Material fitness

UK researchers have begun a joint project with a German university and Rolls-Royce to develop a cheaper and more accurate means of testing aerospace materials for metal fatigue. It is hoped the two-year project will provide a scientific breakthrough that will help make air travel safer by providing aeronautics engineers with more information about how fatigue affects aircraft components.

It was not until a spate of Comet aircraft disasters in the early 1950s that aerospace engineers first began to understand the catastrophic dangers of metal fatigue caused by the stresses and strains of high-altitude flight. Areas of the fuselage around the windows had steadily degraded over time, it was discovered. For the past 30 years researchers at Portsmouth University have been working with Rolls-Royce to study this phenomenon, particularly looking at the performance of its titanium alloy fan blades and nickel-based alloy turbine discs.

However, full-scale material testing is extremely expensive and so the aerospace industry is keen to move further towards using computer software to model the degradation of its components.

Unfortunately, according to Prof Jie Tong from the university's department of mechanical and design engineering, these computer models are not accurate enough to be relied upon entirely. Tong is leading the new research project that teams her department with colleagues from Germany's Siegen University.

'At the moment computer modelling can cut down on testing to some degree but we are just not confident enough in the computer models,' said Tong. 'The future of the industry depends on computer modelling so we can reduce these large expensive tests. Then only a certain number of tests have to be done to calibrate the model and that's it.'

Portsmouth has a unique array of equipment at its disposal for rig-testing of mechanical aerospace components. It owns a large machine that is able to simulate not only the stresses of take-off and landing but also the entire flight cycle using an electronic 'shaker', which can recreate these unique in-flight vibrations on the materials.

A current is passed through the sample and the team has developed a technique that allows it to pick up the electronic signals as cracks appear in materials and measure the voltage change as the crack grows. However, although this technique is well advanced in the UK it is still not accurate enough for Tong and her colleagues.

'We do these sorts of tests to get the global response of the material under certain loading conditions, on which we then base our computer models. This is just a specimen response, however,' she said.

Researchers from the Institute of Materials Technology at Siegen plan to use their expertise using powerful transmission electron microscopes to look in extraordinary detail at the materials during testing to see how the structure is being damaged over time. Features of the microstructures of a material, including its dislocation characteristics, are known to be related to the damage caused by fatigue and creep.

'During testing what you get is a strain response, looking at the number of cycles to failure,' said Tong. 'Ultimately this is a really simplistic approach as we don't actually know when a defect has already developed and we are unsure at a microscopic level what happens to the material. This means that our models tend to be semi-empirical. The work from Siegen could help us in this.'

According to Tong, this will mean their computer models could then be more physically based, making them far more accurate. The results could not only improve air safety but also greatly reduce maintenance costs, she said.