Experimental investigation of strain sensitivity for surface bonded fibre optic sensors

The strain sensitivity coefficient for a surface-mounted fibre optic sensor is strongly affected by the type of adhesive material used for its installation. In the present study, appropriate strain correction factors have been investigated to modify the strain sensitivity coefficient for surface mounted fibre Bragg gratings to achieve accurate strain data. The experimental study adopts a fibre optic sensor (FOS) mounted on a carbon fibre reinforced polyphenylene sulphide specimen using two adhesive types, i.e. a cyanoacrylate and an epoxy-resin. For comparison purposes strain data was also captured using strain gauge, contact extensometer and digital image correlation instrumentation. The experiments reveal that the predicted correction factors vary considerably with the adhesive material used for the mounting of the FOS. Further analysis demonstrates that a 37 % deviation in the value of the correction factor results in a 27 % variation in the strain output. In addition, the microscopic image analysis confirmed that the thickness of the adhesive layer between FOS and the substrate affects the strain sensitivity of FOS and plays a crucial role in transmitting the deformation of the host material to the sensing element.