Image conscious

Researchers claim optical technology is an easier, more accurate way of testing the strength of welded components. Siobhan Wagner reports.

The structural integrity of welded components could potentially be calculated more easily and accurately with a new residual stress measurement system under development at TWI, the UK materials technology specialist.

Researchers are developing a technique that uses Digital Image Correlation (DIC) in a simpler way.

DIC is an optical technology for determining residual stress. In a typical DIC process, a high-resolution camera captures a speckled image of a specimen before and after a hole is drilled through it. The images are loaded into a computer and the difference between them is calculated to measure the displacement of the speckles in the image.

The strain on the specimen is obtained from the displacement measurement, and when that information is fed into analytical or finite element models, the residual stress can be calculated.

Residual stress measurement is used to determine the built-in stresses that are often induced in large welded components when they are manufactured. This is widely quantified in industry using the hole-drilling technique, but the method often uses strain gauges to measure the stresses.

The strain gauges have many limitations. They require the specimen’s surface to be extensively prepared and their measurements are only accurate if the hole is precisely drilled. So there is an increasing interest in non-contact methods for measuring strain.

DIC is becoming an increasingly popular non-contact method but the process is complex and the technique can only be carried out by professionals with expertise in mechanics and materials.

Jianxin Gao, of TWI, said his organisation is trying to avoid that problem with its easy-to-use method. The technique, called Deformation Pattern based Digital Image Correlation (DPDIC) still requires two speckle-like images but the information within these images is extracted in a different way.

While conventional methods require the displacement and the strain of a component (or some linear equations) to be determined before calculating the residual stress, DPDIC takes information about the stress of a component directly from the image.

‘The stress components are treated as the direct variables during digital image correlation,’ said Gao.

The principle was conceived earlier this year, Gao added, and TWI researchers have been spending the last few months perfecting new software that can calculate residual stress directly.

There has been extensive testing of DPDIC over the year, and the results measured by the new technique have been compared with those obtained from strain gauges.

‘These validation tests showed that the technique works,’ Gao said, adding that there are still some improvements to be made.

For instance, he said, the technique cannot yet measure depth variant residual stress.

Residual stress can be measured at various depths by taking multiple images of a specimen as a hole is incrementally drilled through it.

Gao said he hopes DPDIC for surface residual stress measurement will be commercially available in about six months and that a system for measuring depth-variant residual stress will be available slightly later.

TWI is currently seeking commercial partners. The team believes there is a market for such a technology. Gao said residual stress quantification is of crucial importance in mechanical design engineering to prevent structural failure during operation, for example in the aerospace industry.

‘With this system, the cost and time needed for residual stress measurement will be reduced,’ he added.