Low frequency ultrasound NDT of power cable insulation

Ultrasound technology is a very well-known effective technique used in Non-Destructive Testing (NDT). Of the advantages, one can name its relatively low cost and that it is safe. At high frequencies it offers the capability of generating high resolution imagery. Unfortunately at high frequencies ultrasound waves are highly attenuated in air and within materials attenuation also worsens as the frequency increases. Additionally, transmission in air results on a considerable impedance mismatch between the propagating medium and the materials under inspection. Thus a coupling medium is used that limits its applications to materials that can be either immersed in water or be in touch with coupling gel. In this paper we explore the use of 25 KHz frequency transducers used in air with no coupling material. We trade the benefits of high resolution imaging for effective fault detection with a system that offers its own benefits. By choosing such a low frequency we alleviate in part the attenuation of high frequency sound waves within the material and air but by not using a coupling medium the transmitted power to the sample is highly attenuated. Our main application, detection of faults within the insulation material of power cables requires such a system. We report that detection is possible.