High Accurate Stereo-Digital Image Correlation Method in Liquid Nitrogen Immersion Conditions with Single Camera

Background

Stereo-digital image correlation (DIC) has found widespread application in experimental mechanics due to its full-field, non-contact deformation measurement capabilities. However, in liquid nitrogen environments, challenges arise from the difficulties associated with observation under cryogenic and within liquid media, as well as the resulting calibration issues. As a result, existing methods have yet to be effectively applied in such conditions.

Objective

To enable in-situ stereo-DIC in liquid nitrogen environments, and to address the challenges of visualization and calibration in extreme conditions.

Methods

A specially designed bi-prism was developed to solve the challenges posed by boiling bubbles, window frosting, and spatial constraints in liquid nitrogen immersion. To address the calibration issues in stereo-DIC systems in cryogenic, an accurate theoretical model based on a bi-prism-based pseudo-stereo refraction optical path was developed, and a two-step calibration method based on the epipolar constraint was established. In this method, the bi-prism does not need to be precisely positioned. Instead, by pre-calibrating the internal parameters of a single camera, the spatial position parameters of the bi-prism can be determined through the speckle features and the epipolar geometric relationship, fully determining the optical system.

Results

At room temperature, the diameter measurement error of the ping-pong ball was 0.57%, and the displacement error was less than 0.03 mm. In liquid nitrogen immersion, displacement detection of an aluminum alloy sample and accurate measurement of the bending deformation of a polycarbonate rod were successfully performed.

Conclusions

This study presents a novel bi-prism-based single-camera stereo-DIC method, achieving in-situ measurements in a liquid nitrogen environment. The reliability and practicality of the proposed method were validated under various experimental conditions, demonstrating its significant potential in extreme environments.