Thermal Imaging: An NDT technology that is evolving rapidly

Thermal Imaging

An NDT technology that is evolving rapidly

By Joe Escobar
Photos courtesy of Thermal Wave Imaging, 2001

November 2001

imageIn 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. imageThermal image of a Boeing 757 spoiler using Thermal Wave Imaging’s MOSAIQ program.

Basic theory
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."

Seeing the big picture
Some of the aircraft structures that can be inspected by thermal imaging can be fairly large. But the area that can effectively be inspected on each image shot is only around one square foot. In the past, a person using a pulsed thermography system would have to take the images one at a time until the whole surface was inspected. For every shot taken, the inspector had to analyze that shot, make a decision, then move on to the next one. If he wanted to get a big picture of the entire component, he would have to put together a patchwork quilt of the images using all those single shots.
Thermal Wave Imaging has recently improved it’s pulsed thermography system with the addition of their MOSAIQ software. With MOSAIQ, the inspector gets the whole picture of the structure, with the benefit of a greatly enhanced view of subsurface features. Basically, just a little input is required by the inspector.
Dr. Shepard explains, "All the technician has to do is tell the computer what inspection path he is going to take. Once he gives it the pattern and tells it how many shots he anticipates taking, the software knows exactly what to do. It uses advanced signal processing techniques we have developed to compile all the images into one complete image and remove the blurring normally associated with thermography, so that a precise image of the subsurface structure of the entire part is created."

Thermal imaging has several advantages. First of all, it is non-invasive. Unlike X-ray where you need access to the back side of the part, an inspection can be done without any disassembly.
It is also a clean inspection. No couplants or penetrants are needed. In addition, it is a relatively fast process where large areas can be inspected in a matter of minutes.
Another advantage is being able to get the complete picture of the structure, especially with the new MOSAIQ software. Other NDT methods rely on point-to-point inspection and that can be time consuming.

As with any type of NDT inspection method, thermal imaging has its limitations. It relies on the ability of the part being inspected to absorb heat. If the material is shiny, such as an un-painted aluminum skin, the heat will not be absorbed, but merely reflected. In this case, it is sometimes possible to apply a water-based temporary coating on top of the metal for the inspection.
Rubber-coated surfaces, like leading edges with de-ice boots, are impossible to inspect due to rubber’s insulating properties. In addition, very thick parts like landing gear are difficult, if not impossible, to inspect using thermal imaging.
Thermal imaging has seen increased use and greater acceptance in recent years. With the abilities it offers in widening our NDT capabilities, it is likely to see even more use in our industry in the future.

The Source
Additional resources....

American Society for Nondestructive Testing

Nacelles and Thrust Reversers Division
6911 N. Whirlpool Drive
Tulsa, OK 74117
(918) 878-4504

Thermal Wave Imaging Inc.
845 Livernois St.
Ferndale, MI 48220
(248) 414-3730

NDT Listing
The following is a listing of companies that provide NDT products and services.
Please contact them directly for more information.

AEI North America Inc.
Old Mill Bldg., 10 Orange St., Marcellus, NY 13708, (315) 673-0164,
Manufacturer and distributor of remote visual inspection systems that include rigid borescopes, flexible fiberscopes, videoscopes, pole/pushcams, crawlers and related accessories such as fiber-optic illumination, video monitoring and recording systems.

Cardinal Electronics Inc.
4405 Wagon Wheel, Lansing, MI, 48917, (800) 344-9469 or (517) 321-6618
Cardinal Electronics Inc.'s Digital Optical Tachometer works like a camera inside or outside the cockpit. Unit looks like a propeller and displays speed to 1 rpm accuracy and resolution once per second. It is self-contained, easy to use, works on singles and twins, works to 15ft. and adds nothing to the propeller.

Centurion NDT
707 Remington Rd., Ste. 9, Schaumburg, IL 60173, (847) 884-4949,
Centurion NDT designs, manufactures, markets, and services eddy current and ultrasonic test instrumentation. The Model ED-520 is among the most recognized and used eddy current flaw detectors in the aviation industry.

Command Electronics Co.
Box 17842, Kansas City, MO 64134, (816) 761-3514,
Manufactures and distributes digital and analog airdata testers. Digital Model 803 is only tester whose accuracy can be verified right in shop. Traceable to N.I.S.T. Also can be calibrated in-house.

Everest VIT
199 Hwy 206, Flanders, NJ 07836, (888) 332-3848 or (973) 448-0077,
Introducing the new 60 watt light source (ELSV-60) with video pass through. The high intensity light box attaches to borescopes and fiberscopes to provide bright, white light and true-to-life color for better detection of cracks or corrosion during inspections.

