Table of Content
- Fundamentals of Eddy Current Testing
- Eddy Current Testing Procedure
- History of Eddy Current Testing
- Advantages of Eddy Current Testing
- Disadvantages of Eddy Current Testing
- Eddy Current Testing Methods
- Eddy Current Testing Equipment
- Applications of Eddy Current Testing
- Eddy Current Array
- Eddy Current Measurements - The Road Ahead
- Final Thoughts
- Key Takeaways
The emergency landing of Aloha Airlines Flight 243 after an unprecedented decompression caused by the breakage of the fuselage is a significant event in the history of aviation. The most probable cause of this incident was multiple site fatigue cracking that negated the safety aspects of the fuselage. The maintenance team failed to detect these failures during the inspection of the flight. How can such cracks be detected?
Finding a tiny crack inside such a humongous structure like an aircraft is almost impossible. But, with the help of Eddy current inspection, it is possible. Eddy current testing is most commonly used to inspect surfaces and tubes. It is an incredibly sensitive testing method and can identify even very small flaws or cracks in a surface or just beneath it.
The alternating current produces a fluctuating magnetic field through electromagnetic induction and generates eddy currents nearby. The variations in this eddy current are analyzed to detect any flaws present. Variations in electrical conductivity, the test part's magnetic permeability, or the existence of any discontinuities will affect the eddy current, which will vary the measured current's phases and amplitude in turn. Defects are found by interpreting the changes as they are displayed on a screen.
Fundamentals of Eddy Current Testing
The basic principle behind eddy current inspection is Electromagnetic induction. An alternating magnetic field forms around a conductor, such as a copper coil, when an alternating current passes through it. The magnitude of this field varies according to the changes in the alternating current.
This fluctuating magnetic field around the coil passes through the material. According to Lenz's Law, when the coil is then brought close to another electrical conductor, it produces an eddy current in the conductor. The eddy current, in turn, creates a magnetic field of its own. This "secondary" magnetic field opposes the "primary" magnetic field. Any changes in the conductivity of the material being studied, such as near-surface flaws or variations in thickness affect the intensity of the eddy current.
Detection of this change in the eddy current using either the primary coil or the secondary detector coil forms the foundation of the eddy current testing method. A material's permeability determines how easily it can be magnetized. The depth of penetration decreases with an increase in permeability. At the surface, eddy current density is higher, and defect sensitivity is highest; these characteristics diminish as depth increases.
The metal's "permeability" and "conductivity" affect the rate at which the value decreases. The conductivity of the material influences the depth of penetration. Metals with high conductivity have stronger eddy current flow at the surface. The alternating current's frequency can be changed to control the depth of penetration; the lower the frequency, the deeper the penetration. As a result, deep faults can be detected using low frequencies, whereas near-surface imperfections can be found using high frequencies.
Eddy Current Testing Procedure
An inspector performs eddy-current tests on tubes by running a probe along the length of the tube to look for minute flaws. With the aid of pulsed eddy current testing tools, it can also assist in locating defects on the material's surface. This is how tubes are tested using eddy currents:
- Probe:
An inspector begins the test by using a probe, such as the alternating current single-element ETC probe. A conductive wire that has been wound into a coil makes up the eddy current test probe.
- Production of a Magnetic Field:
The probe will produce an alternating magnetic field after it is electrified.
- Introducing the field to the Object:
The inspector will move the field through the object they wish to inspect.
- Generating Eddy Currents:
Upon applying the magnetic field through the material, it produces currents that run counter to the currents in the probe. These are the eddy currents.
- Data Collection:
Any flaws in the material will alter these eddy currents. Inspectors collect this data after introducing the probe during the eddy current test.
- Analyze the Data:
It is necessary to assess the data once it has been collected to find any flaws in the object. It should be noted that because data analysis and data collection involve different degrees of training and certification, the inspector who collects the data may not always be the same inspector who analyzes the data.
History of Eddy Current Testing
Although François Arago, a scholar, first noted the eddy current phenomena in 1824, Léon Foucault, an inventor, is given the credit for its discovery. Michael Faraday, who established the idea of electromagnetic induction in 1831, contributed to Foucault's 1855 discovery, which was partly based on his work.
As a result of Faraday's observation that a magnetic field will pass through a conductive material in a way that changes over time as an electric current passes through it, this principle describes the link between electric currents and magnetic fields. Despite these early findings, it wasn't until 1879 that David Hughes, a scientist, discovered Eddy Current Testing Applications. Hughes was able to show how different types of conductive materials may alter the characteristics of a coiled, conductive wire.
During World War II, German professor Friedrich Förster started looking into the commercial applications of eddy current NDT testing. Förster developed the technique and increased its possible applications after the war by founding the Foerster Group, a business that produced tools for eddy current tests. Today, there are many Eddy Current Testing Companies all over the world. Eddy current test is currently one of the NDT techniques that inspectors utilize the most frequently, and it has a long history of producing accurate results.
