Table of Content
- Introduction
- What Is an Eddy Current Probe?
- How an Eddy Current Probe Works, Step by Step
- The Interior of the Probe: Coil, Core, Shielding and Housing
- Coil Configurations: Absolute, Differential, Reflection and Hybrid
- Probe Geometries and Where Each Is Used
- What Controls Probe Performance
- Reading the Probe Signal: The Impedance Plane
- Limitations and Common Causes of Missed Indications
- FAQ
- Conclusion
Introduction
Eddy current testing (ECT), a popular electromagnetic non-destructive testing technique, is used for detection of surface and near surface discontinuities, material property measurement and monitoring of conductive components. The eddy current probe is the core element of all conventional ECTs.
The probe specifies the way that the electromagnetic energy is injected into the test sample and the manner in which the response is detected. Its coil configuration, geometry, shielding and operating frequency all affect inspection sensitivity and coverage.
The knowledge of working principles of eddy current probes is essential for selecting a probe, interpreting the indications and planning an effective inspection procedure.
What is an Eddy Current Probe?
The sensing element of an eddy current testing system is an eddy current probe. It usually contains one or more coils that generate an alternating magnetic field, together with a means of detecting the changes produced by electromagnetic interaction with a conductive test part.
An alternating current flowing through the probe coil creates a fluctuating magnetic field. When the probe is brought near a conductive material, a set of circulating electrical currents, called eddy currents are induced. These currents and the electromagnetic response measured at the probe change whenever there is a change in the conductivity, permeability, thickness, geometry, or the presence of a discontinuity.
The probe therefore performs two linked functions: electromagnetic excitation of the material and detection of the resulting response.
Another related electromagnetic sensor is an eddy current proximity probe, which gages the distance between the probe tip and a conductive target. The gap signal is most frequently permanently installed in rotating machinery, where it is utilized to monitor shaft vibration, radial position, axial thrust, and rotational speed instead of being used for general dimensional measurement. Therefore, the goal is to monitor the state of the machinery, and the means is the displacement measurement.
This is a focused look at the probe itself — for the full eddy current testing method and process, see our complete guide.
→ Read: Eddy Current Testing: A Complete Guide
How an Eddy Current Probe Works, Step by Step
The operation of an eddy current probe can be understood as a chain of electromagnetic interactions, from excitation of the coil through to the signal displayed on the instrument.
Step 1: AC flows through the Probe Coil
The probe coil receives an alternating current from the ECT apparatus. Together with the material's conductivity and permeability, the frequency of this current serves as the main regulator of the penetration depth and the ratio of surface resolution to subsurface sensitivity. The depth to which the eddy currents penetrate is unaffected by the current's amplitude, which controls the strength of the created magnetic field and, consequently, the overall signal level.
Step 2: The Coil Produces a Primary Magnetic Field
The alternating current produces a continuously changing magnetic field around the coil. This field is transmitted into the conductive part when the probe is close to it.
Step 3: Eddy Currents Are Induced
As the magnetic field changes, the conductive material experiences electrical currents that circulate around it. They are known as eddy currents.
They depend on material conductivity, permeability, frequency, geometry and distance between the probe and surface.
Step 4: Eddy Currents Generate a Secondary Magnetic Field
The induced currents generate their own magnetic field which opposes the change that induced them. The detection is based on the interaction between the primary field and the secondary field.
Step 5: A Discontinuity Changes the Electromagnetic Response
Normal eddy current flow can be disturbed by a crack, corrosion, material variation, thickness change or geometric feature. This alters the secondary magnetic field of the probe.
Step 6: Probe Impedance Changes
The probe coil is electromagnetically coupled to the test material and changes in the response of the material change the electrical properties of the coil, especially its impedance.
Step 7: The Instrument Produces a Signal
The impedance change is passed on by the ECT instrument and displayed as an inspection signal. Amplitude and phase data can be useful to separate or distinguish relevant indications from lift-off, geometry, etc.
This complete chain: from AC-excitation, magnetic coupling, induced currents, impedance change and the output of the signal is the basic operating principle of eddy current testing probes.
The interior of the Probe: Coil, Core, Shielding and Housing
An eddy current probe design looks simple on the outside but is painstakingly constructed on the inside to meet the sensitivity and coverage specifications needed.
Coil:
The main element for excitation and sensing is a coil. The sensitivity and operating frequency is dependent on its wire size, turns, diameter and electrical properties.
Core:
In some probes, the electromagnetic field is controlled and concentrated around a core material that is either magnetic or non-magnetic. The core is designed to enhance coupling and guide the field into specific inspection areas.
