Strain monitoring in FRP laminates and concrete beams using FBG sensors Kin-tak Lau a , Libo Yuan a , Li-min Zhou a, * , Jingshen Wu b , Chung-ho Woo a a Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong b Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong Abstract The ®bre-optic Bragg grating (FBG) sensor is broadly accepted as a structural health monitoring device for ®bre reinforced plastic (FRP) materials by either embedding into or bonding onto the structures. The accuracy of the strain measured by using the FBG sensor is highly dependent on the bonding characteristics among the bare optical ®bre, protective coating, adhesive layer and host material. In general, the signal extracted from the embedded FBG sensor should re¯ect the straining condition of the host structure. However, due to the existence of an adhesive layer and protective coating, part of the energy would convert into shear deformation. Therefore, the mechanical properties of these materials would aect the resultant strain measured by embedding a FBG sensor into the structure. This paper presents a theoretical model to evaluate the dierential strains between the bare ®bre and host material with dierent adhesive thickness and modulus of the protective coating of the embedded FBG sensor. The results are then compared with numerical analysis by using the ®nite element method (FEM). Experimental work was conducted for both glass ®bre composites and FRP strengthened concrete beams with embedded FBG sensors. Externally bonded strain gauges were used to compare the results obtained from the FBG sensors. The theoretical predictions reveal that the axial strain measured at the ®bre- core region is lower than the true strain of the host material with increasing thickness of adhesive layer. A thick adhesive layer and low modulus of coating material would enlarge the shear stress concentration area at the bonded end region. An experimental investigation also shows that the FBG sensor can be con®dently used with sucient bond length. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fibre-optic Bragg grating; Strain sensing; Fibre reinforced composites; Concrete structure 1. Introduction The ®bre-optic Bragg grating (FBG) sensor has been extensively adapted as a new non-destructive evaluation (NDE) technique in monitoring strain and temperature pro®les of structures under service conditions [1]. The speciality of using FBG sensor for strain sensing appli- cation is that it is able to measure strains locally with high resolution and accuracy. As the physical size of an optical ®bre is extremely small compared with other strain measuring components, it enables it to be em- bedded into the structures for determining the strain distributions without in¯uencing the mechanical prop- erties of the host materials [2]. Recent research reports showed that optical ®bre sensors could be eectively used for damage detection and cure monitoring of ad- vanced composite materials (ACM) [3]. Du et al. [4] demonstrated that the FBG sensor was capable of measuring the strain at the inter-laminate layers of tex- tile structural composite (TSC) beams. Hong et al. [5] also found that the strains could be remotely ``real-time'' measured by using the FBG sensing technique for graphite±epoxy (Gr/Ep) composite plate when subjected to a tensile loading. Recent researchers have started to pay an attention to the utilisation of FBG sensor in civil concrete structures. Saouma et al. [6] and Holton [7] adhered FBG sensors on to a reinforced steel bar of cast-in-place concrete structures in order to monitor the strain response in situ. In recent years, many research eorts have concen- trated on using ACM for concrete rehabilitation and retro®ts [8±12]. A high strength ®bre reinforced plastic (FRP) laminate has been adhered to the concrete surface in order to improve the stiness and tensile strength of damaged structures. The attraction of using FRP in the construction industry is their high strength, low weight, and resistance to corrosion and high fatigue life. www.elsevier.com/locate/compstruct Composite Structures 51 (2001) 9±20 * Corresponding author. Tel.: +852-2766-6663; fax: +852-2365-4703. E-mail address: mmlmzhou@polyu.edu.hk (L.-M. Zhou). 0263-8223/01/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 8 2 2 3 ( 0 0 ) 0 0 0 9 4 - 5