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Off-epicentral measurement of laser-ultrasonic shear-wave velocity and its application to elastic-moduli evaluation.

This study investigates the off-epicentral measurement system and methodology of laser-ultrasonic shear waves in the ablative regime, subsequently focusing on its application to the elastic-moduli evaluation. The proposed scheme facilitates the measurement of the shear-wave velocity by using the crosscorrelation of successive shear-wave pulse echoes, ensuring high reliability and reproducibility of the calculation results. Since the geometric diffraction induced by the finite dimensions of laser-ultrasonic source and receiver can increase the measurement error, far-field diffraction correction for the off-epicenter detection has been derived theoretically. Results have shown that the far-field diffraction correction error is of the same order as random error mainly caused by the finite observation time, signal-to-noise ratio and frequency bandwidth, and therefore cannot be neglected. The feasibility of this approach is then demonstrated by inspecting annealed commercial purity titanium plates. Good agreement of experimentally measured and theoretical values of shear-wave velocity confirms the validity of the proposed approach. The ultrasonic measurement-determined shear modulus shows a maximum deviation of ＋3.6% from the standard value of an isotropic material.