Dynamic compressive micro-macro fracture mechanism with the water-saturated strengthening and weakening effect in brittle rocks

The dynamic compressive mechanical properties of water-saturated brittle rocks are of significant practical importance for assessing the stability of deep underground rock masses during excavation. The coupled dynamic effect of free water and microcrack extension within the rocks severely affects the dynamic compressive mechanical properties of saturated rocks. However, there is a significant lack of research on the mechanisms relating the microcrack evolution to the macroscopic deformation behavior of saturated brittle rocks under dynamic compressive loading. This article aims to propose a feasible micro-macro fracture model to explain the enhancement and weakening mechanisms of free water on the dynamic mechanical properties of rocks. Based on the stress-strain constitutive model of dry rock under quasi-static crack extension action, the stress-strain constitutive model of water-saturated rock under quasi-static action is obtained by combining the change of mechanical parameters of rock with free water. Then the quasi-static fracture toughness and dynamic fracture toughness relationship, the crack extension rate and crack opening rate relationship and the crack extension rate and strain rate relationship are introduced and combined with the Stefan effect to derive the stress-strain constitutive model for water-saturated brittle rocks under dynamic compression. And the reasonableness of the theoretical model is verified by the experimental results. The changes in the intercrack friction coefficient mu, initial damage D0 and quasi-static fracture toughness KICQ due to free water have a weakening effect on the dynamic mechanical properties of the rock. The alterations in dynamic fracture toughness amplification factor KV resulting from changes in the quasi-static elastic modulus E and density rho, along with the Stefan force FS effect, contribute to the enhancement of the dynamic mechanical characteristics of the rock. The changes in mechanical parameters and the Stefan effect together constitute the strengthening and weakening mechanisms through which free water affects the dynamic mechanical properties of the rock. And discusses the effects of confining pressure and strain rate on the dynamic compressive strength and crack initiation stress of water-saturated rock. These findings provide theoretical support for the stability analysis of saturated rock masses during deep underground excavation.