PipeWrap

April 20, 2017
The Truth Behind Composite Inspection Methods: Pulsed Eddy Current and Radiographic Testing

Posted by: Casey Whalen

In this article, our third in the series about composite inspection techniques, we review two methods that are commonly used for pipe inspection. Pulsed eddy current and radiography are nondestructive testing (NDT) methods that are popular in use and provide the types of reliable data needed to find or monitor damage in a composite, between the composite and substrate or in the substrate itself.

It should be noted that the implied need for a “special composite technology” is required to enable the inspection of a composite is simply not true.  This series of articles details the specifics of composite inspection methods. Previously, we discussed the use and effectiveness of two ultrasonic testing methods, phased array and electromagnetic acoustic transmitter (EMAT) testing. While these two methods were linked by way of their basic operating principals — guided wave — pulsed eddy current and radiography have much less in common with each other but their commonality in the industry make them worth grouping together.

Pulsed eddy current, a form of electromagnetic NDT, is a method based on Faraday’s Law of Magnetic Induction. This principle states that when an electrical conductor is in the presence of a moving magnetic field, it will generate a current. Conversely, when a current is run through a conductor, it will generate a magnetic field. Pulsed eddy current uses a transmitter to produce alternating currents that induce looping currents, or eddy currents, in nearby conductive materials — in this case the substrate.

This method detects defects that interfere with the flow of the eddy current. As the current flows around the defect, the current weakens and decreases the amount of current flowing through the coil of the transmitter. While a handheld pulsed eddy current transmitter can be oriented to find cracks and other defects in the substrate, this method is primarily used to measure the thickness of the substrate walls and detect wall loss. The data presented of the tested area by pulsed eddy current can be shown as a colored grid (though this may vary between inspection companies). Areas with greater wall loss will show up as a color corresponding to the severity of loss. This method can be used where a composites exists unless it is made of carbon fiber —its conductivity produces too much electromagnetic interference. 

Performed with a single transmitter, the pulsed eddy current method requires only the portion of pipe that is being tested to be accessible. While pulsed eddy current is not used for defect detection within the composite itself, this method is commonly used in addition to other NDTs.

Radiography, or X-ray, is a method that is more expensive, time-consuming and involves greater safety risks, but provides more specific results and data than pulsed eddy current. Radiography methods require the use of a transmitter and receiver, placed across from one another on either side of the pipe being inspected. This method usually requires the pipe to be completely exposed, which can mean longer preparation time and an expensive labor cost if the pipe needs to be dug up to access it.

The transmitter sends radiation through the pipe in the form of electromagnetic rays such as X-rays or gamma rays; gamma rays have higher energy and a shorter wavelength than X-rays. Operators are dealing with radiation, so safety is high priority with this method. Waves that have not been interrupted by the atoms within the pipe or composite material make their way to the receiver and form an image that gives an actual photographical representation of the insides of the pipe and composite, unlike the grid visualization of the pulsed eddy current method. While it is possible to inspect the composite and the bondline as well with radiography, it can be more difficult than substrate inspection. Radiography is very sensitive to the inspection angle and if a disbond or delamination is not aligned parallel to the radiation rays then it is not likely to show up. This means that you will need to rotate the machine around the entire pipe in order to find defects in the composite or bondline.  

The implication that only some composites are inspectable via radiography is a myth. With the proper inspection angle, disbonds and delaminations within any composite repair system can be detected. Consider the following thought experiment to help visualize the proper detection angle:

Imagine a tall block of composite material. There is an air pocket randomly between one of the layers to create a delamination. The delamination will be orientated such that it will be longer and wider than it is tall. Now imagine looking at a side on cross section of the composite block. Looking along the direction of blocks height you’ll see that there is an equal amount of material through the delamination as there is through the rest of the block. But if you look in either the width or length directions, you’ll see that there is actually less composite material when going through the air bubble because it displaces some of the material out of the way. The radiation rays need to travel parallel to the delamination or disbond in order to be detected.

While no NDT method is perfect, each one does provide its own unique advantages and data when inspecting composites or piping structures through composites. However, the point of this series is to provide usable information for determining how to inspect a composite and the relevant information needed for improved asset management.

Be sure to read our introduction to this series HERE and our look at ultrasonic NDTs, EMAT and phased array, HERE. For more answers to your composite inspection-related questions, contact Milliken Infrastructure’s Jim Souza or Myles Johnson.

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Category: Feature Articles, Pipe Wrap

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