Fishy business

A technology based on the way that hammerhead sharks hunt for food could soon help engineers discover hidden flaws in the materials used to build bridges, aircraft wings and pipelines.

Researchers at Warwick University's School of Engineering, together with industrial partners Airbus UK, the BP Research Centre and Shell Global Solutions, are investigating a technique for non-destructive evaluation (NDE) known as capacitive imaging.

The two-year project, which has received funding of £160,000 from the EPSRC, is due to start in October.

When a hammerhead shark is searching for food buried just under the sand on the ocean floor, it uses small electroreceptors located along either side of its head to receive signals from the ocean bed. The shark swings its head from side to side, and if it detects certain changes in the signals, it knows food is present.

The technique being investigated uses a set of electrodes placed close to the surface of the material being studied. As the electrodes are moved over the object, interactions between the the material and the electric field between the electrodes produce slight changes in the field. These can be analysed to see whether there are any flaws in the material.

The approach does not require contact with the object under examination and is capable of inspecting a wide range of material types from metals to insulators, including polymers and composites. It can be used in underwater and hostile environments, including extremely high or low temperatures or even a radioactive environment. Furthermore, the system only requires access on a single side to be able to carry out a test and is unaffected by any magnetic properties of the material.

Ultrasound is a popular NDE technique. However, this is usually carried out using a water bath or requires contact with the object being examined, meaning that it is not suitable when the material can absorb or be damaged by the water or gel used.

X-rays are an alternative, but they are relatively expensive and also have health and safety implications for those using them, meaning that a simpler, safer, non-contact alternative is highly desirable.

'While Airbus are interested in looking for damage in the composite materials increasingly being used to make aircraft parts, BP and Shell would use this to measure corrosion in pipelines, among other things,' said project co-ordinator Prof David Hutchins. 'We have developed a simple version of what nature has allowed the shark to do.'

All the companies involved need a device that can be used to examine large amounts of materials and view any changes in the structure beneath its surface.

A feasibility study funded by the UK Research Centre in Non-Destructive Evaluation, the £8m EPSRC-sponsored collaboration between academia and industry, demonstrated promising results with a wide range of materials.

The project will therefore investigate the fundamental properties of the technique further. Particular areas of interest will include the inspection of aerospace structures and the detection of corrosion under insulators.

The researchers have already scanned a motorway bridge using a prototype device, searching for holes in the concrete used to support the bridge.

'We have used a device to search within a metre-think piece of concrete,' said Hutchins. 'Owing to the thickness of the material we used large electrodes.'

The eventual device will be handheld, and will be able to inspect materials between a few millimetres and several metres thick.

While some materials will be easier to examine using an electrostatic field, depending on their electrical conductivity, the researchers have discovered that by changing the frequency of the electrical field and the electrode size, shape and spacing, they can control the depth of the field's penetration.

Examinations on a thin metal sample and a Perspex plate with a side-drilled hole have also been successfully carried out. Such metallic conductors and electrical insulators are particularly hard to examine using existing NDE methods.