Modelling of induction heating of thermoplastic composites using microscopic level modeling

摘要

Simulation and analysis of Electromagnetic (EM) induction heating of continuous conductive fiber based composite materials is used to (in)validate a set of hypotheses on the physics dominating the heating process. The behavior of carbon fibers with and without surrounding polymer in an alternating electromagnetic field was studied at a microscopic level using the ANSYS Maxwell software code using solid loss to quantify heat generation in the composite material. To limit the number of elements, fibers were modelled with a polyhedron cross-section instead of a circular cross-section. The simulations indicate that samples with fibers oriented in layers of 0 and 90 orientation yield a substantial higher solid loss than layers of fibers oriented in the 0 orientation only. The solid loss in both cases is however not sufficient to explain the level of heating observed in practice. Filling the volumes between fibers filled polymer results in far greater solid loss than samples with fibers having no medium in between the fibers, at equal fiber volume fraction. This effect is stronger with increasing fiber volume fraction. Also, in these cases no direct electrical contact between fibers was modelled. Data for the conductivity of the polymer was derived from laboratory testing. The lab tests showed relatively low finite resistance values for the plies in the transverse direction, indicating that the polymer in a composite should not be considered an isolator at the thickness levels found of the polymer layer surrounding each fiber in a properly consolidated laminate. Analytical equations were derived and compared with simulation results to verify these findings. The simulations seem to justify the conclusion that heating of thermoplastic composites in an alternating magnetic field rely on currents through the polymer without the need for contacting carbon fibers. The carbon fibers are required though to create the electromagnetic force that induces the current through the polymer. It is the product of current density squared times the resistivity of the polymer that determines the heating.