Third-order acoustic nonlinearity parameter measurement techniques in pulse-echo mode

School of Mechanical Engineering, Hanyang University, Seoul, South Korea

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Abstract

Conventional nonlinear ultrasonic techniques have primarily relied on the measurement of the second-order acoustic nonlinearity parameter () in through-transmission (TT) mode, which measure the amplitudes of the fundamental frequency component and its second harmonic component transmitted through a medium to evaluate the degree of nonlinearity. Although this approach is highly accurate under controlled laboratory conditions, it is unsuitable for field applications or in-service structures where access to both sides of the structure is restricted. The pulse-echo (PE) mode offers the only feasible solution to overcome these limitations, as it allows one-sided inspection. However, a technique for measuring nonlinear parameters based on the PE configuration has not been established thus far, because the second harmonic component in the reflected signal is canceled owing to the phase inversion effect. This presentation introduces a new technique for measuring acoustic nonlinearity parameters in PE mode, in which the third harmonic component in the back-wall echo is analyzed to determine the third-order acoustic nonlinearity parameter (). It covers both relative measurement, which uses the voltage amplitude of the received signal, and absolute measurement, which converts the amplitude of the received current signal into displacement amplitude through calibration.

Relative measurements were conducted using a single PZT transducer in a PE setup, and the results were correlated with the microstructural changes observed in thermally aged Al6061-T6, INCONEL 690, and Hastelloy C276 specimens. The measured third-order acoustic nonlinearity parameter exhibited clear trends with increasing heat-treated time and was highly correlated with microstructural changes. For absolute measurements, the PE method was designed such that calibration and the characterization of system nonlinearity were implemented concurrently during harmonic measurement in a PE setup. System nonlinearity was compensated for by analyzing the 1st and 2nd echoes, eliminating the requirement for multiple specimens or additional setups. After applying this compensation, diffraction and attenuation corrections were applied to ensure that the evaluated represented intrinsic material nonlinearity. When the corrected methodology was applied to Al6061-T6 specimens with different thicknesses, the uncorrected values, which initially increased with the increase in propagation distance, became consistent with theoretical expectations. Furthermore, when applied to thermal aged Al6061-T6 specimens, the proposed method successfully distinguished varying degrees of degradation; therefore, the validity of the proposed technique was confirmed.

The proposed method is expected to apply to real field inspections and in-service structures, providing a practical and effective tool for the nondestructive material assessments.