Double frequency piezoelectric transducer design for harmonic imaging purposes in NDT

Harmonic imaging (HI) has emerged as a very promising tool for medical imaging, although there has been little published work using this technique in ultrasonic non-destructive testing (NDT). The core of the technique, which uses nonlinear propagation effects arising in the medium due to the microstructure or the existence of defects, is the ability to design transducers capable of emitting at one frequency and receiving at twice this frequency. The transducers that have been used so far are usually double crystal configurations with coaxial geometry, and commonly using a disc surrounded by a ring. Such a geometry permits the design of broadband transducers if each transducer element is adapted to the medium with its corresponding matching layers. Nevertheless, the different geometry of the emission and reception apertures creates difficulties when resolving the images. In this work, a new transducer design with different emission and reception apertures is presented. It makes use of the traditional construction procedures used to make piezocomposite transducers and the well-known theory of the mode coupling in piezoelectric resonators when the lateral dimensions are comparable with the thickness of the piezoceramic. In this work the design, construction, and characterization of a prototype to be used in NDT of metallic materials is presented. The acoustic field is calculated using water as a propagation medium, and these theoretical predictions then are compared with the experimental measurements. The predicted acoustic performances for the case of propagation in stainless steel are shown.