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April 06, 2017
The Truth Behind Composite Inspection Methods: Ultrasonic Testing with Guided Waves

Posted by: Casey Whalen

In our first article we answered the original question, “Are your composites inspectable?”

We explained that not only are the pipe composite repair materials manufactured by Milliken Infrastructure inspectable, but that most, if not all, composite repair products used in today’s oil and gas pipe industry are capable of being inspected by a number of common inspection methods.

In this article we’ll go over two of today’s common composite non-destructive testing (NDT) inspection methods, how they work and what type of damages they are capable of inspecting.

To understand guided wave inspection, it’s important to understand the basics of ultrasonic testing (UT). Ultrasonic testing is a process based on the propagation and reflection of sound waves as the means to detect flaws in the repair system. While guided waves can find defects in both the pipe and composite, its strength is detecting disbondment.

With UT, a transducer probe is used to emit a high frequency sound wave into the material being tested. The sound waves emit an echo after reaching the back wall of the material (in this case the pipe).When the waves encounter a boundary between two mediums, the damage in the form of a void or delamination, the damage will appear on the scan as an minor echo when compared to the larger echo of the waves reflecting off the back wall of the pipe. The location and depth of the damage can be determined by knowing the reflection time and the speed of the sound through the object. The kind of transducer probe used separates the types of guided wave methods.

Phased array and EMAT are two ways to produce the specialized waveforms used in guided wave technology, a method of UT that screen for defects faster than conventional UT method. But this is where the two methods begin to differ.

Phased array testing uses several transducers positioned in a ring around the outside of the pipe. The transducers can all be individually directed to scan various areas and angles of the pipe at the same time. Using piezoelectric transducers, which require a fluid coupling on the surface, the phased array method is capable of detecting defects within the pipe, voids between the pipe and the composite repair and voids or delamination within the composite itself. The ring of transducers runs along the outside of the pipe and transmits the results of its findings digitally, which can be viewed in either a 2-D top down view or in what is known as a C-scan, a 2-D side on view also known as a B -scan, or a processed 3-D image.

EMAT, which uses a two coil design (one for transmitting and one for receiving, as opposed to the piezoelectric method of phased array) sends out electromagnetic waves that only interact with materials that are cable of giving off their own electromagnetic waves. This means that EMAT will only work with conductive materials. However, unlike phased array, EMAT does not need to come in direct contact with the area being inspected and can be used outside the pipe, or as in inline testing method to avoid having to dig up any assets to test them. EMAT combines the electromagnetic waves produced by the transmitter and the material to create a sound wave that travels the length of the pipe and identifies defects within the pipe or delamination between the pipe and the composite. Results are digitally transmitted and offered in a 2-D or 3-D image.

Though these two methods operate under a similar UT principle, they come with their own unique advantages and disadvantages. Phased array and EMAT represent the tip of the iceberg when it comes to composite testing methods, more of which will be covered in coming articles as we continue to dispel the misunderstanding that not every composite is inspectable.

You can read the first article in this series 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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