Enabling non-contact deformation measurements at cryogenic temperatures for large deformations

The growing use of cryogenic technologies in applications such as nuclear fusion and aviation has created a growing need for mechanical testing methods capable of characterising material behaviour at low temperatures. This work presents a cryogenic mechanical testing methodology that integrates stereo digital image correlation (DIC) with forced-convection cooling, enabling full-field strain measurements from room temperature down to 123 K in a conventional environmental chamber. Key experimental challenges associated with convection-cooled cryogenic environment, including frost formation and convection-induced optical distortions, were mitigated, allowing DIC measurements at cryogenic temperatures during large deformation. A robust speckle pattern fabrication technique based on airbrush-deposited acrylic paint was developed for cryogenic DIC measurements. The resulting pattern remained stable up to 80% elongation at 123 K without cracking or loss of correlation. The performance of the experimental setup was demonstrated through tensile tests on 316L stainless steel specimens at room temperature and 123 K, including an assessment of DIC measurement uncertainty and full-field deformation behaviour under cryogenic conditions. The system enabled full-field observation of spatially heterogeneous deformation was consistent with slip-dominated plasticity reported in the literature. At 123 K, DIC measurements captured a transient phase of necking-like strain localisation during plastic flow prior to second-stage strain hardening, demonstrating the capability of the approach to resolve evolving deformation behaviour under cryogenic conditions.