Seeing Infrared
The use of infrared (IR) thermography for nondestructive evaluation (NDE), characterization and quality assurance of ceramic components and coatings has grown significantly in the past few years. Thermography allows fast, wide-area inspection and does not require immersion or physical contact with the sample. It can be used on flat or curved surfaces, and it does not require extensive reprogramming or setup to inspect new parts or geometries.
The basic principle behind thermography is quite simple. An IR camera records the surface temperature of a sample as it is heated with a light pulse lasting only a few milliseconds. The sample cools as heat diffuses from the surface to the interior. However, internal features or flaws affect the diffusion process. For example, a subsurface void obstructs the flow of heat and causes a transient temperature increase to occur in the vicinity of the flaw. Additionally, conditions such as porosity affect the thermal properties of the sample and the rate at which heat diffuses. Although these changes can be subtle, dedicated software can analyze the data from the IR camera and isolate likely defects. The software can also measure part or coating thickness, flaw depth and size, and thermal diffusivity.
Modern Systems
Early thermography systems were only able to qualitatively show defects such as delamination, which would appear as "hot spots" in the IR image. However, modern systems provide precise, quantitative results and are capable of detecting and measuring features that would be invisible to an operator viewing the unprocessed image sequence from the IR camera (see Figure 1).Many of the advances in thermography can be attributed in part to the evolution of the underlying technologies, including the IR camera, PC and communications protocols, and hardware that allows real-time transfer of data between camera and computer. These enabling advances have allowed researchers to acquire and study high-precision data, and they've facilitated the development of advanced models for the heat transfer process in complex material systems and geometries.
Signal processing has played a major role in this evolution. In early thermography systems, data from the IR camera was treated similar to a movie, which the operator viewed in slow motion to visually identify anomalous features that might indicate subsurface defects.
In modern systems, each camera pixel is treated as an independent entity that represents the behavior of a point on the sample. In one patented technique,* the time-dependent temperature response of each pixel to thermal excitation is expressed as an equation that represents the degree to which that point on the sample conforms to "ideal" behavior. In effect, the entire image sequence is reduced to a set of equations in which time varying noise from the camera or the immediate environment is eliminated.
The sequence, which might comprise hundreds-or even thousands-of discrete frames, can be reduced to a set of noise-free equations, enabling rapid mathematical manipulation of the sequence and advanced analysis capabilities. Put simply, the pixel signal acquired by the IR camera is processed mathematically to enhance weak signals that would normally be undetectable.
*Thermographic Signal Reconstruction® (TSR), developed by Thermal Wave Imaging, Inc., Ferndale, Mich.
Return to Flight
The Return to Flight effort for the NASA space shuttle in the aftermath of the Shuttle Columbia accident offers an excellent example of the dramatic advances that have taken place in thermography. The leading edge of the shuttle orbiter wing, where the failure occurred, is a reinforced carbon-carbon (RCC) structure that poses a formidable challenge to most inspection technologies. It is a thick, porous, multilayer structure with an irregular shape that varies over the length of the wing. Prior to the accident, it was inspected visually and by "touch testing."After the accident, NASA undertook a comprehensive survey of NDE technologies for the inspection of the RCC leading edge. As a result, an infrared NDE system* was selected as the primary inspection system. The system offers fast, non-contact, wide-area inspection of flat or curved structures, and it can measure the length, depth and area of subsurface defects. Additionally, automation features allow the system to communicate with other programs and devices, and the systems are optimized for ease of use and for handling the large volume of data generated by each inspection.
*EcoTherm®, available from Thermal Wave Imaging, Inc., Ferndale, Mich.
Power Generation
Thermography has become particularly useful as an inspection tool for both airborne and land-based turbine components, such as blades and vanes, where thermal barrier coatings (TBCs) are widely used to allow higher operating temperatures. In manufacturing, thermography is used to confirm TBC adhesion, identify contamination and measure thickness.Figure 2 illustrates the difference between an optical image (left) and a thickness map generated through infrared NDE (right) for a TBC combustor liner. Measurement accuracy of modern systems is on the order of 1 mil. Once the blade is in service, thermography is used during periodic inspections to identify thin TBC areas or delaminated regions where spalling is likely to occur (see Figure 3).
Looking Deeper
Large, near-surface flaws might be detectable using a simple system (e.g., a heat gun and IR camera) without computer enhancement. However, the repeatable, quantifiable detection of deeper, more subtle features requires the additional sensitivity that an infrared NDE provides.For more information about thermographic NDE, contact Thermal Wave Imaging, Inc., 845 Livernois St., Ferndale, MI 48220; (248) 414-3730; fax (248) 414-3764; e-mail sales@thermalwave.com ; http://www.thermalwave.com .