May 11, 2017
The Truth Behind Composite Inspection Methods: Microwave and Thermography

Posted by: Jaclyn Deter

In the last two articles of this series we’ve covered a number of different composite inspection methods, looking at the basic principles, operations, benefits and drawbacks. However, throughout the series we kept the same emphasis in mind — no matter the composite, there is always a type of inspection method that will appropriately and positively be able to identify defects and damages. No one type or brand of composite repair is uniquely qualified as “inspectable.”

This article, which covers our final two inspection methods for this series, will look at the basic components of microwave and thermography composite inspection. We’ve grouped these non-destructive testing (NDT) methods together for two reasons. First, both these inspection methods are non-contact methods, meaning no physical connection between the pipe and the inspecting instrument is needed.

Second, both methods have a similar breadth of defect or damage detection. While the majority of NDTs we’ve covered in previous articles are capable of detecting defects within the substrate itself, even through an applied composite, these two methods are primarily used to detect and analyze delaminations or disbondment within the composite repair or between the composite repair and the substrate, and are less capable of inspecting the substrate itself.

Microwave testing methods, which came about in the 1990’s making it fairly new, was originally developed as a way to inspect dielectric materials. This makes most composite repairs, other than carbon fiber composites, prime candidates for this type of inspection. With the transmitter and receiver built in the same device, this method is reasonably simple to operate compared to others. Backscattering microwaves sent from the transmitter pass through a composite with a dielectric constant. The waves are reflected back when meeting a void or discontinuation in the material. These voids are identified as areas of delamination, disbonding or gouged composite and cause the microwaves to be reflected back to the inspection instrument at a different rate. In a 2-D color or grayscale representation (depending on the inspection equipment), inconsistencies in the reflected rays represent the defects within the composite.

Thermography inspection, the more difficult of these two methods based on the amount of equipment involved, analyzes the change in thermal activity within the composite in order to detect defects. This method requires an infrared camera as well as equipment to apply energy to the inspection area. As this energy encounters a void or discontinuity, it is dissipated as heat at a different rate than the rest of the material. The infrared camera is then able to detect these hot zones by recording the surface temperature of the composite and the defects. While this method does allow for quick inspection of larger areas, it does have a significant drawback. Because the applied energy diminishes as it progresses deeper in to the composite, thermography inspection works best for defects closer to the surface. The thicker the composite, the more difficult it will be to identify deeper defects.

With these two inspection methods, we’ve rounded out the top six most common forms of composite inspection used in the field today. Over the course of this series we’ve tried to dispel the myth that only certain composites are inspectable, and hopefully after reading about these vastly different methods, asset owners approach the topic of composite inspection with a different mindset. Instead of asking if a particular composite is inspectable, they ask, “What inspection method is best for my particular composite repair?”

You can begin this series from the beginning HERE, or read about ultrasonic inspection in Part 2 and pulsed eddy current and radiographic inspection in Part 3. For more answers to your composite inspection related questions, contact Milliken Infrastructure’s Jim Souza or Myles Johnson

A special thanks to Donald McNicol at Evisive for providing his technical expertise on Microwave inspection.

Category: Feature Articles, Pipe Wrap


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