ABSTRACT:

The mechanisms of fracture in a brittle material under a compressive load are examined. Three dimensional crack growth experiments and numerical simulations are used to identify the stable processes of fracture up to the point of failure. In 2-D, a single flaw can cause failure of the sample, whereas, in 3-D it is usually many flaws that cause failure. New experiments using a brittle transparent material are employed to illustrate the evolution of three dimensional crack growth. Different arrangements of flaws of various sizes produce either an explosion-like or a splitting-fracture type of failure. Two dimensional modeling of crack growth from a single inclined flaw closely matches experimental results, but cannot capture the true nature of three dimensional cracking. A three dimensional analytical model of a single inclined crack under compression predicts crack growth which resembles that seen in the experiments. A numerical examination of the stress field around a developed three dimensional single crack explains both types of failure seen in the experiments, explosive-collapse and splitting.

1. INTRODUCTION

An understanding of the mechanisms of brittle fracture is an essential element for the prevention, or at least the mitigation of the effects, of natural and human-induced hazards such as earthquakes and rockbursts. Crack propagation from pre-existing internal defects--the nucleation and propagation of secondary fractures--has been established as a dominant mechanism in the deformation and failure of brittle rock. Numerous studies of cracks growing from inclined slots or elliptical cavities in plates have demonstrated this, along with the stable nature of crack growth under compressive loading (see the references in the reviews of Lockner et al., 1995, and Wang and Shrive, 1995). These essentially two-dimensional studies do not adequately describe the true three-dimensional nature of rock mass deformations. Recent experiments on 3-D crack growth in various materials such as casting resin and Portland cement (Dyskin et al., 1994), PMMA, and silica glass (Germanovich et al., 1994) demonstrate the qualitative differences from 2-D. In 2-D, the growth of secondary wing cracks from a single inclined flaw can lead to sample splitting, whereas, in 3-D, wing crack growth is restricted to, at most, a size comparable to the initial flaw. This is insufficient to cause failure in most cases. As the compressive load is increased, many other small (usually invisible to the naked eye) defects can become overstressed. This leads to further secondary cracking followed by splitting or shear failure in rocks or an explosive-like collapse in homogeneous brittle materials like glass or frozen plastic. Experiments (Dyskin et al., 1994, Germanovich et al., 1994) show that the dominant mechanism behind the formation of macrofractures is the interaction between wing cracks as they increase in concentration. This paper reports further experimental, numerical, and analytical modeling aimed at studying the mechanisms of three-dimensional crack growth and interaction.

2. LASER INDUCED CRACK PROPAGATION IN POLYESTERRESIN

Samples were made from transparent casting polyester resin "Polylite 61-209" (Cadillac, Australia). A multitude of internal cracks were created using a laser and a focusing lens (Figure 1).

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