How smart composites could protect planes from invisible damage

Throwing baseballs at the exhibits is generally frowned upon at conferences. But at the iSurface stand at Advanced Engineering, discussions between engineers and visitors are frequently punctuated by the sharp crack of a hard ball smashing into a replica aircraft part.

Next to the aeroplane tail section, a computer display updates each time the model is hit. A virtual representation of the part shows exactly where the ball connected with a grid-like ‘smart’ composite section, and how hard it hit.

The patch, on display at the NEC in Birmingham on 29 October, is made of iSurface material. Developed as part of a two-year, Innovate UK-funded project involving English firms Munro Technology and Z Prime, and Swiss partners Axalp Technologies and the University of Applied Sciences and Arts Northwestern Switzerland, the material is designed to make commercial aviation even safer than it already is by detecting ‘beyond visible impact damage’ (BVID) that composites are susceptible to.

From violent bird strikes and hail ice to dropped tools, ground vehicles and debris on the runway, BVID can come from many different sources, said Munro Technology director Dr Ata Yoosefinejad to an audience earlier that day. Impacts can cause undetected damage that travels throughout surfaces.

Inspection methods to locate and identify BVID already exist, but they require aircraft to be on the ground – and planes cannot make money if they are not in the air. Instead, Yoosefinejad asked, what if non-destructive testing could be done by the materials themselves?

The partners believe that iSurface could be the solution. Based on Munro’s fibrous enTex toughened composite, the system is designed to both reinforce structures and sense damage using integrated electrically conductive nano-scale fibres. It uses an AI-enhanced predictive monitoring platform from Z Prime to detect and characterise damage in real time, and could either be implemented from the start of an aircraft build or retrofitted to existing planes.

While the system is aimed at cutting maintenance costs, the addition of functionalised fibres and an AI platform could raise the possibility of higher material costs – but Yoosefinejad tells Professional Engineering that savings elsewhere would more than compensate.

Maintenance can currently take anything from a day to three weeks, he says at the iSurface stand, “and one-third of it is poking around trying to find where the problems are. With structural health monitoring, that part of it is obsolete because you know where your problem is.” With savings on costs of millions of dollars per plane, the total amount could quickly add up for entire fleets.

But he told the audience earlier: “Profit is not the only thing. Safety is a big, major issue for operators, passengers, regulators, suppliers and everyone else. And over the past 50 years, the number of accidents in aircraft has reduced by 12-fold. We’re hoping that by using iSurface, we are able to improve even on that safety level.”

The multidisciplinary team, which said the product is at technology readiness level six and slightly above, aims to scale up following the conclusion of the project this year, with plans to implement the sensor on a flying aircraft operated by an OEM partner.

The presentation by Yoosefinejad was one of several talks at Advanced Engineering covering the potential for composites to make aerospace structures smarter and safer.

“What if we have a material that already has the sensors in, so we don’t bother with installation?” asked Dr Asimina Manta, technical fellow at the University of Sheffield Advanced Manufacturing Research Centre.

“We can put it in our components and we can wirelessly get information from that sensor. So we avoid extra cables, avoid extra data acquisition systems that add weight and more cost, especially in parts in motion. And with these sensors, [we would] be able to monitor every single moment of the life of a component, from ‘birth’ in manufacturing to service maintenance, repair and the final ‘death’ of components in reuse and recycling.”

That is what Manta and colleagues set out to achieve in the Horizon Europe Infinite project, which embedded glass-coated microwires into aerospace-grade composites known as non-crimp fabrics. The system uses field coils to generate an electromagnetic field that changes the impedance of the microwires, allowing engineers to detect stress development, ply movement and local strain.   

The result is “a very simple, cheap and light data acquisition system”, Manta said, which the team hopes could ensure the safety of composite structures while also improving their sustainability by optimising their end of life.