While honeycomb, glassfibre- and carbonfibre-reinforced plastics have found increasing application across aircraft over the past decades, repairing the synthetic material has remained a labour-intensive, manual task for highly skilled mechanics. And it is still the case for much of the metal airframe work too. Nevertheless, manufacturers and maintenance providers are developing automated processes to make composite repairs more efficient and, crucially, to extend their scope for critical parts.
LHT's Rapid Repair fixes primary composite structures and non-critical components in the same way
Lufthansa Technik (LHT) has been working on an automated repair system for primary composite airframe structures since 2009. The first phase of the six-year research and development programme was completed in April, after the maintenance, repair and overhaul (MRO) company had applied for a patent for a robotic system named Rapid Repair earlier this year.
The objective is to repair primary composite structures in the same way as non-critical components, such as fairings and engine cowlings, where it is possible to remove damaged fibres and replace them with a bespoke repair patch. The inherent risks in the labour-intensive manual work - for example inadvertent damage to intact fibres during the material removal or reduced bonding due to surface contamination in the repair area - are not acceptable for primary structures, however. Critical parts are thus typically fixed with bolted doubler repairs, where the damaged section is reinforced with additional composite or titanium material.
Together with a number of partners, including EADS subsidiaries Cassidian and Eurocopter, LHT has developed a robot-based system that can detect damage, remove the broken fibres, determine surface contamination, and implement a replacement patch. The automation will lead to reliable, replicable results, which should consequently allow the technique to be applied for primary components. However, it should also improve efficiency by up to 60% over today's manual processes.
While an initial, stationary system should become ready next year, the aim for the second three-year phase - named composite adoptable inspection and repair (Caire) - is to develop a mobile version for field repairs. Designing a small, transportable unit with the required accuracy, which is versatile enough for different damage scenarios across an aircraft, will be a challenge, says Jan Popp, project manager new technologies and innovation at LHT. He adds that the focus is on fixing ground damage, such as impact from service vehicles.
Rapid Repiar uses bespoke patches
The system will not only need to function under different environmental conditions but also ensure that no harmful substances can escape. This is particularly important as carbon dust is harmful to electronic equipment onboard the aircraft.
Determining potential contamination in the repair area is crucial to ensure full bonding strength between the existing and new material. Popp points out that while water break-free testing is a reliable, long-established method to test metals for contamination, this may not be so straightforward for composites. He adds that water can behave unexpectedly on sanded composites surfaces. An experienced engineer can use his judgement and "feel" during a manual repair, but for an automated process any contamination needs to be verified beyond doubt.
In the first project phase, the designers were able to develop qualitative methods to check whether and by which substances a surface is contaminated. This will now be extended to include also a quantitative assessment. "The clear objective is to find out the limits [for potential contamination]," says Popp.
Composite specialist GKN Aerospace, also UK-based is working on an automated repair process at its facility in Cowes, on the Isle of Wight. The project focuses on the removal of damaged material and area preparation for a stepped scarf repair. But the crucial difference is that instead of using mechanical cutting tools to remove the existing material, GKN employs a pulsed laser.
This is much more gentle to the composite structure than, for example, a grinding process, says John Cornforth, vice-president technology. The laser vaporises only the epoxy resin but does not penetrate the carbon material. Once the fibres have been laid bare, they form a brittle fluff, which can be brushed away, so laser ablation and automated brushing phases alternate as the system cuts ply by ply through the composite material. With a penetration depth of 0.012mm, the laser even needs several cycles to expose the carbon fibres in one ply.
The advantage is that unlike grinding, the laser does not affect fibres adjacent to a cutting area. The ablated surface feels slightly rougher, but the fibres of the boundary layer of the existing material, which form the interface with the replacement patch, remain intact because there is no adverse effect by the laser's thermal impact on the carbon material. As a consequence, Cornforth says that the technique leads to increased lap shear strength.
GKN is aiming its pulled laser at the automated repair process
GKN is not working on an integrated repair cell that can accomplish all tasks from damage detection to implementing the replacement patch. However, Cornforth says the manufacturer is running a specification programme for such a potential future system to ensure that the laser ablation technology could be included. He estimates that the laser technique could reduce costs for the labour-intensive repair preparation by up to 50%.
It has thus far only been tested on flat panels, but the system will now be modified to work for three-dimensional components, such as skin-stringer combinations, as well as differently structured material, such as honeycomb. This which will require more complex laser movement. "Are we able to manipulate the laser in the right way to achieve the same results? That's what the next phase is all about," says Cornforth.
Although the technology is not yet mature enough for operational use, the goal is to produce a system that can be used for load-bearing primary structures, he adds.
Composite inspection technology is also an area where - despite decades of experience with the synthetic fibres - there is still much need for improvement. Working with Bad Oldesloe-based Automation Technology and the Technical University of Hamburg, LHT has focused on active thermography, which has long been employed for metal structures, as a potential technique to make damage detection more efficient.
The method measures a component's response to a brief thermal impulse from a heat source, such as an infrared lamp. The heat wave travels through the material but if there is any internal damage, such as a delamination, the wave cannot progress. This leads to a heat congestion and temperature difference in the respective surface area, which can be visualised by a highly sensitive thermal imaging camera.
The main advantage of the technique is that it can assess an area of 0.25-0.5m² (2.7-5.4ft²) with a single impulse, allowing speedy inspection of large areas. Other technologies, such as phase-array ultrasound equipment, may provide a more detailed picture of internal damage, but can only scan a small area.
Thermography can be used for composite structures which are up 4-5mm (0.16-0.20 inches) thick, says Dietmar Strohmeyer, manager product engineering and airworthiness management. This translates into approximately 80% of the 787's outer fuselage and wing area. Stronger sections, such as door surround structures, would need to be inspected by other methods.
The developers have also refined the filter algorithms in the thermal imaging software to detect smaller defects and even indicate the depth of internal damage. But they stress that its purpose is to give indication of internal damage, which may then need to be investigated further through other methods.
Strohmeyer is confident that the technology can be scaled to serve different applications, ranging from large, stationary set-ups for scanning complete fuselages in sections to mobile equipment for field use as well as miniature units which could, for example, be employed for internal inspections in thrust reversers.
The four-year project began in 2009 and has thus far evaluated the basic suitability of the existing technology for composites. Strohmeyer says the second half focuses on how it can be used for more complex structures, such as three-dimensional components or parts made of honeycomb. This phase will also determine the system's robustness against environmental influences, such as humidity, temperature and precipitation. But the engineers also need to find out how painted surfaces can influence the inspection.