| This dissertation presents a theoretical study of the effective stiffness reduction for [90n/thetam]s composite laminates containing matrix cracks through 90° layers and [theta m/90n]s laminates under transverse cracking and delaminations. The analytical model is based on the displacement method. The variational theory and principle of minimum potential energy are used to find governing equations. In the model, crack geometry as well as the residual longitudinal Young's modulus Ex and energy release rate G is a function of crack density and delamination length. There are two different composite materials in the study: one is a glass/epoxy material system and the other is AS4/3502 carbon/epoxy material system. The lamination sequences of the materials considered include cross-ply and symmetric balanced configurations, [0m/90n]s, [90 n/0m]s, [+/-15/904]s, [904/+/-15]s, [+/-30/904] s, [904/+/-30]s, [+/-40/904] s and [904/+/-40]s. The changes in the longitudinal Young's modulus Ex and energy release rate G are examined theoretically. Analytical results indicate that local delaminations induced by transverse cracking have significant influence on Ex and G, especially for glass/epoxy composite laminates. Additionally, results also show that under the same configuration and crack density, the propagation of local delaminations is more stable in carbon/epoxy laminates than that in glass/epoxy laminates when theta ≤ 40°. |
