Predicting Shrinkage in Polyester Reinforced by Glass Fabrics

Abstract

Polyester is one of the most common resins used in contact lay-up method because of its low cost, room-temperature curing, wide availability, ease of handing, etc. However, the main disadvantage of this resin is the large volumetric shrinkage after curing (up to about 0.5%). This represents a major problem because it can cause unexpected defects in the molded composite parts such as warpage, distortion, rippled surface, etc. The effect of resin shrinkage on composite deformations is very complex because of the anisotropic properties induced by the fibers, especially in woven fabric composites with interlacing yarns. Moreover, in many applications, when the part geometry has a double curvature, the forming process usually results in significant in-plane shear deformation of the interlaced yams. The angle between the fill and the warp threads is no longer orthogonal because the fabric must follow the shape of the mold. In this work, an approach to measure the shrinkage coefficients of the interlaced yarns of fabric structure has been developed. The recently proposed sub-plies model has been used to predict deformations due to resin shrinkage in woven fabric composite. Resin shrinkage can lead to an expansion in the laminates with specific angles between undulated yams, due probably to a straightening effect on the fibers. Expansions due to matrix shrinkage were verified on several woven laminates. Prediction of deformations due to matrix shrinkage by the sub-plies model is in good agreement with experimental measurements.