Fracturing and AE characteristics of matrix-inclusion rock types under dynamic Brazilian testing

Abstract

The fracturing properties and acoustic characteristics of heterogeneous and matrix-inclusion rocks are crucial to the safety of underground infrastructures and mining projects under extreme loading conditions. In this study, elliptically shaped matrix-inclusion specimens were fabricated with cement as the matrix and three rock materials (basalt, marble and sandstone) as inclusions which cover igneous, metamorphic and sedimentary rocks with different strength ratios to the cement. Dynamic Brazilian tests were conducted on the specimens with various inclusion orientations β (i.e. angles between impact direction and inclusion major axis) in the range of 0° and 90° using a split Hopkinson pressure bar (SHPB). By combining digital image correlation (DIC) and acoustic emission (AE) techniques, the integrated system demonstrates well-synchronised dynamic forces, DIC strain fields, and AE signals. The detailed procedures were presented to interpret dynamic mechanical properties, fracturing process and AE characteristics of specimens. Generally, for stronger inclusion materials with higher stiffness and strength, dynamic peak forces, threshold orientation angles of transgranular cracks, and AE counts and duration indicate higher characteristics. For instance, the fracture pattern of basalt inclusions transforms from interfacial crack to transgranular crack with increasing orientation angle β, and the specimens with transgranular cracks exhibit higher dynamic peak forces. It is observed that transgranular cracks are mainly induced by tensile stress with a higher AE peak frequency (200–250 kHz), whereas interfacial cracks contain additional shear cracking having a relatively lower peak frequency (100–150 kHz). Meanwhile, finite element modelling demonstrates that stress fields are affected by the elastic mismatch between inclusion and matrix, while failure modes (i.e. interfacial and transgranular cracks) are mainly governed by the tensile strength of inclusions.