12.2 Infrared/Thermal & Other Methods

Infrared and Thermal Testing (IRT)

Infrared and thermal testing (IRT), also called thermography, maps the surface temperature distribution of a part by measuring the infrared radiation it emits. A subsurface condition that changes how heat conducts, convects, or radiates produces a thermal contrast at the surface, which the camera renders as a hot or cold region. IRT is a surface-reading, area-coverage method: it sees the outer skin over a wide field at once, so it is efficient for scanning large components with little or no contact.

The physics in one paragraph

Radiant emittance follows the Stefan-Boltzmann law: emitted power per unit area equals emissivity (epsilon) times the Stefan-Boltzmann constant times absolute temperature to the fourth power. Because of the fourth-power relationship, doubling absolute temperature raises radiated power roughly sixteen-fold, so small hot spots stand out strongly. The camera actually measures apparent temperature, which must be corrected for emissivity and reflected apparent temperature (per ASTM E1933). A polished, low-emissivity metal reflects its surroundings and images falsely cool, one of the most common thermographic errors. Detectors are typically uncooled microbolometers in the long-wave band (about 7 to 14 micrometers) or cooled photon detectors such as InSb or HgCdTe in the mid-wave band (about 3 to 5 micrometers) for higher sensitivity.

Passive versus active thermography

Passive thermography relies on a naturally occurring thermal gradient already present in the part or process. No external heat is added; the inspector simply reads temperatures produced by service. Typical targets are overheated electrical connections, motor and bearing wear, missing or wet insulation, refractory breakdown, steam-trap function, and building/roof moisture. Passive work is fast and non-contact but only reveals conditions that generate a temperature difference during operation.

Active thermography deliberately injects heat with a flash lamp, heat lamp, or other energy source, then watches the transient cooling of the surface. A subsurface defect such as a delamination or disbond interrupts heat flow, so the region over the defect cools at a different rate and appears as contrast. Common variants:

• Pulsed thermography applies a short heat pulse and records the cooling transient; fast but sensitive to surface emissivity and uneven heating.
• Lock-in thermography applies periodic (modulated) heating and extracts phase, giving good depth discrimination and noise rejection.
• Vibrothermography (sonic IR) injects mechanical vibration so friction at a tight crack self-heats, revealing cracks that are hard to see thermally otherwise.

Active thermography is a leading choice for composite delamination, honeycomb disbond, coating and bond integrity, and impact damage in aerospace laminates, because it images a broad area quickly with essentially no contact, complementing UT for large panels. Depth of detection is limited by thermal diffusivity and the heat pulse: the deeper the defect, the weaker and later the surface contrast, so IRT is best for near-surface subsurface conditions rather than deep flaws. Practical limits also include uneven heating, surface emissivity variation, and reflections from surroundings, all of which can mimic or mask a real defect if not controlled.

Other and Emerging Methods

Several specialized methods fill gaps left by the mainstream six. A Level III should recognize each at the capability level.

Neutron radiography (NR)

Neutron radiography is a penetrating-radiation method like RT, but neutrons attenuate very differently from X-rays: they are strongly attenuated by low atomic number, hydrogen-rich materials (water, oil, plastics, adhesives, explosives) and pass readily through many metals. This makes NR the complement to X-ray RT, imaging organic material or hydrogenous residue trapped inside metal assemblies, such as adhesive bond lines, O-rings, and pyrotechnics in a metal housing. Its major limitation is the need for a neutron source (reactor or accelerator), which restricts it to specialized facilities.

Microwave and terahertz testing

Microwave (and terahertz) methods use electromagnetic energy that penetrates dielectric, non-conductive materials. They are used on composites, ceramics, foams, and coatings to find delaminations, moisture, and thickness variation. They do not penetrate metal, so they are niche complements rather than general-purpose tools.

Laser shearography

Laser shearography is an optical, interferometric method that measures surface strain response to a small applied load (vacuum, thermal, or pressure). A subsurface disbond or delamination deflects the surface differently under load, producing a fringe pattern. It is non-contact, whole-field, and fast, widely used on composite aircraft structure, honeycomb, and tires.

Magnetic flux leakage (cross-reference)

Magnetic flux leakage (MFL) is covered with the electromagnetic methods in Chapter 9. In selection terms, MFL magnetizes a ferromagnetic part and detects the flux that leaks at wall loss or pitting, making it the standard fast-screening tool for aboveground storage-tank floors, pipeline in-line inspection, and wire rope. It is included in the ASNT Basic method list alongside VT, PT, MT, ET, UT, RT, LT, AE, and IRT.

How these methods fit the Basic outline

For the exam, you do not need to operate these methods; you need to recognize what each one uniquely answers and where the mainstream six fall short. IRT covers broad-area, near-surface conditions with no contact; neutron radiography covers hidden low-Z organics inside metal; microwave/terahertz covers dielectrics; laser shearography covers composite disbond under a small load; and MFL covers fast ferromagnetic wall-loss screening. A recurring exam pattern presents a target the standard methods handle poorly, then rewards the answer that names the specialized method matched to that material and defect. Keep the capability summary, not procedural detail, at your fingertips.