Ultrasonic dual element probes revisited – complex wave phenomena as a basis for transducer design and optimization

Ultrasonic dual element transducers consist of two angled piezoelectric elements housed in the same case but separated by an acoustic barrier. While one element excites transient waves into a delay line, the other element acts as a receiver for the echoes produced in the component under test. The main advantages of this type of transducer compared to a single-element pulse-echo probe are a better near-surface resolution and a reduced direct back-scattering noise in heterogeneous or coarse grained materials. Numerical time-domain simulations reveal that the ultrasonic wave phenomena in a dual element probe are surprisingly rich and complex which may lead to an unexpected spatial redistribution of energy in the exciting wave front and - as a consequence - to a deviation from the expected exit angle according to Snell’s law. Moreover diffraction effects at the interface between the acoustic barrier bridge and the tested component turn out to be very sensitive to small geometrical changes of the delay line which in turn affects the near-surface resolution and the acoustic crosstalk to the receiver channel. In the present contribution it is demonstrated how the described wave phenomena can be used for a systematic design and optimization of dual element probes.