## 4 Experimental Validations
### Inspection of Large Flaws  
Several notches and holes have been fabricated in a planar specimen (Figure 10a). In order to study the influence of the orientations of both probes and flaws, two 6.35 mm diameter probes emitting 45\({}^{\circ}\) P-waves at 2.25 MHz have been positioned in a TOFD configuration with a 60 mm Probe Center Separation and rotated from the 0\({}^{\circ}\) skew angle to the 34\({}^{\circ}\) skew angle. Figure 10b,c present a typical case of the 11\({}^{\circ}\) skew.  
Measurements have been carried out on a rectangular 0\({}^{\circ}\) flaw and three defects (with vertical disorientation of 10, 20 and 30\({}^{\circ}\) for the top edge) and calibrated against a 2 mm diameter SDH. The resulting experimental B-Scan is shown in Figure 11a. The variation in the amplitude of the echo from the top tip (equivalent to that of an embedded flaw) with the vertical disorientation is displayed in Figure 11b. The results simulated by different models are compared in terms of echo amplitude: (1) an older model based on the so-called 2.5D GTD [5], which involves the projection of the incoming and scattered wave vectors on the plane normal to the flaw edge and 2D GTD coefficients related to these projections, (2) 3D PTD model and (3) the hybrid 3D FEM model.  
The differences between the 3D PTD model, the hybrid 3D FEM model and experimental data are less than 1 dB, except for the 30\({}^{\circ}\) misoriented flaw, for which the signal-to-noise  
Figure 11: (**a**) Experimental B-scan obtained when scanning the side drilled hole (SDH) and the three defects (with vertical misorientation of 10\({}^{\circ}\) to 30\({}^{\circ}\) for the top edge); (**b**) validation of the 2.5D GTD, 3D PTD and the hybrid 3D FEM models against the measured echoes from the top tip of misoriented backwall breaking flaws.  
Figure 10: (**a**) A planar component containing disoriented backwall breaking flaws and a 2 mm diameter SDH; the TOFD configuration with the 11\({}^{\circ}\) probes’ skew: (**b**) top view and (**c**) side view showing the flaw used for simulating the top edge of the 30\({}^{\circ}\) disoriented notch.  
ratio is low (see Figure 11b). Note that the 3D PTD-based model provides a slight improvement over the 2.5D GTD model in that configuration.  
In the view of studying more important 3D effects, a second experimental validation has been performed to evaluate the effect of skew flaw angle on edge diffraction amplitude. Tests have been carried out on a planar rectangular backwall breaking flaw (10 mm high and 40 mm wide) using a pair of transducers (2.25 MHz, 45\({}^{\circ}\) longitudinal waves, 6.35 mm diameter) in contact mode. The probes have been positioned in a TOFD configuration with a 60 mm Probe Space Center and with flaw skew angle varying from 0\({}^{\circ}\) to 70\({}^{\circ}\) (Figure 12a).  
In Figure 12b, it can be seen that the influence of the skew angle on the measured diffraction echo amplitudes is minor. The 3D hybrid FEM model leads to a good prediction with a maximal disagreement of about 2.5 dB with measure; the 3D PTD also effectively surpasses the older 2.5D GTD model (available in a previous CIVA release), which breaks down for important skew (>40\({}^{\circ}\)). It can be noticed that the 3D PTD and hybrid 3D FEM models lead to close results for this configuration involving a flaw of both large height and extension.  
The previous experimental validations have thus confirmed that the 3D hybrid method provides in the case of a large flaw a prediction in very good agreement with experiments and also with the other CIVA 3D model (PTD). The hybrid method correctly simulates larges flaws and 3D configurations.