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UKAEA Fusion Technology’s Materials Technology Laboratory uses photography to get materials properties from small samples.

Determining the mechanical properties of potential fusion materials is essential to power plant design. One of the biggest challenges in this area of work is obtaining this information from very small specimens, which is necessary to avoid the hazards associated with larger, irradiated samples.

Engineers at the Materials Technology Laboratory at Culham Centre for Fusion Energy have come up with a non-contact solution. They take small samples and deform them at high temperature while taking photographs to measure the material’s properties by comparing the pixels in successive images.

This process is called digital image correlation (DIC), which compares hundreds of images of the material as it is stretched.

The DIC system uses two cameras to capture images of the sample surface before, during and after the testing process. The DIC algorithm is then able to calculate the displacement of the pixels imaged on the material surface to determine the updated 3D surface coordinates. This enables engineers to create a map of the various displacements the material has undergone and, from this, the material’s mechanical properties.

UKAEA material engineers Rory Spencer and Allan Harte have tested small-scale samples of fusion materials such as EUROFER, up to its expected operating temperature of 550 degrees Celsius. The material’s properties they obtain are in excellent agreement with more standard tests on larger specimens. Their findings will help to develop materials and components that could be used in future fusion power plants, such as STEP and DEMO.

Comparison between small and standard sized samples

Comparison between small and standard sized samples

The success of the work lies in the fact that the team were able to measure the local true stress-strain over an entire test at high temperatures, something which is not possible using standard techniques.

Rory said: “Testing small specimens is vital to get the maximum amount of data from scarce or novel materials. Using the 3D data from the DIC, we have been able to determine true stress-strain behaviour up to failure.

“The question we are keen to find out is what happens to the properties of a material as the temperature increases, in other words, how much force can it withstand and how local will the deformation be?

“Doing this work in a repeatable way on small specimens will help to determine the choice of steel or other materials for a fusion power plant.”

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