eddy-current-testing

name	eddy-current-testing
description	Eddy current testing (ECT) — electromagnetic induction fundamentals, Lenz's law, skin depth and frequency selection, absolute vs differential probe configurations, impedance plane analysis, lift-off effect, phase angle analysis, coil design (surface, encircling, internal), sensitivity to crack orientation, material conductivity and permeability effects, multifrequency ECT, pulsed ECT, ASNT/ASTM E309 standards, flaw sizing and calibration.
metadata	{"priority":7,"promptSignals":{"phrases":["eddy current","ECT","eddy current testing","impedance plane","skin depth"],"minScore":3}}

Eddy Current Testing — Complete Skill

Electromagnetic Fundamentals

Faraday's Law and Eddy Current Generation

Faraday's Law: EMF = −dΦ/dt [induced EMF proportional to rate of change of magnetic flux; Φ = B × A]

Eddy current generation: AC coil → time-varying magnetic field B → induces eddy currents in nearby conductor (Lenz's law: currents oppose change in flux) Eddy currents create secondary magnetic field opposing primary → total impedance of coil changes Discontinuities, conductivity changes, geometry changes → alter eddy current flow → detectable impedance change

Coil impedance: Z = R + jωL [R = resistance; ω = 2πf; L = inductance] In air (no specimen): Z₀ = R₀ + jωL₀ Near specimen: Z = R_effective + jωL_effective [changed by coupling to specimen] Display on impedance plane: X-axis = resistance; Y-axis = inductive reactance (ωL)

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Skin Depth and Frequency Selection

Skin Effect

Skin depth (standard penetration depth): δ = √(ρ/(π × f × μ)) [m] Where: ρ = electrical resistivity [Ω·m]; f = test frequency [Hz]; μ = absolute permeability = μ₀μᵣ [H/m] μ₀ = 4π × 10⁻⁷ H/m; μᵣ = 1 for non-magnetic materials (Al, Ti, Cu, austenitic SS)

Alternative form: δ = 503/√(f × σ × μᵣ) [mm; σ in MS/m]

Rule of thumb: signal amplitude decays to 37% (1/e) at depth δ; effectively negligible at 3δ

Frequency selection: For surface crack detection: f_high → δ small → concentrate energy near surface → high sensitivity to surface flaws For subsurface/through-wall: f_low → δ large → deeper penetration Test depth ≈ 1.5–2δ practical limit for flaw detection with SNR

Example: Aluminum 2024-T3: σ = 17.8 MS/m; μᵣ = 1 δ at 1 kHz: 503/√(1000 × 17.8 × 1) = 503/133.4 = 3.77 mm [good for subsurface Al] δ at 100 kHz: 503/√(100,000 × 17.8 × 1) = 503/1334 = 0.377 mm [surface crack inspection]

Carbon steel: σ ≈ 10 MS/m; μᵣ ≈ 100–500 (ferromagnetic) → δ ≈ 0.5 mm at 1 kHz Ferromagnetic materials: permeability dominates; skin depth very shallow; magnetic noise masks flaw signals Solution: magnetize to saturation (removes permeability variation) before ECT, or use remote-field ECT (RFECT)

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Probe Configurations

Surface Probes

Absolute probe: single coil; measures total impedance; sensitive to gradual property changes (conductivity, thickness); drifts with temperature/lift-off Differential probe: two coils in opposition; cancels gradual changes; sensitive to rapid changes (pits, cracks); insensitive to gradual taper

Pencil probe: small diameter (1–6 mm); high spatial resolution; use for fine cracks, corrosion pitting; limited scanning speed Pancake probe: larger diameter; average over larger area; use for conductivity mapping, large area scanning Reflection probe: driver-pickup configuration; driver coil generates field; pickup coil measures induced signal; separates drive from receive

Encircling (Feed-Through) Probes

For tubing and bars: Coil encircles specimen; test speed high (0.1–1 m/s); 360° coverage for OD flaws Fill factor η = (d_specimen/d_coil)² → η closer to 1 = better coupling; η typically 0.7–0.95

Internal (Bobbin) Probes: For heat exchanger tube inspection; probe travels inside tube Through-transmission or reflection configurations; detect OD, ID, and wall thickness anomalies Rotating probe: adds rotational scan for axial flaws (circumferential cracks missed by bobbin alone)

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Impedance Plane Analysis

Phase Angle and Signal Interpretation

Impedance plane diagram: Operating point on plane determined by: material conductivity, permeability, geometry, lift-off, frequency Moving along "lift-off line" (probe raised from surface): signal moves left along near-horizontal path Crack signal: deviation from lift-off line; phase angle between crack signal and lift-off line used for flaw characterization

Phase angle for depth estimation: Phase lag with depth: β = z/δ [z = depth below surface; δ = skin depth] Phase shift ≈ 57° per skin depth (1 radian per δ) Deeper flaw → greater phase lag → use phase angle to estimate depth (calibrated on reference notches)

