thermography-testing

name	thermography-testing
description	Infrared thermography NDT — active thermography (flash, lock-in, step-heating, vibrothermography), passive thermography, Planck's law, emissivity correction, thermal diffusivity and defect detection depth (1D heat diffusion), thermal contrast, lock-in phase analysis, pulsed thermography blind frequency, subsurface defect sizing (resolution limits), FLIR/InSb/MCT detectors, calibration, and ASTM E2582/E1897 standards for aerospace and structural inspection.
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Infrared Thermography NDT — Complete Skill

Fundamentals of Thermal Radiation

Planck's Law and Emissivity

Planck's law (blackbody spectral radiance): B_λ(T) = (2hc²/λ⁵) × 1/(exp(hc/(λkT)) − 1) [W/(m²·sr·m); h = 6.626×10⁻³⁴ J·s; c = 3×10⁸ m/s; k = 1.381×10⁻²³ J/K]

Wien's displacement law (peak wavelength): λ_max × T = 2,898 μm·K Room temperature (T = 300 K): λ_max = 9.66 μm → long-wave infrared (LWIR, 8–14 μm) Engine surface (T = 600 K): λ_max = 4.83 μm → mid-wave infrared (MWIR, 3–5 μm)

Stefan-Boltzmann (total blackbody power): M = σ_SB × T⁴ [W/m²; σ_SB = 5.67×10⁻⁸ W/(m²·K⁴)] At 300 K: M = 459 W/m²; at 600 K: M = 7,348 W/m²

Real surface radiance: M_real = ε × σ_SB × T⁴ [ε = emissivity; 0 < ε ≤ 1; ε = 1 for blackbody]

Emissivity values:

Material	Surface	ε
Aluminum (polished)	Metallic	0.04–0.06
Aluminum (oxidized)	Oxidized	0.30–0.40
Steel (unoxidized)	Metallic	0.10–0.20
Steel (oxidized)	Rusted	0.70–0.90
Carbon fiber composite	CFRP	0.85–0.95
Epoxy paint	Painted	0.90–0.95
Concrete	Building	0.85–0.95
Human skin	Biological	0.98–0.99

Temperature error from wrong emissivity: ΔT_error ≈ (T_true / 4) × (Δε / ε) [linearized Stefan-Boltzmann] Error at T = 300 K; ε_true = 0.9; ε_used = 0.8 (10% off): ΔT_error ≈ 75 × 0.111 = 8.3 K

Reflected radiation correction: T_measured = ε^0.25 × T_surface + (1−ε)^0.25 × T_background [approximate; camera applies correction when ε and T_background entered]

IR Camera Technology

Detector Types

Cooled detectors (quantum detectors): InSb (Indium Antimonide): 3–5 μm MWIR; cooled to 77 K (LN₂ or Stirling); NETD = 10–20 mK; highest sensitivity MCT (Mercury Cadmium Telluride, HgCdTe): 2–14 μm tunable; cooled 77 K; high detectivity; used in scientific/military InGaAs: 0.9–1.7 μm SWIR; thermoelectric cooled; high speed; lower cost

Uncooled detectors (bolometers): Microbolometer (VOx or amorphous Si): 7.5–14 μm LWIR; room temperature operation; NETD = 50–80 mK Lower cost; no cryogenics; suitable for most industrial inspection FLIR Lepton, Teledyne FLIR A series, FLIR T series

Key specifications: NETD (Noise Equivalent Temperature Difference): minimum detectable ΔT; cooled: < 25 mK; uncooled: 50–80 mK Array size: 640×480 to 1,280×1,024 pixels; 14-bit digital output typical Frame rate: 25–200 Hz (standard); up to 10,000 Hz (specialized high-speed)

Heat Diffusion Fundamentals

1D Transient Heat Conduction

Thermal diffusivity: α = k / (ρ × cₚ) [m²/s; k = conductivity; ρ = density; cₚ = specific heat]

