Issue34

M. Scafidi et alii, Frattura ed Integrità Strutturale, 34 (2015) 622-629; DOI: 10.3221/IGF-ESIS.34.68 625 To analyze the plate, a B-scan map has been built by regularly shifting, along a 100 mm straight line, the laser system and storing the signal for each step. Fig. 5 shows the B-scan map obtained with the panel of Fig. 4. The laser source position defines the abscissa x G of the B-scan map. Two perturbations can be noted due to the presence of the defects. Figure 5 : B-scan map obtained for the plate of fig. 4 with indication of the L-wave negative and LL-wave positive peaks. The B-scan map is shown in false-colours to highlight the peak-to-peak amplitude of the signals. Similarly to the case of the waves diffracted by the crack tip [6, 7, 13], the perturbation shows a parabolic-like pattern in the B-scan. In general, the defect affects the shape of the perturbation then, analyzing this last, the defect characteristics can be determined. Unfortunately, if the presence of the perturbations on the B-scan map can clearly make in evidence the presence of defects, the definition of their size, position and shape requires a more complex analysis. In the next sections, the proposed procedures for defining the size, the position and the shape of the defects are described. B- SCAN ANALYSIS : S IZE AND POSITION DEFINITION he LL-wave (red line in Fig. 5) is interrupted twice in correspondence of the defect D1. These interruptions are caused by the interposition of the defect along the LL-wave path, as shown in the Fig. 6. From the B-scan, it would seem that the defect interrupts also the L-wave. Actually, this effect is caused by the superposition of the longitudinal wave, L-wave, and the reflected wave due to the defect, LR-wave . In this case, in fact, due to the little depth (y) of the defect D1, the LL-wave and the LR-wave have similar path lengths. A different case, shown in Fig. 7, is represented by the defect D2 that has a greater depth. The LL-wave is interrupted only one time but for a bigger length. For the defect D1, the length of the interruptions depends on the defect size while for the defect D2, the length of the interruption depends also on the depth. As general case, measuring the length of the interruptions of the LL-wave directly on the B-scan map, information about the dimension of the defect can be obtained, while information on its depth can be hardly extracted. For a better definition of the size and position of the defect, the draft of an image of the section, considering the information extracted by the B-scan, is here proposed. This section image consists in drawing, for each scan step, a broken line describing the path of the LL-wave. These path-lines are drawn in a gray colour whose intensity is proportional to the amplitude of the corresponding acquired signal. Each pixel of the section image is obtained as intersection of the path-lines of two different steps. The biggest intensity of the (two) intersecting lines is assigned to the pixel. In this way, a bright pixel belongs to one not-interrupted path whereas a dark colored pixel is part of two T