It is essential that technicians be familiar with both the advantages and limitations associated with the eddy current testing process.
Eddy current testing (ECT) is a widely accepted nondestructive testing (NDT) method for the inspection of various electrically conductive materials. Inspections are performed to detect discontinuities or other material variables (hardness, conductivity, lift-off, and/or thickness). The discontinuities detected by ECT are either surface or very near surface, as this is not a volumetric NDT method. Although nonferrous materials are typically used in the aviation industry, there are other occasions when ferrous alloys may be inspected.
Welds may be inspected using ECT. Many eddy current equipment manufacturers supply weld inspection kits for that specific purpose. Tubing inspection in heat-exchangers also utilizes unique eddy current testing techniques and special equipment. In eddy current testing, it is essential that the NDT technician be familiar with both the advantages and limitations associated with the eddy current process.
This discussion is limited to basic ECT using a surface probe for weld inspection. Advanced multifrequency techniques may be used to improve such inspections to achieve specific results.
As with ultrasonic testing (UT), ECT is a comparison test that requires precise reference and/or calibration samples. It is essential that the proper reference or calibration sample be used dependent on the application.
Eddy current testing relies on the principles of electromagnetic induction. In the ECT technique, a coil (probe) is excited with a sinusoidal alternating current, with frequencies from 50Hz to more than 10 MHz. This establishes what are called eddy currents, which travel in closed loops and exist only in conductive materials. The change in coil impedance, Z, that results due to distortion of eddy currents at regions of discontinuities (defects, material property variations, surface characteristics, etc.) and associated magnetic flux leakages, is measured, and correlated with the factor producing it, i.e., a discontinuity. Other variables can be measured or correlated using ECT, including mass, geometry, lift-off (spacing), conductivity, hardness, etc.
Many parts or structures are made of mild steels or other ferrous alloys, and therefore present issues for the eddy current testing processes. It may be that such materials could be examined using some other method of NDT such as ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), or radiographic testing (RT). However, if ECT is to be used for such inspections, saturation coils or multifrequency applications may be necessary, especially for tube inspection. Saturation may be accomplished by applying either a DC electromagnet or a permanent magnet to the component being tested. This practice minimizes the varying effects of permeability on the test article. Permeability is determined by the following equation:
μ = B/H
where μ = permeability, H = magnetizing current, and B = flux density.
Permeability must be considered when the item being examined is a ferromagnetic material, meaning any material that can be magnetized or support magnetic lines of flux. Typically, carbon steels are considered ferromagnetic, and therefore could present some problems in evaluating the condition of the part under test.
Changes in the magnetic characteristics of the material and/or changes in the amount of current applied will greatly affect the permeability value and likewise alter the distribution of the eddy current field.
A DC field can be applied to a part until the material reaches magnetic saturation. By reaching magnetic saturation on the B-H curve, the permeability (μ) becomes ≈1 or unity. At this point, the material takes on the same characteristics of nonferrous materials. The result is that the signal is significantly improved. It is at this saturation point that the permeability variables become less significant during the inspection process.
Due to the many variables encountered in eddy current testing applications, it is always in the best interest of the NDT technician to eliminate as many of them as possible and isolate the one item of interest in an effort to accurately provide a disposition of the component or part. When the variable of permeability is considered, and can be minimized, it is always best to do so. Some of the other material variables that will be encountered include conductivity, geometry, hardness, and alloying.
Of course, the material variables are not the only things that must be considered, there are system variables that influence the eddy current inspection. When accounting for system variables, things such as frequency, probe type, filtering, and current are factors. The frequency selection will have a tremendous impact on the inspection and should be one of the first things considered. These factors are controlled by the NDT technician and should be adjusted so as to provide the best possible test results within the scope of the referenced specification. This can be accomplished by using a reference standard, a calibration block that is like the part being inspected.
Probe coil selection is also important as absolute, differential, and hybrid designs have various capabilities. Surface probes are usually used for weld inspection and therefore must be appropriate for the inspection performed. Since weld geometry can cause indications, it would be best to select a weld probe that can minimize erroneous signals, due to probe wobble or lift-off.
The cross section of the material being inspected, or thickness of the part can also have an impact on the results of the eddy current process. Since the depth that the eddy current field will penetrate a part is a function of what will be evaluated on the display, it is often necessary to determine what effect this “depth of penetration” will have on the test results.
As permeability increases, the standard depth of penetration will decrease; therefore, inspection of ferromagnetic materials is typically a surface inspection.
When inspecting welded areas on ferrous alloys, the welding process can also cause material variations in the heat-affected zone (HAZ). The signal observed in the HAZ may be different from the base material at some distance away from the area, as well as the weld itself. This can be attributed to the localized heating changes that will alter the material properties, including the localized conductivity of the area being inspected.
Frequency selection for inspections of ferromagnetic materials can have a significant impact on the effectiveness of the test. Generally, frequencies selected for inspection of ferrous materials will be lower than those selected for inspection of nonferrous materials. However, when inspecting for surface-breaking discontinuities, higher frequencies may be used on ferrous materials.
Since there are so many variables encountered in ECT, an appropriate NDT procedure and technique should be provided for any eddy current inspection. Such a procedure should take into consideration the variables related to the inspection and provide for a thorough examination.
Presently, there are few codes or standards that address eddy current flaw detection specifically, and therefore the need for inspection specific technical data is essential. Unlike many NDT methods for which codes and standards are provided, eddy current requires considerable attention to the process and development of the technical data on an almost individual basis. The following is a small sample of some of the technical codes and standards related to a few other NDT methods:
• Ultrasonic Testing: ASME Boiler and Pressure Vessel Code, Section V, Articles 4 and 5; ASTM E-388; ASTM E-2375
• Radiographic Testing: ASME Boiler and Pressure Vessel Code, Section V, Article 2; ASTM E-94
• Liquid Penetrant Testing: ASME Boiler and Pressure Vessel Code, Section V, Article 6; ASTM E-1417.
There are many other codes that address other NDT methods or disciplines for numerous applications, but few that address eddy current. When they do address eddy current, it is often for other applications such as conductivity measurement or tubing inspection.
Eddy current testing of ferrous metals, including welds, will present its own challenges. The display will be quite different as compared to nonferrous materials. When inspecting nonferrous materials such as aluminum, a very clean and consistent signal will be observed. Not so for a standard eddy current test on ferrous welds; there will be a much different presentation. However, when utilized appropriately, eddy current can be a valuable means of NDT for weld inspection of ferrous materials.