Detection of degradation in opaque dielectrics by ultrasound

摘要

The applicability of ultrasound methods to the detection ofndegradation in opaque dielectrics was investigated. The ultrasonicnequipment used was a digital ultrasonic flaw detector with fivennarrow-band frequency ranges and a broadband range of 0.5 MHz and 15nMHz. The majority of experimental samples were made either fromnpolyester resin or polyethylene. The polymers with the best ultrasonicnvisibility were rigid unfilled synthetic resins whose use in HVn(high-voltage) insulation, though limited, is expanding. In suchninsulation small channels, cracks, and voids were detected with a goodnresolution at depths of up to 70-80 mm in the insulation. When resinsnwere filled, their ultrasonic visibility was generally diminished. Epoxynresins varied greatly in their characteristics. CT200 has a high Young'snmodulus and hence had very good ultrasonic visibility, whereas CY1311, anmuch less rigid resin used for potting, had very poor ultrasonicnvisibility. Polyethylene cables, which are generally made fromnlow-density cross-linked material, had fair ultrasonic visibility. Thenhigher density polyethylenes, often used for cable protection, had muchnbetter propagation characteristics, making detection of debondingnbetween the two layers very easy