This study experimentally investigated dynamic fracturing behavior of brittle material under blasting loading. In the test, a detonator charged with DDNP (Diazodinitrophenol) and a PMMA ((Poly methyl methacrylate) specimen, a transparent and homogeneous material, were used. A high speed camera was employed to observe the cracking behavior, and it successfully provided obvious evidences to understand the crack propagation of brittle material under blasting loading. Two kinds of crack, namely shock-wave driven and gas-driven crack, were observed during the test. We qualitatively identified the initiation and propagation processes of the cracks. The results show that the gas-driven fracture in an ear-like shape was more predominant than shock-wave-driven fractures. Experimental results (i.e., crack length and the number of cracks) reasonably matched with the analytical solution based on fracture mechanics.

1 Introduction

Borehole fracturing has been widely used in tunneling, mining, gas production and geothermal projects. The fracturing is classified into two types; the ‘static’ and ‘dynamic’ fracturing according to the loading speed and method. Blasting and hydraulic fracturing are dynamic fracturing methods induce fractures through the detonation of explosives and the stimulation with air or water, respectively. It is important to predict the extent and orientation of cracks to achieve the purpose of a project, such as control of the fragment size and estimation of the damaged zone at excavation surfaces.

Many experimental studies have been carried out to investigate the crack propagation characteristics induced by dynamic loading. Kutter & Fairhurst (1971) categorized the fracture mechanisms in blasting into stress-wave-generated and gas-expansion-generated types. The respective pressures of the stress-wave and gas-expansion were simulated by underwater spark discharge and pressurized oil, respectively. The plexiglass and rock specimens were used for the test. The study identified the characteristics of crack propagation during the test, and they noted that the gas-generated fractures play an important role in the fragmentation of blasting, and stress-wave-generated fractures play as precondition of gas-generated fractures. Daehnke & Knasmillner (1996) investigated the propagation of dynamic fractures in PMMA (Poly methyl methacrylate) specimen which was loaded with Lead Azide. They found that the stress wave rapidly outpaces the slower dynamic fractures and the majority of fracturing occurred due to pressurization by detonation gas. Yang et al. (2012) investigated the effect of the pre-exist notches around blasthole on the characteristics of crack propagation. They used PMMA specimen having two blastholes, and created artificial notches with two kinds of modes around the blasthole. The blast loading was simulated with multi-spark discharge. They observed the changes in dynamic stress intensity factor, dynamic energy release rate, and crack velocity during the test, and analyzed the effect of the notches on the test results.

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