The required elevation of temperature for curing of composite repairs is usually achieved by resistance heating blankets, controlled through spot temperature measurements. Although this method is simple and works adequately for small repairs, when larger areas are heated, cold or hot spots frequently occur, due to variable heat losses, caused by geometrical asymmetry etc. For this reason, autoclave curing of large repairs is currently used, as the most reliable curing methodology, which is however accompanied by high cost, wastes and energy consumption. The scope of this proposal is to enable the extension of heating blankets technology for the treatment of large repairs (e.g. reversers), currently requiring autoclave heating.
A five step process is proposed:
- Thermal transfer simulation, to retrieve the “thermal signature” of the part and experimental validation using heat flux sensors.
- Manufacturing of thermal blankets, namely adapted Variable Heating Elements (VHE), with denser or coarser heating elements inside, and standard Compensation Heating Elements (CHE), in order to compensate for dissimilar heat losses.
- Development of universal Advanced Heating Control Unit (AHCU), with innovative programmable closed loop control.
- Development of a specialized High Power Supply Unit (HPSU.
- Process validation.
It is expected that, this innovative methodology will allow the performance of structural repairs, either totally out-of-autoclave or in combination with autoclave treatment, as required, thus improving the overall heating process and achieving large scale economies compared to current autoclave solutions. Given that specified repairs are generally standardized, steps (a) and (b) will only need to be performed once per P/N to be repaired, while the developed equipment described in steps (c) and (d) will be totally reusable, thus significantly decreasing the non-recurring cost encountered per repair.
GMI AERO, based in Paris, is a worldwide recognized expert in technology for advanced composite structural repairs. Right from its creation, back in 1980, GMI has initially developed its capabilities towards composite structure fabrication processes. Specific sensors and control equipment have greatly helped in investigation of critical quality parameters and improved productivity. When composite repairs became a first preoccupation, GMI started early in 1984 to innovate with the equipment and toolings for repair and conduct processes in all steps from NDT to bonding. These works have been the result of regular cooperation both with aircraft manufacturers and end-users (i.e. airlines and MROs). Today, the business activity of the company is worldwide, cooperating with major aeronautical stakeholders in Europe, USA, Canada and China. The innovation of equipment has been the result of both internal development and extensive cooperation with researchers coming the National Technical University of Athens, with which a “strategic cooperation” is in place, participating together in several EU funded R&D projects. Recently, CleanSky GRA, ECO and SFWA R&D projects have offered to the company the opportunity to focus its research activities in order to further mature innovations towards a higher TRL.
Within ECO Design ITD a project titled “Advanced heating system and control mode for homogeneous high temperature curing tf large composite repairs – ADVANCED” has been recently finished by GMI and the NTUA, Topic Manager being Aircelle (Group SAFRAN). ADNANCED concerned the development of innovative solutions for the application of very large composite repairs, to be performed outside autoclaves. Even though achieving the very strict temperature tolerances (usually in the area of (+/-5 at 180 or 225oC) for repairs of several m2 is rather challenging, the expected benefits are very significant, as reduction of autoclave utilization induces direct reductions both to the overall repair cost and to the CO2 footprint of the repair, as the energy requirements for out-of-autoclave curing is minimal, compared to autoclave curing. Detailed 3D FE thermal transfer simulation of the full repair case was performed, in order to retrieve “thermal signature” of the repair, thus achieving customization of heating blankets design, while reduction of number of heating zones was achieved by using non-uniform heating generation elements. A 48 KW Power Supply and Control Unit has been developed, capable of heating up to 18 heating zones, together with the associated software for simultaneous data acquisition from eighty (80) control & monitoring thermocouples, using innovative control algorithms with increased flexibility in defining control mode. User friendly HMI was applied (similar to those of standard GMI ANITA EZ heating consoles), for immediate transition of operating personnel.
The developed equipment has been successfully tested and approved at industrial environment, on an extremely demanding application (A380 reverser).