Volume fraction gradient induced warpage in curved composite plates

Curved composite right-angle brackets have been shown to warp during manufacture due to the response of the part geometry to the large differences in the in-plane and out-of-plane shrinkage. In addition, it has been suggested that warpage in flat, uniaxial composites can be related to through-thickness volume fraction gradients which yield a significant degree of laminate asymmetry. The combination of this geometry effect and a graded volume fraction is investigated in an effort to explain differences in the amount of warpage in curved right-angle brackets produced on convex tooling versus on concave tooling. A computerized metallographic image analysis technique is used to measure the laminate homogeneity and the volume fraction gradients are determined for composites produced on each tooling geometry. Based on these experimentally measured values of curvature and volume fraction gradient, it is shown that the observed tooling geometry dependent differences in warpage noted in many components can be explained. In addition to explaining the tooling geometry dependent manufacturing warpage, this improved understanding of the mechanisms involved suggests the use of concepts of functionally graded materials to help minimize component distortion.