Numerical investigation of near and far field ultrasound transducer radiation characteristics in concrete

Concrete is the most frequently used material in civil engineering. Therefore, it also plays an important role for nondestructive testing methods like ultrasonic testing. In the past, Dry Point Contact transducers have proven to be very efficient transmitters and receivers for ultrasonic testing of concrete. The individual element consists of a
spring-loaded ceramic tip, driven by a piezoelectric bimorph and oscillating either normally or laterally with respect to the component surface. In order to predict the resulting sound fields quantitatively, appropriate modelling is required. Analytic formulas for the wave fields derived from elastic wave theory are available.
However, they usually only cover bulk waves (body waves) in the far field and discrepancies between calculated and measured directivity patterns are reported in the literature. We therefore conduct numerical simulations using the staggered-grid finite difference code EFIT to investigate radiation patterns of US transducers covering surface and bulk waves in the near and the far field. The simulations are performed in a heterogeneous, three-phased medium representing the material parameters and the mesostructure of concrete as well as in a homogeneous medium which has the material parameters for concrete. Sources used in the simulations were normal as well as shear force point sources. With this model, 2D and 3D radiation characteristics were evaluated at different distances to the source and compared to the existing theoretical and experimental literature. With the help of the simulations, a possible explanation between the reported differences in experiment results and theory was found.