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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). UT is a process based on the propagation and reflection of sound waves as the means to detect flaws in the repair system. There are variations within UT methods, but Phased Array and EMAT are the most applicable to composite repairs. Phased array can find defects in the composite and the bondline, while EMAT can find defects in both the pipe and the bondline.

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, a void or delamination, the damage will appear on the scan as a 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 type of transducer probe and waveforms used separates the different UT methods.

Phased array and electromagnetic acoustic transducer (EMAT) are two methods to utilize UT inspection, but in very different ways. Phased array is a conventional UT method, which means that it scans a small section of the pipe near the transducer. EMAT is a guided wave UT method, which can scan the entire pipe over longer distances.

Phased array testing uses many transducers that can all be individually directed to scan various areas and angles of the pipe simultaneously. Using piezoelectric transducers, which require a fluid coupling on the surface, the phased array method can detect defects within the pipe, voids between the pipe and the composite repair and voids or delamination within the composite. The equipment transmits the results of its findings digitally, which can be viewed in either a 2-D top down view (C-scan), a 2-D side on view (B-scan), or a processed 3-D image. The type of image produced can vary between inspection companies.

EMAT is one of several guided wave techniques. Guided waves are different from conventional UT because guided waves utilize the structure itself to create waveforms that can inspect at longer distances. There are 3 types of guided wave transducer types: EMAT, piezoelectric, and magnetorestrictive. EMAT has the advantage of being non-contact, and available in inline formats as well to avoid having to dig up any assets to test. EMAT, which uses a two-coil design (one for transmitting and one for receiving), sends out electromagnetic waves that only interact with materials that are capable of giving off their own electromagnetic waves. Therefore, EMAT will only work with conductive materials like steel pipe. EMAT can detect defects in the bondline through measuring the attenuation at the pipe wall. 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 top down (C-scan) 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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