Application: Verifying the integrity of bonding between the silicone skirts and the composite core of composite high voltage power line insulators.

Background: The high voltage pylon insulators used for power transmission lines and railroad traction power wires have traditionally been made of glass or ceramic, but many users are now switching to composite insulators because of their advantages with respect to weight, durability, and environmental performance. These insulators consist of an FRP composite core coated with a layer of silicone rubber and bracketed with metal end fittings. Good bonding between the silicone and the FPR is essential for proper electrical performance, since bond failures may result in flashovers. Ultrasonic flaw detectors can be used to quickly and nondestructively check the integrity of these bonds, both at the manufacturing stage and in service.

Equipment: Any of the Olympus NDT EPOCH series flaw detectors can be used for this test, along with a small diameter contact transducer such as a V112-RM (10 MHz, 0.25"/6.25 mm diameter) that fits in the space between the flared skirts.

Procedure: The phase or polarity of an echo reflected from the boundary between two materials depends on the relative acoustic impedances (density x sound velocity) of the materials. With Olympus NDT flaw detectors, when the first material has an acoustic impedance higher than the second, the echo is negative polarity. When the first material has an acoustic impedance lower than the second, the echo is positive polarity. Silicone has an impedance lower than fiberglass but higher than air. Thus a bonded silicone/FRP joint returns a positive echo, and a disbonded silicone/air boundary returns a negative echo. In this test, the flaw detector is set to RF display mode. The operator identifies the echo from the silicone/FRP boundary, sets it in the center of the screen, and sets gain to approximately 80% screen height. The display patterns should then be as follows:

Bonded joint - positive peak	Disbonded joint - negative peak

If the thickness and/or concentricity of the silicone coating are of interest, that can be measured at the same time by calibrating the instrument to the velocity of silicone (typically about 1.48 mm/uS or .058 in/uS) and placing a measurement gate on the echo.