Relative measurement of the acoustic nonlinearity parameter using laser detection of an ultrasonic wave

Abstract

The relative acoustic nonlinearity parameter beta' is defined by the amplitudes of the fundamental and second-order harmonic components in the detected ultrasonic wave signal, in which a piezoelectric transducer is conventionally used for detection. Although it is not identical to the absolute parameter beta defined by the displacement amplitudes, beta' has been widely used for the relative comparison between the same materials since its ratio is identical to that of the absolute parameter when the detected signal amplitude is proportional to the displacement amplitude. However, beta cannot be replaced by beta' for the comparison between different materials due to the difference of respective acoustic impedance. This study aims to show that a laser detection method using a photorefractive interferometer can be used for the relative comparison of acoustic nonlinearity between different materials because its output is independent of the acoustic impedance. To demonstrate, two similar materials, Al6061-T6 and Al2024-T4, and one different material, copper, were tested. Experimental results showed that the ratio beta (2024) / beta (6061) , measured via the proposed method and the conventional piezoelectric detection method, were in good agreement with the references. However, in the case of beta (Cu) / beta (6061) , only the proposed method showed good agreement with the references.