the cracks). Results were generated for (0m/90n)s, (0/+ or 45/90)s and (0/90/ + or - 45)s graphite/epoxy laminate configurations. Analytical predictions were compared with experimental results for the two quasi-isotropic laminate configurations. Both analytical and experimental results showed that microcracks reduced the effective stiffness of the 90 deg plies, thus causing the laminates, thermal response to be more like that of a (0) laminate. The change in alpha(y) was a function of lamina material properties, microcrack density, fiber orientation, and stacking sequence. A combination of classical lamination theory and finite element analysis was used to predict the effect of microcracks in both the 90 deg and 0 deg plies. Analytical results showed that the addition of microcracks in the 0 deg plies do affect alpha(y), but to a lesser extent than those in the 90 deg plies.