Material	k [W/mK]	ρcₚ [J/m³K]	α [m²/s]
Steel	50	3.8×10⁶	1.32×10⁻⁵
Aluminum	237	2.43×10⁶	9.77×10⁻⁵
CFRP (through-thickness)	0.7	1.7×10⁶	4.1×10⁻⁷
Glass-epoxy (GFRP)	0.35	2.0×10⁶	1.75×10⁻⁷
Concrete	1.6	2.1×10⁶	7.6×10⁻⁷

Thermal penetration depth (characteristic depth): δ = √(α × t) [m; t = time after heat application] At t = 1 s: CFRP δ = √(4.1×10⁻⁷ × 1) = 0.64 mm; Steel δ = 3.6 mm

Defect at depth d detectable at time: t_detect ≈ d² / α [time for thermal front to reach defect and return signal to surface] CFRP defect at 2 mm: t ≈ (0.002)² / 4.1×10⁻⁷ = 9.8 s

Active Thermography Methods

Pulsed Thermography (Flash Thermography)

Excitation: xenon flash lamps or halogen lamps; pulse duration 1–20 ms Energy deposit: Q_flash = tens to hundreds of kJ/m²

Surface temperature after flash (1D semi-infinite): T(z=0, t) = Q_flash / (e_eff × √(π × t)) [e_eff = √(k × ρ × cₚ) = thermal effusivity; Q = energy per area] Thermal effusivity e = √(kρcₚ) [J/(m²·K·s^0.5)]

Contrast from subsurface defect: A defect at depth d alters heat diffusion → surface thermal contrast appears at t_contrast ≈ d²/α Contrast C(t) = (T_defect(t) − T_sound(t)) / T_sound(t) [dimensionless]

Blind frequency (lock-in): f_blind = α / (π × d²) [Hz; defect at depth d detected at this frequency for lock-in] CFRP; defect at 2 mm: f_blind = 4.1×10⁻⁷ / (π × 0.004) = 3.3×10⁻⁵ Hz → period T = 8.5 h → use step-heating instead

Lateral resolution: determined by PSF of thermal diffusion Δx_min ≈ 2 × d [defects smaller than twice their depth may be missed laterally] CFRP delamination at 2 mm: Δx_min ≈ 4 mm → minimum detectable defect area ≈ π(2mm)² = 12 mm²

Depth quantification (log-log method, ASTM E2582): Plot ln(T_surface) vs. ln(t): slope = −0.5 for sound material; deviation identifies defect depth d = √(α × t_knee) [t_knee = time of departure from −0.5 slope; thermal effusivity analysis]

Lock-In Thermography

Excitation: sinusoidal heat input at frequency f; illumination sources modulated Detect at frequency f using lock-in amplifier (phase-sensitive detection)

Phase image: phase shift φ(d) = −d × √(π × f / α) [depth-dependent phase shift at modulation frequency f] Phase image independent of non-uniform illumination and emissivity → more robust than amplitude

Optimum frequency for depth d: f_opt = α / (π × d²) [same as blind frequency — detect at this f for maximum phase contrast] CFRP; d = 1 mm: f_opt = 4.1×10⁻⁷ / (π × 10⁻⁶) = 0.13 Hz → period 7.7 s

Phase resolution: σ_φ ≈ NETD / (ΔT_surface × √N) [N = number of lock-in cycles; higher N → better SNR] Typically: N = 5–10 cycles; total measurement time = N/f_opt

Frequency sweep: scan f from 0.01 to 1 Hz → generate depth-profiling phase images at multiple depths

Step Heating (Long Pulse) Thermography

Constant power applied for duration t_s then removed: Surface T rises as √t during heating; deeper defects appear later Used when flash energy insufficient (thick composites, concrete) t_s = 1–60 s typical; observe cooling curve after step

Vibrothermography (Ultrasound Excited)

Excitation: high-power ultrasound (20–40 kHz, 100–1,000 W) → friction/rubbing at crack faces → localized heating Detection: IR camera captures temperature rise at cracks Particularly sensitive to tight fatigue cracks (even with no gap — friction generates heat) Limitations: coupling-dependent; complex thermal signal; not quantitative for depth

