LAUSR

The crack initiation and propagation induced by loading finally lead to the failure of rocks. An identification of crack evolution has been widely investigated in quasi-static tests (Hoek and Martin 2014), which is essential to prevent the catastrophic rock engineering disaster. A consensus in those studies is that the crack process has several stages corresponding to several stress thresholds. The validity of these thresholds was majorly verified by the interpretation of acoustic emission (AE) method (Eberhardt et al. 1998; Moradian et al. 2016; Zhao et al. 2015). Xue et al. (2014) summarised the studies of uniaxial compression tests to evaluate similarities and differences of stress thresholds among igneous, metamorphic and sedimentary rocks. Using the volumetric strain–stress and petrographic technology, Nicksiar and Martin (2013) discussed the relationship between the mineral composition and crack initiation stress. The formation and propagation of microcracks, and fracturing mechanisms are also dependent on Poisson’s ratio of geomaterials (Gercek 2007). With the increased accuracy in computational input for numerical analyses, a precise knowledge of the Poisson’s ratio is of importance. In quasi-static tests, however, the lateral strain is substantially challenging to determine (Swamy 1971). The strain gauge is widely used, but large strain will generate when the stress is above its peak value. Due to the influences of localised failure, it is desirable to measure the lateral strain as an average over a part instead of one point of the specimen. Such measurements include chain extensometer or fluid-field vessel by the fluid level change, but these contact methods to some extent lead to a confinement. Therefore, non-contact optical techniques have substantial advantages. The digital image correlation (DIC) has been proved robust in dealing with multiple deformation conditions (Xing et al. 2017). Cui et al. (2016), Pan et al. (2015) and Pritchard et al. (2013) used 2D-DIC to investigate the axial and transverse strain of composite material or alloy with a thin-section shape in static tests. Munoz et al. (2016) applied the 3D-DIC to determine the crack evolution stages of sandstone in quasi-static uniaxial compression tests. In addition to the static condition, rock engineering may encounter high-strain-rate loadings from explosion, impact or seismic activity (Barla and Zhao 2010; Zhou and Zhao 2011). To the best of our knowledge, there is no report on the crack thresholds or direct measurement of dynamic Poisson’s ratio of rock under high-strain-rate conditions. The primary challenge is that the lateral strain cannot be accurately acquired by traditional methods such as the strain gauge and extensometer under dynamic loading conditions. In this study, stress and strain thresholds of crack development of sandstone under different high strain rates with a split Hopkinson pressure bar (SHPB) were characterised by the high-speed 3D-DIC technique. The real-time evolution of Poisson’s ratio was extracted together with its strain-rate effect according to strain fields in 3D-DIC.