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The numbers make grim reading for the industry, but some engineering advances have helped to reduce costs in recent years. One of the most prevalent is structural health monitoring (SHM), which uses a mix of sensors and actuators, ‘smart’ materials and energy harvesting to detect minor damage to aircraft structures before it becomes bigger and more dangerous.
“The widespread adoption of SHM could both significantly improve safety, and reduce maintenance and repair expenses that are estimated to be about a quarter of an aircraft fleet’s operating costs,” says the 2016 book Structural Health Monitoring (SHM) in Aerospace Structures, edited by Fuh-Gwo Yuan.
Use of SHM can quickly target damage, helping to maintain safety and maximise time in the air. Commercial devices include the Smart Layer from Acellent Technologies. The device is an extremely thin dielectric film with an array of networked piezoelectric sensors, which generate a voltage when deformed. It includes strain gauges, thermometers and fibre-optic sensors to detect localised damage and delaminations – fractures into layers – as well as impacts.
Another option is CVM from Structural Monitoring Systems, which allows realtime monitoring of crack initiation and propagation. The sensor measures the differential pressure between fine ‘galleries’ containing a low vacuum and galleries at atmospheric pressure. If no flaw is present, the vacuum stays at a stable level. If a flaw develops, however, air flows from the atmosphere to the vacuum galleries and is measured by a transducer.
Embedded sensors
The devices can either take the form of self-adhesive polymer sensors, or can form part of a component. Embedded sensors have been the subject of research in recent years for their use in diagnosing complex failures in composite materials.
The US Army Research Laboratory, for example, is investigating embedded strain sensing and damage monitoring within polymers with 3D-printed carbon nanotubes. “The conductive nature of these materials could lead to an unprecedented level of multifunctionality, with potential in realtime SHM through embedded strain sensing and damage monitoring that will lead to safely and accurately assessing the remaining life in vehicle components,” said Dr Yelena Sliozberg in a research announcement.
Open minds
Professor Nico Avdelidis is head of the Integrated Vehicle Health Management Centre at Cranfield University. Rather than foreseeing increased investment in sensors post-Covid, Avdelidis predicts a shift in focus towards robotics and automation. “The Covid restrictions have forced the industry to be open-minded,” he says.
With increased pressure on workers, he forecasts further investment in robotic inspection. Drones and ground-based devices, such as the four-wheeled Vortex Robot that uses suction to climb around and inspect aircraft, could keep workers out of hazardous areas – and reduce costs.
Sensors used for SHM will become smarter as well, and Avdelidis says they will increasingly be embedded within structures. The main challenge, as he sees it, is how to process and use the data.
“You have millions of collection points from the sensors, so how do you process that data?” he asks. “If you know how to process that properly, you can make the correct decisions about maintenance.”
Artificial intelligence and machine learning are the most promising solution, he says. The more data can be fed into a system, the more accurate its results become over time, so data-heavy SHM is a perfect candidate.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.