Test Instruments for Aviation
Frequently Asked Questions
Use Ctrl+f to search for FAQs pertaining to Eddy Current Testing.
What is NDT?
Nondestructive testing (NDT) is the process of testing or inspecting a material, component, or system without causing damage to the test subject. There are several varieties of nondestructive testing, including traditional ultrasonic flaw detection, phased array testing, and eddy current testing.
NDT includes a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. • Other terms commonly used the describe NDT are: Non-destructive examination (NDE) Non-destructive inspection (NDI) Non-destructive evaluation (NDE)
NDT does not permanently alter the material being inspected. • Time and cost effective as any problems can be rectified before becoming a terminal issue. • NDT is used as a safety monitor in many industries including ensuring the planes and trains we travel on.
​
What are the benefits of nondestructive testing?
​Nondestructive testing provides an abundance of advantages across a range of industries. Since NDT enables the tester to inspect materials such as metals, concrete, ceramics, plastics, and composites—even if only one side of the inspected part is accessible—it can be employed in both quality assurance and in-field testing of materials without needing to damage the test subject. NDT testing can be used to inspect pipes, bridges, dams, and other materials that require routine inspection.
​
What industries can eddy current apply to?
​NDT is used in a variety of settings that covers a wide range of industrial activity. • Automotive • Aviation / Aerospace including Engines, Propellers, turbines, Outer Shell & Wheel Inspection. • Power Plants • Construction including Structures, Bridges • Manufacturing • Machine parts • Castings and Forgings • Industrial plants such as Nuclear, Petrochemical, Power, Refineries, Pulp and Paper, Fabrication shops, Mine processing. • Pressure vessels • Storage tanks • Welds • Boilers • Heat exchangers • Pipelines • Leak Detection • Rail • Tubular NDT, for Tubing material • Amusement park rides • Submarines and other Naval warships • Medical imaging applications.
​
What is eddy current testing?
​Eddy Current Testing (ECT) is a non-destructive testing (NDT) method used to detect surface and sub-surface flaws in conductive materials. It works by inducing electrical currents (eddy currents) in the material, which are affected by any flaws or changes in the material's properties.
​
Who discovered eddy currents?
The phenomenon of eddy currents was discovered by French physicist Leon Foucault in 1851, and for this reason eddy currents are sometimes called Foucault currents. Foucault built a device that used a copper disk moving in a strong magnetic field to show that eddy currents (magnetic fields) are generated when a material moves within an applied magnetic field.
Eddy current testing began largely as a result of the English scientist Michael Faraday's discovery of electromagnetic induction in 1831. Faraday discovered that when a magnetic field passes through a conductor (a material in which electrons move easily)-or when a conductor passes through a magnetic field-an electric current will flow through the conductor if there is a closed path through which the current can circulate.
In 1879, another breakthrough was made when another English scientist, David Hughes, demonstrated how the properties of a coil change when placed in contact with metals of different conductivity and permeability. However, it was not until the Second World War that these developments in the transmitting and receiving of electromagnetic waves were put to practical use for materials testing.
​
Beginning in 1933, in Germany, while working for the Kaiser-Wilhelm-Institute, Professor Friedrich Förster adapted eddy current technology to industrial use, developing instruments for measuring conductivity and for sorting mixed-up ferrous components. In 1948, Förster founded his own company in Reutlingen, a business based on eddy current testing that continues to this day. Other companies soon followed. Many advances were made throughout the 1950s and 1960s, especially in the aircraft and nuclear industries.
There have been many recent developments in eddy current testing, leading to improved performance and the development of new applications. Eddy current testing is now a widely used and well-understood inspection technique for flaw detection as well as for thickness and conductivity measurements.
​
What phenomena is eddy current testing based on?
Eddy current testing is based on the physical 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, such as a metal test piece, a circular flow of electrons known as an eddy current begin to move through the metal like swirling water in a stream. That eddy current flowing through the metal in turn generates its own magnetic field, which interacts with the coil and its field through mutual inductance.
