Eddycurrent Testing Level 2 Constantia Kloof

What is Eddy Current Testing?

Eddy Current Testing (ET or ECT) is a non-destructive testing (NDT) method that uses electromagnetic induction to detect and characterize surface and near-surface flaws (e.g., cracks, corrosion, pitting) in conductive materials (metals like aluminium, copper, titanium, stainless steel). It also measures thickness, conductivity, coating thickness, and material properties without contact or surface preparation.

How It Works:

1. Probe Activation: Alternating current (AC, typically 50 Hz–6 MHz) flows through a test coil in a probe, generating an alternating magnetic field.
2. Eddy Current Induction: When the probe is placed near the conductive material, the magnetic field induces circular eddy currents in the material (per Faraday’s law).
3. Disruption Detection: Flaws or property changes alter eddy current paths, causing changes in coil impedance (amplitude and phase shift).
4. Signal Analysis: The instrument displays signals on an impedance plane (X-Y plot: resistance vs. reactance) or time-based waveform. Calibration with reference standards sets baselines.
5. Skin Depth (penetration): δ = 1 / √(π f μ σ) (where f = frequency, μ = permeability, σ = conductivity) → Higher frequency = shallower penetration (surface focus); lower frequency = deeper (up to ~10 mm in low-conductivity materials).

Applications:

• Surface flaw detection (cracks in aircraft, welds, rails).
• Tubing inspection (heat exchangers, boilers).
• Corrosion mapping (under paint/coatings).
• Thickness/coating measurement (non-conductive layers on metals).
• Material sorting (conductivity, heat treatment, alloy ID).
• Industries: Aerospace, oil & gas, power generation, automotive, manufacturing.

Advantages:

• Fast and portable (real-time results, no couplant).
• Non-contact (works through coatings up to ~5 mm).
• Versatile (flaws + measurements).
• No radiation or chemicals (safe, clean).
• High-speed scanning (arrays cover large areas quickly).

Limitations:

• Conductive materials only (not plastics/composites).
• Limited penetration (skin effect: ~1–10 mm max).
• Affected by permeability (challenges with ferromagnetic steels unless RFT used).
• Skilled interpretation required (signals from geometry/lift-off can mask flaws).
• Surface roughness or edges may interfere.