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. It's coil configuration, geometry, shielding and operating frequency will 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 includes one or more coils of wire that create a magnetic field that changes in intensity in an alternating fashion; it also includes a detector that responds to the changes in the magnetic field caused by the 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.
Thus, the probe is able to excite the material electromagnetically and detect the response it produces, which are two related functions.
An eddy current proximity probe is a related electromagnetic proximity sensor, but the main use is to measure distance/displacement between the proximity probe and a conductive target. It is generally employed not for the conventional displacement measurement but for monitoring the condition of machinery.
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
An eddy current probe is understood to be a series of electromagnetic interactions.
Step 1: AC flows through the Probe Coil
Alternating current is provided to the probe coil by the ECT instrument. This current is used to influence the penetration and the sensitivity of the inspection.
Step 2: The Coil Produces a Primary Magnetic Field
This alternates the current, which in turn produces a 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 flow 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. Core Design to enhance coupling and guide the field into specific inspection areas.
Shielding:
Electrical/magnetic shielding can eliminate undesirable interaction of electromagnetic components with other components nearby. It is especially beneficial in close proximity to edges and/or complex geometries or other conductive structures.
Housing:
Housing contains the internal components and defines the exterior shape of the probe. It has to be built in a way that will resist the mechanical and environmental conditions of inspection.
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
There are generally two types of absolute probes – one type relies on a single coil to sense changes in the electromagnetic properties of the product under the probe, while the other type relies on multiple coils to sense changes in the electromagnetic properties. It can react to a slow change or localised discontinuities.
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.
The set up is good for detecting localized defects, but for minimizing the effect of material variation over time.
Reflection Probes
The separation of the function of excitation and receiving is performed through a reflection configuration between different coils or coil elements. One of them is responsible for producing the electromagnetic field, and the other is responsible for measuring, or sensing, the response of the field.
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 a shorter time period than would be possible with a single coil or sensing element, 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.
There is a trade-off of higher frequencies versus better surface resolution and less depth, and lower frequencies with greater depth and lower surface sensitivity.
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
Lift-off is the distance between the probe and test surface. The amount of lift-off increases generally with decreasing electromagnetic coupling and changing probe response. Thus, changes in the probe position can produce signals that mask true indications.
Fill Factor
Bobbin and encircling probes require special consideration with regard to fill factor. It refers to the degree of closeness of the probe's effective sensing area that encircles the test component. Low fill factor may lead to low coupling and sensitivity.
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.
A secondary eddy current flow may be fairly small and difficult to detect in a crack parallel to the main eddy current flow. The direction of the probe and the direction of the scan, therefore, are important when conducting inspection for directionality of cracks.
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 taken on their own.
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
Eddy currents must have conductivity. Traditional ECT is therefore mostly used for conductive materials like aluminum, copper, nickel alloys and steels.
Limited Penetration Depth
The number of eddy currents decreases as the depth increases. Frequency can be lowered to improve penetration but ECT is most effective for surface and shallow 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 or lower coverage can be achieved due to 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.
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 produces one or more coils to create an alternating magnetic field, and measures any changes associated with electromagnetic interaction with a conductive material.
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?
Because the induced eddy currents are influenced by the orientation of the probe, this interaction with the discontinuity will vary. A secondary eddy current flow perpendicular to the primary flow could give a stronger indication than a flow parallel to the primary 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 material coverage desired. 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.
