Your Comprehensive Guide to Eddy Current Testing

Plant Inspection May 2, 2023

Eddy current testing, also known as ECT, is a non-destructive testing (NDT) method that uses electromagnetic induction to detect flaws and evaluate material properties in conductive materials. In addition to fault detection, this technique can measure material and coating thickness and determine heat treatment condition.

The eddy current method of non-destructive testing is widely used in a number of industries, including mining, aerospace, automotive, power generation, manufacturing and in the military. The technique is popular as it provides accurate results and is fast in comparison to other NDT methods.

NDT eddy current testing can be used to detect surface and near-surface flaws, such as cracks and corrosion. It also checks for changes in material properties like thickness, conductivity, and permeability.

If you’re in need of experts to conduct non-destructive testing services in Perth or eddy current testing specifically (or are considering getting it done, but aren’t sure if it’s the best method to use), then this article can help.

This guide provides a comprehensive overview of eddy current testing, including its principles, applications, equipment, advantages, and limitations.

What Is Eddy Current Testing and How Does It Work?

If you’re unfamiliar with eddy current testing, the foremost questions in your mind are likely to be: What is eddy current testing? How does eddy current testing work?

ECT is a versatile inspection technique that can be used for a number of material checks, including the detection of surface and near-surface flaws, the measurement of metal and coating thickness, and for classifying the grade and hardness of metals.

Eddy current testing uses electromagnetic induction to find flaws in both ferrous and non-ferrous materials by creating an eddy current field inside the test specimen.

Principles of Eddy Current Testing

Eddy current testing is based on Michael Faraday’s law of electromagnetic induction, which states that a magnetic field that changes in strength or direction induces an electric current in a conductor.

In ECT, a coil carrying an alternating current is placed near or on the surface of the conductive material being tested. The magnetic field generated by the coil induces eddy currents in the material which, in turn, generate their own magnetic field that opposes the original field. This opposition produces a change in the impedance of the coil, which can be detected and measured by a receiver.

The magnitude and phase of the impedance change depend on the properties of the material being inspected, as well as the presence of flaws or discontinuities in the material.

For example, a crack or void in the material will disrupt the flow of eddy currents and produce a larger impedance change than a defect-free piece. Similarly, a change in material thickness or conductivity will alter the impedance change and indicate the presence of a flaw or change in the material properties.

[ Image depicts AME inspector in the process of using the Eddy Current Testing method ]

Eddy Current Testing Steps and Procedure

While the eddy current testing procedure is perceived to be complicated, conducting it is quite simple and straightforward. Below are the general steps involved in the process:

1. Equipment setup: The first step in eddy current testing involves setting up the equipment. This typically involves connecting the testing instrument to the testing probe, selecting the appropriate probe frequency, and calibrating the instrument.
2. Surface preparation: Before testing can begin, the surface of the material to be tested must be prepped to ensure reliable results are acquired. Preparation may involve cleaning the surface of the piece to get rid of any dirt, oil, or other contaminants that could interfere with the test.
3. Probe placement: Once the material is ready, the testing probe is then placed on its surface. The probe typically comes in the form of an induction coil that generates a magnetic field when an alternating current is passed through it.
4. Test signal generation: Putting the probe in place results in a test signal. This signal causes the probe to generate an electromagnetic field that induces eddy currents in the material being tested.
5. Signal detection: As the eddy currents flow through the material, they generate their own magnetic fields. These fields interact with the magnetic field generated by the probe, thereby causing changes in the test signal. The testing instrument detects any changes in the test signals and uses this data to identify the presence of any defects in the material.
6. Analysis and interpretation: The final step in eddy current testing is the analysis of the test data and interpreting the results. This involves comparing the test results to known defect signatures and using the results to identify the location, size, and type of defects detected.
   By following the above steps carefully, engineers and technicians can use eddy current testing to ensure the quality and reliability of a wide range of components and materials.

Applications of Eddy Current Testing

When it comes to the applications of eddy current testing, you will find that it actually has many uses.

Eddy current testing is used in a variety of applications, including:

• Mining: ECT is used in the mining industry to detect cracks, corrosion, and other defects in metal components of mining equipment, such as drill pipes and casings, to ensure integrity and prevent costly downtime or safety hazards.
• Aerospace: In the aerospace industry, ECT is used to inspect aircraft components such as engine parts, landing gears, and airframes for cracks, corrosion, and other defects.
• Automotive: ECT is also used in inspecting various automotive parts, including crankshafts, camshafts, and suspension components for cracks and other material defects.
• Power generation: ECT is used to inspect power generation equipment, such as steam turbines, generators, and transformers for signs of damage or to reveal flaws or defects.
• Manufacturing: ECT is used to inspect metal products such as pipes, tubes, and wires for defects. It may also be used to measure material properties like thickness and conductivity.

