SiGnal Processing Techniques For Nondestructive Evaluation Using Laser Ultrasonics
Laser ultrasonics is a form of ultrasonic evaluation in which a pulsed laser generates an ultrasonic wave in a sample which interrogates a specific feature. When this wave returns to the surface a separate laser interferometer detects the small resulting displacement. This technique is strictly noncontact and is thus suitable for in-process inspection of parts at high temperature or moving at high speed. In many applications the acquired temporal signals have features which differ from those acquired using conventional transducers. In this paper we will describe these features and discuss techniques which have been used to process these signals.
Laser ultrasonic TOFD method
We applied laser ultrasound for the time of flight diffraction (TOFD) measurement. To find an effective testing condition of TOFD method, laser ultrasonic transmission around the slit in glass specimens was observed using the photoelastic ultrasonic visualization system. After the observation we constructed the laser TOFD system and applied to the slit depth measurement of aluminum plate, and then, obtained highly accurate results.
Improved Performance of Laser-Ultrasonic F-SAFT Imaging
The spatial resolution of laser-ultrasonics depends upon the spot sizes of the generation and detection lasers and may be inadequate for detecting small and buried flaws. The use of a broad laser spot at the surface of the specimen to produce an ultrasonic beam with little divergence gives a spatial resolution limited by the spot size. In the opposite case, focusing the laser beam to a small laser spot yields a strongly diverging acoustic wave, leading also to poor spatial resolution. As in conventional ultrasonics, spatial resolution can be greatly enhanced by the use of the Synthetic Aperture Focusing Technique (SAFT) [1,2]. In addition, the coherent summation performed by SAFT yields an improved signal-to-noise ratio (SNR). Originally developed in the time domain, SAFT can be advantageously implemented in the frequency domain (F-SAFT). F-SAFT is based on the angular spectrum approach [2–4], which allows a significant reduction in processing time as compared to time-domain SAFT.