## 6 NDT Applications
### Application of the Bragg Networks to Detect the Wave Propagation in Prismatic Wave Guided  
Techniques of damage detection based on the elastic wave propagation are attractive due to the low level of intervention over the structure analyzed. In particular, only one point of access to emit and receive the wave is enough to prospect a region of interest.  
Several techniques can be applied to measure NDT; one of them is the use of optical fiber [45]. The fiber can conduct the light even through long distances and one special arrangement, known as Bragg's network (FBG--fiber Bragg grating), cause the light to be reflected. If the Bragg network suffers a spatial perturbation, the reflected light changes. With a calibration, it is possible to observe under the Bragg network how a displacement changes in a spatial region [46]. The great advantage of using optical fiber in NDTs is that fibers are inert and may be in contact with potentially explosive materials such as petroleum or its derivatives.  
Figure 16 shows the experimental setup used to evaluate the response of an optical fiber sensor when the wave longitudinal mode passes through the bar. The setup consists of a cylindrical piezo-ceramic, coupled at the end of the rod used in the generation of the longitudinal mode. The Bragg network is bound at 350 mm away from the excitation. The system that measures the perturbation over the fiber Bragg grating (FBG) consists of a circulator, an optical filter, and the laser emitter. Basically, the light excites the laser emitter, and then it passes through the circulator dividing itself partly into the Bragg network and partly into the optical filter. The Bragg network was calibrated and reflected more light when it is deformed.  
Figure 15: Scattering curves of three different sections height \(\times\) width. (The blue dots in the graph indicate no fundamental modes). Notice that in this graph the wave number is defined as \(\mathrm{k}=2\pi/\Delta\left[\mathrm{m}^{-1}\right]\).  
There are two main difficulties in using FGB sensors to capture high-frequency events: interrogation technology, with adequate acquisition frequency (\(>\)10 times the frequency of the signal), and the signal/noise ratio. There are currently interrogators on the market that can reach 7.1 MHz of acquisition [47].  
These devices do not allow acquisition with averages. This practice is very usual in eliminating signal noise when the noise is a stochastic phenomenon. In the present work, an interrogator that uses a national acquisition system is used, which allows both a high acquisition rate and the possibility of using a trigger to link the excitation with the signal collection and repeat the event and take averages to neutralize the noise.  
An application example of an FGB/piezo system can be found in [48], where a mechanical wave is excited on a composite material plate. However, the frequency of acquisition of the system is around 2 kHz, to below when it is necessary to work with metallic materials.  
Figure 17 shows the signal obtained with this arrangement where the time was converted in the distance, using the propagation velocity (**c = f/k**), extracted from the dispersion curves, and by the velocity relation (distance/time). Notice that the distance from the emitting source to the place where the fiber Bragg grating (FBG) was glued is known. In the case of Figure 17, the displacement perturbation was emitted artificially by using a piezo ceramic but it could be produced by a mechanical wave reflecting over a defect such as a crack or loss of thickness by corrosion.  
Figure 16: Experimental setup used in the assessment made using optical fiber sensors to capture the propagation of mechanical waves in a rectangular rod.  
Figure 17: Collected Signal with the fiber Bragg grating with a tone burst excitation (57 KHz and 7 cycles).