Eddy Current Testing in Mining Operations

ECT is an indispensable NDT method used in the mining industry. Some specific examples of how eddy current testing is used in mining include:

• Inspection of conveyor belts: ECT is used for detecting defects in conveyor belts used to transport materials in mining operations. By detecting flaws early, operators can prevent costly downtime and avoid safety risks.
• Evaluation of metal components: ECT can also be used to inspect metal components, such as valves, pipes, and tanks, for signs of wear and tear. Identifying potential issues early on allows maintenance teams to schedule repairs or replacements before a failure occurs.
• Assessing the condition of mining equipment: ECT aids in assessing the condition of mining equipment, such as drills, excavators, and trucks. When potential issues are detected before a failure occurs, operators can reduce downtime and improve safety.
• Weld inspection: The inspection of welds on metal structures, such as mine shafts and equipment frames is an important function of ECT. It can help detect cracks or other defects in the welds that could weaken the structure and make operations unsafe.

Overall, eddy current testing is a valuable tool for ensuring the safety and reliability of mining operations, as it enables the early detection of defects and potential problems before they lead to equipment failure or accidents.

Different Types of ECT Tests

Depending on the industry, material and purpose of the test to be conducted, there are different types of tests that may be applied using eddy currents.

Eddy current conductivity testing

The eddy current conductivity test is done to determine the electrical conductivity of non-magnetic metals. Metals with a flat or slightly curved surface, as well as metals with or without a thin non-conductive coating, are all eligible for this test.

• The eddy current conductivity testing method is done to determine the following:
• Type of metal or alloy
• Kind of heat treatment (for aluminium, this evaluation should be done in conjunction with a hardness study)
• Aging of the alloy
• Heat damage

Eddy current weld testing

Many large-scale manufacturing, infrastructure, and transportation projects depend on welds to operate successfully and safely.

Welds can be found in a variety of structures, including building steel supports, oil storage tanks, and in steam turbines that produce electricity in both fossil and nuclear power plants. High-quality welds are crucial to the security and optimal operation of processes that provide a range of services and needs essential to modern society.

This is where eddy current weld testing comes in as the inspection of welds is a vital step in ensuring the integrity of welds and the safety of plants and equipment.

Eddy current hardness testing

Testing materials or products for hardness is a common practice in many industries for ensuring that the items being tested can withstand normal wear and tear. Eddy current hardness testing is one of the best NDT methods used for this purpose.

Analysts can utilise eddy current for hardness testing to accurately depict an asset’s or material’s design form, especially when they combine ECT with high-quality technology.

Eddy current array testing

Several eddy current coils positioned next to one another in the same probe assembly are driven electrically in eddy current array testing (aka ECA testing).

Using an eddy current array allows for the replication of the majority of conventional eddy current flaw detection methods. However, one of the exceptional benefits of ECA technology is that it enables better inspection capabilities and considerable time savings.

Other benefits of ECA testing technology include:

• Covering a larger area that can be scanned in a single-probe pass, with the assurance of a high resolution
• Elimination of the need for complex robotics for moving the probe
• Better flaw detection and sizing through imaging
• Customisation of the probes to enable the inspection of components with complex shapes or forms

Pulsed eddy current testing

A method of inspection used to check for corrosion under insulation (CUI) is referred to as pulsed eddy current testing (PECT). It allows for the inspection of materials without the need for direct contact with carbon steel structures, including pipes, vessels, tanks, and spherical tank legs.

By using PECT, it becomes possible to monitor spot percentage changes in the thickness of steel through any non-conductive and non-magnetic material between the sensor and the surface of the steel component, including air, concrete, plastics, paint, coatings, seawater, marine growth, deposits, oil, and so on.

Eddy current testing of tubes

In the power generation, petrochemical, chemical, fertilizer, and air conditioning industries, eddy current testing of tubes is a useful way to evaluate the condition and lifespan of tubes.

Eddy current testing of tubes is used to look for signs of corrosion, pitting, fractures or cracks, erosion, and other surface changes on both the interior and exterior of the component.

Eddy Current Testing Methods

There are three eddy current testing methods, namely crack testing, measuring coating thickness and examining the microstructural properties of a material.

• Crack testing: Using eddy currents is one of the best methods of finding defects on surfaces, such as cracks or pores. Crack testing may be static or dynamic depending on the component being checked.
• Coating thickness measurement: Eddy currents are used to check the thickness of coating materials, such as non-ferromagnetic coatings like copper or zinc and non-electrically conductive coatings like paint, varnish, anodizing, etc.
• Examination of the microstructural properties of a material: This eddy current test method may involve hardness testing and or material mix testing, depending on the purpose of the assessment being conducted.

Advantages of Eddy Current Testing

There are several advantages of eddy current testing over other NDT methods which include the following:

• Non-destructive: ECT is a non-destructive testing method that does not damage the material being tested.
• Fast and efficient: You can inspect large areas and complex geometries quickly and efficiently by using ECT. This helps to reduce downtime and maintain or increase productivity.
• Versatile: ECT can be used to detect a wide range of flaws and changes in material properties, including surface and near-surface cracks, corrosion, and changes in the thickness, conductivity, and permeability of a component.
• Portable: Being generally portable, ECT equipment is easy to use in the field or on-site.
• Cost-effective: ECT is often more cost-effective than other NDT methods because it does not require expensive consumables or extensive preparation.

