RF Processing of Thermoset and Thermoplastic Composites

Abstract

ABSTRACTGraphite fiber-thermoset epoxy resin composite materials are frequently used in aerospace and related industries. Electromagnetic heating may have some advantages over the present autoclave techniques, especially for large objects. However, the loss factor of graphite composites at microwave frequencies (915 MHz and 2.45 GHz) is so high that the depth of penetration is no more than about 0.1 mm. Arcing may be encountered at high heating rates due to strong transverse E-fields. Also, since the curing process is exothermic and the surface is at elevated temperature, thermal runaway may be encountered in the interior spaces and result in over-cured (thermally damaged) material.We have investigated the heating obtained at radio frequency (nominal 27 MHz) in magnetic induction fields in both experiments and numerical models. Inductive coupling has the advantage that arcing is virtually eliminated, even at high heating rates, for typical cylindrical geometries. Also, induction heating easily accomplishes lap seam welds in thermoplastic composites owing to favorable electric field boundary conditions. Transient temperatures were recorded during heating by optical fiber probes and heating distribution was measured, where possible, using thermographic imaging. Finite difference electrical and thermal model results illustrate the dominance of conduction heat transfer in these materials.