The Macroscopic Elasticity of 3D Woven Composites

The elastic properties of graphite/epoxy composites with three-dimensional interlock weave reinforcement have been measured over length scales somewhat greater than the characteristic length of the weave pattern. Orientation averaging models similar to those developed elsewhere over the last twenty years provide estimates of elastic constants that are in fair agreement with the experimental data. However, in-plane Young's moduli are consistently too high and properties related to the through-thickness reinforcement show considerable scatter. Most of the discrepancies can be attributed to waviness and other geometrical irregularities in nominally straight tow segments. Much improved agreement with in-plane properties is obtained by measuring and accounting for the out-of-plane waviness of in-plane tows. Other observed distortions of in-plane tows and irregularity in through-thickness tows are very difficult to quantify experimentally. There results a significant and apparently unavoidable uncertainty in predictions of properties that depend strongly on the through-thickness reinforcement. Nevertheless, the utility of orientation averaging models in applications where the in-plane properties are paramount is clearly verified.