Abstract
The Laser ultrasonic testing method is effective for the location and evaluation of small surface-breaking cracks. This paper presents a bi-grating laser acoustic spectrum method for precise evaluation and imaging of surface cracks. With a bi-grating laser source, the reflection and transmission coefficient of the SAWs at the crack can be directly obtained with a single measurement by monitoring the amplitude ratio of the two frequency SAW in the frequency domain. It provides a self-reference method with good reliability, high SNR, and high resolution. Simulation results show that the surface crack can be precisely positioned from the ultrasonic reflection/transmission coefficient, and the characteristic parameter increases linearly with the surface crack depth when crack depth is smaller than the wavelength of the surface wave. Moreover, the feasibility and capability of the method are verified by experiment validation. The ultrasonic reflection/transmission coefficient is taken to obtain the C-scan imaging. The experimental results show that the proposed method is effective to locate and evaluate the small surface crack depth, and promising in the in-situ detection and imaging for surface cracks.
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