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The difference between the thermal expansion properties of thermoplastics and high modulus fibers such as graphite and Kevlar® aramid is large. A significant level of residual thermal stresses is expected in thermoplastic matrix composites and these residual stresses will probably play a role in their properties. We first discuss the thermal expansion properties of the two classes of thermoplastics—amorphous thermoplastics and semicrystalline thermoplastics. The relevant thermal expansion data for prediction of the magnitude of the residual stresses in composites is the zero (atmospheric)-pressure thermal expansion data; these data are given for polysulfone and poly(ethylene terephthalate). By following the curvature of cross-ply composite strips, we investigated the build up of residual thermal stresses in thermoplastic matrix composites. In polysulfone composites (that is, amorphous thermoplastic matrix composites), the residual stresses begin to build up at the glass transition temperature. The cross-ply composite strip curvature can be accurately calculated from component properties and the bimetallic strip equation. In poly(ethylene terephthalate) composites (that is, semicrystalline thermoplastic matrix composites), the residual stresses begin to build up at the beginning of crystallization. A calculation of the cross-ply composite strip curvature using the bimetallic strip equation agrees well with the data except for deviations at low temperatures. The low temperature deviations occur when the differential shrinkage is sufficient to cause matrix cracking.
residual stresses, thermal stresses, amorphous thermoplastics, semicrystalline thermoplastics, composites, thermoplastic matrices
professor, University of Utah, Salt Lake City, UT
professor, University of Colorado, Boulder, CO