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Experimental study on uniaxial mechanical properties and crack propagation in sandstone containing a single oval cavity

Huang Y. H., Yang S. Q., Hall M. R., Tian W. L., Yin P. F.

Archives of Civil and Mechanical Engineering

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2018

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Vol. 18, no. 4

1359--1373

EN

The geometry and orientation of cavities in a rock mass has a significant influence on the failure mechanism and fracture propagation. The theoretical study of these behaviors is of great importance to enable fundamental understanding of unstable failure in rock mechanics. A very limited number of experimental studies have been conducted for sandstone specimens containing oval cavities. Sensitive parameters include the short axis (b) to long axis (a) ratio. Here, we enhance the present understanding of crack coalescence behavior around an oval cavity by applying uniaxial compression tests to sandstone cores containing manually inserted single oval cavity, combined with photographic and acoustic emission monitoring. The b:a ratio was varied from 1/4 to 1/1, thus the cross-sectional cavity shape evolved from oval to a fully circular opening. The experimental results showed that the stress–strain curves of specimens containing cavities exhibited multiple stress reductions prior to peak strength, which led to a sudden increase in the accumulated AE count-rate. As the b:a ratio was increased, the uniaxial compressive strength, crack initiation stress and peak strain of cavity specimens all decreased linearly. Tensile fractures, shear fractures, and compound shear-tensile fractures were all observed to emanate from the cavities. Compared to the intact specimen, the failure mode of cavity specimens comprised a mixture of tensile and shear cracks, emanating from the tips (or surfaces) of the oval cavity. For specimen with a cavity angle of 45°, when the b:a ratio was below 1/2, no tensile crack from the surface of short axis was initiated, whereas tensile crack was observed clearly after b:a = 1/2. This study not only increases our understanding of mechanical failure behavior in fractured rock, but also enhances our knowledge of underground tunneling or roadway stability.

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Cracking behavior of three types granite with different grain size containing two non-coplanar fissures under uniaxial compression

Tian W. L., Yang S. Q., Xie L. X., Wang Z. L.

Archives of Civil and Mechanical Engineering

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2018

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Vol. 18, no. 4

1580--1596

EN

Granite, an excellent medium for deep geological disposal projects, geothermal systems and geological carbon storage, may be affected by its grain size and pre-existing flaws. Thus, three types of granite specimens with different grain sizes containing two non-coplanar fissures under uniaxial compression were investigated experimentally and numerically. Notably, the ligament angle had more effect on the peak strength than the grain size. With an increasing ligament angle, the angle between the crack and the principal stress did not show an obvious trend for [...], whereas the angle between the crack and the principal stress increased when [...]. This trend is similar to that for the peak strength. For the same ligament angle, the angle between the crack and the principal stress increased as the grain size decreased. Based on the micro-structure of the crack surface in the ligament [...], the hypothesis that a shear crack is more difficult to initiate in coarse granite specimens than in fine granite specimens was presented and verified by experiments and numerical simulations.

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Physical and mechanical behavior of granite containing pre-existing holes after high temperature treatment

Huang Y. H., Yang S. Q., Tian W. L., Zhao J., Ma D., Zhang C. S.

Archives of Civil and Mechanical Engineering

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2017

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Vol. 17, no. 4

912--925

EN

To understand the high temperature effects on the mechanical and failure behaviors of rock, uniaxial compression tests were carried out on granite specimens containing three pre-existing holes using a rock testing system. Based on the experimental results, the influences of testing temperature on the physical and mechanical parameters of granite were analyzed in detail. An obvious color change of tested granite occurs from gray at room temperature to reddish after 450 °C and to red-brown after 900 °C high temperature treatment. The granite volume increases, mass decreases and density decreases with increasing testing temperature. As the temperature increases, the peak strength first increases and then decreases, while the elastic modulus decreases. However, the peak strain changes slightly before 450 °C, increases dramatically up 450 °C. As the bridge angle increases, the mechanical parameters of granite specimens first decrease and then increase. And then, the crack initiation, propagation and coalescence behavior of granite specimens after high temperature exposure was investigated using an acoustic emission (AE) and photography monitoring technique. The cracking process shows that the propagation of crack from the surface of holes leads to the coalescence between adjacent holes. A large AE count and a stress drop are observed during the crack initiation and propagation. The failure modes can be generally classified into three categories: splitting mode, shear mode and mixed mode and they are closely related to heat treatment temperature and bridge angle. Finally, the mechanism causing the differences in the mechanical parameters observed with increasing temperature was discussed based on the SEM observations.