​
Changes in metal thickness or defects, such as near-surface cracking, 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.
​
What are the advantages of Eddy Current Testing?
ECT is fast, accurate, and doesn't require contact with the material, making it ideal for inspecting difficult-to-reach areas. It can detect cracks, corrosion, and thickness variations without damaging the component.
​
Which industries use Eddy Current Testing?
ECT is widely used in the aerospace, automotive, power generation (including nuclear), and manufacturing industries. It's commonly used to inspect aircraft components, steam generator tubes, heat exchangers, and welds.
​
Can Eddy Current Testing detect internal defects?
Yes, ECT can detect some subsurface defects, but the detection depth depends on the material's conductivity and the frequency of the test. It is most sensitive to surface and near-surface flaws.
​
What is the difference between Eddy Current Testing and Ultrasonic Testing?
ECT is better for detecting surface cracks in conductive materials, while Ultrasonic Testing (UT) is more effective for finding internal flaws and can be used on a wider range of materials, including non-conductive ones. Traditional ultrasonic testing (UT) uses the propagation of ultrasonic waves through the test subject to determine if any flaws or defects exist within the subject. Eddy current testing (ECT), on the other hand, uses electromagnetic induction to determine whether there are any internal flaws in conductive materials.
​
How are eddy currents used to find flaws?
Eddy currents are fields of alternating magnetic current that are created when an alternating electric current is passed through one or more coils in a probe assembly. When the probe is placed close to the part under inspection, the alternating magnetic field induces eddy currents in the test part. Discontinuities or property variations in the test part change the flow of the eddy current and are detected by the inspection probe, enabling material thickness measurements or the detection of defects such as cracks and corrosion.
​
What are some common types of eddy current probes?
Some probes that are commonly used for ECT are listed below with a description of the flaw types they target and parts they are usually used to inspect.
​
Surface probes: Used for identifying flaws on and below metal surfaces, surface probes usually have a large diameter to accommodate lower frequencies for deeper penetration or for scanning larger areas.
​
Pencil probes: Smaller diameter probes housing coils built for high frequencies for high resolution of near-surface flaws.
​
Bolt hole probes: Designed to inspect the inside of a bolt hole, these probes can be rotated by hand or automatically using a rotary scanner.
​
Donut probes: Designed to inspect aircraft fastener holes with the fasteners in place.
​
Sliding probes: Also used in testing aircraft fastener holes, sliding probes offer higher scan rates than donut probes.
​
ID probes: Used for the inspection of heat exchangers and similar metal tubing from the inside, internal diameter (ID) probes are available in a variety of sizes.
​
OD probes: Used for inspection of metal tubing and bars from the outside, with the test piece passing through the coil.
​
What is eddy current array (ECA) testing?
Eddy current array (ECA) is a nondestructive testing technology that provides the ability to electronically drive multiple eddy current coils, which are placed side by side in the same probe assembly. Each individual eddy current coil in the probe produces a signal relative to the phase and amplitude of the structure below it. This data is referenced to an encoded position and time and represented graphically as a C-scan image. Most conventional eddy current flaw detection techniques can be reproduced with ECA inspections; however, the remarkable advantages of ECA technology allow improved inspection capabilities and significant time savings.
​
ECA technology includes the following advantages:
-
A larger area can be scanned in a single-probe pass, while maintaining a high resolution.
-
Less need for complex robotics to move the probe; a simple manual scan is often enough.
-
C-scan imaging improves flaw detection and sizing.
-
Complex shapes can be inspected using probes customized to the profile of the part being inspected.
​
What are some common applications of eddy current testing?
Eddy current testing is widely used in the aerospace industry and in other manufacturing and service environments that require inspection of thin metal for potential safety-related or quality-related problems. In addition to crack detection in metal sheets and tubing, eddy current testing can be used for certain metal thickness measurements such as identifying corrosion under aircraft skin, measuring conductivity and monitoring the effects of heat treatment, and determining the thickness of nonconductive coatings over conductive substrates. Both field portable and fixed system instruments are available to meet a wide variety of test needs.