The specific advantages of eddy current testing over dye penetrant or magnetic particle testing depends on the application and the type of defect being detected. However, some general include:

• ECT is a non-contact method, which means it does not require physical contact with the test material, making it safer and less likely to cause damage to the material.
• ECT is faster than dye penetrant and magnetic particle testing since it does not require the application of a liquid or magnetic particles and subsequent cleaning.
• ECT can detect surface and subsurface defects without any surface preparation, which can be time-consuming and expensive.
• ECT is more sensitive to small cracks and other defects than dye penetrant and magnetic particle testing.
• ECT is less affected by surface roughness, paint, and coatings than dye penetrant and magnetic particle testing, making it more versatile.
• ECT can be used to inspect welds that are difficult to access, such as those in pipes and other structures.
• ECT can be used to inspect welds made of non-magnetic materials, which cannot be inspected using magnetic particle testing.
• ECT does not require the use of hazardous chemicals or materials, making it safer for the operator and the environment.
• ECT provides digital output, making it easier to store and analyse data, and less prone to human error.
• ECT is less affected by ambient light and temperature variations than dye penetrant and magnetic particle testing, providing more consistent and reliable results.

Limitations of Eddy Current Testing

Although ECT offers a number of benefits, there are also limitations that should be considered, including:

• Limited penetration depth: ECT is limited to detecting flaws and changes in material properties near the surface of the material being tested. It cannot detect defects that are deeper than a few millimetres.
• Low applicability to non-conductive materials: Eddy current testing is generally applicable to conductive materials, such as metals. Its effectiveness as a testing method is extremely limited when it involves non-conductive materials like ceramics or plastics.
• Requires skilled operators: To get accurate, reliable results, ECT must be conducted by skilled operators who are trained to interpret the results and identify potential flaws or changes in material properties.
• Sensitivity to surface conditions: ECT can be affected by surface conditions such as roughness, scale, and paint, as these can interfere with the eddy currents and produce false readings.
• Environmental factors: The application of ECT can be affected by environmental factors such as temperature, humidity, and magnetic fields, as the presence of these can influence the eddy currents and produce inaccurate readings.

What Equipment Is Required for Eddy Current Testing?

To perform ECT, an eddy current testing machine made up of various parts is required. These eddy current testing equipment, parts or tools work together to generate and detect eddy currents, analyse the results, and identify any defects or inconsistencies in the material being tested.
Here are some of the most common equipment used in conducting ECT:

• Eddy current probes: The probes are the most important piece of equipment for eddy current testing. They comprise coils of wire that generate a magnetic field when an alternating current is passed through them. There are different types of probes available, such as absolute, differential, and rotating probes, which can be used to detect different types of defects in a range of materials.
• Eddy current testing instruments: These ECT instruments are used to generate and detect eddy currents in the material being tested. They consist of a signal generator, a probe driver, and a signal analyser. The signal generator creates a test signal that is sent to the probe driver, which then generates an electromagnetic field in the material being tested. The signal analyser detects any changes in the signal caused by eddy currents and displays them on a screen for analysis.
• Reference standards: Reference standards are used to calibrate the eddy current testing equipment and to verify the accuracy of the test results generated.
• Accessories: Various accessories are required to perform eddy current testing, such as cables, adapters, and connectors. These accessories allow the testing equipment to be connected to the material being tested and to be adjusted to meet specific testing requirements.
• Computer systems: In modern eddy current testing, computer systems are used to store and analyse test results. They also help in processing data in real-time and provide automated defect detection and sizing information.

Eddy Current Testing Standards

To ensure that the testing is conducted consistently and accurately, various standards have been developed. These standards outline the procedures, techniques, and acceptance criteria for eddy current testing.

Some of the commonly used eddy current testing standards (or standards that affect ECT implementation) are as follows:

• ASTM E426-16 (2021) Standard Practice for Electromagnetic (Eddy Current) Examination of Seamless and Welded Tubular Products, Titanium, Austenitic Stainless Steel and Similar Alloys
• SNT-TC-1A 2020 Personnel Qualification and Certification in Non-destructive Testing
• ASME Boiler and Pressure Vessel Code
• AWS D17.1 Specification for Fusion Welding for Aerospace Applications
• ISO 9712:2021 Non-destructive testing – Qualification and certification of NDT personnel
• EN 10246-3: 1999 Non-destructive testing of steel tubes – Part 3: Automatic eddy current testing of seamless and welded (except submerged arc-welded) steel tubes for the detection of imperfections

These standards specify the type of equipment to be used, the test procedures, the defect detection criteria, and the qualifications of the personnel performing the tests. They are designed to ensure that eddy current testing is conducted consistently and accurately, and that the results it yields are reliable and reproducible.

Adhering to these standards is critical in ensuring that the testing meets the quality requirements for the industry in which ECT is used.

AME Eddy Current Testing Services

ECT is one of the most popular non-destructive testing methods as it can be used in a wide range of applications and industries.

If you require eddy current testing services, please reach out to us.

Besides implementing ECT and other NDT methods, AME provides a range of asset management services.

Find out more by getting in touch with us today.