## 4 Experimental Validations
### Inspection of Small Flaws Compared to the Wavelength
#### 4.2.2 Inspections with Shear (S) Waves  
Since the numerical validations have shown that differences are more important between the numerical and analytical models for shear waves, experimental acquisitions employing transversal waves have also been carried out in pulse-echo and TOFD configurations. Figure 16a shows the configuration of ultrasonic pulse echo NDT of the embedded planar electro-eroded slots (depicted in Figure 13). Transducers acting in immersion and generating oblique transversal waves at 45\({}^{\circ}\) at 0.5 and 1 MHz have been used. The experimental BScan at 0.5 MHz shown in Figure 16b highlights the emission echo versus time in blue, the echo from the entry surface, the response of the four slots and the flat bottom holes. The echogenic signature of the slot include the direct, corner and indirect echoes. All these echoes can be mixed when the slot backwall ligament is low (two slots at left); in that case, the corner echo amplitude is higher. As seen in Figure 17a, the FEM model leads to the better prediction. The gap between PTD and FEM reaches 8 dB at 0.5 MHz, PTD losing its effectiveness for small flaw heights with respect to the wavelength. The prediction of the echo signal also seems better when using the FEM model (Figure 17b,c). The previous validation confirms that the FEM-based 3D hybrid model could bring an interesting contribution to the simulation of small defects compared to the wavelength for shear waves. The last experimental validation deals with the S45\({}^{\circ}\) TOFD inspection of the specimen at both 0.5 MHz and 1 MHz, whose configuration is schematized in Figure 18a. One can observe chronologically on the experimental BScan of Figure 18b for the 0.5 MHz case the lateral wave, the echo due to the specular reflection on the entry surface, the echo from the defect and the echo due to the S waves reflection on the backwall. Figure 16: **(a)** Configuration of ultrasonic pulse echo NDT of the embedded planar electro-eroded slots (depicted in Figure 13); **(b)** experimental True BScan using the immersion probe at 0.5 MHz. The entry surface and backwall of the component are indicated in grey. The slot utilized for validation is shown thanks to the blue arrow, whereas the yellow ellipse indicates the component right corners and the flat bottom holes. Figure 15: For the TOFD P45\({}^{\circ}\) waves inspection at 1 MHz: **(a)** experimental Bscan; **(b)** Bscan simulated using the 3D hybrid FEM model in simulation. The table of Figure 19a presents the comparison with measure of both the 3D hybrid FEM method and the PTD model simulations of the maximal echo amplitude from the validation defect. The 3D hybrid FEM gives rise again to a better prediction in amplitude for the 0.5 MHz case corresponding to the smallest flaw height/wavelength ratio. The measured and simulated normalized Ascans are presented in Figure 19b at 0.5 MHz. One can notice the effectiveness of the simulation which models all these waves of very different natures both in terms of wave forms and of amplitude ratios (notably relatively to the lateral wave and entry surface echoes for the FEM model). The experimental validation has to be pursued on small flaws notably in 3D configurations, but the first numerical and experimental validations show that the hybrid method provides a much better prediction than analytical models for small flaws (especially with shear waves). Figure 19: **(a)** Gap between simulation and experience for the 3D hybrid FEM model and for the 3D PTD model; **(b)** Superimposition of the normalized experimental and simulated AScans at 0.5 MHz. Figure 17: **(a)** Disagreements with measurements obtained with the 3D FEM and PTD simulations; **(b)** Predicted and measured signals with the 0.5 MHz probe; **(c)** the same as **(b)** with normalization on the maximal amplitude. Figure 18: **(a)** S45\({}^{\circ}\) TOFD immersion inspection of several embedded planar electro eroded slots; **(b)** experimental BScan obtained when scanning the notches and the flat bottom holes at 0.5 MHz.