Issue34

M. Scafidi et alii, Frattura ed Integrità Strutturale, 34 (2015) 622-629; DOI: 10.3221/IGF-ESIS.34.68 627 Figure 9 : Binarized section images obtained with a threshold value of 40% (a) for D1 and 35% (b) for D2. The analysis of the section images allows to determine the positions of the defects by identifying the center of gravity ( CG ) of the dark zones. The procedure consists in calculating the coordinates of the CG by the following equations: i g x x N   (1) i g y y N   (2) where x i and y i are the coordinates of the generic dark pixel, while N is the total number of pixels in the dark zones. From the analysis of the section image, the x coordinates of the center of the defects are x 1 =40.2 mm and x 2 =77.7 mm, respectively for D1 and D2. Consequently, the calculated distance between the two defects is L =37.5 mm. This value is in a very good agreement with the real one, equal to 37.26 mm, and the error is about 0.6%. The y positions (depth) of the center of the defects result y 1 =1.62 mm for D1 and y 2 =2.32 mm for D2. The errors on the depth are, respectively, about +5.2% and -5.3%. B- SCAN ANALYSIS : D EFECT SHAPE DEFINITION he shape of the defects has been determined by analyzing the B-scan map perturbations due to the defects (Fig. 5). Because of the regular shape of the drilled holes, the perturbation is not caused by the diffraction [4-9] but is due to the reflection of the longitudinal waves at the smoothed boundary of the hole. As shown in Fig. 10, for each scan step, a longitudinal wave ( LR-wave ), starting from the generation point, hits the defect and is reflected to the receiver point. Figure 10 : Path of a generic LR-Wave (left) and corresponding point in the B-scan image (right). At each step, the laser receiver acquires the signal of the LR-wave reflected by the closer point of the defect. In this way, the bottom boundary of the perturbation on the B-scan map is defined. Extracting the coordinates x G of the perturbation boundary points from the B-scan, the time of arrival t L ( x G ) of the LR-waves can be determined (fig. 10). Known the wave velocity v L of the longitudinal wave, the length of the path L L ( x G ) covered by the LR-wave can be determined by the following equation:     L G L L G L x v t x   (3) As seen in Fig.10, the path of the LR-wave is composed of two parts: the first one goes from the generation point to the reflection point, the second one from this last point to the receiver. Since the initial direction of the LR-wave and the T