Defect detection in thin plates using S o Lamb wave scanning

Abstract

This paper describes work towards the development of a Lamb wave scanning method for the detection of defects in thin plates. The approach requires the generation of an ultrasonic S o -Mode Lamb wave using an incident transmitter excited with a tone burst centered at a near non-dispersive frequency. A pair of receiving transducers, with a fixed relative separation, remotely scans line sections of the thin plate. The global position of the receiver pair is moved to cover a large plate area. The arrival time information coming from incident and reflected waves contain information associated with the location of reflection surfaces or potential flaws. The cross-correlation between the excitation signal and the receivers' waveforms is obtained and subsequently demodulated using a quadrature amplitude method in order to facilitate the determination of arrival times. Distances from the source, to the reflection surface and to the receivers are found from the arrival times of the reflected waves and the Lamb wave phase velocity. The distances and the source and receiver locations are incorporated in an elliptical solution to find coordinates of the reflection points. In a line scanning the set of predicted reflection points define the extent of the defect. The Lamb wave scanning approach is tested using 1.6 mm-thick Aluminum plates with notches of various lengths and orientations from 0, 22.5 and 45 degrees with respect to the far edge of the plates. The results are summarized with defect maps that compare favorably to the actual notch locations.

Geometric approach to detecting and locating cracks in thin plates by Lamb wave reflection: case of moving transducer

“…, and R 1 S 1 =d 1 ; therefore, the Law of Cosines leads to (6) Similarly, (7) Subtraction (3) from (4), we conclude that: (8) hence for (9) we get the formula:…”

“…The So and A o mode propagates at a phase velocity of 5,280 m/s and 2,300 m/s respectively at this frequency. The scanning strategy selected was based in the visualization of Lamb wave scattering and reflections from notch and hole combination conducted by Osegueda et al [8], and DeVilla et al [9]; these animations served as a visualization tool.…”

“…The approach is documented in reference [9]. However this approach required iterative methods to solve the resulting geometric problem, as well as decreasing the signal energy as the receiver moves away from the receiver.…”

Detection of cracks at rivet holes in thin plates using Lamb-wave scanning

“…The main problem is to determine the coordinates of points E 1 and E 2 that form a tangent line to both ellipses. The solution is documented in [10] and is not repeated here. A similar equation exists for the second triangle using the prime notation.…”

“…The approach is documented in reference [10] and considers a fixed transmitter with a fixed focal source at F and two receivers at locations F 1 and F 2 as shown in Figure 11. All focal coordinates are known.…”