What is Eddy Current Testing or ECT?

What is Eddy Current Testing?

Eddy current testing is one of many NDT methods that uses the electromagnetism principle for flaw detection in conductive materials. A specially designed coil energized with an alternating-current (AC) is placed in proximity to the test surface, generating a changing magnetic field that interacts with the test-part and produces eddy currents in the vicinity of it.

Variations in the changing phases and magnitude of these eddy currents are then monitored through the use of a receiver-coil or by measuring changes to the alternate current flowing in the primary excitation-coil.

The electrical conductivity variations, the magnetic permeability of the test-part, or the presence of any discontinuities, will cause a change in the eddy current and a corresponding change in phases and amplitude of the measured current. The changes are shown on a screen and are interpreted to identify defects.

ECT testing
Figure 1. Eddy Current Testing

Application of Eddy Current Testing

Eddy current instruments can be used in a wide variety of tests. Some of the most common are listed below.

Weld Inspection– Many weld inspections employ ultrasonic NDT for subsurface testing and a complimentary eddy current method to scan the surface for open surface cracks on weld caps and in heat affected zones.

Conductivity Testing – Eddy current testing’s ability to measure conductivity can be used to identify and sort ferrous and nonferrous alloys, and to verify heat treatment.

Surface Inspection – Surface cracks in machined parts and metal stock can be readily identified with eddy current. This includes inspection of the area around fasteners in aircraft and other critical applications.

Corrosion Detection – Eddy current instruments can be used to detect and quantify corrosion on the inside of thin metal such as aluminum aircraft skin. Low frequency probes can be used to locate corrosion on second and third layers of metal that cannot be inspected ultrasonically.

Bolt Hole Inspection – Cracking inside bolt holes can be detected using bolt hole probes, often with automated rotary scanners.

Tubing inspection – Both in-line inspection of tubing at the manufacturing stage and field inspection of tubing like heat exchangers are common eddy current applications. Both cracking and thickness variations can be detected.

How Does Eddy Current Testing Work?

Eddy current testing (ECT) is based on the physics phenomenon of electromagnetic induction. In an eddy current probe, an alternating current flows through a wire coil and generates an oscillating magnetic field. If the probe and its magnetic field are brought close to a conductive material like a metal test piece, a circular flow of electrons known as an eddy current will begin to move through the metal like swirling water in a stream. That eddy current flowing through the metal will in turn generate its own magnetic field, which will interact with the coil and its field through mutual inductance. Changes in metal thickness or defects like near-surface cracking will interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field. This in turn affects the movement of electrons in the coil by varying the electrical impedance of the coil. The eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used by a trained operator to identify changes in the test piece.

Eddy current density is highest near the surface of the part, so that is the region of highest test resolution. The standard depth of penetration is defined as the depth at which the eddy current density is 37% of its surface value, which in turn can be calculated from the test frequency and the magnetic permeability and conductivity of the test material. Thus, variations in the conductivity of the test material, its magnetic permeability, the frequency of the AC pulses driving the coil, and coil geometry will all have an effect on test sensitivity, resolution, and penetration.

There are many factors that will affect the capabilities of an eddy current inspection. Eddy currents traveling in materials with higher conductivity values will be more sensitive to surface defects but will have less penetration into the material, with penetration also being dependent on test frequency. Higher test frequencies increase near surface resolution but limit the depth of penetration, while lower test frequencies increase penetration. Larger coils inspect a greater volume of material from any given position, since the magnetic field flows deeper into the test piece, while smaller coils are more sensitive to small defects. Variations in permeability of a material generate noise that can limit flaw resolution because of greater background variations.

While conductivity and permeability are properties of the test material that are outside of the operator’s control, the test frequency, coil type, and coil size can be chosen based on test requirements. In a given test, the resolution will be determined by the probe type while detection capability will be controlled by material and equipment characteristics. Some inspections involve sweeping through multiple frequencies to optimize results, or inspection with multiple probes to obtain the best resolution and penetration required to detect all possible flaws. It is always important to select the right probe for each application in order to optimize test performance.

Eddy Current Instruments

ECT machine

Eddy current instruments can be purchased in a large variety of configurations. Both analog and digital instruments are available. Instruments are commonly classified by the type of display used to present the data. The common display types are analog meter, digital readout, impedance plane and time versus signal amplitude. Some instruments are capable of presenting data in several display formats.

The most basic eddy current testing instrument consists of an alternating current source, a coil of wire connected to this source, and a voltmeter to measure the voltage change across the coil. An ammeter could also be used to measure the current change in the circuit instead of using the voltmeter.

Advantages & Limitations of Eddy Current Testing

Advantages of Visual Testing: 

  • Able to detect surface and near-surface cracks as small as 0.5mm.
  • Able to detect defects through several layers, including non-conductive surface coatings, without interference from planar defects.
  • Non-contact method making it possible to inspect high-temperature surfaces and underwater surfaces.
  • Effective on test objects with physically complex geometries.
  • Provides immediate feedback.
  • Portable and light equipment.
  • Quick preparation time – surfaces require little pre-cleaning and couplant is not required.
  • Able to the measure electrical conductivity of test objects.
  • Can be automated for inspecting uniform parts such as wheels, boiler tubes, or aero-engine disks.

Limitations of Visual Testing:

  • Can only be used on conductive materials.
  • The depth of penetration is variable.
  • Very susceptible to magnetic permeability changes – making testing of welds in ferromagnetic materials difficult – but with modern digital flaw detectors and probe design, not impossible.
  • Unable to detect defects that are parallel to the test object’s surface.
  • Careful signal interpretation is required to differentiate between relevant and non-relevant indications.

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