Detection of Defects in plate boards using non-contact ultrasound and Lamb waves

The study presents the application of non-contact ultrasound for the non-destructive detection of defects in two plastic boards made from polyamide and polyethene. Studies were carried out on the boards with synthetic defects generated to resemble those occurring in actual materials, such as delamination and cracking at the board edge, and cracking (discontinuity) in the middle part of the board. The defect size ranged from 1 mm to 15 mm. Ultrasonic transducers operating at 30 kHz were used to excite Lamb waves in the boards. The measurement method enabled the excitation and recording of this specific Lamb wave mode across the entire width of the tested materials (approximately 1 meter). The study's originality lies in detecting the presence of defects near the board edges, i.e., in the areas where they most often occur during production or transportation. This issue is often omitted in the literature due to the problematic interpretation of the results of ultrasonic tests because, in these areas, two overlapping effects are observed: (i) reflection of the Lamb wave from the edge and (ii) diffraction of the Lamb wave on defects. As a result, significant changes in the amplitudes of the recorded signals are observed as a function of the scan position at the beginning and end of the plates, while they are not observed in the central parts of the plates. The effect of the boundary disappears with distance from the plate edges, and in the central parts, the wave amplitudes are almost constant. It was shown that the method is highly sensitive to even the slightest changes in the material's integrity. A significant decrease in the signal amplitude, even for defects of a few millimetres in length, was observed. This technique's advantage is that it does not require any liquid coupling between the probes and the material being tested, making it a highly practical and efficient method for controlling board quality.