Prediction of fiber orientation and microstructure of woven fabric composites after forming

The forming of fabric composites into complex parts such as double-curvature shells leads to complex redistribution and reorientation of fibers in the composite. This results in variations in thermal and mechanical properties of the material, which, in turn creates complex residual stresses and deformations of the part as a function of temperature change. Consequently, it is usually very difficult to perform any structural analysis of these composites. In order to predict the thermo-mechanical behavior of fabric composites, a numerical program has been developed in this work to predict the redistribution of the woven fibers and the resin in parts with complex shapes. A brief description of the approach used for the calculation and display of the variations in microstructure throughout the part is presented in this paper. The change in microstructure is discussed in terms of various possible mechanisms such as fiber orientation, fiber slippage, thickness variation, fiber volume fraction throughout the molded part. In parts containing double curvatures, the intraply shear deformation that occurs during the forming process involves a surface area reduction of the ply. Consequently, in single-die molding, there is a ply thickness increase and a variation of fiber volume fraction. The comparison between numerical prediction and experimental measurements is presented by examining the microstructure of different composite systems with a room curing-temperature, molded by hand lay-up on a rounded top cone die. The results revealed that the prediction agrees with the actual variation in the properties of the molded composites. However the accuracy of computation is limited by the variations of the composite's characteristics due to handling and processing. In the structure investigated, these variations can be as high as 20%. (C) 2001 Elsevier Science Ltd. All rights reserved.