Assistant professor, Department of Mechanical Engineering, and director, Composite Materials, Manufacture and Structures Laboratory, Colorado State University, Fort Collins, CO
Autoclave cured uni-axial carbon fiber/epoxy samples are observed to warp during production even though classical laminated plate theory predicts that such composites should have no such tendency. Particularly in thin laminates the observed convex up curvature can be quite pronounced, but measurable warpage does exist even in thicker composites. Distorted uni-axial laminates produced by top bleed autoclave cure have been analyzed using quantitative optical microscopy and have shown volume fraction gradients related to laminate through-thickness position. The results of these preliminary quantitative observations indicate that many laminates are measurably resin-rich at the composite/tool interface, and resin-poor at the laminate top surface near where bleeding takes place. However, the present metallographic technique does not yield a precise through-thickness resin fraction profile. Modeling of the magnitude of this warpage has been undertaken to investigate the likely through-thickness volume fraction profiles. The approach used has included a number of volume fraction variations in a classical laminated plate analysis and determines the mid-plane curvatures predicted based on general composite thermal expansion and matrix shrinkages. The results of this analysis for long uni-axial carbon fiber/epoxy sample strips of varying thickness match the curvature experimentally observed and substantiate the idea that the volume fraction gradient is not linear through the laminate thickness, but rather is locally resin-rich near the tooling, uniform through most of the thickness, and resin-poor at the top surface adjacent to the bleeder. Further, the results show that volume fraction gradients induced during top bleed autoclave cure are an important component of composite part warpage.
Paper ID: CTR10381J