Effect of Fracture Dynamics in Thin Plate on the Waveform and Radiation Pattern of the Lamb Waves

Since the acoustic emission (AE) due to fractures in thin-wall structures is monitored as Lamb waves, advanced signal analyses are required for evaluation. The present study aims to estimate the fracture kinematics (crack normal and displacement discontinuity vectors) in thin plates by radiation pattern analysis of the zeroth-order symmetric Lamb waves ( S 0 -Lamb). We monitored in-plane profiles of the S 0 -amplitudes and polarities produced by six types of surface and buried Mode-I, II and mixed-mode fractures simulated with a line pulse laser and slender piezo-elactric Pb(Zr-Ti)O 3 (PZT) elements. The Mode-I fracture showed an elliptical amplitude profile with the largest amplitude in the crack-opening direction, while the mixed-mode fracture as well the Mode-II fracture showed a connected rugby-ball-shaped pattern, or a two-quadrant profile of S 0 -polarities with the largest amplitude in the displacement discontinuity direction. The radiation pattern was demonstrated to be utilizable for estimating the fracture kinematics. For studying the fracture kinetic or crack generation velocity, we determined the overall transfer function of the system by the Gauss-Zeidel deconvolution of the wave generated by PZT elements whose out-of-plane displacement was measured using a laser interferometer. The source dynamics (kinematics and kinetics) of hydrogen-induced blistering (HIB) in a ferritic steel plate was estimated by radiation pattern analysis and source simulation. HIB was found to be caused by surface-normal cleavage fracture with source rise times from 0.6 to 1.0 µs. The estimated crack size agreed fairly well with the postmortem blister size.