Experimental Investigation on the Mechanisms of Fiber Bragg Gratings to Monitor the Failure Processes of Pre-Cracked Sandstone Specimens

Real-time monitoring of internal fracture evolution in fractured rock masses using fiber Bragg grating (FBG) technology can help mitigate geotechnical hazards. This study employed FBG, acoustic emission (AE), and digital image correlation (DIC) to analyze pre-cracked sandstone under uniaxial compression. During the failure of the pre-cracked specimens, the FBGs experienced non-uniform stresses. In the initial loading phase, the stress concentrations at the crack tips and the wing-crack development were dominated by tensile stresses, and the maximum tensile strain was 1.01%. After the initial yield strength was reached, the crack-propagation process transitioned to shear-stress dominance, and a maximum shear strain of 6.45% was exhibited. During multiple stress decreases (180–250 s), the FBG-measured local shear and tensile strains reflected stress variations that were associated with shear-locking effects and failure stages. Before the tensile-crack initiation, the FBG-detected principal-strain concentration zones exhibited prolonged incubation periods, whereas the shear-crack initiation was preceded by shorter incubation periods. The evolution curves of the damage variable, which was defined by the FBG coupling strength, could be categorized into three distinct stages: initial damage accumulation, damage acceleration, and final damage. When the initial yield strength was reached, the damage variable rapidly increased, particularly during the two stress decreases.