# The Effects of Adhesive and Bonding Length on the Strain Transfer of Optical Fiber Sensors
## 1 Introduction  
Optical fibers are widely applied in various sensing systems, owing to the excellent properties, such as small dimensions, flexibility, embeddability, high temperature endurance, dielectric nature, immunity to corrosion, as compared with traditional sensors, and being light weight [1]. Optical fiber sensors have been developed to measure a variety of physical quantities such as strain, temperature and vibration. Smart structures which integrate both sensors and actuators have the ability to sense environmental changes within or around the structure and are able to interpret and react to these changes. Optical fiber sensors have enabled the implementation of smart structure systems [2] by (1) providing sensors that are small enough to be integrated directly into materials; (2) enabling sensors to be multiplexed in substantial numbers along a single line; (3) providing electrical isolation and immunity to electromagnetic interference; and (4) supporting the high bandwidth necessary for large numbers of high performance sensors. A number of optical fiber-based sensor systems have been proposed for structural health monitoring; two sensor designs tend to predominate, namely fiber Bragg gratings (FBGs) and interferometric sensors. The sensing principle of FBGs is based on tracking Bragg peak shift due to the changes of environmental conditions, in particular, both strain and temperature in the grating region. FBG sensors have received considerable attention in recent years as powerful strain measurement tools [3]. They have been employed in a variety of engineering applications involving solidification strain measurement in photocurable resin [4], thin film stress measurement during the deposition [5], monitoring the residual strain in thermoplastic composites [6], and estimating thedynamic structural displacements in a cantilever beam [7]. Interferometric type optical fiber sensors are based on the phenomenon of optical interference between two identical light waves. The light waves in one of these fibers is used as reference signal, where the second arm acts as the sensing arm of the interferometer. They have the advantages of high sensitively and high spatial resolution. Based on single mode optical fibers, the three most widely employed interferometric configurations are: Mach-Zehnder [8], Michelson [9] with a two-arm interferometer and Fabry-Perot [10; 11; 12] with a single arm interferometer. Fiber optic sensors are considered one of the most promising methods for monitoring of process-induced residual strains. [PERSON] [13] employed an FBG sensor to measure the residual strains in photocurable resins. He found that the resin experienced a non-uniform compressive strain due to the photo-polymer solidification. [PERSON] _et al._[14] investigated the effect of the cladding diameter on the strain measurement. They reported that the longitudinal force F is proportional to the outer surface of the optical fiber, _i.e._, F \(\propto\) cladding diameter. The strain of the optical fiber induced by the longitudinal force is equal to \(\varepsilon=\dfrac{F}{EA}\) where the cross section area A is proportional to the square of the cladding diameter. Thus, the strain sensitivity of the optical fiber sensor is inverse proportional to the cladding diameter. Optical fibers can be either embedded or surface bonded to the host structure. The capability of an optical fiber sensor to monitor the strain in a structure depends on the bonding characteristics which includes the bonding length, cladding diameter, adhesive and protective coating. As a sensor, it is expected that the strains between the optical fiber and host structure are the same. However, due to the existence of the adhesive layer and protective coating, part of the energy is converted into shear deformation. Thus, the strain of the optical fiber is different from that of the host structure.