The combined propagation and interaction of pre-existing cracks are important factors for rock mass overall instability and failure. Many experiments have been conducted to investigate the influence of 3D internal crack on strength and crack propagation pattern in rock samples. However, several issues like the influence of cracking spacing on the cracking pattern of rock subjected to tensile stress remain unsolved. In this study, a new type of rock-like material and a novel direct tensile apparatus are developed. Uniaxial tensile experiments on artificial rock samples containing two internal elliptical cracks with different crack spacing are conducted. Experimental results indicate that the tensile stress-strain curves of different samples exhibit 4-stage behaviors. Crack spacing has obvious influence on the tensile peak strength (sP) of sample. sP decreases gradually with the increase of crack spacing when 0d/2a=1 (where d represents crack spacing, and 2a is the length of crack), but increases as crack spacing increases when 1d/2a=1.25. Under tensile stress, wrapping wing crack and secondary crack initiate sequentially near the front of long axis of each pre-existing crack, and the lateral growth initiates near the front of short axis.


Fracture universally exists in rock masses, playing an import role in the strength and failure behaviors of rock masses. With the increase of external loads, pre-existing cracks in a rock mass will initiate, grow and join with other neighbouring ones [1]. The growth and interaction of cracks change the stress field distribution in a rock mass, and cause local stress concentration which leads to the failure of rock mass [2-4]. Therefore, the existence of cracks in a rock mass is a potential threat to the safety of rock structures, such as tunnel and dam. At present, the mechanism of crack growth and coalescence in rock mass remains one of the most fundamental and promising problems in both academic and engineering aspects. A number of studies have been performed to investigate the growth rules of 2-dimensional crack, and many important conclusions have been obtained [5-9]. The simplification of 3D cracks into 2D cases may bring convenience to analysis, but would result in great deviations from the actual conditions. Adams and Sines [10] performed experiments to investigate the propagation pattern of 3D elliptical crack in PMMA material under compression, and they observed the initiation and propagation processes of squamous cracks. Through compression tests, Dyskin et al [11-12] investigated the growth rules of 3D single crack in various materials, including transparent casting resin, cement, and mortar, taking into account the influence of shape and location of cracks on their growth, and proposed a 3D model of wing crack propagation and interaction. To observe the growth process of internal crack, Li et al [13] conducted uniaxial compression tests on brittle ceramic samples using a real-time CT scanning apparatus. They obtained some important parameters such as the sample strength and the length of wing crack. Liang et al [14] studied the failure process of heterogeneous rock by using a RFPA3D code.

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