Reliable testing technology: non-destructive residual stress measurement using ultrasound

Residual stresses have a considerable effect on component properties. They influence fatigue strength and fracture toughness, among other things. Both the magnitude and the sign of the residual stress matter – in other words, whether the stresses are tensile or compressive. Compressive residual stresses typically improve component properties, whereas tensile residual stresses can impair them. Reliable knowledge of the residual stress state is therefore essential when assessing operational safety and service life.

Methods for measuring residual stresses

Various measurement methods are available for determining residual stresses. One established approach uses strain gauges. This generally involves locally cutting the component, so the method is usually destructive. X-ray goniometers allow residual stresses to be measured near the surface of components and are particularly suitable for analysing surface layers.

Polarised ultrasonic waves, by contrast, allow non-destructive measurement across the entire cross-section. This method opens up new possibilities, particularly for safety-critical components with complex stress states.

Practical example: freight wagon wheels

Railway wheels on freight wagons are a particularly sensitive application. These wheels are often braked using tread brakes that act directly on the running surfaces.

Braking generates considerable heat in the wheel rims. This thermal loading causes tensile residual stresses to build up. As a result, fatigue strength and fracture toughness can be reduced to such an extent that, in extreme cases, wheel fractures occur.

The residual stress state of these wheels must therefore be monitored continuously. Although established ultrasonic testing techniques exist, they reach their limits with certain wheel types and advanced states of wear.

If the stress state cannot be determined reliably, scrapping the affected wheels is often the only option for safety reasons, with corresponding economic consequences.

Ultrasonic testing technology for challenging cases

W.S. Werkstoff Service GmbH has examined this problem in depth and developed an ultrasonic testing technique that enables reliable residual stress measurement even in these challenging cases.

This development is based on our long-standing qualifications in non-destructive ultrasonic testing and our expertise in mechanical and technological testing. The latter is particularly important for correctly calibrating the testing system, as valid stress values depend on precise referencing.

Beyond wheels: a versatile testing technique

A key advantage of our in-house development is that, unlike many established testing systems, it is not limited to railway wheels. Other metal components can also be examined using W.S. testing technology.

The price of this flexibility is the high level of expertise required of testing personnel: correctly acquiring and interpreting measurement data requires sound specialist knowledge of materials science, stress analysis and ultrasonic testing. This is precisely where our expertise comes in.

W.S. Werkstoff Service – expertise in NDT services

The W.S. MaterialtestCenter applies these newly developed capabilities both in its NDT testing services and in training testing personnel.

Interested in NDT training? Further information about our training programmes is available under non-destructive testing courses.