Dynamic unsaturated effect of microcrack extension on brittle rock fracture under impact compression

Abstract

Deep underground engineering rocks are typically unsaturated, and their dynamic mechanical properties exhibit significant variations with water saturation. Microcrack propagation within rock matrices critically influences dynamic compressive behavior. However, existing research on the micro-macro fracture mechanics relationship of brittle rocks under dynamic compression remains insufficiently explored, particularly for varying water saturation conditions. This study proposes a novel theoretical framework to quantify the unsaturated effects on the dynamic micro-macro fracture mechanical properties of brittle rocks. The model integrates multiple key mechanisms: 1) the wing microcrack dynamics, 2) the K_ICQ - K_ICD transition relationship between quasi-static and dynamic fracture toughness, 3) the crack extension rate (v_l) dependency on opening rate (v_h), 4) the strain rate (ε₁') sensitivity of crack extension rate, 5) the Stefan force (F_S) governed by crack opening rate, and 6) the saturation-dependent mechanical parameters (initial intercrack friction coefficient μ, initial damage D₀, quasi-static fracture toughness K_ICQ, elastic modulus E, and density ρ). A dynamic stress-strain constitutive model for unsaturated brittle rocks, triggered by microcrack growth, is formulated and experimentally validated through laboratory tests, demonstrating robust predictive accuracy. The results reveal that free water induces competing strengthening/weakening mechanisms under dynamic loading, leading to distinct trends in dynamic fracture toughness, compressive strength, and crack initiation stress as functions of saturation at varying strain rates. Furthermore, systematic investigations into confining pressure effects demonstrate its critical role in modulating the relationships between water saturation, dynamic compressive strength, and crack initiation stress across strain rate regimes. These findings advance fundamental understanding of unsaturated rock behavior under extreme conditions and offer critical insights for stability assessment of deep underground engineering excavations.