Optimization and Validation of Dual Element Ultrasound Transducers for Improved Pulse-Echo Measurements of Material Nonlinearity

In nonlinearity parameter (beta) measurements of solids, the pulse-echo method is more desirable than the through-transmission method. 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 because such configuration provides an improved second harmonic generation and dependence on the total correction in the pulse-echo mode. For accurate and reliable measurements of beta for relatively 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 optimized transducer should provide the largest possible second harmonic reception and a total correction value close to one. In this paper, optimization of the dual element transducer is performed for 1 cm thick Al specimen at 5 MHz fundamental frequency, and the results are presented for the received second harmonic amplitude and the total correction. 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. The experimental results validate the transducer optimized for pulse-echo beta measurements on 1 cm specimens, and provide a quantitative correlation between the beta and the microstructural change.