Characteristics of thermally-induced transverse cracks in graphite-epoxy composite laminates

Abstract

The characteristics of thermally-induced transverse cracks in T300/5208 graphite-epoxy cross-ply and quasi-isotropic laminates were investigated both experimentally and analytically. The formation of transverse cracks and the subsequent crack spacing present during cooldown to -250°F (116K) and thermal cycling between 250 and -250°F (116 and 394K) was investigated. The state of stress in the vicinity of a transverse crack and the influence of transverse cracking on the laminate coefficient of thermal expansion (CTE) was predicted using a generalized plane-strain finite element analysis and a modified shear-lag analysis.

It is shown that a majority of the cross-ply laminates experienced transverse cracking during the initial cool-down to -250°F whereas the quasi-isotropic laminates remained uncracked. All cross-ply laminates and the [0/±45/90]_s quasi-isotropic laminate exhibited transverse cracking following 20 thermal cycles. The uniformity of crack spacing increased with an increasing number of thermal cycles. The cross-ply laminates exhibited a rather sharp drop in CTE at crack densities less than 50 cracks/in. (19.7 cracks/cm) whereas the quasi-isotropic laminates exhibited a smaller decrease in CTE. The in situ transverse strength of the 90° layers was more than 1.9 times greater than the transverse strength of the unidirectional 90° material for all laminates investigated.