Abstract
A digital imaging correlation (DIC) test setup was developed to accurately record the contraction of a Bi-2223 superconducting racetrack coil conduction cooled to 154 K. The coil was gripped at both ends to simultaneously measure the load and displacement induced by its own contraction. Postprocessing of the DIC data derived the displacement and strain fields from two areas of interest. The global deformation of the coil structure was influenced by how it was restrained, but the stress-strain curve in the y-direction of the HTS coil was established, and its stiffness when under thermally induced strain was found to closely match that of the epoxy resin. The coefficient of thermal expansion determined from the strain-temperature curve was in the same order of magnitude as dilatometer measurements performed upon a sample removed from a similar coil and the difference attributed to small variability in the temperatures across the coil and the mechanical restraint placed on the coil.
Highlights
THE thermo-mechanical stresses in superconducting coils are usually modelled and rightly focused upon evaluating their impact upon the conductor
Non-invasive optical techniques can be applied to measure the global response of the coil, e.g. techniques like Digital Speckle Pattern Interferometry (DSPI) have been applied to study deformations of HTS tapes [4]
This novel piece of work has confirmed that Digital Imaging Correlation (DIC) can be successfully deployed at cryogenic temperatures
Summary
THE thermo-mechanical stresses in superconducting coils are usually modelled and rightly focused upon evaluating their impact upon the conductor. Models are normally validated by mechanical tests performed on coupon size specimens removed from sample coils and instrumented with strain gauges [1,2,3]. Non-invasive optical techniques can be applied to measure the global response of the coil, e.g. techniques like Digital Speckle Pattern Interferometry (DSPI) have been applied to study deformations of HTS tapes [4]. DIC is a well-established technique that tracks the displacement of pixelated blocks from a series of digital images to measure surface deformation down to one part per million of the field of view [5]. The application of DIC at cryogenic temperatures is almost untried [9] and presents new challenges, to assess the robustness of speckle patterning and to develop an appropriate cooling technique
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