Frequent flyer

Aircraft safety could be boosted by faster and more frequent inspections of wing structures if researchers can demonstrate that their methods are reliable. They plan to speed up the detection of defects in wings by monitoring the behaviour of low-frequency ultrasound.

The use of ultrasonics for non-destructive testing (NDT) and structural health monitoring is spreading, but inspectors tend to use frequencies of 5MHz. ‘Our guided ultrasound uses much lower frequencies, at about 100kHz,’ said Dr Paul Fromme, project leader at UniversityCollege, London. ‘Then it’s like throwing a stone in water and seeing how the ripples spread.’

Fromme’s team, which is collaborating with a major European aerospace company, will be working with two or three layers of aircraft-grade aluminium plates, with a combined thickness of up to 10mm, to see if it is possible to detect flaws in the materials or the fasteners. ‘Our interest is in areas of the wing and possibly in the area where the wing is attached to the body,’ said Fromme.

Aircraft are subject to cyclic loading during take-off, landing, manoeuvring and bad weather. Stress concentrates at the bolt holes of the fasteners that keep the wing materials together, and fatigue cracks can start to develop. so regular NDT is needed — but it takes time and is expensive. The guided ultrasound may be able to inspect several metres of wing at a time, speeding up the process, reducing the cost and so allowing it to happen more frequently.

Although it will be a lab-based project, the team will be working on real, complex, multi-layered aircraft structures to find ways of dealing with the complexity of the guided wave propagation and scattering.

The team will monitor fatigue crack growth during cyclic loading and investigate the method’s sensitivity and reliability in detecting the cracks in the different layers.

Fromme believes the three-year project could lead to commercial applications in a decade. ‘The theory was developed 50 years ago, but at the time they lacked facilities to pursue it,’ he said.

‘In the past 10 years there has been progress with the technology for pipeline inspection and now the whole aircraft business is getting more concerned about safety. That’s why it’s happening now.’