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.
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
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.