Chirpy outlook

Anglo-Indian project aims to adapt bats’ echolocation system to quickly and accurately inspect composite structures for unseen cracks and damage. Siobhan Wagner reports.

A technique adapted from the echo- location method used by bats and used to improve radar performance could prove to be an effective way of detecting dangerous defects in aircraft, racing cars and spacecraft.

An Anglo-Indian project will analyse the efficiency of a thermography method called Frequency Modulated Thermal Wave Imaging. Thermography involves examining heat as it flows across an inspected part. If a crack that could lead to failure is present, it will trap heat and this will show up on infrared camera scans.

The technique, dubbed ‘chirp modulation’ after the sound emitted by bats, involves heating up an inspected part by shining a light modulated over a range of frequencies on to it.

The project, involving Bath University and the Indian Institute of Technology in Delhi, will compare the effectiveness of various forms of thermography. The aim is to establish a quick and accurate way of inspecting for cracks and other damage; this is even more esssential today as more aircraft, racing cars and other engineering structures are made from carbon composite material rather than metal to reduce weight.

Unlike metals, composite fibres can receive an impact — such as an aircraft being hit by a stone on take-off, or by a hailstone in flight — and show no obvious sign. Yet they may be extensively weakened beneath the surface.

This is because the composite consists of layers that are glued together. Any impact can make the carbon layers come apart from the glue, which is beneath the surface and invisible to the eye. If the ‘chirp modulation’ method proves effective, then the aircraft industry, for one, could well be interested in using it.

Project leader Darryl Almond, of Bath’s department of mechanical engineering, said chirp modulation could be an improvement on one current thermography method that uses modulated light at only one frequency.

‘The problem with that method is you tend to detect defects at only one particular depth,’ he said. ‘If you know that a defect is going to occur at a millimetre beneath the surface, you do a small calculation to see which is the best frequency to use. But often in testing you don’t have the luxury of knowing what depth the defect is going to be.’

Defects can be any number of depths, so users have to carry out the test at more than one frequency to make sure they haven’t missed anything.

Chirp modulation operates at a range of frequencies and thus eliminates the depth limitations. While this technique might seem common sense, there were many technical challenges that stood in the way before it could be put into practice. For one thing, there was how to collect and analyse all the data the technique produced.

‘You’re essentially taking an image, which is formed by 320 x 240 pixels, which is an awful lot of individual image points,’ said Almond. ‘We have to take every one of those points and compare it with a chirp frequency changing signal. To do that, computationally, is a long job. It has only been recently that we’ve been able to do this in a reasonable amount of time.’

Now all the major technical challenges have been met, Almond claimed it shouldn’t be long until chirp modulation will be ready for use by industry.

He said he knows of at least two companies that manufacture flash thermography equipment and periodic thermography equipment that would be able to easily take the ‘chirp’ technology on board and incorporate it into their existing systems.

Almond said they would simply need to change their technology’s excitation system, so they have a technique that drives heating in a chirp fashion. Also, they would have to incorporate a new software program for analysing the data images. ‘It’s not a huge job,’ he said.

The collaboration, which is scheduled to begin in April, has received £70,000 funding from its partner institutions and the UK India Education and Research Initiative (UKIERI). Their final results will be out within the next three years.