Appropriate curing of the patch resin and the adhesive layer is an essential step to secure the integrity and the sound mechanical performance of bonded composite repairs. Today, the required temperature elevation is achieved by means of resistance heating blankets, conducting heat to the repair area. This method is simple and works adequately for small repairs, but when larger areas need to be heated, discrepancies, like cold or hot spots, occur, due to variable heat losses, caused by “heat sinks”.
The scope of this proposal was to change the currently used heat generation principle, from conduction to induction heating for large repair cases (i.e. 65x65cm or larger). Heat was generated within the repair by induced electrical currents (eddy currents) through a coil placed on top of the patch, creating an alternating electromagnetic field to the conductive elements of the repair (susceptors). The system was made of several flexible coils, driven by induction equipment employing multi-channel temperature control, to compensate for differences in temperature development across the structure. The process was monitored by special software, capable of compensating temperature differences by variable heat generation algorithm. The susceptor was one or a combination of the following elements:
- the lightning protection metallic mesh placed on top of composite repairs;
- the carbon fibres of the patch and surrounding structure;
- a special metallic mesh embedded into a silicon mat, placed directly on top of the patch.
The induction heating method achieved better temperature homogeneity and process efficiency compared to conventional conduction heating equipment and methods, resulting in direct improvement of the repair quality. As induction heating is a non-contact method, equipment and consumables requirements were minimised, making induction heating more robust, reliable, faster, cheaper and easier to apply, compared to conduction heating techniques.