Eddy current testing (ECT) is a nondestructive inspection method that uses electromagnetic induction to detect cracks, corrosion, and other defects in metal parts without cutting into or damaging them. It works on any electrically conductive material, requires minimal surface preparation, and can scan at speeds up to 72 inches per second, making it one of the fastest inspection techniques available in industry.
How Eddy Current Testing Works
The basic setup is straightforward: an alternating current flows through a small coil (the probe), which generates a fluctuating magnetic field. When this probe is brought near a conductive material, that magnetic field penetrates the surface and induces circular electrical currents in the metal. These are the “eddy currents,” named for the way they swirl like eddies in water.
Those eddy currents immediately generate their own secondary magnetic field, which pushes back against the original field from the probe. The probe’s electronics monitor this interaction continuously. When the eddy currents flow through uniform, defect-free material, the signal stays stable. But when the currents hit a crack, a void, a change in material thickness, or a region with different conductivity, they get disrupted. That disruption changes the secondary magnetic field, and the probe picks up the change as a shift in electrical impedance.
Skilled operators read these impedance changes on what’s called an impedance plane display, where the horizontal axis represents one component of impedance and the vertical axis represents another. Different types of flaws produce distinct signal patterns on this display. A surface crack, for example, forces eddy currents to take a longer path around it, which shifts the signal in a characteristic direction. Changes in material conductivity or the gap between probe and surface (called “liftoff”) each produce their own recognizable signal trajectories. The angle of the signal relative to a reference line even correlates with how deep a flaw sits below the surface, since phase lag increases linearly with depth.
What It Can Detect
ECT excels at finding surface and near-surface defects in conductive metals. Its primary targets include surface-breaking cracks, subsurface cracks, pitting, gouges, voids, and corrosion-related wall thinning. Recent probe designs can reliably detect cracks as narrow as 0.1 mm in width and subsurface flaws as shallow as 0.5 mm deep in aluminum.
Beyond flaw detection, ECT can measure material conductivity, coating thickness, and even assess heat treatment conditions, since changes in alloy hardness or residual stress alter how eddy currents behave. This versatility makes it useful not just for finding damage but for verifying that parts meet material specifications.
Compared to ultrasonic inspection, ECT has a clear edge for certain defect types. In heat exchanger tubing inspections, ECT detects pitting at a 90% rate versus 67% for ultrasonic methods, and catches cracks at 93%, while ultrasonic techniques often fail to detect cracks entirely in non-ferromagnetic materials like stainless steel, brass, and titanium.
Materials and Limitations
The one hard requirement is that the material must conduct electricity. Plastics, ceramics, and composites won’t produce eddy currents and can’t be inspected this way. Metals like aluminum, copper, titanium, stainless steel, and carbon steel are all candidates, though each behaves differently.
Three properties of the test material shape the inspection: electrical conductivity, geometry, and magnetic permeability. Highly conductive metals like copper and aluminum generate stronger eddy currents, which means cracks produce larger, more distinct signals and are easier to separate from background noise. Ferromagnetic materials (those that can be magnetized, like carbon steel) are trickier because their high magnetic permeability concentrates eddy currents very close to the surface and can mask flaw signals.
Penetration depth is perhaps the most significant practical limitation. Eddy currents don’t reach deep into a material. They concentrate near the surface and decay exponentially with depth. The characteristic penetration depth depends on the test frequency, the material’s conductivity, and its magnetic permeability. Lower frequencies penetrate deeper but sacrifice sensitivity to small flaws. In practice, defect detection beyond about one to three times the calculated skin depth becomes unreliable, and for many common alloys at typical inspection frequencies, that translates to a few millimeters at most. Deep internal flaws require a different method, like ultrasonic or radiographic testing.
Another challenge is that multiple variables affect the signal simultaneously. A change in liftoff, temperature, conductivity, or geometry can all shift the impedance reading, and untangling which variable is responsible requires training and careful technique.
Types of Probes
Probes come in several configurations, each suited to different inspection tasks.
- Absolute probes use a single coil that both generates eddy currents and senses changes. They measure the total impedance at any point, making them versatile for conductivity measurements, thickness checks, liftoff readings, and general flaw detection. The tradeoff is that they respond to every variable at once, including temperature drift, so many commercial versions include a reference coil to compensate for ambient conditions.
- Differential probes use two active coils wound in opposition. When both coils sit over identical material, the signals cancel out and the reading is zero. When one coil passes over a defect and the other doesn’t, a signal appears. This design is highly sensitive to localized cracks while ignoring gradual changes in temperature or dimensions. The downside: if a flaw is longer than the spacing between the two coils, only its leading and trailing edges register, because both coils sense the defect equally and cancel again.
- Reflection (driver/pickup) probes separate the functions entirely. One coil drives the eddy currents while a second, often much smaller, coil detects changes. This lets each coil be optimized independently. The driver coil can create a strong, uniform field while the tiny pickup coil resolves very small defects.
For inspecting the inside of tubes, such as in heat exchangers and condensers, a common choice is the bobbin probe, which is pulled or pushed through the tube bore. Bobbin probes work well for detecting significant holes and long axial flaws, but they struggle with smaller discontinuities and can’t pinpoint exactly where around the tube’s circumference a defect sits. Array probes, which use multiple coils arranged around the circumference, fill that gap by providing more detailed imaging.
Where ECT Is Used
Aerospace is one of the largest users of eddy current testing. Aircraft structures endure repeated stress cycles over decades, and fatigue cracks around fastener holes, in wing skins, and on turbine components must be caught early. ECT’s speed, portability, and sensitivity to surface cracks make it a natural fit for routine aircraft maintenance inspections.
Power generation and petrochemical plants rely on ECT heavily for heat exchanger tube inspections. A single facility may have thousands of tubes that need periodic checking for corrosion and cracking. At inspection speeds that can cover nearly 1,000 tubes per shift, with minimal cleaning required beforehand, ECT is far more practical than ultrasonic methods that demand thoroughly cleaned surfaces and run roughly 30 times slower.
Manufacturing quality control is another major application. ECT can verify that metal parts meet conductivity specifications coming off a production line, check coating thickness on plated components, and screen for cracks in welds or machined surfaces. Because the probe doesn’t need to contact the surface directly, it works well on parts with coatings or thin paint layers.
Training and Certification
Interpreting eddy current signals is not intuitive. The impedance plane display requires understanding how different variables produce different signal patterns, and distinguishing a crack from a liftoff variation or a conductivity change takes practice. International standards, particularly ISO 15549, define the general principles for performing eddy current examinations in a repeatable and reliable way. Personnel performing ECT must be certified under ISO 9712 or an equivalent standard, which establishes multiple qualification levels based on the technician’s ability to set up equipment, conduct inspections, and evaluate results.