Digital Image Correlation (DIC) is a popular, non-contacting technique to record full-field displacements from the surface of a deforming object. In brief, the technique works by applying a high-contrast  speckle pattern to the surface of the object, and using digital cameras to track displacements of the pattern through time. The derivatives of the displacements are then often used to compute strain. The quality of DIC measurements depends closely on the scale and contrast of the speckle pattern applied to the specimen. Most common methods of speckling involve paint, but some researchers have reported success using a native surface pattern in place of an externally applied speckle pattern. Native patterns are especially important when paint is difficult to apply, for example in post-irradiation evaluation (PIE) when irradiated materials are difficult to safely handle.

 

This research explores the viability of using text as a speckle pattern by collecting text from 5 different seminal DIC documents: (1) “Image Correlation for Shape, Motion, and Deformation Measurements,” a textbook by Michael Sutton et. Al; (2) The user manual for Vic-3D v7, a commercial DIC algorithm by Correlated Solutions Inc.; (3) “The Good Practices Guide for DIC.” A standardization document by the International DIC society edited by Elizabeth M.C. Jones; (4) “The Art and Application of DIC,” a series of 24 editorials by Phillip Reu; and (5) a paper by Robert Hansen, a graduate student in our lab, which was published in Review of Scientific Instruments. Specimens were created by placing text from the first paragraph of each document on wooden craft sticks. For comparison, a 6^th^ pattern was created with spray paint to form a traditional random speckle pattern. Greyscale histograms of the 6 patterns showed all patterns to have comparable contrast, and a Virtual Strain Gauge (VSG) study was performed on the random speckle pattern to determine which common DIC parameters to apply across all experiments.  The specimens were subjected to a series of three tests. First, each specimen was translated by a known, uniform, in-plane displacement; after which 2D-DIC was used to compare how well the mean DIC measurements agreed with the applied displacements. Second, each specimen was subjected to an in-plane rotation, and similarly compared between 2D-DIC measurements and the applied motion. Finally, each specimen was deformed out-of-plane under 3-point bending, and heterogeneous displacements and strains were compared against an analytical solution derived using beam theory. The study also explores the effect of font type and line spacing on the utility of the text as a speckle pattern.


Using Text as a Native Speckle Pattern in Digital Image Correlation

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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