Three-dimensional evolution of damage in sandstone Brazilian discs by the concurrent use of active and passive ultrasonic techniques

Active and passive ultrasonic techniques were used to study three-dimensional evolution of damage in sandstone discs under Brazilian test. A standard disc, 50 mm in diameter and 25 mm in thickness, was instrumented with a 3D sensor array containing 12 Nano30 sensors and was diametrically compressed under line loads quasi-statically. P-wave velocities along 64 ray paths within the disc sample were measured before and during the experiment, which were found to be strongly anisotropic and were then used to construct a time-dependent transversely isotropic velocity model. Twelve channels of full waveform data were recorded continuously by acoustic emission (AE) monitoring until final failure, from which discrete AE events were triggered. Based on the constructed velocity model, a total of 1775 AE events were located successfully by the collapsing grid search algorithm. Two different classification methods were applied to the decomposed moment tensors, revealing a mixture of tensile and shear microcracks clustered in the crack initiation point. Orientations of P and T axes were also found to be a not perfect but passable indicator of stress state of the disc sample. Furthermore, a similar Brazilian test was performed with a 2D sensor array for comparison. Array analysis indicated that the 3D sensor array generally performs better in the three-dimensional space due to its full-size spatial coverage, while the 2D sensor array has higher location accuracy on the plane where sensors are. Both cases were found to have similar crack initiation positions, about 10 mm away from the disc centre, suggesting that the obtained tensile strength may underestimate the true value.