Search

The tensile strength of rocks is affected by loading conditions, specifically direct and Brazilian tension, as well as water saturation. But the fundamental reasons for the differences between direct tensile strength σ td and Brazilian tensile strength σ tb , and their degradation induced by water saturation, are not yet fully understood. Here, we report a series of direct tensile and Brazilian tests performed on marble and granite rocks under both dry and water-saturated conditions. We employed the acoustic emission (AE) technique and further analyzed the recorded AE waveforms using a methodology based on the statistical analysis of dominant frequency to infer the intrinsic failure process of studied rocks under different water and loading conditions. Our results show that water saturation causes substantial reductions in both σ td and σ tb , with the reduction in σ tb being greater. A correspondence between AE waveforms distributed in high and low dominant frequency bands (H-type and L-type waveform) and rock failure types is found, that is, the H-type waveforms correspond to the micro-shear failures, and the L-type waveforms correspond to the micro-tensile failures. Based on this, the generation of abundant micro-shear failures is the cause of enhancement in σ tb in rocks compared to its σ td , and the reductions in σ td and σ tb for saturated rocks result from increasing micro-tensile failures. Furthermore, the friction-weakening effect induced by lubricating water films is responsible for the tensile strength reduction of saturated marble and granite under direct and Brazilian tension. The pronounced friction-weakening effect resulting from the widespread friction condition in the Brazilian test facilitates the σ tb loss. In addition to the friction-weakening effect, stress corrosion could also be a secondary cause of the tensile strength reduction of saturated granite. Highlights: The corresponding relationship between acoustic emission waveform types with micro-failure patterns is built. Reasons for the difference in measured tensile strength under direct and Brazilian tension are proposed. Water-weakening effects on the tensile strength of rocks under direct tension and Brazilian loading are revealed. [ABSTRACT FROM AUTHOR]

Wave propagation is widely used in geologic mining and geotechnical engineering to acquire the structure information of rock masses. As rocks, by their nature, are filled with cracks and pores, wave propagation in rocks shows its clear dependence on stress. To describe the stress dependence of wave propagation, a nonlinear viscoelastic model consisting of a modified Hooke Model and Kelvin Model was proposed based on experimental observations on sandstones under uniaxial loads. With this model, the nonlinear behavior of rocks at the early stage of compression is attributed to the deformation of voids with a variable tangent modulus. Thus, wave velocity and wave attenuation can be altered, which change with the closure of voids and act as a function of stress. The proposed model was validated by comparison with experimental measurements from ultrasonic transmission at varying uniaxial stresses. It is shown that the effects of applied stress on the stress–strain relationship, wave velocity and wave attenuation for P-waves are well describe by the developed viscoelastic model. Changes in wave attenuation are also analyzed in terms of viscosity-like parameter defined by this model, which is found to be negatively proportional to the uniaxial stress. Highlights: The wave propagation in porous sandstones under uniaxial loads was investigated in experimental and analytical ways. A viscoelastic model was proposed to incorporate the nonlinear mechanical and acoustic behaviors observed at the early stage of compression. Both the nonlinear stress-strain relationship and the stress dependence of wave velocity and wave attenuation can be well described by the proposed model. [ABSTRACT FROM AUTHOR]