Paint Speckle Application Recommendations for Digital Image Correlation Analysis of Brazilian Tensile Strength Tests on Low-Porosity Rocks

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.