Abstract

Experiments have been performed with Mancos Shale under Brazilian tensile test conditions, addressing the effect of the angle between layer or bedding planes and the loading direction. A high-speed camera with digital image correlation software is used in combination with acoustic emission recording to monitor the fracture initiation and growth processes during loading. Although a clear anisotropy is observed in the variation of the P-wave velocity with the inclination angle, a significant effect on the Brazilian tensile strength is not observed. The mode of failure depends however on sample orientation. For all specimens, a main diametrical central fracture is induced first. It originates in the middle of the specimen and grows as a straight line or as a zig-zag line, depending on the orientation of the sample with respect to load direction. The zig-zag fracture is then a combination of a fracture along the weak direction and in other directions. Its evolution is an order of magnitude slower than that of a brittle straight diametrical central fracture.

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