Geometric approach for detecting and locating cracks in thin plates by Lamb wave reflection

This thesis portrays work for the development of a Lamb-Wave scanning method for the detection of defects in thin plates. The approach requires the generation of an ultrasonic Ao and So-Mode lamb Wave using an incident transmitter excited with a tone burst centered at a near non-dispersive frequency. With a fixed relative separation both transmitter and receiving transducer, remotely scan a specific line section of the plate. The arrival time information related with incident and reflected waves contain information associated with the location of reflection surfaces or potential flaws. The Hilbert-Huang transform is applied to the intrinsic mode functions which permit the computation of the signal energy as a function of time, proportional to the square of the amplitude of the analytical signal. The arrival times and amplitudes of the notch-reflected energy are used to calculate by means of two geometric methods; Geometric Method I and Geometric Method II the coordinates of the source of the reflections. The resulting coordinates outline the extent and relative direction of notches in two different scenarios. One is having notches in a 0 to 22.5 degree orientation in respect to the far edge of the plate; the second considers notches ranging in size from ⅛&inches; to 1&inches; in length at a single rivet hole. Results of experiments conducted on 1.6 mm thick Aluminum rectangular plates, with an arrangement of notches as described compare favorably with the actual notches