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If you work in an industry that requires nondestructive testing (NDT) to meet certain specifications and regulations, it’s important that you understand testing methods that are available, and which equipment is most efficiency and fits your budget. One such testing method is eddy current testing (ECT). In its simplest form, a single-coil eddy current testing probe uses a copper wire injected with an electrical current. The wire produces a magnetic field that oscillates at the same frequency as the current running through the copper wire. When you move the testing probe near conductive materials, currents opposed to the ones in the copper wire are induced in the material. These are known as eddy currents. Any defect in the test material disturbs the paths of these eddy currents and can be detected using the probe. This technology is commonly used in industries such as automobile and aerospace manufacturing and engineering.
Nondestructive testing equipment uses different technologies to gain results, and it’s important to know the benefits and limitations so you can make the best decision for your business. In this article, we will discuss the advantages and disadvantages of eddy current testing.
Benefits of Eddy Current Testing
Using technology that detects defects in materials is vital in industries where health and safety are involved. In this section, we will discuss the benefits of eddy current testing:
Sensitivity to surface defects: The smallest defects can weaken materials such as carbon or steel, compromising certain safety standards. Eddy current testing equipment can accurately detect defects of 0.5 mm (0.02 in.) in length under favorable conditions.
Detects through different layers: If you’re working with materials that have a multilayered structure, eddy current equipment can detect defects up to 14 layers deep without interference from the planar interfaces.
Quick and simple: Eddy current is a quick, simple, and reliable inspection technique, which is ideal if you’re conducting regular testing and speed is important.
Measures material conductivity: Eddy current testing can be used to measure a material’s ability to transmit heat or electricity. This can be a critical factor in the selection of a material for a given application.
Measures of nonconductive coating: Coating measurements require precision. Accurate measurements are crucial because the loss of coating material can often compromise safety standards in industries such as automotive and aerospace. Eddy current testing is a practical method for measuring nonconductive coatings because of its versatility—it can test anodic coatings in addition to most nonconductive coatings.
Fast hole inspection using high-speed rotating scanner and surface probes: Effective eddy current hole inspection requires a rotating scanner, rotary probe, and an instrument that records and displays the resulting data. Eddy current equipment is popular choice to conduct NDT bolt hole inspections because of its speed and efficiency.
Portability: Eddy current testing equipment is small and light, making it easy to carry around on the job.
Equipment can be automated: Relatively uniform parts can be inspected quickly and accurately using automated or semiautomated equipment.
Contactless inspection: Eddy current testing equipment doesn’t need to be in contact with the part under inspection to take an accurate measurement.
Limitations of Eddy Current Testing
We have discussed the advantages of eddy current testing, but it’s also important to look at the disadvantages to understand if it’s what your industry requires. Here are the limitations of eddy current testing:
Susceptible to magnetic permeability changes: Small changes in permeability can make testing of welds and other ferromagnetic materials difficult.
Only effective on conductive materials: Eddy current can only be used to measure materials that support the flow of electrical current.
Will not detect defects parallel to the surface: The flow of eddy currents is parallel to the surface. This means eddy current test equipment cannot detect defects that do not cross or interfere with the current.
Signal interpretation required: There are a number of factors that can affect eddy current testing. This requires careful interpretation of signals to distinguish between relevant and nonrelevant indications.
Not suitable for large areas: Eddy current testing on large areas requires the aid of a scanning device. Complex geometry also makes it more difficult to differentiate between defect signals and geometry effect signals.
Benefits of Eddy Current Array Testing
Eddy current array (ECA) is a form of nondestructive eddy current testing that involves electronically driving eddy current coils aligned next to each other in a testing probe. Each coil produces a signal, and the strength of the signal depends on the phase and amplitude of the test material. This signal can be measured, and the data is recorded to carry out tests.
Eddy current array has several advantages over other eddy current testing methods:
Faster inspection time: Using a multicoil probe, the user can scan the material in a single pass. This means the scan takes just a few minutes and is quicker than conventional ECT and certain other NDT methods, such as magnetic particle testing (MPT).
Covers a larger area in one pass: Using multiple coils enables the user to complete the inspection in a single pass. Single-coil probes require the technician to scan the material multiple times.
Simplifies inspection: Using eddy current array reduces the complexity of mechanical and robotic scanning systems required to move the probe.
Inspects complex shapes: Eddy current array probes can be customized to the profile of the part being inspected. This reduces inspection limitations caused by complex test materials.
Real-time cartography: Eddy current array technology provides real-time cartography of the inspected region, facilitating data interpretation.
Increased accuracy: Using eddy current array probes improves the reliability and probability of detection (POD).
Eddy current array testing is used in a range of different industries. It can be used to measure the thickness of steel and to detect corrosion. The unique benefits of eddy current array mean multicoil probes can be used on diverse materials, such as vessels, columns, storage tanks and spheres, piping systems, and structural applications.
This article should help you decide if eddy current testing is the right method for your industry. If you’re interested in eddy current testing equipment, see Olympus’ range of eddy current flaw detectors and probes.
