Thermography

Thermography (manufacturing-quality/ndt/thermography)

Use when the task is planning or interpreting an infrared thermography inspection on an aerospace part: choosing the excitation mode, computing the thermal response of a subsurface defect, and sizing the acquisition parameters that make the inspection valid.

Domain quick reference

• Active versus passive thermography: active methods apply a controlled thermal stimulus (flash lamp, hot air, induction) and record the material response; passive methods image temperature differences that already exist, for example heat from a running engine or an in-service thermal gradient, without applying any stimulus.
• Pulsed (flash) thermography: a short, high-power light pulse heats the surface; subsurface features that trap heat (disbond, delamination, void) cool more slowly than sound material and appear as hot spots in the image sequence. Suited to thin structures, large areas, and contact-free scanning of composite and metallic skins.
• Lock-in thermography: the heating source is modulated at a fixed frequency and the camera records the periodic surface temperature; amplitude and phase images are extracted by correlation (lock-in) analysis. Phase images tolerate emissivity variations and nonuniform heating better than amplitude images, and lowering the modulation frequency probes deeper.
• Semi-infinite solid response: for a constant surface heat flux q on a semi-infinite solid, the surface temperature rise is delta_T = (2 * q / k) * sqrt(alpha * t / pi), where k is the thermal conductivity, alpha = k / (rho * c) is the thermal diffusivity, and t is the time. The rise scales with sqrt(t): fast right after the pulse, slower later.
• Thermal contrast: contrast(t) = T_defect(t) - T_sound(t), the temperature difference between the defect region and the sound region; the normalized contrast is (T_defect - T_sound) / T_sound. An indication is visible when the contrast exceeds the camera and analysis noise floor, commonly at a signal-to-noise ratio of 2 or more.
• Time of maximum contrast: the temperature difference between a flat subsurface defect at depth z and the sound region peaks near t_max ~ z^2 / (2 * alpha). The diffusion time z^2 / alpha bounds the observation window: record from roughly half the diffusion time to a few times it, before lateral heat spreading washes the contrast out.
• What thermography finds: disbonds and delaminations (air gaps with low thermal conductivity trap heat), voids, and corroded or thinned regions. Detection depends on the defect being close enough to the surface to build contrast inside the observation window; deeper defects need longer observation times and lower lock-in frequencies.
• Inspection parameters: heating pulse energy (raise the surface a few kelvin without damage; more energy density improves contrast but risks overheating the part), acquisition rate (the frame rate must resolve the fastest expected contrast peak, typically 50 Hz to 100 Hz for thin high-diffusivity skins), and the observation time window (set from the defect depth and the thermal diffusivity, with z^2 / alpha as the reference time).
• Emissivity and surface treatment: the camera measures radiance, not temperature; a uniform, known emissivity (matt paint or tape) is required or the contrast is corrupted. Dull coatings are preferred; bare shiny metal is a poor emitter.
• Comparison with other methods: thermography is fast, large-area, non-contact, and safe (no radiation), and suits composites; it is limited to near-surface defects (roughly the first centimeters), gives lower resolution than ultrasonic, and depends on surface emissivity. Ultrasonic penetrates deeper with better depth and size resolution but needs couplant and scan time; radiography finds volumetric flaws but needs radiation safety controls and two-sided access; eddy current is fast and quantitative for surface and near-surface cracks in conductors but only on electrically conductive materials. The approved NDT procedure selects the method by defect class, material, and access, not by preference.
• Standards framing: AS9100 clause 8.5.1.3 treats NDT as a special process under controlled procedures, qualified personnel, and records; thermography work follows the same control discipline as every other NDT method.

Workflow

1. Define the inspection goal: defect type (disbond, delamination, void, corrosion), material and thickness, expected depth, and the acceptance criteria from the approved NDT procedure.
2. Choose active or passive excitation, then pulsed or lock-in mode: pulsed for fast large-area screening of thin skins, lock-in for depth discrimination and phase analysis on composites.
3. Compute the surface temperature rise with surface_temperature_rise() for the planned heating pulse energy and check that it stays inside the part damage limit; size the pulse with heating_pulse_energy_density() when a target rise is given.
4. Estimate the time of maximum contrast with time_of_max_contrast() and the diffusion time with characteristic_diffusion_time() to set the observation window and the acquisition rate.
5. During the inspection, extract defect and sound region temperatures, compute the contrast with thermal_contrast() and normalized_thermal_contrast(), and compare with the noise floor using detectability_verdict().
6. Report the contrast, the peak time, and the disposition, and record the inspection under the special-process control the procedure requires.

Pitfalls

• Routing method-selection questions here: choosing among NDT methods by defect class and material belongs to ndt-method-selection; this leaf assumes thermography was already selected and plans the inspection.
• Routing ultrasonic questions here: time of flight, transducer, and near-field calculations belong to ultrasonic-inspection.
• Routing radiography questions here: exposure, film, and density calculations belong to radiographic-inspection.
• Ignoring emissivity: radiance contrast is not temperature contrast unless the emissivity is uniform and known; bare shiny metal and untreated carbon surfaces need surface treatment.
• Observing too short or too late: a defect at depth z peaks near z^2 / (2 * alpha); stopping the recording before the peak or after lateral spreading hides the indication.
• Undersampling the peak: an acquisition rate too low for the material smears the contrast peak; thin high-diffusivity skins need high frame rates.
• Using the semi-infinite formula for thin parts: the sqrt(t) solution assumes a semi-infinite solid; once the thermal wave reaches the back wall the response departs from it.
• Overheating the part: pulse energy that raises the surface past the material limit damages the part; verify the rise with surface_temperature_rise() before firing.
• Comparing contrast at different times: contrast evolves with time; compare frames at the same time after the pulse, near the expected peak.
• Forgetting the reference region: a hot spot is only an indication when the sound region baseline is established; nonuniform heating and varying thickness create false hot spots.

Behavior contract (gate 3)

The thermography math is exercised by the gate 3 contract test: scripts/test_thermography.py against scripts/thermography_logic.py (stdlib unittest, offline). Run: python3 scripts/test_thermography.py

Compliance

• Standards referenced, not reproduced: AS9100 clause 8.5.1.3 frames NDT as a special process requiring controlled procedures, qualified personnel, and records; the formulas and practice above are common infrared thermography methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.