Behavior of intact brittle rocks during loading is characterized by a micro-cracking process, which occurs due the presence of flaws in their microscopic structure, propagating through the intact rock leading to shear fracture. This fracturing process is of fundamental importance as it affects the mechanical properties of the rock. The purpose of the present study is to investigate the fracture mechanism and specifically detect the crack initiation and crack damage stresses in a granite rock. For this purpose, stress-strain data were recorded and the acoustic emission method was used in uniaxial compression tests. The results from both methods correlate well and show that the crack initiation and crack damage thresholds are about 46% and 78% of the uniaxial compressive strength of the intact rock respectively, slightly higher than the results reported in literature from previous studies in granite.

1. Main text

The micro-cracking process of granites during loading is well documented in literature, e.g. Lac du Bonnet granite [1, 2]. In the present study, granite from Attica region in Greece was tested in the laboratory in order to study its crack initiation and crack damage process. Usually, crack initiation and damage stresses are detected using the stress-strain data, while acoustic emission has also been used for this purpose. Using the acoustic emission during the uniaxial compression test, it was found that the crack initiation and damage thresholds can be correlated with the wave events and characteristics recorded.

In the present study, both approaches were used during uniaxial compression tests in order to determine the crack initiation and crack damage stresses and study the cracking behavior of the tested granite.

The progressive deformation of intact rock is characterized by a fracturing process composed by the following stages [3,4]:

  • Crack closure

  • Linear elastic deformation

  • Crack initiation and stable crack growth

  • Crack damage-Critical energy release and unstable crack growth

  • Failure

Griffith [5, 6] suggests that crack initiation occurs at the tips of microscopic pre-existing flaws of the intact rock when tensile strength is exceeded. These flaws are represented as elliptical cracks. Griffith's theory led to the development of a discipline known as Linear Elastic Fracture Mechanics (LEFM). Anderson [7], distinguishes three types of crack loading, namely Mode I in pure tension, Mode II in pure shear and Mode III in out of plane shearing. There is an extensive series of published papers on the fracture initiation and crack propagation under compressive stresses. All attempted to overcome the mathematical difficulties to express the stress field along the crack flanks under compressive stresses, as compression closes the gap between them resulting in contact of them, not satisfying the boundary conditions of the Mode I loading of cracks developed by classical theory [8]. Zuo et al. [9] examined further the conditions of micro-crack growth. Their results agreed with Griffith's model of wing cracks in the tips of pre-existing micro-cracks in the rock [10]. Based on these results, they suggested a new failure criterion in a biaxial compressive strength field.

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