CFRP Inspection (Primary Application)

Defects detectable: Impact damage (barely visible impact damage, BVID): delaminations at 1–5 mm depth Disbonds: skin-stiffener separation; honeycomb sandwich face-sheet separations Porosity: only very high porosity (> 3%) detectable; AUT better for volumetric porosity Moisture: wet areas appear cooler due to higher thermal effusivity

ASTM E2582: standard practice for infrared flash thermography of composite panels Specifies: flash energy; standoff; image acquisition; depth calibration using flat-bottom holes Acceptance criteria: disbond area > 25 mm diameter (reference defect)

Quantitative Analysis

Defect Sizing

Defect area: lateral extent from thermal contrast map at t = 1.5 × t_contrast Threshold: C > 20% of peak contrast (ASTM E2582 approach) Accuracy: ±1–3 mm at defect edge for d < 3 mm in CFRP

Defect depth: from t_knee or lock-in phase: d = √(α × t_knee) Accuracy: ±0.3–1 mm for d < 5 mm in CFRP

Missing bond (disbond): thermal resistance analysis: R_th,defect = L_air / k_air [thermal resistance of air gap; L_air = gap thickness] Contrast proportional to R_th,defect × Q × √(α/πt) (first-order estimate)

Passive Thermography

No external excitation: observe natural temperature distribution Applications: electrical hot spots (circuit breakers, bus bars); building energy audit; process monitoring; predictive maintenance of rotating machinery

Building envelope inspection: Interior survey (night, ΔT_inside-outside > 10°C): identifies insulation gaps (hot spots), air leaks Exterior survey (morning): identifies thermal bridges (cold spots) ASTM E1897: standard for passive building inspection

Electrical inspection: Hot spots in panels, motors, transformers: temperature rise above ambient > 40°C = critical NFPA 70B: electrical inspection standard (references IR thermography for predictive maintenance)

Calibration and Standards

Camera calibration: Blackbody reference: certified NIST-traceable blackbody source (ε = 0.99) at known T Accuracy: ±1–2°C for calibrated system; ±0.1°C for high-precision research Non-uniformity correction (NUC): one-point or two-point calibration applied to each pixel

Emissivity measurement: Apply matte black paint (ε = 0.95) to compare → determine surface ε by back-calculation Heated surface comparison with thermocouple reference

Standards

Standard	Scope
ASTM E2582	Flash thermography for composites
ASTM E1897	Passive building thermography
ASTM E1316	Terminology for NDT (includes thermography)
ASTM C1046	Heat flux measurements (calibrated thermographic systems)
ISO 6781	Building thermal insulation — passive thermography
ASNT SNT-TC-1A	Qualification of thermography NDT personnel
EN 13187	Building passive thermography

Output

Provide: inspection objective (material: CFRP/steel/concrete; defect type: delamination/disbond/crack/hot-spot; depth range d [mm]; minimum detectable defect size [mm]), thermography method selection (pulsed/lock-in/step-heating/vibrothermography; basis: depth, material diffusivity, defect type), camera selection (cooled MCT for high sensitivity; uncooled microbolometer for field; MWIR for high-T; LWIR for room-T; NETD requirement ≤ [value] mK), material thermal properties (α [m²/s]; e = √(kρcₚ); t_contrast = d²/α [s] for target depth), excitation parameters (flash energy Q [kJ/m²]; flash duration [ms]; or lock-in frequency f [Hz] = α/(πd²); step heat duration [s]; standoff distance [m]), phase or contrast analysis (lock-in: phase image φ = −d√(πf/α) [rad]; pulsed: t_knee method for depth; acceptance threshold C > 20%), resolution limits (lateral Δx ≈ 2d [mm]; minimum detectable area [mm²]; depth accuracy ±[mm]), image interpretation (color map scale; defect map overlay; sizing at 150% t_contrast), calibration (blackbody T and ε; emissivity of target surface; reflected temperature; NUC correction), and applicable standard (ASTM E2582 for CFRP; ASTM E1897 for buildings; ASNT Level II/III qualification).