Most rocks in nature are affected by a certain degree of anisotropy. They contain cracks mostly with certain distributions. Therefore their mechanical behaviour is dependent on the direction of the strength distribution, which is different in each direction. We can distinguish the directions in which the rock is relatively firm and conversely the directions in which it is weak. The strength anisotropy and the direction of weakening is useful information for engineering applications. Metamorphic foliation with the specific orientation of rock grains also influences the mechanical behaviour, and the mechanical properties, i.e. strength, are changed in different directions of loading. This change of strength is called the strength anisotropy. The main purpose of this research is to analyze the effect of the orientation of metamorphic foliation distinguished within the rock specimens on their strength anisotropy. In the article, the anisotropy of a mechanical property, i.e. the fracture toughness, of migmatized gneiss is discussed. For this purpose, series of semi-circular bend tests were carried out on specimens of migmatized gneiss with a metamorphic foliation. The methodology used in this research is effective for evaluating the strength anisotropy of a brittle rock.

It is well known that a directional variation of rock properties is one of the key aspects of rock mechanics. It is relatively uncommon to find a rock mass with completely isotropic properties. The regular distribution of the internal fabric of rocks is mostly disturbed by the presence of planes of weakness, such as discontinuities, microstructures or any mechanical fractures. Besides, the internal fabric of the rock in many cases presents a predisposition to mechanical anisotropy [1]. Metamorphic rocks were subject to extreme loading and high temperature in the course of their development. These processes led to their particular properties, where the orientation of grains in one direction forms the specific features of the rock [2]. When mechanical properties change in different directions of loading, this is defined as strength anisotropy.

The strength behaviour of rocks can be characterized with an evaluation of mechanical parameters. The uniaxial compressive strength, shear strength, and tensile strength determine the macroscopic strength behaviour. Fracture toughness is one of the most important material properties of rocks in their microscopic strength behaviour. It expresses the resistance of a rock material to the initiation and propagation of cracks. It supposes a linear strength- deformation relationship with no plastic flow in the rock mass [3]. It describes the fracture behaviour of rocks in loading conditions and it is closely related to the macroscopic strength. The mechanical properties that express the deformation behaviour of rocks are very important for rock mass classification in mines or underground constructions.

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