Can FBG sensors measure rock deformation under hydrostatic pressure?

ABSTRACT: The typical way of measuring radial strain in hydrostatic tests involve the use of radial gauges devices placed at the middle height of the rock sample, which read the sample radial displacement. In homogenous materials, these readings provide a reasonable estimate of the average radial strain undergone by sample. However, in heterogeneous materials, those central readings may not represent the average behaviour of the tested sample. In the recent years, new methods have been proposed to increase the accuracy for hydrostatic strain measurement. In this study, a new protocol that incorporates Fiber Bragg Grating (FBG) sensors is suggested and tested on an axisymmetric triaxial stress vessel, via the performance of two hydrostatic tests. The materials used in the test campaign were Berea Grey sandstone and Savonnières limestone samples. Results confirm that FBG sensors can be used as an alternative tool for radial strain measurement, with higher precision and ability for 3D strain mapping.