A close-up view of a worker in red protective gear and a white hard hat inspecting a white metal structure using a handheld electromagnetic testing device.

Electromagnetic Testing (ET): An Essential Method for NDT

Discover the advantages of the electromagnetic testing (ET) method, understand the basic principles of ET, and explore the variety of techniques for applying this highly adaptable method in nondestructive testing across industries.

What Is Electromagnetic Testing and How is it Used in NDT?

Electromagnetic testing is an NDT method that uses electromagnetic fields to detect and measure discontinuities in industrial components. ET operates on the principle of inducing electric currents or magnetic fields in a material and analyzing the resulting electromagnetic response to gather information about the internal structure of the test part.

The versatility of this method allows it to be applied through various techniques, making it useful in many industries and environments.

A skilled NDT specialist uses field experience and knowledge to apply the right technique for the job—and can communicate findings through a variety of reporting formats.

The primary objectives of ET in industrial applications include:

  • Detecting Surface and Subsurface Defects: ET is effective in identifying surface and near-surface discontinuities by analyzing changes in the electromagnetic field.

  • Material Characterization: ET can assess material properties such as conductivity and permeability, which are critical for quality control.

  • Thickness Measurement: ET can measure the thickness of non-conductive coatings on conductive substrates.

Advantages and Limitations of Electromagnetic Testing in NDT

ET is a versatile method applicable in many industries, including manufacturing, aerospace, oil and gas, and more. Its ability to perform inspections without direct contact makes it invaluable for ensuring the integrity and safety of equipment and structures.

Advantages of Electromagnetic Testing

  • Non-contact Measurement: ET allows for the measurement of material properties and detection of discontinuities without direct contact with the test material.

  • Speed: ET provides nearly instantaneous measurements, often in a fraction of a second, making it suitable for high-speed production lines.

  • Versatility: ET can be used to measure thickness, detect material discontinuities, identify material composition, and evaluate heat treatment conditions, among other applications.

  • Cost-Effectiveness: Modern ET techniques offer low-cost methods for high-speed, large-scale testing of metallic materials, which is economically beneficial for industries.

  • Automation: ET systems can be automated, allowing for mass testing of similar parts at high rates, which enhances efficiency and reduces labor costs.

  • Detection of Surface and Subsurface Defects: ET is effective at detecting both surface and near-surface discontinuities, making it a valuable tool for quality control and maintenance.

  • Portable Equipment: Small, portable ET instruments allow for manual quality tests by individual operators and monitoring of material deterioration in service.

Limitations of Electromagnetic Testing

  • Material Restrictions: ET is primarily effective for conductive materials, such as metals. It is not suitable for non-conductive materials like plastics, ceramics, or composites.

  • Surface Condition Sensitivity: ET results can be significantly affected by the surface condition of the test material. Rough surfaces, coatings, and surface contaminants can interfere with the accuracy of the test.

  • Limited Applicability for Certain Defects: ET may not be effective for detecting certain types of defects, such as very small cracks or defects in complex geometries. Other NDT methods may be more suitable for these applications. ET is also generally limited to detecting surface and near-surface defects, and it may not be effective for identifying deeper subsurface defects.

  • Complexity of Interpretation: The interpretation of ET signals can be complex and requires skilled and experienced personnel. Variations in material properties, geometry, and other factors can produce signals that are difficult to interpret.

  • Sensitivity to Noise: ET is sensitive to electromagnetic noise and interference from the environment, which can affect the accuracy and reliability of the test results. Proper shielding and filtering are necessary to minimize these effects.

  • Equipment Complexity and Cost: Advanced ET systems can be complex and expensive, requiring significant investment in equipment and training. Maintenance and troubleshooting of sophisticated ET equipment can also be challenging.

How Electromagnetic Testing Works: Basic Principles

In the ET method, alternating current (AC) is introduced into a coil, creating a varying electromagnetic field. Technicians place an eddy current probe on the material and the electromagnetic field can penetrate the material directly.

The probe generates an alternating magnetic field, which induces eddy currents in the conductive material being inspected. These eddy currents create their own secondary magnetic field, which opposes the primary field generated by the probe.

