Strain monitoring in steel rock bolts using FBG sensor arrays

Rock bolts replace timber sets and steel arcs in securing the structural stability of underground cavities in order to reduce the volume of excavation. Rock bolts used in the mining industry are made of steel allowing up to 20% relative elongation of the bolt. Strain measurements using Fibre-Bragg-Gratings (FBG) are limited to relative strains in the order of 1%. In order to allow measurements of up to 20% relative strain, special arrangements of the FBG sensors had to be found to extend their measuring range. While a rock bolt elongates under stress its circumference contracts. An oblique line on the cylinder shell can be found where these effects cancel each other. A FBG sensor attached on the surface of the rock bolt in an inclination slightly steeper than this neutral angle will be elongated in a predictable ratio to the overall elongation of the bolt. This ratio changes while the bolt is being elongated. To ensure a:measuring range of the system of up to 20% relative elongation combined with suitable resolutions of its sensors we have devised an array of three FBG sensors. The sensors are arranged in inclinations that yield relative initial strain translation factors of -0.15, 0 and 0.15 with respect to the overall tensile str-ain in the rock bolt.