An NDT technology that is evolving rapidly
By Joe Escobar
Photos courtesy of Thermal Wave Imaging, 2001
In the day to day routine of aircraft inspection, non-destructive testing (NDT) plays a vital role. One of the newer technologies available in NDT is thermal imaging. It has evolved in just a few years into an effective inspection tool that can perform highly repeatable, quantitative measurements of defects that are comparable to more traditional inspection techniques like X-ray and ultrasound. In this article, we will take a look at this fairly new technology and its uses in defect detection. Thermal image of a Boeing 757 spoiler using Thermal Wave Imaging’s MOSAIQ program.
In its simplest form, thermal imaging works by applying heat to the area being tested and then acquiring a thermal image of the sample surface as the heat dissipates into the part. Flaws in the material such as cracks, water intrusion, material dis-bonds, or corrosion will result in a different rate of heat dissipation compared to normal structure. But, thermal imaging is not just a matter of investing in a high-intensity light and an infrared camera. With that basic equipment and some practice, one may get lucky and eventually learn how to detect gross surface defects. However, critical sub-surface flaws will be impossible to detect by this simple method because the difference in heat dissipation is so minute and so quick. Steven M. Shepard, Ph.D., President of Ferndale, Michigan-based Thermal Wave Imaging, explains, "If you just look at an aircraft or an aircraft part with an infrared camera, you get an interesting picture, but that picture doesn’t necessarily tell you much about what’s inside. If you go to the next step, which is using a heat source to heat it up, you begin to get an inkling of what’s inside, but again it’s difficult to control, difficult to interpret and nearly impossible to replicate. The next step is to be more precise about how you heat the part and how you analyze the data. And at that point, you start to get pretty serious about learning what’s inside."
Taking it to the next level
The next level of accuracy in thermal imaging is pulsed thermography. With a pulsed thermography unit, the process starts off by heating the surface of the part with a brief, light pulse from a high intensity flash lamp. The heat from the surface then flows into the sample. Any flaws in the structure block this heat flow and act similar to an insulator. As the area around the defect cools, the surface temperature above the defect temporarily appears hotter.
In a pulsed thermography setup, the flash lamps and infrared camera are located within a shroud on an imaging head. The shroud serves to focus the heat flash towards the part being inspected and allows for a focal point for the IR camera to monitor the rapid cooling of the surface of the part.
The heating and subsequent cooling of the part can occur in a fraction of a second. By just viewing an infrared camera as this happens, one will probably miss defect indications. That is where the computer and software system play such a vital role in defect detection. In Thermal Wave Imaging’s system, the data that is collected by the IR camera is sent to a computer. The computer then processes the data as a sequence of images. With the ability to analyze the images, the software has significantly increased the systems ability to detect defects with repeatability and accuracy.
Bob Ducar, a Level III thermography inspector for NORDAM Nacelles and Thrust Reversers Division in Tulsa, Oklahoma explained the relative ease of using the system. "For someone just getting into this, learning how to set up the equipment and running it takes about two weeks max to learn. The software is extremely easy to use. Basically, you turn on the computer, bring up the program while the camera is running up, calibrate the camera, and you are ready to go. Where it takes time is the data analysis and understanding the internal structure of the parts that you’re looking at. Once you have that down, then it’s easy."
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