Advantages of Eddy Current Testing
Eddy current testing is an NDT inspection method used for a variety of purposes, including flaw detection, material, and coating. Advantages are as follows:
- Helps to identify cracks as small as 0.5mm located on the surface and near the surface.
- Able to identify flaws over several layers, including non-conductive surface coatings, without interference from planar defects.
- Being a non-contact technique, it enables the inspection of underwater and high-temperature surfaces.
- It can be used successfully on test objects with physically complicated geometries.
- Provides instantaneous feedback
- Involves short preparation time since couplant is not necessary and surfaces only need to be lightly pre-cleaned
- Inspection of uniform parts, such as wheels, boiler tubes, or aero engine disks, can be automated.
Disadvantages of Eddy Current Testing
- Eddy current testing can be used with conductive materials only.
- Testing welds in ferromagnetic materials is difficult due to their high susceptibility to changes in magnetic permeability, but it is not impossible, thanks to new digital fault detectors and probe designs.
- Unable to detect flaws parallel to the surface of the test object.
- It requires a power supply to function.
- A flaw that isn't directly in contact with the ECT current can't be detected because the current always runs parallel to the surface of a material, which means that some defects might go undetected.
- Its magnetic permeability may fluctuate, making it challenging to check ferromagnetic material components. Due to the permeability effects on the welds experienced by ECT equipment, it is also non-conductive when used with ferromagnetic materials.
Eddy Current Testing Methods
There are several different eddy current testing methods. Here are some of the most common ones:
1. Eddy Current Array
An array of electrically charged coils is used in Eddy Current Array testing to develop a sensitivity profile designed to find defects in the material. Inspectors must exercise caution during this type of testing to prevent mutual inductance between the different coils.
2. Heat Exchanger Testing
One of the most common applications of Eddy current Testing is for heat exchanger testing. In this kind of testing, inspectors utilize eddy currents to detect flaws in metal tubes and provide instantaneous data after just one pass with the probes.
3. Lorentz Force ETC
Lorentz force eddy current testing involves using multiple DC magnets to try and overcome the skin effect. Along with multiple magnets, the Lorentz force eddy-current testing utilizes relative motion to help inspectors conduct quick, accurate eddy-current tests.
4. Surface Array Testing
Surface array testing is frequently employed in the aerospace sector because it offers highly accurate conductivity and corrosion/wall thickness measurements. This kind of testing is particularly flexible and can locate flaws in challenging-to-inspect locations where other inspection techniques would fail.
Eddy Current Testing Equipment
There are two categories of eddy current testing equipment—probes and instruments. Generally, probes collect the data and instruments convert that data into interpretable results. Here are the different types of probes used in eddy current testing:
1. Probes
Eddy current probes come in a wide range of sizes and forms. The ability to custom-design probes for a wide range of applications is one of the main benefits of eddy current inspection. The design and the mode of operation of the test coils serve as the basis for categorizing eddy current probes.
How the coil or coils are packaged to best "couple" to the test area of interest is often referred to as the probe's configuration. Bobbin probes, which are inserted into a piece of pipe to inspect it from the inside out, as opposed to encircling probes, in which the coil or coils wrap the pipe to inspect it from the outside in, are examples of different designs of probes.
- Absolute Probes
Absolute probes typically include a single test coil to create eddy currents and detect changes in the eddy current field. As Alternating Current passes through the coil, an expanding and contracting magnetic field is created both inside and outside the coil. Eddy currents are produced inside conductive materials when the probe is placed close to them because of the fluctuating magnetic field.
Eddy current generation consumes energy from the coil, which manifests as a rise in the coil's electrical resistance. The magnetic field of the coil is opposed by the magnetic field created by the eddy currents, which alters the coil's inductive reactance. Flaw detection, conductivity measurements, liftoff measurements, and thickness measurements can all be done with absolute coils.
They find frequent applications because of their adaptability. Commercially marketed absolute probes frequently feature a fixed "air-loaded" reference coil that corrects for changes in the surrounding temperature.
- Differential Probes
Two active coils are often wound in opposition to differential probes, although they could also be wound in addition and produce comparable results. Since the two coils are investigating the same material when they are over a test sample without flaws, no differential signal is created between the coils.
However, a differential signal is generated when one coil is over a defect and the other is over non-defective material. They have the advantage of being extremely sensitive to flaws while being generally insensitive to qualities that change slowly, like modest changes in temperature or size.
With this probe type, probe wobbling signals are also diminished. The use of differential probes has drawbacks as well such as the signals could be challenging to decipher.
- Reflection Probes
Reflection probes include two coils, however, one coil is used to create eddy currents and the other to detect changes in the test material. They are also called driver/pickup probes. Reflection probes have the benefit of allowing the driver and pickup coils to be individually adjusted for their respective functions.
The pickup coil can be made very small so that it will be sensitive to very small flaws, and the driving coil can be constructed to provide a strong and uniform flux field close by. Probes can also be classified as shown below:
- Handheld Probes
One of the most popular types of probes used in eddy current testing is the handheld probe. They are employed in numerous industries and frequently have interchangeable probe tips and handles.
