Strain measurement with multiplexed FBG sensor arrays: An experimental investigation

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 con-ventional strain gauges and radial cantilevers. FBG sensors have small size, multiplexing capa-bility, 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 (Savonnie`res limestone) and one acrylic Poly Methyl Methacrylate (PMMA) plug, all of 38 mm diameter, were prepared. The acrylic plug and one of the Savonnie`res samples plugs were subjected to Unconfined Compressive Strength (UCS) tests. The second Savonnie`res plug was subjected to a hydrostatic test up to 20 MPa confining at room temperature. FBG sensors of 125 & mu;m 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 prop-erties derived from the FBG readings for samples are comparable to readings from the conven-tional 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 Savonnie`res sample was consistent with benchtop measurements conducted on the Savonnie`res 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.

Automated summary

Conventional rock mechanics testing may not accurately measure rock strain if the tested rock has significant heterogeneity. Fibre Bragg Grating (FBG) sensors have been introduced as a versatile alternative for strain measurement during rock mechanics testing. This study aims to explore the capabilities of FBG sensors for strain measurement during rock mechanics testing, including under confining conditions.