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Application Notes

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Bond Testing in Composite Power Line Insulators

power lines

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.

Example of sectioned composite insulator
Example of disbonding

sectioned composite insulator

example of disbonding

Equipment: Any of the Olympus 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 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
bondede 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.

Products used for this application

Contact Transducers

A contact transducer is a single element longitudinal wave transducer intended for use in direct contact with a test piece.

EPOCH 1000 Series

The EPOCH 1000 is an advanced conventional ultrasonic flaw detector that can be upgraded with phased array imaging at an authorized Olympus service center. Key features include: EN12668-1 compliant, 37 digital receiver filter selections, and 6 kHz pulse repetition rate for high speed scanning.


The EPOCH 600 is mid-level, handheld ultrasonic flaw detector. Weighing only 1.68 kg (3.72 lb.), its horizontal case is built to withstand the rigors of very harsh environments. EN12668-1 plus features such as 400 V PerfectSquare tunable square wave pulser, digital filtering for enhanced signal-to-noise ratio.


The EPOCH 650 is a conventional ultrasonic flaw detector with excellent inspection performance and usability for a wide variety of applications. This intuitive, rugged instrument is a continuation of the popular EPOCH 600 flaw detector with additional capabilities.


The EPOCH LTC is a mid-level, handheld ultrasonic flaw detector in a compact 2.12 lbs (0.96 kg) vertical case. It is a full-featured instrument with EN12668-1 compliance and a wide variety of standard features as well as specialized options to meet your inspection needs.


Performs eddy current array testing inspections. The test configuration supports 32 sensor coils (up to 64 with an external multiplexer) working in bridge or transmit-receive mode. The operating frequency ranges from 20 Hz to 6 MHz with the option of using multiple frequencies in the same acquisition.

Ultrasonic Transducers

Ultrasonic transducers are used for applications as diverse as flaw detection, thickness gaging, materials research and medical diagnostics. More than 5000 ultrasonic transducer types come in many styles, element diameters, frequencies, and connector styles.
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