To analyse the fracture coalescence behaviour of rock, rectangular prismatic sandstone specimens (80 9 160 9 30 mm in size) containing three fissures were tested under uniaxial compression. The strength and deformation behaviours of the specimens are first analysed by investigating the effects of the ligament angle b 2 on the peak strength, peak strain and crack initiation stress of the specimens. To confirm the sequence of crack coalescence, a photographic monitoring technique is used throughout the entire period of deformation. Based on the results, the relationship between the real-time crack coalescence process and the axial stress-strain curve of brittle sandstone specimens is also developed, and this relationship can be used to evaluate the macroscopic deformation characteristics of pre-cracked rock. The equivalent strain evolution fields of the specimen, with a = b 1 = 45°and b 2 = 90°, are obtained using the digital image correlation technique and show good agreement with the experimental results of pre-cracked brittle sandstone. These experimental results are expected to improve the understanding of fracture mechanisms and be used in rock engineering with intermittent structures, such as deep underground excavated tunnels.
Shale swelling can
cause a series of problems during the exploitation
of shale gas. However, because of the complex fabric of shale, a thorough
understanding of shale swelling has not been gained. In this work,
an experimental multiscale approach is employed to characterize the
swelling properties of a shale sample from Sichuan basin of China.
Wetting/drying tests are performed by environmental scanning electron
microscopy (ESEM), and deformation of the material is measured by
analyzing the ESEM micrographs using digital image correlation (DIC)
techniques. Investigations are conducted at three distinct scales,
enabling the macroscopic behavior to be linked to the microscopic
mechanisms (e.g., the role of inclusions, the behavior of clay particles).
The macroscopic swelling of the tested shale is less than 2%. This
limited swelling is mainly due to the presence of nonswelling inclusions
as well as the low swelling capacity of the clay inside the shale.
Microcracking is evidenced along both the wetting and the drying paths.
It is demonstrated that the clay particles swell rather undirectionally
perpendicular to their orientation. Moreover, a preferred orientation
of the clay particles is found parallel to the bedding plane, which
finally leads to a moderate anisotropy of the macroscopic swelling.
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