Damage Evolution and Splitting Failure Mechanism of Layered Rock under Dynamic Pulse Load

Abstract

In order to study the damage evolution and splitting failure characteristics of shale with different bedding orientations under dynamic pulse load, different impact velocities (40, 50, 60, 70 m/s) are selected to carry out dynamic pulse load on D-type and S-type shale samples. Ultrasonic testing, Brazilian splitting test [combined with digital image correlation technology and acoustic emission (AE) monitoring], and nuclear magnetic resonance are carried out on shale samples before and after impact. The results show that the P-wave velocity damage coefficient and splitting mechanic parameters of D-type shale show exponential attenuation, while those of S-type shale show linear attenuation. With the increase of impact velocity, the time-domain spectra of D-type and S-type shale show different degrees of waveform distortion. The frequency-domain spectra present a change characteristic of "high-frequency decrease, low-frequency increase", the total energy of energy spectra decreases, and the main frequency band shifts from medium frequency sub-band (fourth to sixth) to low frequency sub-band (especially first). The AE characteristics of D-type shale are peak distribution with obvious brittle fracture characteristics, while S-type shale is uniformly distributed with progressive failure characteristics. The horizontal strain cloud map shows a band-like stress concentration distribution. Before and after impact, the total porosity of D-type shale decreases, while that of S-type shale increases. The fractal dimension of mesopores generally increases, while the fractal dimension of macropores remained basically unchanged. The existence of the bedding surface is the main reason for the difference in fracture mechanism of different types of shale, and the weakening of the bedding surface caused by impact load is an important factor that complicates the complexity of this difference.