- Surface Array Probes
In surface array testing, surface array probes are frequently used to find flaws in surfaces that aren't flat. It finds application during the inspection of complex geometries like turbine dovetails. It can detect even very tiny surface defects.
- Tubing Array Probes
Inspectors frequently use this probe, a sort of eddy current tube testing apparatus, to gather all the information they require from a tube in a single pass.
2. Instruments
The following are the commonly used instruments in eddy current testing:
- Eddy Current Testing Handheld Instrument
Inspectors have more flexibility during testing thanks to handheld eddy current testing equipment, which also gives them a portable device for collecting ETC data.
- Modular Eddy Current Testing Units
A modular ETC unit is a smaller, more portable eddy current testing device that just uses instruments. Mostly condensers and steam generators are inspected using this equipment.
Applications of Eddy Current Testing
Eddy current is used for surface defect detection for tubes/bars/wires/bearings, etc. On surfaces, ETC can be done with both ferromagnetic and non-ferromagnetic materials. Here are the types of flaws eddy current testing is generally used to find:
- Cracks
- Corrosion
- Wear (in tubes, often due to erosion)
- Freezing-related damage (in tubes)
- Lack of fusion
- Pitting
- Wall loss/thickness loss
Other common applications include the following:
- Weld Inspection
Many weld inspections include complimentary eddy current methods to scan the surface for open surface cracks on weld caps and in heat-affected zones (HAZ) and ultrasonic NDT for subsurface testing. Eddy current testing may detect conductivity, which can be used to identify and classify ferrous and nonferrous alloys as well as confirm heat treatment.
- Surface Examination
Eddy current Non-destructive Testing makes it simple to spot surface cracks in metal stock and manufactured items. This includes checking the vicinity surrounding aircraft fasteners.
- Corrosion Detection
Using eddy current equipment, corrosion on the inside of thin metals, like the aluminium used in aircraft skin, can be found and measured. When corrosion is present on the second and third layers of metal but cannot be seen using ultrasound, it can be found using low-frequency probes.
Eddy Current Array
A single eddy current sensor or coil is multiplexed in a certain order to create an eddy current array (ECA) with the required sensitivity. Eddy current probes come in a wide range of configurations, such as flexible, cushioned, and spring-loaded fingers. The user can determine the most effective central frequency for the inspection using the multi-frequency technique, making it feasible to inspect a test object with multiple central frequencies simultaneously. After results analysis, the equipment's software allows for the application of filters and other procedures to highlight or obscure particular aspects.
Eddy Current Measurements - The Road Ahead
Eddy current measurement methods are still being improved through extensive research. Here’s a glimpse of the research being conducted in this field.
- Photoinductive Imaging
Photoinductive imaging (PI), a method that was invented at CNDE, is a potent, high-resolution scanning and imaging tool. Standard-sized eddy-current sensors can provide microscopic resolution. The creation of photoinductive (PI) imaging probes and equipment is based on the employment of a medium-power (5 W nominal power) argon ion laser. The use of this probe to examine cracks, welds, and diffusion bonds in metallic specimens has produced high-resolution images.
- Pulsed Eddy Current Testing
Pulsed eddy current (PEC) testing is a technology that is being widely studied. Using this method, corrosion and cracking in multi-layer aluminum Aircraft Structures can be found and measured. Due to the skin effect, pulsed eddy-current signals have a spectrum of frequencies, which means that each pulse signal comprises data from a variety of depths within a particular test specimen.
Additionally, the pulse signals have remarkable depth penetration due to their rich low-frequency nature. The pulse signals, as opposed to multi-frequency techniques, make for easy analysis. In multi-layer aluminium structures, Corrosion Testing has shown how material loss may be identified and measured.
Recent research on three- and four-layer constructions demonstrates the capability to identify cracks originating from fasteners. Ferromagnetic materials have also been subjected to pulsed eddy-current experiments. In recent studies, the case depth in samples of hardened steel has been measured.
Final Thoughts
Eddy Current test remains one of the most commonly used NDT Techniques used for detecting surface and sub-surface flaws of conductive materials. This non-contact method works on the principle of electromagnetic induction and has a wide variety of applications ranging from flaw detection to verification of heat treatment.
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Key Takeaways
- Eddy Current Testing is a widely used Non-destructive Testing technique to detect flaws or defects inside conductive materials.
- It involves the use of electronic probes powered by alternating current to create eddy currents for flaw detection.
- Eddy current inspection is based on Electromagnetic induction, creating a fluctuating magnetic field to induce eddy currents in conductive materials.
- Changes in conductivity or near-surface flaws affect the intensity of eddy currents, forming the basis of flaw detection.
- Eddy Current Testing methods include techniques like Eddy Current Array, Heat Exchanger Testing, Lorentz Force ETC, and Surface Array Testing.
- Handheld probes, surface array probes, and tubing array probes are commonly used for Eddy Current Testing.
- Applications include flaw detection, corrosion assessment, weld inspection, and material classification.
References
- Wermac
- TWI Global
