# Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring
## 5 Fiber Optics
### Fiber Bragg Grating  
FBG sensors (Figure 6) can measure strain, deflection, temperature, damage, the dynamic behavior of structures, etc. An FBG is a periodic refractive index variation, which is formed in the core of an optical fiber by exposure to an intense UV interference pattern [36]. The FBG sensor comprises a distributed Bragg reflector, which has a higher refractive index than the rest of the glass core. There is also a sensing interrogator that transmits light to the reflector [33]. The distance between each Bragg reflector is constant, and for this reason, only one type of wavelength of light is reflected back to the interrogator. The interrogator receives the wavelength and transfers it to a digital signal. Strain, temperature, pressure or acoustic signals can change the distance between each Bragg reflector. When this distance changes, the reflected wavelength of light (Bragg wavelength) also changes. Thus, the optical sensing interrogator indicates the change of wavelength (wavelength shift) and converts it to digital data and computer analyses and records the sensing information. Figure 6: FBG sensor [37]. There are expressions that relate the change of wavelength to strain, temperature, pressure or acoustic signals. For example, the relationship between wavelength shift, \(\delta\lambda_{B}\), and strain, \(\varepsilon\), for a homogeneous and isotropic material is the following [36]:  
\[\frac{\delta\lambda_{B}}{\lambda_{B}}\ =\ (1-p_{e})\varepsilon \tag{1}\]  
where \(\lambda_{B}\) is the Bragg wavelength and \(p_{e}\) is given by the following expression:  
\[p_{e}\ =\ \frac{n^{2}}{2}[p_{12}-\mu(p_{11}+p_{12})] \tag{2}\]  
where \(p_{\rm ij}\) are he fiber Pockel's coefficients and \(\mu\) is the Poisson ratio. The Bragg wavelength \(\lambda_{B}\) is given by the following equation:  
\[\lambda_{B}\ =\ 2n_{\rm eff}\Lambda \tag{3}\]  
where \(n_{\rm eff}\) is the modal index and \(\Lambda\) is the grating period. FBG sensors have been extensively used in sensor technology and SHM. [PERSON] et al. [38] have used FBG sensors to measure the inelastic cyclic response of reinforced concrete sections of tunnel linings located in seismic prone areas. A full-scale specimen that represented the lining sections of a tunnel specimen and five substructure specimens were tested. Different sets of FBG packaging were used, including bonded and un-bonded sensors in concrete and embedded and external sensors to monitor the inelastic strains of the specimens. They concluded that un-bonded external FBG packaging is suitable for SHM of civil infrastructures exhibiting an inelastic response. The seismic damage response of reinforced concrete structures has also been monitored using FBG sensing technology [39]. Shake table tests were performed to simulate the seismic response of a three-story reinforced concrete building. The FBG sensors were embedded in the structure and were utilized to measure dynamic strain response. This research concluded that FBG sensors are very accurate and capable of detecting the initiation of a crack and the damages that occurred in the beams [39]. Moreover, long-gauge FBG sensors were applied to measure the deflection of tied-arch steel bridges by [PERSON] et al. [40]. A new method was proposed based on long-strain measurements to determine the deflection of tied-arch bridges under both static and dynamic loading. The accuracy of the proposed method was verified through the experimental results of a real model of a tied-arch bridge. [PERSON] et al. [41] investigated the application of FBG sensors in monitoring strains in concrete dams subjected to earthquakes. A novel friction type FBG sensor was used to measure strains during shaking table tests of small gravity dam models. The FBG strain sensors were embedded in the dam models and the packaging method used in this paper can prevent damage of FBG sensor during the pouring process of concrete.