Issue34

M. Scafidi et alii, Frattura ed Integrità Strutturale, 34 (2015) 622-629; DOI: 10.3221/IGF-ESIS.34.68 624 As shown in Fig. 2, the surface skimming longitudinal wave, L-wave, travels just below the surface of the plate; the longitudinal back-wall wave, LL-wave , reflected by the opposite plate surface in accordance with the Snell’s law, travels with an orientation  that depends on the distance D . Being the longitudinal wave velocity v L about twice than the shear and surface wave velocities [14] and considering that the L-wave path (length D ) is the shortest between the generation point and the receiver point, the L-wave is the first that reaches the receiver. If the LL-wave travels along a path with length D LL =2D, the surface wave, the shear wave and the LL-wave reach the receiver at about the same time. This condition occurs when the triangle of the wave path is equilateral ( θ =30°) but, in this case, it is impossible to identify the three different waves apart. Choosing a propagation angle θ =45°, instead, the distance between the two lasers is D =2 T and the total length of the LL-wave path is D LL =2 T √2=2.83 T  1.4 D . This choice ensures that the LL-wave can be entirely acquired, cutting off the surface wave and the shear wave from the time window. Furthermore, the intensity of the out-of-plane component of the LL-wave for θ =45° is about 70% of the LL-wave intensity. In this way, in line with the purposes of this research, the distance D and the time-window have been then optimized to visualize only the L-wave and the LL-wave. In Fig. 3, a signal acquired by the receiver, in a T =5 mm aluminum plate without defect and a source/receiver distance of D =10 mm, is shown. Figure 3 : Typical signal of the longitudinal waves without defect. In Fig. 4, the section of the analyzed plate with the laser layout scheme is shown. Figure 4 : Section of the analyzed aluminum plate and laser layout (measures in mm). In the plate two circular holes, D 1 and D 2 , were drilled. Their diameters are, respectively , 2.10 mm and 2.09 mm. The distances of the defects centers from the scanning surface ( y -depth) are, respectively, y 1 =1.54 mm and y 2 =2.45 mm. The distance, L, between the centers of the two defects is 37.26 mm.