​
Eddy current NDT can examine large areas very quickly, and it does not require the use of coupling liquids. In addition to finding cracks, eddy current testing can also be used to check metal hardness and conductivity in applications where those properties are of interest, and to measure thin layers of nonconductive coatings, such as paint on metal parts. At the same time, eddy current testing is limited to materials that conduct electricity and thus cannot be used on plastics. In some cases, eddy current and ultrasonic testing are used together as complementary techniques, with eddy current having an advantage for quick surface testing and ultrasonics having better depth penetration.
​
Listed below are some of the most common eddy current testing applications:
​
Weld inspection—Many weld inspections use ultrasonic NDT for subsurface testing and a complementary eddy current method to scan the surface for open surface cracks on weld caps and in heat-affected zones (HAZ).
​
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 such as heat exchangers are common eddy current applications. Both cracking and thickness variations can be detected.
​
What are reference standards used for in eddy current testing?
An eddy current system consisting of an instrument and a probe must always be calibrated with appropriate reference standards at the start of a test. This process involves identifying the baseline display from a given test piece and observing how it changes under the conditions that the test is intended to identify. In flaw detection applications, this calibration process typically involves the use of reference standards of the same material, shape, and size as the test piece, containing artificial defects such as saw cuts, drilled holes, or milled walls to simulate flaws. In thickness measurement applications, the reference standards would consist of various samples of known thicknesses. The operator observes the response from the reference standards and then compares the indications from test pieces to these reference patterns to categorize parts. Proper calibration with appropriate reference standards is an essential part of any eddy current test procedure.
​
What are pencil surface probes used for?
These are the probes normally used for surface-crack detection, also known as high-frequency eddy current (HFEC) probes. They have a small coil that can be made shielded or unshielded. Most are absolute types, although they can be made with the balance coil built into the probe body to ensure good balance and increased frequency range. There are many types available, both in straight and angled versions, to match any requirements. They are also available with flexible shafts that can be adjusted to different shapes.
​
Pencil probes can be designed to operate at various frequencies, depending mostly on the material to be tested. For aluminum, 100 kHz is the most popular, enabling use of up to 200 kHz or more, depending on the balance coil and the instrument used. Higher frequencies will give better angle to liftoff, although as the probe approaches 500 kHz it becomes more liftoff sensitive and will not penetrate as much material. Because of this, it is normally preferable to stay at lower frequencies.
​
It has become common to use pencil probes at lower than 100 kHz when looking for first-layer cracks that originate in the opposite side of the layer and are growing but have not broken the surface yet (such as with clad skins). A frequency between 20 kHz and 50 kHz will penetrate the cladding and detect a defect that is only 50% through the thickness. Some standard 100 kHz probes can be operated at 50 kHz provided that we compensate for this by using higher gains; however, it is better to use probes designed for the lower frequencies, even if we have to accept a slightly larger diameter.
​
For low-conductivity materials, such as titanium or stainless steel, it is necessary to choose a frequency of 1 MHz to 2 MHz to improve the sensitivity and phase angle to surface breaking cracks. For magnetic steels, frequency is not as critical of a factor; although to minimize permeability variations, good results are often obtained at 1 MHz or 2 MHz. When the material is cadmium-plated, lower frequencies are needed to minimize its effect and sometimes a frequency of 25 kHz to 50 kHz is the best, although a bigger probe diameter is required.
​
What are eddy current spot probes used for?
Also known as low-frequency eddy current (LFEC) probes, spot probes are used at low frequencies for subsurface detection of cracks and/or corrosion. They are available in 100 Hz and higher (to penetrate the thicker structures), in both shielded and unshielded versions. Shielded probes are more popular as they concentrate the magnetic field under the probe and avoid interference from edges and other structures; however, they are more sensitive to small defects. Reflection types are also widely used because they offer lower drift and often higher gain for more demanding applications. Spring-loaded bodies are useful to maintain a constant pressure when needed, such as when spot testing for conductivity differences.
