Abstract
In this paper, the interaction of a broadband Rayleigh wave generated by a laser and an artificial rectangular notch is analyzed theoretically and experimentally. For the theoretical analysis, a Gaussian function is adopted to analyze the modulation of notch depth on the frequency spectrum via reflection and transmission coefficients. By the finite element method, the Rayleigh wave generated by pulsed laser beam irradiation and its scattering waves at cracks are calculated. A curve with a slope close to 4 fitted by crack depth and critical wavelength of the threshold phenomenon is obtained by the wavelet transform and Parseval’s theorem according to simulated and experimental results. Based on this relationship, the critical frequency at which the threshold phenomenon happens due to energy transformation of transmission/reflection Rayleigh waves is adopted to determine the size of sub-wavelength surface crack. The experimental results of artificial notch depth estimation on aluminum alloy specimens consistent with theoretical analysis validates the usefulness of the critical frequency method based on a broadband Rayleigh wave generated by laser ultrasonic.
Highlights
Without prompt detection, sub-wavelength surface cracks can develop into open and deep cracks, resulting in structural fracture failure
Rayleigh waves based on lasers are broad band acoustic waves which contain a great deal of individual components with different frequencies
The modulation of surface crack size on the frequency spectrum of acoustic wave is theoretically analyzed by Gauss and power functions
Summary
Sub-wavelength surface cracks can develop into open and deep cracks, resulting in structural fracture failure. Quantitative determination of sub-wavelength surface cracks in the aerospace, aviation, and other fields is important to effectively evaluate mechanical properties. Rayleigh waves, whose energy penetration depth is approximately one wavelength during their propagation, are ideally suitable for detecting surface cracks, especially cracks with depths less than the incident Rayleigh wavelength. The reflected, transmitted, and diffracted Rayleigh waves, whose time and frequency characteristics provide a substantial amount of crack size information, are generated by the interaction between the surface crack and the acoustic wave. Contact transducers in the conventional detection technique are main method to generate and receive Rayleigh waves. Compared to incident wave wavelength, detected cracks are divided into two kinds that are long cracks with depths greater than at least one wavelength and sub-wavelength cracks whose depth is less than the incident wavelength
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