When eddy currents encounter a discontinuity—a material with different electrical conductivity or magnetic permeability—they are disrupted, causing changes in the secondary magnetic field. By analyzing these changes, skilled operators can determine the presence and characteristics of discontinuities, such as cracks, corrosion, or inclusions.

NDT practitioners leverage a comprehensive understanding of various probe types, frequencies, and display options, enabling the ET method to be flexible across many kinds of requirements, environments, and applications.

Eddy Currents in ET

Eddy currents are named for their circular, swirling motion, similar to water currents in a stream. These currents are strongest on the surface of a material and gradually weaken as they go deeper, eventually becoming too weak to be useful. Surface defects cause the most disruption to these currents.

Changes in the material, like thickness or conductivity, affect how the magnetic fields interact, and this shows up on the testing instruments. The results can be displayed in different ways, but the most common is the impedance plane display. This process helps identify issues in the material without causing any damage.

Diagram showing the relationship between eddy current density, depth, and factors like frequency, conductivity, and permeability, with test coils producing alternating current above the material.

Electromagnetic Fields Interaction with Materials

Electromagnetic fields interact with materials primarily through the induction of eddy currents, which are influenced by the material's electrical conductivity and magnetic permeability. Discontinuities disrupt these currents, altering the electromagnetic field in detectable ways.

The depth at which eddy currents penetrate a material depends on factors like frequency, conductivity, and permeability.

Frequency

How many times a wave goes up and down. Higher frequencies reduce penetration depth but increase sensitivity. Frequency is adjusted to get the desired balance between penetration and sensitivity.

Conductivity

How well a material lets electricity flow through it. Materials with higher conductivity create stronger eddy currents but with less penetration. Lower conductivity results in weaker currents but deeper penetration.

Peremeability

How easily a material allows magnetic fields to pass through it. Materials with high permeability generate strong magnetic fields that keep eddy currents near the surface, which limits inspection depth.

A worker wearing a white hard hat and dark protective gear is performing electromagnetic testing on a large white tank in a snowy outdoor setting.

Electromagnetic Testing Techniques Used in NDT

Electromagnetic Testing (ET) employs a variety of techniques to detect and measure discontinuities in materials. These techniques can be categorized based on how the eddy currents are introduced into the material and the specific modes and advanced methods used to enhance flaw detection and measurement.

There are five primary methods used.

Alternating Current Field Measurement (ACFM)

  • Uses an alternating current magnetic field conditioned to be homogeneous in undamaged areas.

  • Detects discontinuities by measuring the disturbance in the magnetic field.

  • Ideal for sizing cracks in offshore structures and other applications where surface-breaking defects need to be accurately measured.

Electromagnetic Acoustic Transducer (EMAT)

  • A non-contact, couplant-free technique using interacting magnetic fields to induce ultrasonic waves into the test object.

  • Suitable for high-temperature and high-speed inspections.

  • Requires special instrumentation and a larger-than-usual probe, making it ideal for specific applications where traditional contact methods are not feasible.

Eddy Current Testing (ECT)

  • The most traditional technique of eddy current testing.

  • Uses a single-coil probe to generate and detect eddy currents.

  • Ideal for detecting surface and near-surface discontinuities in conductive materials.

  • Provides immediate feedback on the presence of defects.

Remote Field Testing (RFT)

  • Uses a probe placed inside a tube to generate a magnetic field that travels through the material.

  • Detects both internal and external defects by analyzing the magnetic field that returns to the probe.

  • Commonly used for inspecting ferromagnetic materials, such as carbon steel tubes.

Pulsed Eddy Current Testing (PECT)

  • Uses a pulsed DC magnetic field to generate eddy currents.

  • Measures the decay of the eddy currents over time to detect corrosion and other discontinuities.

  • Effective for inspecting thick materials and detecting corrosion under insulation.

  • Suitable for testing through intermediate objects like insulation or concrete.

Add Electromagnetic Testing Certification to Your Qualifications

ASNT certifications enable you to become a qualified Level II or Level III in ET.

What Certification Is Right for Me?

