Abstract
Digital Image Correlation (DIC) is a popular non-contact, photography-based strain measurement method in geomechanics laboratory testing of rock. The application of a speckle pattern on specimens is often perfunctory, but it is a critical control for the success of DIC. This study recommends an optimal method for applying sub-millimeter scale speckles to Brazilian Tensile Strength (BTS) rock specimens. Four speckle pattern application methods are optimized for this purpose, including spray paint, airbrush, stamp, and a novel laser paint-burning method. An experimental BTS testing program on Westerly granite specimens equipped with a strain gauge on one side and 2D DIC on the other is used to assess the influence of speckle patterns on the uncertainty and repeatability of the resulting strain measurements. Five BTS specimens were prepared using each speckle pattern application method. The results show that high-quality strain measurements are achievable with speckle patterns prepared using the spray paint, airbrush, and laser methods. However, the consistency of measurements obtained using aerosol-prepared patterns varies, as these methods are more sensitive to environmental factors and subject to practitioner experience compared to the laser method, which utilizes a computer-generated pattern. The novel laser paint-engraving method is recommended as it provides a standardized and robust procedure for preparing speckle patterns on BTS specimens. If a low-cost alternative is desired, the airbrush method is recommended over spray paint due to its higher control and consistency. Overall, the findings underscore the importance of preparing adequate speckle patterns to ensure high-quality DIC strain measurements on rock specimens.
Highlights
-
Accurate Digital Image Correlation strain measurements on Brazilian Tensile Strength rock specimens require high quality sub-mm speckle patterns.
-
A novel laser-paint burning method streamlines the preparation of sub-millimeter speckles on Brazilian Tensile Strength rock specimens.
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Aerosol-prepared speckle patterns produce variable measurement quality due to sensitivity to environmental factors and practitioner experience.
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Airbrushing is recommended over spray painting as a low-cost alternative to the laser due to better consistency in speckle pattern quality.
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Data availability
The data that support the findings of this study are available on request from the corresponding author, EG.
Abbreviations
- #En :
-
Scientific notation (# × 10n)
- BTS:
-
Brazilian tensile strength
- CAD:
-
Canadian dollar
- CO2 :
-
Carbon dioxide
- DIC:
-
Digital image correlation
- DICm :
-
Averaged DIC stress–strain curve
- DICr :
-
Raw DIC stress–strain curve
- DTS:
-
Direct tensile strength
- 2D:
-
Two-dimensional
- 3D:
-
Three-dimensional
- f/:
-
F-stop
- Hz:
-
Hertz
- K:
-
Kelvin
- kN:
-
Kilonewton
- kPa:
-
Kilopascal
- mm:
-
Millimeter
- MP:
-
Megapixel
- MPa:
-
Megapascal
- PPE:
-
Personal protective equipment
- PSI:
-
Pound per square inch
- s:
-
Second
- SD:
-
Standard deviation
- SG:
-
Strain gauge
- TTS:
-
True tensile strength
- W:
-
Watt
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Acknowledgements
This research was financially supported by Canada’s Nuclear Waste Management Organization (NWMO), the Department of Geological Sciences and Geological Engineering at Queen’s University, and the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Undergraduate Student Research Award (held by S.K. Woodland), the Canada Graduate Scholarship (Master’s) Program (held by É. Gagnon), the Alexander Graham Bell Canada Doctoral Graduate Scholarship Program (held by T.R.M. Packulak, CGS-D 535289-2019), and the Discovery Grants Program (held by J.J. Day). Thanks to Prof. Heidi Ploeg and her biomechanics research group at Queen’s University for the fruitful technical discussions. Thanks to CanmetMINING (a division of Natural Resources Canada) for coring the specimens used in this study.
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SW: Conceptualization, Methodology, Investigation, Data Curation, Writing—Original Draft, Visualization; ÉG: Conceptualization, Methodology, Formal Analysis, Investigation, Writing—Original Draft, Writing—Review and Editing, Visualization; TP: Conceptualization, Methodology, Investigation, Writing—Original Draft, Writing—Review and Editing; AD: Conceptualization, Resources; JJD: Conceptualization, Supervision, Writing—Review and Editing.
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Woodland, S.K., Gagnon, É., Packulak, T.R.M. et al. Paint Speckle Application Recommendations for Digital Image Correlation Analysis of Brazilian Tensile Strength Tests on Low-Porosity Rocks. Rock Mech Rock Eng 57, 1495–1507 (2024). https://doi.org/10.1007/s00603-023-03604-9
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DOI: https://doi.org/10.1007/s00603-023-03604-9