Shielding:
The coil's field is concentrated and undesired contact with surrounding features is greatly reduced by using magnetic or eddy current shielding. It reduces the probe's effective field such that scanning near edges, fasteners, nearby conductive structures, and complicated geometry results in less interfering signals, although it does not completely eliminate these effects. Compared to unshielded probes of the same size, shielded probes often have a narrower coverage area and slightly lower penetration.
Housing:
The housing establishes the probe's external shape and houses its interior components. It must be able to endure the inspection's mechanical and climatic conditions. Lift-off stability during manual scanning is directly impacted by the consistency with which the coil can be kept against the surface, which is also influenced by housing material and wear-face design.
The overall design of the eddy current probe is a compromise of electromagnetic characteristics, durability, accessibility and inspection requirements.
If you're ready to compare actual probe hardware built around these components, browse our range of standard eddy current probes.
→ Read: Standard Eddy Current NDT Probes
Coil Configurations: Absolute, Differential, Reflection and Hybrid
Various coil patterns yield different responses and the coil pattern is chosen based on the inspection purpose.
Absolute Probes
An absolute probe measures the absolute electromagnetic state of the substance directly beneath it using a single active detecting coil. A second coil is frequently used as a reference in designs, but it is kept far from the test section to prevent it from coupling with the material. Its sole purpose is to account for temperature drift and instrument instability.
This arrangement reacts to both localized discontinuities and progressive changes like conductivity variation, permeability variation, thickness change, and wall thinning because the coil provides an absolute value rather than a comparison. Its disadvantage is that absolute probes are more susceptible to lift-off, temperature drift, and changes in material properties than differential probes.
Differential Probes
A Differential Eddy Current Probe includes two coils or elements of which the responses are compared. If they come across similar material conditions, their signals can largely cancel each other out. If there are any breaks between, there is some imbalance and it becomes an indication.
This configuration is ideal for mitigating the effects of slow lift-off change, temperature drift, and gradual material fluctuation as well as for detecting brief, localized flaws.
Its primary drawback is the mirror image of that strength: both elements are equally affected by long, uniform, or gradually changing conditions, such as general corrosion, uniform wall thinning, or a flaw longer than the coil spacing, which may cancel out and go undetected. In order to record both localized and gradual circumstances, absolute and differential channels are frequently obtained concurrently.
Reflection Probes
The excitation and sensing operations between various coils are separated by a reflection probe, also known as a driver-pickup or send-receive probe. One or more separate pickup coils detect the electromagnetic field produced by a driving coil.
By separating the two roles, each coil may be optimized for its own purpose: the pickup can be made small for high spatial resolution, while the driver can be designed for field strength and penetration. In comparison to a single-coil design of the same size, this gives reflection probes superior depth performance and more stable behavior across a variety of frequencies. The configuration is commonly employed in array probes for weld inspection.
Hybrid Probes
Hybrid probes consist of different combinations of coil, or sensing technologies, in one probe. To meet several requirements of the inspections, they can be constructed to enhance coverage and sensitivity.
This configuration will vary based on part geometry, flaw type, material, frequency and sensitivity.
Probe Geometries and Where Each Is Used
The types of eddy current probes vary due to the shape, size and accessibility of the components.
- Surface probes or pencil probes: These are small probes that are hand-held to examine surface and near surface areas, such as welds, edges, holes, and complex geometries.
- Bobbin probes: They are commonly used for the inspection of heat exchanger, condenser and boiler tubes.
- Encircling probes: The component is passed through a coil which is wrapped around the circumference of the component. They are widely applied for inspection of wires, rods, tubes and so on, on a continuous basis.
- Rotating probes: A sensor element is rotated about or across a part to create high resolution coverage over a localized area.
- Array probes: Multiple coils or multiple sensing elements are grouped together to scan larger areas in less time than a single coil would allow, while still providing localized detection capability.
Component shape, access, anticipated discontinuity, resolution and inspection coverage dictate probe geometry selection.
Each of these probe geometries is available as a ready-to-use product, see our surface, pencil, angled, and weld-specific probes.
→ Read: Eddy Current Surface Probes
What Controls Probe Performance?
There are a number of electromagnetic and physical properties that influence the ability of a probe to detect discontinuities.
Frequency and Depth of Penetration
One of the most important variables is frequency. Higher frequency tends to focus the eddy currents near the surface and will give greater surface resolution but will have a smaller depth of penetration.
Lower frequencies will give increased penetration, but may give poor sensitivity to very small surface discontinuities.
Standard Depth of Penetration
The relationship between frequency and depth is quantified by the standard depth of penetration (δ) — the depth at which eddy current density has fallen to approximately 37 % of its value at the surface:
δ = 1 / √(π f μ σ)
where σ is the material's electrical conductivity, μ is its magnetic permeability, and f is the excitation frequency.
