عنوان مقاله [English]
This paper focuses on some fracture toughness tests performed on the pre-cracked Brazilian specimens of rock-like materials. Also the effect of rock fracture filling on the fracture toughness was considered experimentally. Fracture toughness is a key parameter for studying the crack propagation and fragmentation processes in rock structures. Fracture mechanics is an applicable tool to improve the mechanical performance of materials and components. It is a comparatively general phenomenon that rock fractures are naturally filled with gouging material, but the impact of fillings on rock fracture toughness has not yet been considered precisely. In the present study, an experimental investigation was made to evaluate the effect of rock fracture fillings on the crack propagation mechanism and fracture toughness of some rock like specimens. For this purpose, several molds are used for preparation of Brazilian disks with straight central crack. In the next stage, three different ratios of ingredients have been used for preparation of three model materials to fill the central pre-crack of the specimen. Diametrical compression load with a rate of 0.3 mm/min in different directions respect to the central crack orientation is applied to the Brazilian disk specimens and the failure loads corresponding to the each test are recorded. Result of laboratory tests indicates that fracture fillers strongly affect the value of rock fracture toughness while fracture filler has no influence on the failure mode of CSCBD specimens. Also the impact of filler cohesion on mode II fracture toughness is more than its impact on mode I fracture toughness.
The ability of rock to resist fracturing and propagation of pre-existing cracks was introduced as rock fracture toughness or critical Stress Intensity Factor (SIF). Rock fracture toughness is an intrinsic rock property that is used as an index for fragmentation processes such as rock cutting, hydraulic fracturing, explosive modeling etc. [1-3]. Numerous experimental and numerical researches have been developed for assessing rock fracture toughness in different conditions of fracture and loading.
It has been reported that fracture toughness of rocks increases with increasing confining pressure [4-6].
Khan and Al-Shayea investigated the effect of specimen geometry and testing method on mixed mode I/II fracture toughness. They found that specimen diameter and crack type have a substantial influence on the measured fracture toughness .
Al-Shayea investigated the trajectories of crack under mixed mode I/II loading in limestone with high brittleness under CSCBD specimen. Furthermore, the effect of confining pressure and temperature on crack initiation and propagation was also studied by him .
Ke et al. presented a systematic procedure for determining fracture toughness of an anisotropic marble using the diametric compression test (Brazilian test) with a central crack on the disks. They developed a new fracture criterion to predict pure mode I, pure mode II, or mixed mode (I/II) fracture toughness of the anisotropic marble .
A new cubic element formulation of the displacement discontinuity method using three special crack tip elements for crack analysis has been developed by Fatehi Marji et al. This analyses are performed based on mixed mode I/II stress intensity factor and LEFM concept .