Analyzing the Validity of Brazilian Testing Using Digital Image Correlation and Numerical Simulation Techniques

Abstract

Characterizing the mechanical behavior of rocks plays a crucial role to optimize the fracturing process in unconventional reservoirs. However, due to the intrinsic anisotropy and heterogeneity in unconventional resources, fracture process prediction remains the most significant challenge for sustainable and economic hydrocarbon production. During the deformation tracking under compression, deploying conventional methods (strain gauge, extensometer, etc.) is insufficient to measure the deformation since the physical attachment of the device is restricted to the size of the sample, monitoring limited point-wise deformation, producing difficulties in data retrieval, and a tendency to lose track in failure points, etc. Where conventional methods are limited, the application of digital image correlation (DIC) provides detailed and additional information of strain evolution and fracture patterns under loading. DIC is an image-based optical method that records an object with a camera and monitors the random contrast speckle pattern painted on the facing surface of the specimen. To overcome the existing limitations, this paper presents numerical modeling of Brazilian disc tests under quasi-static conditions to understand the full-field deformation behaviors and finally, it is validated by DIC. As the direct tensile test has limitations in sample preparation and test execution, the Brazilian testing principle is commonly used to evaluate indirectly the tensile strength of rocks. The two-dimensional numerical model was built to predict the stress distribution and full-field deformation on Brazilian disc under compression based on the assumptions of a homogenous, isotropic and linear elastic material. The uniaxial compression test was conducted using the DIC technique to determine the elastic properties of Spider Berea sandstone, which were used as inputs for the simulation model. The model was verified by the analytical solution and compared with the digital image correlation. The numerical simulation results showed that the solutions matched reasonably with the analytical solutions where the maximum deviation of stress distribution was obtained as 14.59%. The strain evolution (normal and shear strains) and displacements along the central horizontal and vertical planes were investigated in three distinguishable percentages of peak loads (20%, 40%, and 90%) to understand the deformation behaviors in rock. The simulation results demonstrated that the strain evolution contours consistently matched with DIC generated contours with a reasonable agreement. The changes in displacement along the central horizontal and vertical planes showed that numerical simulation and DIC generated experimental results were repeatable and matched closely. In terms of validation, Brazilian testing to measure the indirect tensile strength of rocks is still an issue of debate. The numerical model of fracture propagation supported by digital image correlation from this study can be used to explain the fracturing process in the homogeneous material and can be extended to non-homogeneous cases by incorporating heterogeneity, which is essential for rock mechanics field applications.

Strain Field Evolution Characteristics of Free Surface during Crater Blasting in Sandstone under High Stress

“…DIC is an effective method for measurements of surface displacement and strain, widely used in rock mechanics [22][23][24][25]. The DIC-2D speckle system can monitor the deformations in two directions, u and v, for each point on the free surface.…”

Study of the Influence of Nonlinear Dynamic Loads on Elastic Modulus of Carbonate Reservoir Rocks

“…Due to its simplicity, the use of contact mechanical extensometers for testing rocks is widely known and has found application in many experimental works (for example, [35]). Non-contact measuring systems, such as Vic-3D, are also widely used in mechanical testing of rocks (see, for example, [36,37]). The system operates on the Vic-3D digital image correlation technique (Figure 6).…”

“…perimental works (for example, [35]). Non-contact measuring systems, such as Vic-3D, are also widely used in mechanical testing of rocks (see, for example, [36,37]). The system operates on the Vic-3D digital image correlation technique (Figure 6).…”

Evaluation of a low-cost approach to 2-D digital image correlation vs. a commercial stereo-DIC system in Brazilian testing of soil specimens

“…Other studies focussed on the application of DIC in BTS tests for monitoring crack initiation and propagation [15]. DIC's ability to capture data has led to a wide dissemination of this technique in the validation of BTS numerical models trying to understand the deformation behaviours in the full field [16] or more specifically in the fracture process zone [17]. In contrast with traditional methods, DIC provides a continuous field of displacements across the surface of the specimen without the need for direct contact.…”