Frequency selection is always material-dependent, as explained by the formula. In copper and carbon steel, where high permeability drastically lowers δ, a frequency that provides useful penetration in austenitic stainless steel would behave very differently.
In actuality, two to three times δ is typically considered to be the useful inspection depth. Frequency is often chosen during calibration on a reference standard rather than using the formula alone since beyond that point the signal becomes too weak to interpret with confidence.
Coil Diameter and Resolution
A smaller coil will have a higher spatial resolution and be able to detect smaller features of a region but will only survey a smaller area each scan. Coils with a wider diameter will offer wider coverage but may not be as good at detecting small discontinuities.
Lift-Off
The distance between the probe coil and the test surface is known as lift-off. The electromagnetic coupling between the coil and the component reduces as lift-off increases, altering the impedance of the probe and, consequently, the signal that is shown.
Even minor, inadvertent changes, such as probe wobble, surface roughness, paint, or an uneven scanning hand, might produce signals big enough to obscure real signs since coupling decreases quickly with distance. Lift-off is often controlled by keeping constant probe contact and separating the lift-off direction from the defect response using phase information on the impedance plane.
Fill Factor
The fill factor, which indicates how closely the coil and the component fit together, is applicable to bobbin and surrounding probes. It can be stated as a percentage or as a ratio of cross-sectional areas with a value between 0 and 1.
The fill factor for an internal bobbin probe is calculated by dividing the outer diameter of the coil by the inner diameter of the tube. It is the ratio of the outer diameter squared of the component to the inner diameter squared of the coil for an encircling probe.
Good sensitivity and strong electromagnetic coupling are indicated by a high fill factor. A low fill factor lowers the response to discontinuities and decreases coupling. It might be produced by an inadequate coil, an oversized bore, or excessive clearance.
Edge Effect
Eddy current flow patterns can be disturbed by edges, corners, holes, fasteners or abrupt geometry changes and indicate something other than a material defect.
Flaw Orientation
The paths of the eddy currents are set by the electromagnetic field of the probe. A crack will be easiest to identify if it severely interferes with those paths.
The current is forced to reroute around a crack that is perpendicular to the eddy current flow, which significantly alters the impedance of the probe. The flow is hardly disturbed by a crack that runs parallel to it, and it may show very little or nothing at all.
Thus, it is necessary to organize the probe orientation and scan direction around the anticipated fracture direction. The component is scanned in many directions or an array probe with multiple current directions is utilized when the orientation is unclear or the cracking may be multidirectional.
Reading the Probe Signal: The Impedance Plane
A common representation of the probe response is the impedance plane used by Eddy current instruments, which gives information on the amplitude and phase of the signal.
The amplitude represents the size of the response and the phase represents the angle of the response with respect to a reference. The signal can vary in shape according to cracks, lift-off, conductivity, permeability and geometry.
The benefit of phase information is that it can be used to distinguish a flaw response from other responses like lift-off. Signals should not, however, be interpreted in isolation.
Phase Lag and Defect Depth
In addition to weakening with depth, eddy currents gradually lose phase. Therefore, even though their amplitudes are comparable, a discontinuity at the surface, one halfway through the wall, and one at the far surface will appear at different angles on the impedance plane.
Depth estimation is based on this phase separation. Additionally, it is what distinguishes fault signals from lift-off: as depth increases, flaw responses gradually rotate away from the distinctive direction that lift-off produces a response along.
The angles don't apply to everyone. Before the inspection starts, they must be set on a reference standard with known artificial discontinuities because they rely on the material, frequency, and probe.
Normally calibration is carried out prior to inspection on an appropriate reference standard. The instrument settings are established and the operator is aware as to what relevant indications are expected to be visible due to known conditions or artificial discontinuities in the standard.
The impedance plane should thus be considered a representation of the electromagnetic response, not a pass/fail report.
Signal interpretation is a skill built through hands-on practice, our eddy current testing course covers impedance-plane reading in full depth.
→ Read: Eddy Current Testing Course
Limitations and Common Causes of Missed Indications
Eddy current testing is very sensitive but not all defects can be detected by eddy current probe under all conditions. It is important to know its limitations to have a reliable inspection.
Conductive Materials Only
Conventional ECT is not suitable to plastics, ceramics, or composites without a conductive route because eddy currents can only be induced in electrically conductive materials.
The method works well with non-ferromagnetic conductive materials such aluminum, copper, titanium, austenitic stainless steel, and nickel alloys.
Ferritic stainless steel and carbon steel are examples of ferromagnetic materials that need extra caution. Penetration is severely limited by their high and frequently fluctuating magnetic permeability, which also produces permeability noise that can overpower defect indications.
