Stress management

A clever new coating could help speed the design process by discovering flaws or problems before a part is mass-produced. Christopher Sell reports.

A new luminescent paint, developed by researchers at the University of Florida, could change the face of stress and strain experiments by highlighting flaws in prototype versions of drive-shafts, axles and other parts, allowing them to be produced more quickly and with less chance of breakage.

According to Peter Ifju, associate professor of mechanical and aerospace engineering at the University of Florida, and one of the inventors, the coating means a part can be assessed in two to three days, whereas traditional strain gauges can take months.

Designers spray on the paint, dry it, and allow it to cure overnight under UV lamps. Then, as light is transmitted through the chemicals that make up the paint, its polarisation changes in direct proportion to the amount of strain the part is experiencing. The light-emitting chemicals then transmit these polarised differences — invisible to the human eye — to a digital camera and computer. The result is a graphic, 3D map of the stress levels sustained over the entire coating.

The optical system, Ifju explained, looks for changes in polarisation with the paint as a function of stress levels. ‘Basically, you check the initial image, map out the polarisation effect and from the loaded image you compare the two. It is like having many tiny strain gauges all over the place,’ he said.

One of its biggest advantages is that in a fairly short period of time you can do very accurate assessment of the design of the prototype, which you can then use to validate your finite element analysis or your computational model. ‘It could be significantly cheaper in the long run,’ said Ifju.

Further advantages are that unlike conventional techniques such as photo-elasticity, where sensitivity to measurement is dependent upon the thickness of the paint, the strain-sensitive paint is not affected and gives ‘robust information’ no matter how the paint is used.

Many techniques are used to measure stress, including the strain gauge — a device that functions in a similar way to a miniature scale. However, the majority of these techniques only take a measurement at one specific point, have poor resolution for larger areas, or require expensive equipment. These shortcomings can lead to designers missing a ‘hot-spot’ — an area that may appear normal to the eye and CAD designs, but actually experiences significant and potential failure-level stress.

Ifju, however, believes the new coating will provide designers with a way to accurately measure the stresses applied to large areas or an entire part, which will not only highlight potential flaws in a prototype, but also help them refine their computer models and improve the design.

‘All the hot-spots are visible if you have optical access, so they should be picked up. The coating has a very high spatial resolution, if there is a hot-spot that is 1 or 2mm away from a place that is not a hot-spot, it will be picked up.’

So far the coating has proved effective in experimental tests aimed at identifying previously recognised hot-spots on prototype parts, such as a suspension control arm for a Cadillac sedan, a drive-shaft for a Jeep Cherokee and a strut dome for a Porsche SUV.

Ifju also revealed that the technology could be used for experimental stress analysis and is keen to adapt it for use in other applications such as non-destructive monitoring. This involves painting a part, putting it into service and periodically checking whether there is degradation. Ifju is keen to see this technique applied in the aerospace and automotive industries.

Automotive technology company Visteon, which has spent $2m (£1m) on the research since 1997 and holds the licence for the technology, is keen both to act as a test-bed and to license the technology to interested manufacturers.