Detection of degradation in opaque dielectrics by ultrasound

The applicability of ultrasound methods to the detection of degradation in opaque dielectrics was investigated. The ultrasonic equipment used was a digital ultrasonic flaw detector with five narrow-band frequency ranges and a broadband range of 0.5 MHz and 15 MHz. The majority of experimental samples were made either from polyester resin or polyethylene. The polymers with the best ultrasonic visibility were rigid unfilled synthetic resins whose use in HV (high-voltage) insulation, though limited, is expanding. In such insulation small channels, cracks, and voids were detected with a good resolution at depths of up to 70-80 mm in the insulation. When resins were filled, their ultrasonic visibility was generally diminished. Epoxy resins varied greatly in their characteristics. CT200 has a high Young´s modulus and hence had very good ultrasonic visibility, whereas CY1311, a much less rigid resin used for potting, had very poor ultrasonic visibility. Polyethylene cables, which are generally made from low-density cross-linked material, had fair ultrasonic visibility. The higher density polyethylenes, often used for cable protection, had much better propagation characteristics, making detection of debonding between the two layers very easy