Abstract
Digital Image Correlation (DIC) is a well-established, non-contact diagnostic technique used to measure shape, displacement and strain of a solid specimen subjected to loading or deformation. However, measurements using standard DIC can have significant errors or be completely infeasible in challenging experiments, such as explosive, combustion, or fluid-structure interaction applications, where beam-steering due to index of refraction variation biases measurements or where the sample is engulfed in flames or soot. To address these challenges, we propose using X-ray imaging instead of visible light imaging for stereo-DIC, since refraction of X-rays is negligible in many situations, and X-rays can penetrate occluding material. Two methods of creating an appropriate pattern for X-ray DIC are presented, both based on adding a dense material in a random speckle pattern on top of a less-dense specimen. A standard dot-calibration target is adapted for X-ray imaging, allowing the common bundle-adjustment calibration process in commercial stereo-DIC software to be used. High-quality X-ray images with sufficient signal-to-noise ratios for DIC are obtained for aluminum specimens with thickness up to 22.2 mm, with a speckle pattern thickness of only 80 μm of tantalum. The accuracy and precision of X-ray DIC measurements are verified through simultaneous optical and X-ray stereo-DIC measurements during rigid in-plane and out-of-plane translations, where errors in the X-ray DIC displacements were approximately 2–10 μm for applied displacements up to 20 mm. Finally, a vast reduction in measurement error—5–20 times reduction of displacement error and 2–3 times reduction of strain error—is demonstrated, by comparing X-ray and optical DIC when a hot plate induced a heterogeneous index of refraction field in the air between the specimen and the imaging systems. Collectively, these results show the feasibility of using X-ray-based stereo-DIC for non-contact measurements in exacting experimental conditions, where optical DIC cannot be used.
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Notes
In the literature, X2D-DIC has also been called “digital speckle photography”, “digital speckle radiography”, or “texture correlation”.
Laser-printed patterns on paper can suffer from dithering, where the borders of the speckles are polluted by small droplets of ink. In some situations, this can cause spatial aliasing in digital images of the speckle pattern. However, these spurious features were measured with a high-magnification, high-resolution image to be approximately 85 μm or smaller. Additionally, the lens system resolution was measured to be 7 lp/mm, which translates to a smallest resolvable feature of 70 μm. Therefore, the lens acts as an analog filter, filtering the small features on the edges of the speckles, leading to unaliased speckles in the images used for optical DIC.
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Acknowledgments
The authors gratefully acknowledge David Saiz for measurements of the tantalum speckle coating thickness, Andrew Miller and Dr. Andrew Vackel for creating the tantalum speckle pattern using thermal spray, Andrew Lentfer for X-ray imaging and for creating the tungsten speckle pattern using tungsten powder mixed in paint, Paul Farias for creating the drawings of the mask for thermal spray, and Dr. Kyle Lynch and Dr. Benjamin Halls for insightful discussion.
This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
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Jones, E., Quintana, E., Reu, P. et al. X-Ray Stereo Digital Image Correlation. Exp Tech 44, 159–174 (2020). https://doi.org/10.1007/s40799-019-00339-7
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DOI: https://doi.org/10.1007/s40799-019-00339-7