# Principles and Application of Polyimide Fiber Bragg Gratings for Surface Strain Measurement
## 4 Practicality Validation of Polyimide FBGs
### Reinforced Concrete Specim Internal Explosion Measurement  
An experimental system photo is shown in Figure 22. The system consisted of two cylindrical specimens which were made up of C40 concrete and an HRB335 steel bar. The diameter of the specimen was 3.5 m, and thickness of the circle specimen was 1 m. The specimen was deployed in the explosion test installation. There was a cavity between the two cylinder specimens. Sand bags and a rubber cushion were piled surrounding the specimen in order to seal the cavity. Detonating fuses were deployed at the center of the cavity. Ten optical fiber sensors were symmetrically mounted by different types, along the vertical and horizontal radius of the cylinder. The three kinds of optical fiber sensors used in the experiment were bare polyimide FBGs, the MOI (Micron Optics Incorporation) os-3200 non-metallic optical strain gauge and the JPhotonics JFFBGS-200 concrete surface strain gauge. As shown in Figure 22, Positions 1 and 4 show os-3200 non-metallic optical strain gauges. Positions 7 and 8 show JFFBGS-200 concrete surface strain gauges. Positions 2, 3, 5, 6, 9, and 10 show bare polyimide FBGs. As shown in Figure 23, the central wave length of bare polyimide FBG were 1545 nm and 1550 nm. They were mounted to the surface of the concrete specimen by instant adhesive first, and then covered by E-120 HP epoxy adhesive. The os-3200 is a product from Micron Optics Incorporation, Atlanta, GA, USA while JFFBGS-200 is a product from JPhotonics, an optical fiber sensing incorporation in Hangzhou, China. As shown in Figure 24, the encapsulation body length of MOI os-3200 is 25 mm and the width of it is 5 mm. Figure 23: Photos of bare polyimide FBGs. Figure 22: Reinforced concrete specimen with FBGs without coating and other FBG sensors. The encapsulation material is santoprene. os-3200 has a self-adhesive backing that holds the sensor body in place and protects the FBG while epoxy is injected. The epoxy encapsulates the FBG and bonds it to a structure's surface. The length of JPhotonics JPFBGS-200 is 100 mm. The FBG is encapsulated in a metal tube whose diameter is 10 mm, and the mounted to the surface of the specimen by four screws. The shear modulus of C40 concrete is \(2.0\times 10^{10}\) Pa, and, and its density is \(2.4\times 10^{3}\) kg/m\({}^{3}\). Hence, speed of the shear stress wave is given by:  
\[C_{s}=\sqrt{\frac{2.0\times 10^{10}}{2400}}=2887\,\mathrm{m/s} \tag{20}\]  
The estimated maximum frequency of the shear stress wave in specimen is given by:  
\[f=\frac{C_{s}}{\lambda}=\frac{2887}{10\times 3.5}=82.5\,\mathrm{Hz} \tag{21}\]  
The sampling frequency of SM130 is great enough to obtain data. The experiments were conducted in three groups. During each group, the amount of explosive was increased by 50%. The data of different optical sensors obtained are shown as Figures 25-28:  
Figure 24: Photos of os-3200 non-metallic and JPFBGS-200 optical strain gauges. Figure 25: Data from bare polyimide FBG No. 9. Bare polyimide FBGs can obtain an explosion peak in every group, and the variation of the peak in each group is almost linear to the increase of the explosion. For example, in Figure 25, the peak of group 3 is nearly twice of that of group 2, because amount of the powder was twice that in group 1. In Figures 25 and 26, there is a collision peak. It means that the cylinder specimen collided with the installation surrounding it, and bounced back because the stress of the explosion was too strong. Figure 28: Data from JFBGS-200 concrete surface strain gauges (position No. 7). Figure 26: Data from bare polyimide FBG No. 4. Figure 27: Data from os-3200 non-metallic optical strain gauges (position No. 2). However, in Figures 27 and 28, there is no collision peak. Both os-3200 and JFBGS-200 had a loss in peak in group 3. This is because there are only three strain transfer layers, fiber-adhesive-substrate, in polyimide FBG strain measurement system.