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The uniaxial compressive strength (UCS) test is crucial in determining the strength and stiffness behavior of intact rock and is frequently utilized by industry to determine project site characteristics. A fundamental procedure of UCS testing is strain response measurement. Conventionally, discrete strain measuring devices such as extensometers and...
Citations
... Distributed FOS, such as Brillouin and Rayleigh backscatter systems, measure a fibre's response to strain changes along the entire length of the optical fibre [7][8][9][10]. In contrast, discrete FOS measures the strain response of a rock sample at specific locations alongside the sample, similar to conventional strain gauges [11,12]. Fabry-Perot and Fibre Bragg gratings (FBG) sensors are examples of discrete strain sensing [13,14]. ...
In conventional rock mechanics testing, radial strain measuring devices are usually attached to the sample's surface at its mid-height. Although this procedure provides a realistic picture of the lateral deformation undergone by homogeneous samples, however, this assumption may not be accurate if the tested rock has significant heterogeneity. Fibre Bragg Grating (FBG) sensors have recently been introduced to various rock testing applications due to their versatility over conventional strain gauges and radial cantilevers. FBG sensors have small size, multiplexing capability, and immunity to magnetic interference. The main objective of this study is to explore and understand the capabilities of FBG sensing for strain measurement during rock mechanics testing, including under confining. To do so, two limestone plugs (Savonnières limestone) and one acrylic Poly Methyl Methacrylate (PMMA) plug, all of 38 mm diameter, were prepared. The acrylic plug and one of the Savonnières samples plugs were subjected to Unconfined Compressive Strength (UCS) tests. The second Savonnières plug was subjected to a hydrostatic test up to 20 MPa confining at room temperature. FBG sensors of 125 μm cladding diameter with ceramics (Ormocer) coating were glued on the surface of each sample, spreading across the entire sample's height. Strain gauges and cantilever-type radial gauges were used on the samples submitted to UCS for comparison. Results show that radial strain measurements and calculated elastic properties derived from the FBG readings for samples are comparable to readings from the conventional strain gauges and cantilever-type devices. Apparent bulk moduli based on volumetric strain computed from FBG radial strain readings during the hydrostatic test on the Savonnières sample was consistent with benchtop measurements conducted on the Savonnières sample and another plug extracted from the same parental block, as well as published literature data. Moreover, variations in the calculated elastic properties are interpreted as evidence that the FBG sensors detected heterogeneities in the samples' inner structure, which can be seen in the density profiles computed from x-ray CT images. Such observation confirms the potential of the presented FBG sensors configuration for 3D strain mapping in rock mechanics tests.
The deformation-to-failure process of rock is accompanied by the dissipation and release of energy and is the result of the mutual conversion of energy. The uniaxial compression and uniaxial cyclic loading–unloading testing of rock samples with eight height–diameter ratios (size) was performed using the MTS 816 rock mechanics testing system to determine the effect of the height–diameter ratio on the rock strength, energy accumulation, and dissipation. The influence rules of the height–diameter ratio on the uniaxial compressive strength, deformation parameters, and failure mode of the rock samples were analyzed. The total, elastic, and dissipated energy densities absorbed by the rock samples with various height–diameter ratios were obtained through calculation, and the evolution and distribution rules of the size effect on energy accumulation and dissipation were revealed. The energy density of rock samples with various height–diameter ratios increased nonlinearly with the increase in the cycle index or axial stress, whereas the energy density of the rock sample decreased with the increase in the height–diameter ratio. At the pre-peak stage, the elastic energy accumulated in the rock samples was higher than the dissipated energy, and the proportion of the elastic energy density was greater. With the increase in the height–diameter ratio of the rock sample, the proportion of elastic energy approximately increased, and the proportion of dissipated energy decreased.
This study proposes a novel approach to surface strain measurement for cylindrical rock specimens subjected to uniaxial compression using distributed fibre optic sensing technology. The capability and accuracy of this approach in measuring the full-field strain distribution of a rock specimen have been verified by a series of uniaxial compressive strength (UCS) tests on cylindrical specimens of aluminium alloy, sandstone, and granite. By analysing the experimental results, this new approach also has the potential of being utilized to detect the potential failure locations and sequence through strain localization zone variations and estimation of the development of crack opening displacement and rock fracturing characteristics during the loading and unloading process. Detailed installation procedures are provided for this study for assistance in the use of this new approach. The boundary issue of fibre measurements is identified, and solved by extending the bonding length of the measuring fibre.
