Eddy Current Testing
Eddy Current Testing: Theoretical Foundations
Principles of Eddy Current Testing (ECT)
Professor, how does eddy current testing work?
An alternating current is passed through a probe coil, inducing eddy currents in the test object. If a defect (crack, corrosion) is present, the flow of eddy currents is disturbed, causing a change in the probe's impedance.
The location and size of the defect are estimated from the impedance change $\Delta Z$.
So the inspection depth changes with frequency, right?
Correct. The standard penetration depth $\delta = \sqrt{2/(\omega\mu\sigma)}$ serves as a guideline for inspection depth. Low frequencies inspect deeper regions, high frequencies inspect the surface. In multi-frequency ECT, multiple frequencies are used simultaneously to improve depth resolution.
Summary
- Impedance change $\Delta Z$ — Principle of defect detection
- Inspection depth $\approx \delta$ — Controlled by frequency
- Non-contact & high-speed — Applicable to inline inspection on production lines
Crack Detection in Jet Engine Blades—How Eddy Current Testing Finds "Invisible Flaws"
Have you ever seen a scene where a maintenance technician runs a small probe over the surface of an aircraft engine blade during inspection? That's the eddy current testing (ECT) field in action. When an alternating magnetic field is applied, eddy currents flow on the metal surface beneath the coil. However, if a crack or corrosion is present, the eddy current flow is disturbed, changing the coil's impedance. By picking up this minute impedance change, invisible cracks smaller than 0.1mm can be detected non-contact and non-destructively. Unlike X-rays or UT (ultrasonic testing), ECT has the strengths of being "specialized for conductor surface/subsurface flaws" and "probes are small and light, easily mounted on robots," making it an indispensable inspection method in the aerospace, nuclear, and automotive industries.
Computational Methods for Eddy Current Testing
ECT Simulation Using FEM
What is the significance of simulating ECT using FEM?
It is used for probe design optimization, defect signal prediction, and optimization of inspection conditions (frequency, lift-off). 3D eddy current analysis calculates the interaction between the probe and the defect.
The probe is scanned, and the impedance change $\Delta Z$ at each position is plotted (Lissajous figure).
Isn't the computational cost high for 3D?
Combine the A-V method with edge elements and use fine meshes only near the defect. Also, reduce computational scale using symmetry or Fourier decomposition. COMSOL's Parametric Sweep can automatically scan probe positions.
Summary
- 3D Eddy Current Analysis — Probe-defect interaction
- Impedance Change Calculation — Defect determination via Lissajous figure
- Parametric Sweep — Automatic sweep of probe position
Impedance Plane Diagram—The Technique of Deciphering the "Fingerprint" of Eddy Current Testing
Eddy current testing signals are often displayed as an "impedance plane diagram (Lissajous figure)." Plotting the resistance component on the X-axis and the reactance component on the Y-axis, cracks, conductivity changes, and lift-off (probe lift) each draw vectors in different directions. Experienced inspectors read this "direction and length of the trajectory" to determine the depth and type of the flaw. Recently, research using machine learning for automatic determination has become active, with cases where convolutional neural networks show judgment accuracy equal to or better than veteran inspectors. Signal data from CAE simulations for "flaw-free cases" and "cases with various defects" are used as training data.