Crack Initiation, Propagation, and Coalescence Experiments in Sandstone Brazilian Disks Containing Pre-Existing Flaws

Shijun Zhao,Qing Zhang,Limin Liu

doi:10.1155/2019/9816067

Abstract

This study aims to investigate the fracture behaviors of cracks and crack systems in Brazilian disk (BD) sandstone specimens under compressive line loading. We conduct a series of rock fracture experiments to investigate crack initiation, propagation, and coalescence in samples with one or more pre-existing flaws under radial compressive stress. The displacements and failure loads of the tested specimens are measured. Experimental results show that there are two main types of cracks growing from the pre-existing flaws: wing cracks and secondary cracks. Both initiate mostly from the tips of the pre-existing flaws and propagate in a stable manner. The results reveal that wing cracks appear first and propagate toward the loading direction. Secondary cracks can form in some multiple pre-cracked cases after wing cracks are already present. We also provide a characterization of the observed crack propagation paths and patterns and discuss the influence of pre-existing crack or crack systems. The results help in investigating the failure mechanisms and mechanical properties of rock or rock-like materials.

Highlights

Rock mass is a natural formation of mineral aggregates which may contain voids, cracks, and other defects at different scales. is makes rocks’ internal structure discontinuous [1,2,3], as shown in Figure 1. e presence of pre-existing defects may reduce the fracture toughness of rock mass, and such defects are a source of initiation of new fracture lines or cracks
Over the last few decades, many experiments have been devoted to using semi-circular core in three-point bending (SCB) specimen [4], Brazilian disk (BD) specimen with chevron flaws or other pre-existing flaws [5], radial cracked ring (RCR) specimen [6], and modified ring (MR) specimen [7] to investigate rock fracture toughness and crack propagation
Brace and Bombolakis [8] conducted an experimental study on fracture mechanisms in rock material, and it is considered one of the earliest studies in this field. en, the extensive studies have been performed on cracking failure processes evolution from pre-existing discontinuities that emerged in brittle materials

Summary

Introduction

Rock mass is a natural formation of mineral aggregates which may contain voids, cracks, and other defects at different scales. is makes rocks’ internal structure discontinuous [1,2,3], as shown in Figure 1. e presence of pre-existing defects may reduce the fracture toughness of rock mass, and such defects are a source of initiation of new fracture lines or cracks. Under external and internal pressures, cracks can initiate and propagate from those pre-existing cracks or pores and coalesce into macrofailure of the rock structure. Because of the difficulties of in-situ tests, the laboratory experiment is an important and effective research method to investigate rock failure modes and fracture mechanisms, in the presence of pre-existing defects that can be inserted into the samples. A series of fracture experimental tests were conducted to study crack initiation, propagation, and coalescence under compressive diametric or cyclic uniaxial compression loading using precracked specimens or intermittently joint rock models [9,10,11,12,13,14,15,16]. A series of fracture experimental tests were conducted to study crack initiation, propagation, and coalescence under compressive diametric or cyclic uniaxial compression loading using precracked specimens or intermittently joint rock models [9,10,11,12,13,14,15,16]. e crack propagation patterns usually obtained in previous studies of brittle material contain pre-existing flaws

Methods

Results

Discussion

Conclusion