Foerster Instruments Inc.
140 Industry Dr., RIDC Park West, Pittsburgh, PA 15275, (800) 635-0613,
Non-destructive testing electrical equipment for crack detection, electrical conductivity measurement and determination of material properties. Foerster's equipment is portable, easy to operate, and highly sensitive.

Jet-Care International Inc.
3 Saddle Rd., Cedar Knolls, NJ, 07927, (973) 292-9597,
Jet-Care is the leading expert in trend monitoring and fluid analysis worldwide. Jet-Care holds approvals from most engine manufacturers. Please contact us about out 24/7/365 availability and our scanning electron microscopes.

Lenox Instrument Co. Inc.
265 Andrews Rd., Trevose, PA 19053, (215) 322-9990,
Manufacturers of a complete line of boroscopes both rigid and flexible. All products are video capable. Sizes range from .020-inch to over 100 feet. New as well as used products are offered.

Machida Inc.
40 Ramland Road South, Orangeburg, NY 10962, (845) 365-0600,
Offers a complete line of quality flexible borescopes designed for various industrial inspections. Machida also carries engine inspection kits, blending borescopes, video systems, light sources and borescope accessories.

Midcoast Aviation
St. Louis Downtown Airport, 14 Archview Dr., Cahokia, IL 62206, (800) 222-0422,
Using its new COMSCAN II screening unit, Midcoast checks Gulfstream II and III aircraft for corrosion at the 580 bulkhead in only one day, without having to remove portions of the aircraft skin. The required inspections are accomplished in 8 to 10 hours versus 24 hours needed by the COMSCAN I.

Newage Testing Instruments
147 James Way, Southampton, PA 18966,
Newage manufactures a complete line of hardness testers including automatic, bench and portable models in Rockwell, Brinnell, Microhardness, Durometer, and IRHD scales that are designed to work in the most challenging conditions.

Olympus Industrial America Inc.
414 Airport Executive Park, Nanuet, NY 10954, (845) 425-3100,
Offers borescopic systems and instruments that make it possible to see inside turbines and other difficult-to-access areas. Locate and measure FOD. Perform a wide range of advanced image management functions, including image storage, voice annotation, and email.

64 Technology Park Rd., Sturbridge, MA 01566, (800) 225-7486,
OPTIM’s FS-236 fiberscope provides exceptional optics for aircraft engine inspection where viewing is essential but tear down is impractical. Features bright, high-resolution image, tungsten shaft, water and fuel-tight construction.

Panametrics, Inc.
221 Crescent St., Waltham, MA 02453, (800) 225-8330,
The new EPOCH 4 digital ultrasonic flaw detector is a lighter, easier to use flaw detector used to find cracks, corrosion, and other safety defects in a variety of metals including aircraft engine parts.

PRI Research & Development Corp.
25500 Hawthorne Blvd., Ste. 2300, Torrance, CA 90505, (310) 489-1855,
The MOI is a hand-held visual inspection tool that rapidly finds surface and subsurface cracks and corrosion in aircraft skins. Procedures exist from most major manufacturers.

Quality Material Inspection Inc.
3505 Cadillac Ave., N-3, Costa Mesa, CA 92626, (714) 436-0410,
QMI offers ultrasonic, eddy-current, radiography, liquid penetrant, and magnetic particle inspection services. They specialize in high power non-contact "AIRSCAN" testing of composites and manufacture Automated Ultrasonic C-scan imaging systems and transducers.

Quality Testing Services Inc.
4003 Fee Fee Rd., Bridgeton, MO 63044, (314) 770-0607,
Quality Testing Services provides nondestructive testing services and training in all methods including penetrant, magnetic particle, eddy current, X-ray, and ultrasound, Quality Testing also provides calibration services.

Richard Wolf Medical Instruments Corp.
353 Corporate Woods Pkwy, Vernon Hills, IL, 60061-3110, 800-323-9653,
Richard Wolf manufactures a comprehensive line of rigid boroscopes specifically designed for airframe and powerplant inspections. Sizes range in diameters of 1.9mm to 10mm.

Spectronics Corp.
956 Brush Hollow Rd., Westbury, NY 11590, (800) 274-8888 or (516) 333-4840,
Spectronics Corp's ultra-powerful MAXIMA™ 3500 UV inspection lamp enables inspectors to locate surface defects, corrosion, coating voids and contamination much faster and more accurately than before even in broad daylight.

TEC/Materials Testing Division
10737 Lexington Dr., Knoxville, TN, 37932, (865) 966-5856,
TEC provides non-destructive testing (by x-ray diffraction) systems and services are provided in either lab or field. The lab A2/LA and ISO 9000 certifications.

UE Systems, Inc.
14 Hayes St., Elmsford, NY 10523, (914) 592-1220,
Portable ultrasonic instruments for leak detection and mechanical inspection. Typical applications: cabin pressure, fuel cell, pitot leaks, bearing, gear, valve inspection.