​
What are eddy current ring probes used for?
These are similar to surface spot probes, except that the center has been enlarged (and made into a hole) to encompass the diameter of the fastener head/hole to be inspected. They provide greater sensitivity to cracks, as the fastener/hole interface aids the penetration. This is even more noticeable with ferrous fasteners, but permeability variations can also cause problems. The internal diameter (ID) is an important dimension for probe selection. You should choose an ID that is slightly larger than the fastener head. The outside diameter (OD) is not normally critical, but it should not overlap neighboring fastener heads. The probe height is not critical; however, in cases of limited access, special low-profile types are available where the test coil and balance coil sections of the probe are separated to further reduce the height of the probe.
​
What are eddy current bolt hole probes used for?
Bolt hole probes are designed to inspect the bore of holes after the fastener is removed. They can be divided into two groups:
Manually operated with adjustable collar—The probe is indexed to the right depth and rotated manually. Typical coil configuration used with the manual bolt hole probes is absolute, bridge and bridge differential.
​
Rotating scanner—These are manufactured to mate with the various scanners in use and provide the best coverage and high inspection speeds. Rotating scanner probes typically contain reflection-differential coil configurations as the differential coils are less sensitive to interface and provide better detection of defects. Reflection mode is used to maximize gain, provides a wider frequency range, and minimizes drift, which could be caused by heat build-up in the probe as it rotates at high RPM's.
​
Do I need Level III consulting services for my Eddy Current Testing?
Level III consultants offer advanced expertise in eddy current testing, helping companies with procedure development, training, certification, and complex inspections. If you're facing unique inspection challenges or need to develop an ECT program, consulting a Level III expert can ensure compliance and efficiency.
​
How often should equipment used in Eddy Current Testing be calibrated?
Calibration schedules depend on the type of equipment, usage, and regulatory requirements. Typically, most ECT equipment is calibrated annually, but some industries may require more frequent calibration based on usage conditions.
​
What's the best way to troubleshooting eddy current probes?
When experiencing difficulty in operating a probe, it is advisable to do some simple tests:
​
-
Check that the operating frequency is within the probe’s range. If the probe is not balancing properly, the instrument may have entered into “saturation.” This can be verified easily. If the signals produced by liftoff and the defect (or an edge) superimpose on each other, there is no phase angle and saturation has occurred. The frequency may be too high, or the probe coil and the balance coil are not of the same value. Try lowering the probe drive voltage. Note that some instruments have the capability of very high output values that may be excessive for some probes.
-
Try moving the cable, particularly where it joins the connector or the probe body as these are the vulnerable points. If it shows intermittent operation, the cable needs replacement. Also, it may be necessary to clean the connector contacts. Silicon spray or an electrical contact cleaner will often help.
-
If the dot appears dead or the signals are small and/or distorted, check the filter settings. Many instruments now offer a range of high-pass and low-pass filters. These are very useful, but if set incorrectly will cause various effects.
-
High-pass filters (HPF) will always bring the dot to the balance point and at high settings (as used for rotating scanners), they make the dot appear as static at the balance point. For hand operation, set the high-pass filter to OFF (or 0 Hz).
-
Low-pass filters (LPF) will make the display speed dependent. The best setting for manual use is typically 100 Hz, but if the signal is too noisy it may be necessary to reduce this setting. If so, the scanning speed will need to be kept low enough that it does not to reduce the size of the signals.
-
​
-
Examine the probe test surface. It may be damaged or worn. Watch for exposed wires or other damage. Use Teflon tape at the probe face whenever possible. This reduces probe wear and prevents possible contact with the ferrite, which often produces noise.
-
When confronted with high signal-to-noise ratios (SNR), typically seen when using rotating scanner probes, it is good practice to insert a small piece of sponge or foam rubber to enhance the coil’s contact with the inner surface of the hole. This technique can greatly reduce noise and increase sensitivity.
​
Can Eddy Current Testing be used on non-metallic materials?
