Strain is a critical parameter in the calculation of elastic rock properties, yet its conventional methods for strain measurement has several deficinies. In this paper, we analyze the application of optical methods with Digital Image Correlation (DIC) technique to provide detailed information regarding fracture patterns and strain development with time under Brazilian testing condition. The effect of porosity, rock types, lamination, and saturation on tensile strength will be also discussed.

To examine the effect of rock type, 60 samples of sandstone (Parker, Nugget and Berea) and carbonate formations (Winterset limestone, Silurian dolomite, Edward Brown and Austin Chalk) were testedunder dry and saturated conditions and with regard to lamination angle in laminated samples. A Vic-snap photogrammetry system was employed to monitor the samples in non-contact manner while conducting indirect tensile experiment. DIC is based on the photogrammetry system, which helps to visualize and examine rock fracture pattern from the recorded images of the rock before and after deformation by assessing the strain development in samples.

The experimental results show that - (1) average tensile strength declines while increasing porosity for homogeneous, laminated, and heterogeneous rock specimens. (2) lower tensile strengths are observed in carbonate rock samples compared to the sandstones except Silurian dolomite; (3) saturation reduces the rock strengths, for isotropic samples, highest 28% decline in strength (Berea sandstone) observed; whereas, a larger decrease (65%) was observed in fully heterogeneous Edwards Brown carbonate samples; (4) increase of lamination angle (from 0° to 90°) impacts the tensile strength, average tensile strength was observed for Parker and Nugget sandstone greater in perpendicular to the lamination (9 = 90°) direction compare to that of parallel (9 = 0°); (5) fracture patterns examined for homogeneous rocks are almost centrally propagated and relatively linear; whereas, three different fracture patterns (central fracture, layer activation and non-central or mixed mode) investigated for laminated and heterogeneous samples; (6) Finally, DIC results illustrated the fracture initiation and propagation with consistent strain mapping. The homogeneous samples produced a uniform fracture strain until the diametrical split where for the laminated samples were influenced by planes of weakness, and fully heterogeneous anisotropic rocks produced winding and erratic fractures.

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