Effects of Water Saturation and Loading Condition on Rock Tensile Strength: Insights from Acoustic Emission Analysis

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 \({\sigma }_{\mathrm{td}}\) and Brazilian tensile strength \({\sigma }_{\mathrm{tb}}\), 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 \({\sigma }_{\mathrm{td}}\) and \({\sigma }_{\mathrm{tb}}\), with the reduction in \({\sigma }_{\mathrm{tb}}\) 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 \({\sigma }_{\mathrm{tb}}\) in rocks compared to its \({\sigma }_{\mathrm{td}}\), and the reductions in \({\sigma }_{\mathrm{td}}\) and \({\sigma }_{\mathrm{tb}}\) 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 \({\sigma }_{\mathrm{tb}}\) loss. In addition to the friction-weakening effect, stress corrosion could also be a secondary cause of the tensile strength reduction of saturated granite.