Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

K Liu,J Zhao

doi:10.1007/s00603-021-02408-z

Abstract

Investigation of rock progressive damage under static confinement and strain rates facilitates the generation mechanism of natural fault damage zones. A triaxial Hopkinson bar apparatus is used to perform dynamic triaxial compression tests to examine the damage and degradation process of rocks subjected to multiple impacts. Dynamic mechanical properties are determined under a static triaxial pre-stress of (30, 20, 10) MPa and multiple dynamic loadings, with the repetitive impact velocity of 27 m/s and strain rates from 50 to 150/s. The acoustic characteristics are identified by ultrasonic measurement to qualify the damage values. The micro-crack parameters, including crack area and volumes are detected using synchrotron X-ray micro-computed tomography (μCT) to characterize the progressive damage. In addition, the microcrack orientation, density and fractal dimension are analysed from thin section. Experimental results show that dynamic stress-strain curves can be divided to elastic, nonlinear deformation and unloading phases. Dynamic peak stress, Young’s modulus and ultrasonic wave velocity decrease with increasing impact times. The high frequency of ultrasonic wave is filtered by the induced microcracks. The progressive damage and evolution of fracture networks are associated highly with microcrack initiation, propagation, branching and coalescence. Shear bands are commonly generated in granite, and tensile cracks are dominant in marble, while sandstone is mainly failed by compaction and deformation band. The absorbed energy of rock increases nonlinearly with increasing crack surface and volume. Besides, microcracks propagate primarily along the maximum principal stress; the density and fractal dimension exhibit an anisotropic distribution controlled by true triaxial confinement and dynamic impacts.

Highlights

The physical mechanism of fault zones is always a hot topic in recent years, and is identified as the complex result from geological and mechanical factors, intrinsically and extrinsically
The progressive damage and failure of rocks subjected to repeated impact loading were investigated using split Hopkinson pressure bar (SHPB), and the results show that dynamic stress–strain, elastic modulus, micro-crack evolution and cumulative damage of rocks are highly related to impact energy and numbers (Aben et al 2016; Braunagel and Griffith 2019; Doan and d’Hour 2012; Li et al 2004, 2018; Luo et al 2016; Wu et al 2014)
This study investigates the progressive failure behaviours of rocks under triaxial pre-stress state and high-rate multiple impacts using a triaxial Hopkinson bar system, which allows to control independently both pre-stress and impact loads

Summary

Introduction

The physical mechanism of fault zones is always a hot topic in recent years, and is identified as the complex result from geological and mechanical factors, intrinsically and extrinsically. The fault core consists of highly broken-up fragments or gouge with largest fault displacement, and the associated damage zone contains intense subgrain fracturing with the microns and tens of microns scale (Aben et al 2016; Faulkner et al 2003, 2006; Mitchell et al 2011; Rodríguez-Escudero et al 2020; Whearty et al 2017). The fracturing in fault damage zone affects greatly on the mechanical behaviours [e.g. permeability (Mitchell et al 2017), anisotropy (Zhao et al 2011), chemical change and rock-fluid reactions (Goddard and Evans 1995)] of the faults and earthquake dynamic behaviours, including wave propagation, energy dissipation, rupture directivity and destructive efficiency (Aben et al 2017; Huang et al 2014).

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