Residual Stress Development During Consolidation of Thermoplastic Resin Matrix Composites

Alfred C. Loos
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061-0219

Min-Chung Li
General Motors Corporation
Warren, MI 58090-9055

Fabrication of high quality composite laminates from thermoplastic prepregs requires that the processing temperature and pressure be selected so that intimate contact of the ply interfaces is achieved resulting in the formation of strong interply bonds and process-induced residual stresses are minimized. The purpose of this talk will be to present recent work on modeling consolidation and development of residual stresses during processing of thermoplastic resin matrix composites.

The uniformity of the prepreg microstructure and the tow height uniformity (surface roughness) are critical parameters which govern selection of the processing pressure and time during consolidation. A reasonable model of the prepreg is to assume that the prepreg consists of a central fiber and resin core, possibly containing intraply voids, with resin rich surfaces. Spatial gaps due to prepreg surface roughness are eliminated during processing by localized deformation and flow at the resin rich surfaces. A model of this phenomenon was developed by integration of the prepreg surface topology characterization with a resin flow analysis. A formulation which takes into account the temperature dependent zero-shear-rate resin viscosity was derived to describe the deformation of the prepreg interply interface. Experiments were performed to obtain data which can be used to verify the model. Two-ply unidirectional and three-ply [0,90,0] cross-ply laminates fabricated from graphite-polysulfone and graphite-PEEK prepreg were consolidated using different processing cycles. The degree of intimate contact of the ply interfaces was measured by optical and acoustic microscopy. The agreement between the measured and model predicted degree of intimate contact was very good.

A plane-strain, linear finite element model with temperature dependent matrix properties was developed to analyze residual stresses in a graphite/PEEK composite. The residual stress model takes into account the mismatch of the thermal expansion coefficients and the crystallization shrinkage of the matrix. Good agreement between the reported transverse normal stress data and the modeling result was observed in the analysis of a [040]T APC-2 laminate processed at a 35oC/s surface cooling rate. Furthermore, [010/906]T APC-2 laminates were manufactured at different cooling rates to verify the model. The induced residual thermal deformations were measured by a shadow moiré system. The model estimated the out-of-plane displacement of the non-symmetrical laminates accurately. The optimum processing cycles, which minimize the residual stresses and maximize the mechanical properties of composite materials, were found to be different for different lay- ups.