Stress relief

A portable X-ray device designed to detect stresses in large components such as turbine blades and aircraft wings has been developed by researchers at Oxford University.

The device uses intense x-rays and is capable of analysing large critical components for signs of residual stress without causing damage to the part itself.

Manufacturing processes, such as machining and finishing, produce residual stresses in components which affect the durability of the structures and shorten their lifetimes.

Standard x-ray machines, which take measurements using low energy beams in reflection, are not suitable for stress analysis of large structures as they are not of high enough intensity to penetrate beyond the first few layers of the component, said Dr Roger Welch, project manager at Oxford University’s technology transfer company Isis Innovation.

‘In reflection, all of the x-ray diffraction tends to occur in the first surface layers of the object. This is because in a typical reflection experiment you have a single wavelength of x-ray, so you are throwing away a lot of the rays,’ he said.

The Oxford system uses intense x-rays in transmission, allowing the device to penetrate the entire thickness of the component. ‘Transmission systems use all the X-rays generated so the intensity is much greater and there is a higher chance of the rays getting through,’ said Welch.

To detect residual stresses, manufacturers currently have to take components to a synchrotron or neutron facility, a time-consuming and expensive process. It is, of course, impossible to take a whole aircraft wing to be analysed.

Alternatively, engineers must drill damaging boreholes to test components.Having developed the device, the researchers have been testing it to produce data from an aluminium alloy beam. They claim the device is capable of penetrating to depths of 25mm in aluminium and over 5mm in steel and nickel.

Isis Innovation is talking to European aerospace manufacturers and companies specialising in non-destructive testing with the aim of commercialising the technology.

‘We are getting to the stage where we can open up the technology to many more people. People in engineering should be doing this type of testing but because of the cost and difficulty of going to a synchrotron they are not,’ said Welch.

Instead, many engineers simply imply the stresses on a particular component or structure produced by particular manufacturing processes based on experience, he said.