Crack initiation characteristics of brittle rock under coupled hydraulic pressure and triaxial static-dynamic loading

In this study, a modified hydraulic pressure loading apparatus was designed to apply hydraulic pressure to crack surface and to measure crack initiation stress ( σ i ). Dynamic hydro-mechanical coupled loading tests were conducted on pre-cracked specimens to analyze the effects of crack inclination angle and hydraulic pressure on crack initiation characteristics, including the crack initiation stress ( σ i ) and crack initiation angle ( θ IC or θ IIC ). Based on our K -ratio criterion, the crack initiation mechanism (CIM) of brittle rock under dynamic hydro-mechanical coupled loading was investigated by calculating dynamic hydro-mechanical coupled stress intensity factor (SIF) along arbitrary directions. Research results show that σ i initially rises and subsequently declines as hydraulic pressure increases, and it increases with increasing crack inclination angle ( α ). In the absence of hydraulic pressure, the absolute value of θ IC or θ IIC is increased with α . Under the applied hydraulic pressures, θ IC or θ IIC decreases with the increasing α , except for the specimen with α = 75° (where θ IC or θ IIC remains almost unchanged). In addition, θ IC or θ IIC is less affected by hydraulic pressure (1, 2, 4 MPa). Under dynamic hydro-mechanical coupled loading, all specimens exhibit Mode I crack initiation, except those with α = 75°. Mode II crack initiation only occurs under the condition without hydraulic pressure or with a larger α (e.g. 75°). The theoretically predicted θ IC or θ IIC are in good agreement with the experimental results, confirming the accuracy of the dynamic hydro-mechanical coupled SIF calculation and the reliability of the crack initiation criterion.