Researchers at the University of Bath have developed a method using sound waves to examine aircraft parts and look for damage, while another group of researchers from the Nanyang Technological University in Singapore have used heat to predict results of inspections.

Both methods could lead to massive financial savings for the aerospace industry.

One of the primary objectives for the industry is to keep the weight of aircraft structures as low as possible. Aircraft manufacturers want to use materials such as carbon fibre reinforced polymers (CFRPs) and glass-reinforced polymers (GFRPs) more extensively as they give better performance and fuel efficiency.

But inspection of such light-weight materials is of great importance during the various stages of fabrication. Stones thrown up during take-off, hail-stones or dropped hand tools can cause significant internal damage that doesn't show at the surface.

Researchers needed to devise a fast, reliable and economical technique to find out how this minor damage is amplified by subsequent in-flight wear and tear and if this could lead to eventual catastrophic failure.

The researchers at the University of Bath use acoustography -­ ultrasonic techniques -­ to monitor the growth of defects that lead to fracture.

Their system makes use of a large piezoelectric transducer, a device that produces high frequency mechanical vibrations when driven by a high frequency electrical signal. This transducer then produces a broad beam of ultrasound that is passed through the material being tested, to an acousto-optic imager.

The imager is formed by a layer of liquid crystal material specially formulated to be sensitive to ultrasound. When ultrasound impinges on this layer, liquid crystal molecules are tipped over to provide a visual contrast of the type seen on liquid crystal displays such as in digital watches.

The researchers repeatedly put loads on the material to simulate a plane going on many flights, and found that impact damage grows as time goes on, leading to eventual failure of the whole piece of material.

Their results also showed that this kind of damage growth only occurred when the stress caused by the loading was over a certain critical amount. For loads below this level, the damaged samples could be loaded over a million times without showing any damage growth.

"This is a very reassuring discovery as it indicates that composite materials used in the aerospace industry can be designed to remain safe even when damaged by minor impacts," said Professor Almond of the University of Bath. This research was made possible by funding by the Engineering and Physical Sciences Research Council (EPSRC) and with close collaboration with Airbus UK.

The other detection method, developed by the researchers in Singapore, uses a technique known as lock-in thermography -­ an infrared camera detects the surface temperature of a wave passing through the material, producing a thermal image.

In the past, this technique has only been used in ambient conditions, but for the first time, the researchers use a model to investigate how well this technique works under realistic convective conditions, as convection is an important way of transferring heat.

The experimental results indicate that lock-in thermography is an effective technique for detecting subsurface defects in carbon fibre reinforced polymer structures.

"This model is very valuable for the selection of inspection parameters which would result in great cost savings," said Bai Weimen from the Nanyang Technological University, Singapore.

Measurement Science and Technology is published by Institute of Physics Publishing, a wholly owned subsidiary of the Institute of Physics. The Institute of Physics is a leading international professional body and learned society with over 30,000 members, which promotes the advancement and dissemination of a knowledge of and education in the science of physics, pure and applied.

(Reference: Measurement Science and Technology, Volume 12, Issue 2.)

[Contact: Professor Darryl Almond, Bai Weimen]

18-Jan-2001