Application of Electromagnetic Testing in NDT Across Industries

Electromagnetic testing (ET) is widely used across various industries due to its effectiveness in detecting surface and near-surface defects in conductive materials.

Energy

ET is used in the oil and gas industry to inspect pipelines, storage tanks, and other infrastructure. It helps detect corrosion, cracks, and other defects that could lead to leaks or failures. The power generation industry uses ET to inspect turbines, generators, and other critical components in power plants. It helps ensure the reliability and efficiency of power generation equipment. In the nuclear industry, ET is used to inspect reactor components, fuel rods, and other critical infrastructure. It helps detect defects that could compromise the safety and performance of nuclear facilities.

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"A composite image showcasing various energy sources: solar panels in the foreground, oil pump jacks in the middle ground, and wind turbines and a power plant in the background. The scene illustrates the diversity of energy production methods at sunset.

Aerospace

ET is used extensively in the aerospace industry for inspecting aircraft components, including airframes, engines, and landing gear. It helps detect cracks, corrosion, and other defects that could compromise the safety and performance of aircraft.

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A technician performing maintenance or inspection work on the landing gear of a large commercial airplane inside an aircraft hangar. The scene is illuminated with a blue tint, highlighting the aircraft's engines and the structural details of the hangar.

Transportation

In the automotive industry, ET is employed to inspect critical components such as wheels, axles, and suspension parts. It ensures the integrity and reliability of these components, which are essential for vehicle safety. ET is used in the railway industry to inspect rails, wheels, and other components of trains and tracks. It helps detect defects that could lead to derailments or other safety issues. The marine industry uses ET to inspect ship hulls, propellers, and other components. It helps detect corrosion, cracks, and other defects that could affect the safety and performance of vessels.

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A modern high-speed train moving swiftly through a train station at sunset. The motion blur effect emphasizes the train's speed, with vibrant colors in the sky and station lights creating a dynamic and futuristic atmosphere.

Manufacturing

In manufacturing, ET is used to inspect raw materials, semi-finished products, and finished goods. It helps ensure the quality and integrity of products before they are shipped to customers. ET is used in metal fabrication to inspect welds, castings, and other metal components. It helps ensure the quality and integrity of fabricated products.

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A modern manufacturing facility with robotic arms working on an automated assembly line. The scene is well-lit with blue overhead lighting, showcasing advanced machinery and precision engineering in a clean, industrial environment.

Infrastructure

ET is used in the construction industry to inspect structural components such as beams, columns, and welds. It helps ensure the integrity and safety of buildings and other structures.

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Example: ET in the Real World

Heat exchangers are crucial in power plants and chemical processing facilities, transferring heat between fluids through multiple tubes. Over time, these tubes can degrade from corrosion, erosion, and cracking. Eddy current testing is commonly used to inspect the condition of heat exchanger tubes.

During the inspection, the heat exchanger is taken offline, and the tubes are cleaned to remove any deposits or fouling. An eddy current probe is inserted into each tube to detect changes in the tube wall's thickness or the presence of defects. The recorded signals are then analyzed to identify and characterize any discontinuities, allowing the maintenance team to decide whether to replace affected tubes, preventing potential leaks, and ensuring efficient operation of the heat exchanger.

A worker in a white hard hat and protective gear is inspecting a large industrial heat exchanger with numerous circular openings. The focus is on the structure, with the worker slightly blurred in the background, highlighting the precision and attention required in this inspection task

Deeper Learning About Electromagnetic Testing

ASNT offers both members and nonmembers learning opportunities and resources for NDT specialists certifying in and using ET.

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Electromagnetic Testing (ET) Courses

Advance your skills and knowledge with courses and webinars on electromagnetic testing.

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Programmed Instruction Series: Introduction to NDT

A comprehensive self-study resource for Level I and II candidates covering 16 NDT methods. Includes theory, principles, applications, quizzes, and an online interactive training program.

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Nondestructive Testing Handbook, Vol. 5: Electromagnetic Testing (ET), 3rd ed.

A practical guide on ET for Level II and III inspectors, featuring industry-specific applications, theory, application and instrumentation.

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