No, ECT is specifically for conductive materials such as metals. For non-metallic materials, other NDT methods like Ultrasonic or Radiographic Testing are more suitable.
​
How deep can Eddy Current Testing penetrate?
The depth of penetration in ECT depends on the test frequency and the material being inspected. Higher frequencies are used for shallow inspections, while lower frequencies can penetrate deeper, though with reduced sensitivity. One-quarter of an inch is considered very deep for eddy current testing.
​
What is the role of Eddy Current Level I, II, and III personnel?
-
Level I: Technicians perform standard ECT under the supervision of higher-level personnel.
-
Level II: Technicians can set up and calibrate equipment, conduct tests, and interpret results.
-
Level III: Experts develop procedures, train and certify technicians, and provide advanced solutions for complex inspection problems.
Do I have to use array coils for Eddy Current Testing?
No, you don’t have to use array coils for every inspection. Conventional eddy current (EC) probes can be sufficient for simple inspections or smaller areas. However, if you're inspecting large surfaces or complex geometries, eddy current array (ECA) probes can improve coverage and reduce inspection time.
What are the benefits of using Eddy Current Array (ECA) probes?
Eddy Current Array (ECA) probes allow for faster inspections by covering larger areas in a single pass. They can detect more flaws in less time, provide better mapping of defects, and reduce the need for complex scanning patterns, especially in hard-to-reach areas.
​
Are Eddy Current Array (ECA) probes and instruments more expensive than conventional Eddy Current (EC) testers?
Yes, ECA probes and instruments are generally more expensive than conventional EC systems due to their increased complexity, larger number of coils, and advanced data processing capabilities. However, the cost is often justified by improved efficiency, faster inspections, and more comprehensive data.
​
When should I consider switching to Eddy Current Array (ECA) technology?
Consider switching to ECA if you're performing inspections on large surfaces, complex geometries, or components with tight schedules. ECA is especially useful when high coverage rates, better defect detection, and precise mapping are critical, such as in aerospace or large heat exchanger inspections.
​
How do I go about purchasing Eddy Current probes?
When purchasing eddy current probes, consider the type of inspection you're performing, the material being inspected, and the defect types you expect to find. Reach out to trusted manufacturers or resellers, provide your inspection requirements, and ask for recommendations on the most appropriate probe type for your needs.
​
How do I know whether to purchase absolute probes, reflection probes, or other types of probes?
​The type of probe depends on the specific application:
-
Absolute probes: Best for detecting general material properties and surface defects.
-
Reflection probes: Ideal for detecting subsurface flaws.
-
Differential probes: Sensitive to small surface discontinuities and material variations. Consulting with a Level III expert can help you choose the right probe based on the material, geometry, and flaw types.​
What is the difference between conventional Eddy Current probes and Eddy Current Array probes?
Conventional EC probes use a single coil or a small number of coils, limiting the area covered in one pass. ECA probes, on the other hand, use multiple coils arranged in an array, allowing them to cover larger areas with greater speed and accuracy, and providing detailed data in real-time.
​
Can I use the same instrument for both conventional Eddy Current testing and Eddy Current Array (ECA)?
Some advanced ECA instruments are compatible with both conventional and array probes, offering flexibility in your inspection approach. However, not all conventional EC instruments can handle ECA probes due to the differences in data processing and requirements.
​
What should I consider when purchasing Eddy Current instruments?
When purchasing EC instruments, consider the types of probes you'll be using, whether you'll need support for ECA technology, portability, data processing capabilities, and whether the instrument can be integrated with your existing systems. Cost, ease of use, and after-sales support are also important factors.
​
What are some benefits of conventional 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.
What are the benefits of eddy current array (ECA) 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.
​
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.
​
What is multiplexing?
