Critical composite maintenance

A group of non-destructive evaluation specialists are set to investigate an advanced ultrasonic evaluation technique that monitors the structural integrity of composite aircraft parts.

The team from Bristol and Nottingham universities are working with Airbus and Rolls-Royce on a new technique that would place an array of nearly 100 transducers on large composite structures to build up 3D pictures of their internal structure.

Anthony Croxford, a mechanical engineer at Bristol, said large arrays are currently used to study the internal structures of metal but they are not used with much reliability on composite structures because their internal structure is not uniform.

‘The problem is when you put ultrasonic energy into this structure, it will have different velocity in different directions because of all the fibres being aligned differently,’ he added.

These issues can be avoided when studying thin sections of composites by using higher frequencies. It remains a problem when trying to study thick sections - sizeable enough for use as a safety-critical component - because high-frequency waves become absorbed in the structure.

‘We have to move to lower frequency to inspect a thick structure so we have to start to consider the fact when we put energy into this component it’s going to propagate through that component in a very complex manner,’ said Croxford. ‘A big part of this project is working out a physical model to describe practically and quantitatively how energy will propagate through this structure.’

The researchers will use a large array of transducers to send ultrasonic signals through a structure. Reflected signals will be collected by sensors and sent to a computer.

Croxford said the key challenge for the research team will be developing ways to process the data and extract useful information out of it.

‘We hope to develop a model that will let us quantify and measure things such as polarity, the amount of resin and possible problems in fibre lay-up for thick composite components,’ he added. ‘We want to have in place a way we can model ultrasonic propagation through composite components and use that to locate and detect defects within those components. We want to do that for not just the transducer arrays we have at the moment but for future arrays as well.’

New aeroplanes are being constructed with less metal and more composites as a way to save weight and fuel without affecting structural integrity. The materials are believed to be as strong, if not stronger, than their metal counterparts.

While manufacturers have gained a good deal of experience inspecting the health of metal structures, evaluation of composites is still a relatively new field.

Croxford said non-destruction evaluation techniques for composites currently vary from visual checks for cracks to single ultrasonic probes moved repeatedly over a structural area. The methods are either not precise enough, he added, or they are extremely time consuming.

A better method is needed, Croxford said, with aircraft manufacturers' increasing desire to use composites in safety-critical areas. One example of this is the Boeing 787, which will be the first major commercial plane with a fuselage made completely of composite materials.

‘This is looking at things a significant way down the line,’ he added. ‘It’s a technique that will be required and will be necessary in the future. It’s a timely thing to be doing now.’

Siobhan Wagner