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General Probe Selection Criteria

General Considerations

Like other nondestructive testing methods, eddy current can perform a variety of tests depending on the type of probe being used. Optimal performance therefore is ensured by careful probe selection. This  page offers information and specifications for selecting probes to meet your inspection requirements.​

 

Instruments

​Impedance Plane Display EC instruments take the greatest advantage of the many different probe designs available. Most can be used with bridge and reflection probe types, displaying phase and amplitude changes.  Units featuring enhanced filtering, frequency range, sensitivity and recording should be considered for additional versatility. ​

 

Material and Test Requirements​

The material for inspection often determines whether EC can be used or not. Nonmagnetic conductive material ranging from .6% IACS to 110% IACS offers the best opportunity for a successful EC test. Test criteria are based on penetration depth, sensitivity, signal to noise ratio and scan speed. Detectable flaw size, resolution and accuracy requirements must also be determined. Eddy current is traditionally used to determine material thickness, non-conductive coating thickness, conductivity and   plating measurement, and cracks.  Crack detection of surface and subsurface flaws is the most popular application. Frequency of the instrument and probe is decided by the material being inspected and the size of the discontinuities required for detection. The EC Slide Rule can be of assistance in defining this test parameter by calculating the best depth of penetration, flaw angle and frequency. 

 

​Probe Selection​

 

Advanced eddy current inspection requires the use of many different coil configurations.  Absolute and  differential coils are available in both bridge and driver/pickup (reflection) designs.  Some coils incorporate a balancing load within the coil to eliminate the need for a separate probe for balancing.  Depending on the test, some are very small in diameter and others can be quite large.  A compromise is usually made to give the best depth of penetration, greatest sensitivity to the smallest defects, and smallest coil diameter. In addition to coil size and frequency, the physical shape of a probe contributes to a successful EC test.  Flaw location and part geometry determine whether a standard probe can be used or if a custom design is needed.  Because the coil must pass over the flaw or be in close proximity to it, the probe body requires accurate geometric specifications.

Standard Probe Designs

​Cost, uniformity and availability make it desirable to use standard probes whenever possible.  Many standard designs, with various frequencies, diameters and detection circuits are listed in each probe manufacturer’s catalog.

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Primary applications for standard probes:

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Surface probes- are used for discovering flaws both on and below surfaces.  Available in a broad frequency range, their diameters are usually large for accommodating lower frequencies or scanning large areas.

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Pencil probes- have smaller diameter housing coils built for higher frequencies.  Applications are usually limited to surface flaws because of the small coil diameter and the high test frequencies that are normally used with pencil probes.

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Right Angle Pencil Probes- reach areas inaccessible to pencil probes.  They normally employ the same coils and frequencies as pencil probes.

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Hand Held Bolt Hole Probes- have coils located at right angles to the probe direction, and are rotated by hand with the fastener removed.  Standard and custom diameters are available with absolute and differential coils.

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Scanner Driven Bolt Hole Probes- are used with mechanical devices for automatically rotating and indexing into hole.  Absolute and differential coils are common at higher frequencies of 500 kHz and 2 MHz. They bear close tolerances and are highly reliable.

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Donut Probes- are designed to inspect aircraft fastener holes with fasteners in place, and can be used on uneven fastener holes. Various inside/outside diameters and frequencies for different size fasteners and differing materials are available.

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Sliding Probes- are designed to inspect aircraft fastener holes with fasteners in place, at higher scan rates than donut probes.  Common sizes and frequencies using the reflection coil techniques are often applied in major airframe inspection requirements.

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ID Probes- usually have a frequency range of 5 kHz to 100 kHz made in absolute or differential                 configurations.  Many body styles are available depending on the inside surfaces of tubes to be inspected.  Most probe diameters are made to meet customers' requirements.

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OD Probes- are usually designed for a frequency range of 5 kHz to 100 kHz and available in absolute, differential and switchable configurations.  Tubes to be inspected are passed through the coil for absolute or differential flaw detection at high speeds.

Special Probe Designs*

​Probes are designed to meet customers' special requirements.  The accuracy of a design is tested by placing a test coil over a defect through matching surfaces of the part and the EC probe.

 

Special configurations are used in many applications.  The broadest range is employed in the inspection of aircraft engines and airframes. These are developed in cooperation with the customer and the test instruments' designers.

 

Custom development is done routinely at by probe manufacturers, as probe designers are adept in solving difficult problems maximizes the performance of instrument and probe.  Understanding of Impedance Plane Analysis and the performance of certain EC components are considered by probe designers and results are shown on an EC instrument compatible with the type of probe.  Impedance changes are the direct result of changes in the inductive reactance or resistance of the EC field induced into a part.  Both phase and amplitude changes are viewable on the instrument screen.

 

Coil configurations, diameter and frequency effect sensitivity, but calibration standards are also important to the success of the EC test.  Calibration standards are usually based upon the material to be inspected containing known notches of minimum size for detection.  These notches are made by EDM processing for best results, but a narrow saw cut is adequate in certain situations.  Conductivity measurements can be certified to N.I.S.T. 

Scanner Probe Selection Criteria

Expanding probes have a long wearing plastic tip and stainless steel backshell. They are designed to allow the use of the same probe when holes are reamed to a larger diameter. They are also useful to check unusual hole sizes when a fixed size probe is unavailable. These probes normally use a reflection differential type coil operating in the 500 kHz to 3 MHz range. This makes them suitable for aluminum structures as well as low conductivity materials.

These probes are also available in kits.

 

Non surface riding scanner probes are made from durable stainless steel and normally use a reflection differential type coil operating in the 500 KHz to 3 MHz range. This makes them suitable for aluminum structures as well as low conductivity materials. The standard stock types are identified in fractions (1/2”). The actual probe size is diameter .010” (.25mm) below this size to provide clearance. Identify special orders with decimals (.500”) . These probes will be made to the diameter ordered without under-sizing. The working length (WL) required should also be stated. These probes are also available in kits

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Increased-throw expanding probes have a long wearing plastic tip and stainless steel backshell, and are normally expandable to around 20% above their nominal diameter. They are designed to allow the use of the same probe when holes are reamed to a larger diameter. They are also useful to check unusual hole sizes when a fixed size probe is unavailable. These probes normally use a reflection differential type coil operating in the 500 KHz to 3 MHz range. This makes them suitable for aluminum structures as well as low conductivity materials. These probes are also available in kits

Ring Probes

Ring/ Donut (encircling) probes are made to fit various fastener head diameters. They are used mostly for subsurface crack detection with the fastener in place.

Sliding Probes

Fixed and adjustable types are available. Operating in the reflection mode, they allow inspection of fastener rows for surface and subsurface cracks. Sliding probes are directionally sensitive and feature engraved green lines to assist scan orientation to the expected direction of the cracks. Fixed types are mostly used in riveted joints to detect cracks emanating from holes. Penetration is sufficient for fuselage thickness’ up to.15” (4mm). Adjustable types are used for thicker structures up to .75” (19mm). They operate satisfactorily with various fasteners including magnetic types.

Conductivity Probes

Conductivity probes are designed for sorting metals and aluminum alloys, determining heat-treatment condition or damage. Standard frequency of the probes normally come in either a high or low test frequency.

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