New bonding technology for aircraft parts

Reliable curing of the resin and adhesive layer is essential to ensure the integrity and sound mechanical performance of bonded composite repairs. Conventional heating blankets have hitherto been used to conduct heat to the repair area by using one or two spots for temperature measurement. Although this method provides good results for small repairs, large temperature discrepancies — cold and hot spots — are frequently observed when larger areas need to be heated.

With EU funding of the project 'Flexible conductive composite repair heaters' (CONDUCTOR), scientists have been developing a new heating solution dubbed conductive skin heating (CSH) to minimise inhomogeneous temperature distribution. This method involves lower currents and voltages compared to heating blankets. Consequently, the CSH mats should be much cheaper, and electrical danger for the operators and any peripheral electronic devices will be significantly reduced. In addition, as this method allows heating of very thin foils, the CSH mats should be very flexible and easy to adapt to complex 3D geometries.

Scientists defined the main design parameters and the repair process constraints for preparing the specifications and overall design of the new CSH system. In their bid to select a suitable material for CSH that meets the requirements, they proceeded with a metal-coated material and an expanded metal foil.

Use of available finite element codes helped simulate the thermal transfer mechanism in composite repairs. A representative repair case was modelled to calculate the heat losses through conduction and convection, which are the main causes for steep temperature gradients. This numerical study would help with the choice of the proper CSH material.

Scientists also started preparing the optimised CSH prototype hardware including all elements. This would be used to optimise the CSH parameters and fine tune accordingly the heating control algorithm.

CONDUCTOR is in line with the Clean Sky objectives regarding green repair solutions that further guarantee safe and 'ecolonomic' long-term use of an aircraft structure. The proposed repair process should improve reliability and performance, while also minimising the time an aircraft needs to spend on the ground for repair.