Vibration-based damage identification is a well-known method to support health monitoring of civil engineering structures. Damage in such structures can be identified by measuring changes of the natural frequencies, damping factors or modal displacements of the structure. However, this approach suffers from the low sensitivity of these natural frequencies and modal displacements to certain types of damage. Modal strains and curvatures can be more sensitive to local damage, but direct monitoring of these quantities with sufficient spatial resolution is not possible with current measurement techniques due to the very small strain levels (< 1 microstrain) when considering ambient or operational excitation of the civil structure.
To deal with this issue, the VUB Brussels Photonics Team B-PHOT has developed a novel mechanical transducer equipped with an optical fibre Bragg grating (FBG) sensor that mechanically amplifies the strain experienced by a civil engineering structure and applies that amplified strain to an optical fibre sensor with an amplification factor larger than 30 (for a transducer with outer dimensions of 380mm x 105mm and with a gauge length of 275mm). The principle of operation of the transducer exploits a symmetric cantilever structure that enlarges the strain experienced by the FBG sensor compared to the strain applied to the transducer itself. The dimensions can be adapted in function of the overall gauge length, the relevant frequency range and the targeted strain amplification. The transducer can be mounted on the surface of concrete civil engineering structures to support high-resolution modal strain measurements at low cost.