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

Abstract

Concrete is the most frequently used material in civil engineering. Therefore, it also plays an im-portant 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 modeling 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 di-rectivity 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 cover-ing surface and bulk waves in the near and the far field. The simulations are performed in a strongly heterogeneous, two-phased medium representing the material parameters and the mesostructure of concrete. With this model, the effects of damping as well as scattering at pores and aggregates on radiation characteristics shall be investigated in future. In order to validate EFIT results, measure-ments were conducted on a concrete specimen with an embedded tube as a test feature. As will be shown, a good agreement between measurements and simulations was found. During the evaluation of the simulation and measurement results focus was put on surface waves because a significant part of the wave field’s energy goes into the surface modes. Ultimately, these studies might be used for US transducer sensitivity analysis as well as for US testing reliability assessments in the future.