Dual element transducers consisting of an outer ring transmitter and an inner disk receiver have been found to be an efficient alternative to single element transducers in SHG and pulse-echo beta measurements of solid specimens. The dual element structure provides improved SHG compared to single element transducers mainly because the phase difference other than pi between the two second harmonic components after the stress-free boundary reflection. However, the previous design of the dual element transducer was not appropriate to measure the beta of relatively thin specimens (1 cm thick Al alloy, for instance) because the size of transmitter and receiver was not optimized. For accurate and reliable beta measurements for thin samples, further improvement is needed by optimizing the transmitter and receiver size of dual element transducers for a given sample thickness and frequency.

The absolute nonlinearity parameter (beta) is defined as the product of the uncorrected nonlinearity parameter and the total correction. The total correction is needed to compensate for losses due to attenuation, diffraction and boundary reflection. The optimized dual element transducer should provide the largest possible second harmonic reception and the total correction value close to one. This paper presents the optimization results of the dual element transducer for 1 cm thick Al specimen at 5 MHz, and shows that the optimization can meet both of these conditions or one of them. The optimized dual element transducer is fabricated and experimentally characterized to be applied to determine the absolute  beta of Al specimens whose microstructure changes due to precipitation heat treatment or fatigue. The experimental results validate the transducer optimized for pulse-echo  beta measurements on 1 cm thick specimens, and provide a quantitative correlation between the  beta and the microstructural change.


Transducer Optimization for Pulse-Echo Measurement of Material Nonlinearity

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