Development and application of ultrasonic transducer for intensive focusing of Rayleigh waves

Intensive focusing of Rayleigh waves was realized by the fabrication of a curved PZT (Pb(Zr 2 Ti)O 3 ) piezoelectric element having a new shape. Two- and three-dimensional beam profiles of the focusing transducer employing the curved PZT element were obtained by a contact scan on a steel block with flat bottom holes. The beam intensity was higher, about 35 dB, at the focal distance and the signal-to-noise ratio was higher, about 17 dB, than the focusing transducer employing a flexible PVDF (polyvinylidene fluoride) film in the earlier study. The −6 dB and −3 dB lateral widths at the focal point were 0.61 mm and 0.46mm, and the −6 dB and −3 dB axial widths were about 5.5mm and 4.0mm, respectively. The axial stress field showed a good agreement with the overall pattern estimated by a simplified model. The ratio of the detected smallest flaw size to the wavelength of Rayleigh waves for the center frequency was about 0.29. As an application, a single time scan was performed along the flat bottom holes (FBHs) at the focal distance, and an image to show sharp peaks with amplitude depending on the size of the FBHs was obtained. The focusing transducer showed the characteristics of a narrow lateral resolution, high energy, and high signal-to-noise ratio, and was practical for the detection of surface flaws smaller than the wavelength.