Signal amplitude: Amplitude decreases with depth (exponential decay) Amplitude also depends on flaw volume and orientation perpendicular to eddy current flow

Conductivity effect: Higher σ → operating point moves up-right on impedance plane (higher inductive reactance) Conductivity measurement: calibrate with known standards (ASTM E1004)

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Lift-Off Effect

Probe-to-Surface Distance

Lift-off: spacing between probe and surface; creates large impedance change that can mask flaw signals Lift-off effect more severe at lower frequencies Methods to reduce lift-off sensitivity:

• Use high frequency (shorter skin depth; less coupling change per unit lift-off)
• Use differential probe (common-mode rejection of lift-off)
• Maintain consistent probe-to-surface contact (spring-loaded probe holders)
• Electronic lift-off compensation (phase rotation to align lift-off along horizontal)

Lift-off compensation: Rotate impedance plane display so lift-off signal is horizontal → flaw signals appear as vertical excursions → separate with vertical gate

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Multifrequency and Pulsed ECT

Multifrequency ECT (MF-ECT)

Application: heat exchanger tube inspection Simultaneously apply 2–4 frequencies → characterize flaw type and depth Mix frequencies to suppress specific signals (support plate signals, deposits) while retaining flaw response Frequency mixing: algebraic combination of channels to cancel geometric signals

Typical frequency sets for carbon steel tubes: Primary: 100 Hz (main flaw detection; ~1.5 mm penetration) Secondary: 300 Hz (OD differentiation) Mix: subtract weighted channels to suppress support plate signal

Pulsed ECT (PEC)

PEC principle: pulse excitation → broadband frequency content → single measurement gives depth information from time-domain response Time-gating: early time = surface/near-surface; late time = deep Advantage: works through non-conductive coatings (paint, insulation up to 100 mm with PEC-in-insulation probes)

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Calibration and Standards

Reference Standards

Calibration block: same material, heat treatment, surface condition as test article Reference notches: EDM slots at known depths (e.g., 10%, 20%, 40%, 60%, 80% of wall thickness) Flat-bottom holes: for subsurface detection calibration Circular holes: for conductivity calibration

ASTM E309: ECT standard practice for tubular products (ferromagnetic) ASTM E376: ECT for thickness measurement ASTM E690: ECT for nonferromagnetic heat exchanger tubing ASTM E1004: conductivity measurement by ECT ASTM E2884: eddy current array inspection

Sensitivity and Flaw Sizing

Signal-to-noise ratio (SNR): Minimum SNR for reliable detection: ≥ 3:1 (amplitude); ≥ 6 dB Calibration: set gain so 20% through-wall notch gives screen height of 80% FSH (full-screen height); report any indication ≥ 20% FSH

Flaw length sizing: 6 dB drop method: length = distance between half-amplitude points (-6 dB from peak) ASME Code Case 2863: ECT for heat exchanger tubes; plugging criterion = indication ≥ 20% depth

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Limitations and Applicable Standards

ECT limitations:

• Ferromagnetic materials: permeability variations mask flaw signals → requires magnetic saturation or DC bias
• Orientation sensitivity: cracks parallel to eddy current flow are poorly detected (≤ 30° to current → signal ↓ significantly)
• Depth limit: ≈ 2–3 skin depths practical maximum
• Requires trained operator for impedance plane interpretation

Standard	Scope
ASNT SNT-TC-1A	ECT operator certification levels (I, II, III)
ASTM E309	ECT for steel tubular products
ASTM E690	ECT for nonferromagnetic heat exchanger tubes
ASTM E1004	Conductivity by ECT
ASTM E2884	Eddy current array testing
ASME V Article 8	ECT for ASME code applications
ISO 15548	ECT equipment characterization
EN 1711	ECT for welds

Output

Provide: test configuration (probe type: surface/encircling/bobbin; absolute vs differential; coil diameter [mm]; frequency [Hz]; fill factor η if tubular), material properties (conductivity σ [MS/m]; μᵣ; skin depth δ = 503/√(fσμᵣ) [mm]; effective inspection depth = 1.5δ [mm]), frequency selection rationale (f selected for target depth; δ at f; confirm target depth ≤ 1.5δ), calibration (reference standard: material, notch depths [%WT]; gain setting: reference indication at [%FSH]; SNR ≥ 3:1 confirmed), impedance plane setup (lift-off compensation: phase rotated; flaw gate: amplitude threshold [%FSH]; phase gate if applicable), scan parameters (scan increment [mm]; test speed [m/s] for tubular; total scan coverage confirmed), indications (location; amplitude [%FSH]; phase angle [°]; estimated depth from phase calibration [mm or %WT]; length by 6dB drop [mm]; disposition: accept/reject/further evaluate), and applicable standard (ASTM E309 for ferromagnetic tubes; ASTM E690 for non-ferromagnetic HX tubes; ASME V Article 8 for code applications; ASTM E1004 for conductivity).