Ultrasonic Lamb wave tomography of through hole flaws in isotropic thin plates

A "cross borehole" tomographic reconstruction technique exploited by seismologists is used for quantitative Lamb wave nondestructive evaluation (NDE) in this paper. Combining ray tracing with algebraic reconstruction technique (ART), ultrasonic Lamb wave tomography for through hole flaws is developed. Based on Snell law and Fermat's principle, ray tracing is used to find actual ray paths and calculate travel times of ultrasonic Lamb waves between transmitting and receiving transducers. ART is then carried out to realize the velocity imaging reconstruction for the given set of arrival times. Reconstruction for both simulated and experimental data acquired on aluminum plates with machined through hole flaws is presented with straight-ray and curved-ray. The results demonstrate that the sizes of flaws reconstructed with curved-ray ART appear to be closer to their real size than straight ray ART, and the artifacts of reconstructed images are also reduced when ray-bending is considered.