Limited Penetration Depth
The skin effect refers to the fact that eddy current density is greatest at the surface and diminishes exponentially with depth. The excitation frequency, the material's conductivity, and its magnetic permeability can all be used to compute the standard depth of penetration, which is the depth at which the current density has dropped to roughly 37% of its surface value.
Lowering the frequency can increase penetration, but there is a limit to how much this can be done before surface sensitivity is lost. As a result, eddy current testing is still mostly used for surface and near-surface inspection.
Surface Condition
Lift-off variations or other signals that may be present due to rough surfaces, scale, contamination, coating, and inconsistent positioning of the probe can make signal analysis more difficult.
Operator Dependence
Manual scanning involves consistent probe movement, positioning, orientation and speed. Inconsistent responses and reduced coverage can result from poor technique. Some of these effects can be minimised by automated and array systems, but these need to be set up and calibrated correctly.
Geometry and Access
Holes, access limitations, weld profiles, sharp edges, and complex shapes may change the flow of eddy currents, or make it difficult to position the probe effectively.
Coating Thickness
The distance between the coil and conductive material increases with non-conductive coatings. Some thin coatings are allowed, but too thick and/or too variable can cause loss of sensitivity and lead to lift-off effects.
Probe Selection
The choice of geometry, coil configuration, frequency and orientation of the probe can lead to weak or no indications if they are not used correctly. Therefore, the probe, frequency, scanning method, material and geometry should be taken into account as one inspection system.
Applicable Standards and Practices
Eddy current inspections are normally carried out to a written procedure that references recognised standards covering equipment, technique and personnel qualification. Documents commonly cited include:
- ASTM E243 — Standard Practice for Electromagnetic (Eddy Current) Examination of Seamless Copper and Copper-Alloy Tubes
- ASTM E376 — Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current Examination Methods
- ASTM E571 — Standard Practice for Electromagnetic (Eddy Current) Examination of Nickel and Nickel Alloy Tubular Products
- ASTM E2884 — Standard Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays
- ISO 15548 — Non-destructive testing: Equipment for eddy current examination
- ISO 20339 — Non-destructive testing: Equipment for eddy current examination — Array probe characteristics and verification
- ASNT SNT-TC-1A / ISO 9712 — Qualification and certification of NDT personnel
Procedures, acceptance criteria and reference standards should always be confirmed against the governing code for the specific component, material and industry, as requirements vary between sectors such as aerospace, power generation and oil and gas.
Many of these limitations come down to choosing the right instrument for the job, see our full guide to selecting an eddy current instrument.
→ Read: How to Choose Eddy Current Instruments
FAQ
What is meant by eddy current probe?
The sensing element of an ECT system is an eddy current probe. It measures the changes brought about by electromagnetic interaction with a conductive substance using one or more coils to create an alternating magnetic field.
What are the main eddy current probe types?
These are commonly surface or pencil probes, bobbin probes, encircling probes, rotating probes or array probes. Coil configurations can also be absolute, differential, reflection or hybrid.
What is a differential eddy current probe?
A differential eddy current probe is a probe that compares the responses of two sensing elements. Similar material conditions will cancel, and discontinuities will produce a differential response.
What is the effect of frequency on inspection?
Higher frequencies have a higher sensitivity to the surface defects and lower penetration. Lower frequencies are able to penetrate deeper, but may not be as sensitive for very small discontinuities in the surface.
Can an eddy current probe detect cracks?
Yes. Under proper conditions of probe configuration, frequency, orientation, and inspection conditions, eddy current probes can be used to detect surface or near-surface cracks in conductive materials.
Why is probe orientation important?
Probe orientation is important because direction of eddy currents in the component is determined by the orientation of the probe, and detection relies on how the discontinuity interacts with that flow.
How should an eddy current probe be selected?
The selection should take into account the material, component geometry, type and orientation of defects, inspection depth, surface condition, access, frequency range, and the inspection coverage required. When there are no suitable standard probes for the application, the Eddy current probe manufacturers and NDT equipment suppliers can offer them the application specific design.
Conclusion
An eddy current probe is not just a tiny coil that is attached to an inspection tool. It creates electromagnetic coupling to the test material and transforms the changes in the electromagnetic interaction into measurable information.
It is influenced by the coil configuration, geometry, frequency, shielding, lift-off, fill factor and orientation. It is important to understand these basics so that the inspector can select the correct eddy current testing probes, prevent the common sources of missed indications and get reliable inspection results.
The first step in effective eddy current testing is to match the probe with the material, the shape, the discontinuity anticipated and the purpose of the inspection. If these are all working together, the probe can be an extremely sensitive detector of defects that might otherwise go unnoticed.
