Non-contact laser/EMAT measurement systems for ultrasound B-scan imaging

For non-contact non-destructive evaluation (NDE), a laser/EMAT system is an alternative to a more expensive all optical laser-ultrasound system. Several design options of the electromagnetic acoustic transducer (EMAT) are available, permitting inspection of electrically conducting materials. In this paper, we describe a system capable of monitoring variation in either sample thickness or defects, with time-of-flight diffraction arising from mode-converted ultrasonic waves. In a B-scan imaging configuration, quantitative time-of-flight analysis of laser-generated acoustic waves is shown to be an effective method for measurement. Various images will be presented together with an interpretation of their features. For these images, transient laser pulses with typical energies of ∼18 mJ have been delivered to the material surface via an optical fiber and focused to a line source by a cylindrical lens. Acoustic waves arising from this excitation propagated through the sample to be reflected from the far surface...

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