Inspection of materials and parts often requires sacrificing test pieces – destroying them to probe for defects. If the production and manufacturing processes are consistent and repeatable, then the results for the test pieces apply to a batch of material or parts. Defects are detected either in test pieces or during a failure in operation. For something like an airplane turbine blade, those undetected flaws can translate into a catastrophic failure.
Various non-destructive testing (NDT) methods have been developed to inspect parts without destroying or altering them. NDT technologies such as acoustic emission, dye penetrant, eddy current testing, ultrasonic testing and x-ray testing rely on the physical response of a part to acoustic, optical, electrical, or nuclear stimulus to detect defects. Each of these methods has its advantages and disadvantages. For example, dye penetrant is a relatively simple method appropriate for various materials, but it only easily detects surface defects on non-porous materials.
Infrared thermography is a technology that is fast becoming a new tool in NDT. Infrared thermography first emerged in the 1980s, but with the introduction of new high-speed, high-resolution infrared cameras, it now serves as a valuable tool for detecting defects like surface cracks, faulty welds, and internal structural defects.
This thermography overview is summarized from Development and Application of Infrared Thermography Non-Destructive Testing Techniques by Zhi Qu, Peng Jiang, and Weixu Zhang.
How Infrared Thermography Works
The theoretical principle behind infrared thermography is simple. All materials respond to temperature differences and this temperature gradient can be determined if the intensity of radiation is known. Infrared cameras are designed to detect emitted infrared intensity. If the material’s surface emissivity coefficient is known, infrared cameras also automatically calculate the material’s temperature.
Applying these principles to a simple case—a block of material with an infinitely deep cross-section and a heat sink across its bottom surface and exposing the top surface to a heat source—a linear temperature gradient results. If there is a crack or a defect in the block, there will be a discontinuity in the heat flux and the temperature difference above the crack will be higher. Infrared cameras can detect this difference.
This is theoretically straightforward for a block with a crack perpendicular to the direction of heat flux. In the real world, however, part geometry, interference, material nonuniformity, attenuation and signal processing complicate things. To address complications, various methods of infrared thermography have been developed.
Types of Infrared Thermography
Most of the methods discussed below use some variation of Figure 1. The methods differ in how heat is applied and how the signal is processed and interpreted.
Infrared Pulsed Thermography
Infrared pulsed thermography is the most mature technique with more data to support it than other methods. A pulse of heat from a heat lamp, laser, infrared lamp, or hot air gun thermally excites the part and an infrared camera captures surface temperatures. Using one-dimensional conduction as a theoretical basis, defects like cracks, debonding, corrosion, and fatigue appear as temperature anomalies. This method can be used for metals, non-metals and composites.
This is the simplest of thermography techniques. It’s best suited for flat, uniform parts with minimal geometric complexity. It cannot detect deep defects, so parts must be thin, and it is best for locating defects parallel to the surface i.e. perpendicular to the heat flow.
Infrared Lock-in Thermography
Infrared lock-in thermography relies on modulated heat source phase shifts to detect flaws. Above a defect, a periodic phase difference occurs in the surface’s temperature distribution. The depth of detection is determined by the frequency. Heat sources include infrared or halogen lamps, a laser, ultrasonics, or electromagnetics. Theoretically, excitation modulation should be a sinusoidal wave, but in practice, square pulses work better due to boundary conditions.
This technique is an attractive alternative to infrared pulsed thermography because it does not heat up the part. However, the signal is susceptible to noise and requires trying multiple modulation frequencies to optimize the signal. This trial-and-error approach results in lengthy, intensive inspection times.
Infrared Ultrasonic Thermography
This is an acoustic-optical technology that induces high-frequency vibration ultrasonically and the part must be under hydrostatic load for best results. The vibration causes frictional heating at a defect’s interface which is detected by an infrared camera.
Since the defect itself is generating heat, infrared ultrasonic thermography is not as susceptible to background noise and it has a good depth penetration. However, the pressure requirement can cause damage and may not be suitable for some parts.
Infrared Laser Scanning Thermography
Infrared laser scanning aims to detect cracks perpendicular to the scanned surface using a point or line source for excitation and thermal resistance to detect a flaw. The thermal resistance changes at the flaw’s interface.
This technology is still in its infancy, but it is promising for detecting fatigue cracks. One drawback is that it takes a long time for heat to propagate through a part, so it is most suitable for small parts. Laser excitation may also cause surface damage.
Grating Infrared Thermal Wave Scanning
Grating infrared thermal wave scanning is best for detecting surface defects. The input excitation comes from infrared light projected from a holographic grating. The projection scans across the surface at a constant velocity and the reflected thermal wave is analyzed. When the scanning light hits a crack, the reflection changes.
Grating infrared thermal wave scanning is suitable for detecting surface defects such as fatigue cracks and surface corrosion.
Applications of Infrared Thermography
Infrared thermography is not a recent technology. Infrared cameras are used for everything including HVAC inspection, equipment monitoring, pipeline leak detection, weather forecasting, and medical diagnosis. Now, these principles are being applied to NDT methods:
- Manufacturing inspection: Infrared thermography can detect defective welds. Similarly, it is effective at detecting insufficient bonds, delamination in composites, damage in honeycomb core materials, and problems in carbon or glass fiber fills.
- Structural inspection: Infrared thermography is a preferred method for non-contact concrete structural inspection to detect internal damage and fatigue cracks in welds. Researchers have also been able to examine fire-damaged concrete structures.
- Turbine blade inspection: Turbine blades are exposed to high temperatures, large centrifugal forces, and hot corrosive gases. Minor defects can cause catastrophic failures. Recent advances in infrared thermography have improved turbine blade inspection to detect cracks and internal blockages.
- Stress detection: Infrared lock-in technology is effective at locating areas of high-stress concentration in structural components by spotting temperature anomalies.
Infrared thermography has been successfully implemented in industries such as oil and gas, construction, meteorology, equipment monitoring, and healthcare. As infrared imaging cameras have increased both their resolution and imaging speeds, they have enabled the expansion of infrared thermography into the field of nondestructive testing. Infrared thermography is even a preferred inspection technique for inspecting composites, cement structures, and turbine blades. As processing and interpretation techniques improve, infrared thermography NDT is sure to expand its reach as well.