Appropriate curing of the patch resin and the adhesive layer is an essential step to secure the integrity and the sound mechanical performance of a bonded composite repair. Today, the required elevation of temperature is usually achieved by means of resistance heating blankets, which conduct heat to the repair area, regulated through closed loop temperature control, using one or two spot temperature measurements. Even though this method is rather simple and works adequately for small repairs, when larger areas need to be heated, discrepancies, like cold or hot spots, frequently occur, due to variable heat losses, caused by geometrical asymmetry, “heat sinks” etc.
The scope of this proposal, further to the know-how of the proposing Consortium in conventional resistance heating and induction heating, was to develop a third heating method called Conductive Skin Heating (CSH). The main advantages of CSH are that the voltage and the current could be very low compared to heat mats because of the very good efficiency of this middle frequency heating, which will be about 50 Hz to 20 kHz. Similarly, the voltage range could be limited between 5 to 100 V, which means that the maximum current in the mat will be less than 100 A, depending on the application and the heating rates which need to be achieved. Consequently, the CSH-mats could be much cheaper, while the electrical danger for the operators and any peripherical electronic devices will be significantly reduced. Additionally, as this method allows heating of very thin foils, the CSH-mats will be very flexible and easy to adapt to complex 3Dgeometries, while they could be easily integrated into the vacuum bag. Consequently, the handling of this thin and lightweight CSH-mats will be much easier compared to the other heating systems used for FRP repair, like thermal blanket and induction heating.
Within WP1, the definition of the main design parameters and repair process constraints that need to be taken into consideration for the preparation of the specifications and the overall design of the new CSH (Conductive Skin Heating) system have been specified, as listed in this Deliverable. It is well understood that, given the R&D nature of the project, potential changes to these requirements may be performed during the evolution of this project, as required, after discussion and agreement among involved entities. Moreover, an extensive comparative evaluation study had been performed in order to select the CSH foils material to be used will this project, complying to the requirements set out in Deliverable 1.1. For this reason, several types of carbon veil (foils) had been identified by the consortium as candidate CSH element, together with metallic foils and expanded metallic foils. It has been decided that two (2) materials comply with most of the project requirements, exhibiting corresponding advantages/disadvantages (Nickel Coated Carbon veil & expanded stainless steel foil). As, it was not evident which of these two materials was the most appropriate for fulfilling the CONDUCTOR objectives, it has been decided to proceed with both materials to the following steps of this project.
Within WP2, the optimised prototype of the CSH hardware has started being prepared, including all elements (power supply, flexible foils, connecting parts, conductors etc.) following extensive laboratory tests which was performed on the first prototype equipment. The main application of this prototype equipment was to optimise the CSH parameters and fine tune accordingly the heating control algorithm, as described in the following Task. The outcome of this Task was an optimised prototype of all equipment developed, the specifications of which was frozen during the Critical Design Review (CDR).
Within WP3, the first part of the testing of the developed CSH curing tool took place, in cooperation with the Topic Manager. A reliability and robustness study was prepared and verified against practical results, in order to evaluate the overall performance and robustness of the materials developed and of the methodology used. This Task involved extensive field testing of the developed CSH curing tool equipment and associated equipment and methodology (vacuum bagging specificities, installation requirements, temperature measurement strategy and thermocouple placement guidelines, connectors etc.) for CSH of flat bonded composite repairs. In order to guarantee that all practical application parameters have been taken into consideration, this work was carried out on panels that the Topic Manager has provided and was also demonstrated at the Topic Manager’s facilities.
Within WP4, the second part of the testing of the developed CSH curing tool took place. The main goal of this task was to verify the capability of the CSH blanket to operate on curved surfaces. By taking into account all the lesson learned in WP 3, the equipment was improved in areas such as the control software and the blanket production techniques. On the second step, a complicated aircraft part with medium radiuses was selected as a representative structure to test the limits of the new blankets application and also demonstrate its abilities. As a conclusion, the work performed within the project has generally been in line with the provisions of the Dow, while additional elements and effort has been added, as analysed previously. The CONDUCTOR project has been delayed by approximately nine (9) months and therefore, a request for the extension of the CONDUCTOR project has been submitted and was approved by CSJU, with a new finishing date of 31/8/2014. No other contractual and / or financial provisions were affected, whatsoever.