​
Multiplexing is the process by which multiple analog message signals are combined into one digital signal on a shared medium. When eddy current array data is multiplexed, the individual eddy current coils are excited at different times, allowing the system to excite all of the coils in the probe without ever exciting any two adjacent coils at the same time. An undesirable effect known as mutual inductance (magnetic coupling between coils in close proximity) is minimized with the use of an internal multiplexing system to carefully program the exact time that each coil is excited to transmit its eddy current signal. The signals are then reassembled before being displayed as an image. In addition to the enhanced imaging capabilities of multiplexed data, multiplexing allows any individual coil (data) channel to be analyzed after inspection. Multiplexing allows an increased channel resolution, increased coil sensitivity (through the reduction of mutual inductance), and a reduced noise level. This ultimately leads to an improved signal-to-noise ratio.
​
Can I use Eddy Current Testing on coated or painted surfaces?
Yes, eddy current testing can be performed on coated or painted surfaces, provided the coating is non-conductive and not too thick. The eddy current signal can pass through the coating to inspect the material underneath. However, the depth of flaw detection may be affected by the thickness of the coating.
What are the applications of Eddy Current Testing (ECT)?
Eddy current testing is widely used across multiple industries for different types of inspections and evaluations. Here are several common applications:
​
Conductivity Sorting
ECT can measure the electrical conductivity of materials, making it ideal for sorting different metals or alloys based on their conductivity. For example, in the aerospace industry, it is used to sort high-strength aluminum alloys by their electrical properties.
​
Flaw Detection
ECT is commonly used to detect surface and near-surface cracks or flaws in metal components. In aviation, it's applied to inspect turbine blades or aircraft fuselages for cracks, ensuring structural integrity.
​
Confirming Case Depth in Heat-Treated Metals
ECT is used to confirm the depth of hardened layers in case-hardened steel components, such as gears or bearings. This ensures that parts have the correct hardness and wear resistance.
​
Alloy Sorting
By measuring conductivity and permeability, ECT can help sort different alloys, even when they appear visually similar. This is valuable in recycling industries or quality control when ensuring material consistency.
​
Thickness Testing of Thin Materials
ECT can be used to measure the thickness of non-ferrous materials, coatings, or plating. For example, it's frequently used to check the thickness of aluminum, copper, or non-conductive coatings in manufacturing processes.
​
Weld Inspection
ECT is used to inspect welds for cracks or discontinuities in critical applications such as pipelines, pressure vessels, and structural steel. It helps detect surface-breaking flaws that could compromise weld integrity.
​
Heat Exchanger and Steam Generator Tube Inspection
ECT is widely used for inspecting heat exchanger and steam generator tubes in power plants, including nuclear facilities. It detects thinning, pitting, and cracking in tubes, ensuring their reliability and safety.
​
Corrosion Detection
ECT can detect surface corrosion, pitting, or material thinning in conductive materials, which is vital in industries like oil and gas or marine, where corrosion can lead to significant safety risks.
​
Coating Thickness Measurement
ECT is employed to measure the thickness of non-conductive coatings on conductive substrates. This application is useful in industries like automotive or electronics, where coating thickness is critical for performance and durability.
​
Tube and Pipe Testing
ECT is used to inspect tubing and piping for defects such as cracks, corrosion, and wall thinning. This is crucial in industries like petrochemical and power generation, where the integrity of piping systems is essential.
​
Eddy current testing is often referred to as "black magic" because interpreting the signals can be complex and requires a deep understanding of both the technology and the material being tested. Variations in signals due to material properties, surface conditions, or probe positioning can make it difficult to provide a straightforward, consistent answer, leading to a perception of mystery around the results.
​
Why don’t engineers and managers always trust Eddy Current Testing results?
Eddy current results can sometimes be hard to interpret without expert knowledge. Factors like probe setup, material conditions, or operator experience can affect the outcome, leading to inconsistency or uncertainty in the results. This can cause hesitation or skepticism from engineers and managers who may not fully understand the nuances of the technology or may have seen varied results in the past.
​
Is Eddy Current Testing one of the hardest NDT methods to learn?
Yes, eddy current testing is considered one of the more difficult non-destructive testing (NDT) methods to master. It requires an understanding of electromagnetic theory, material properties, and signal interpretation, all of which can vary depending on the test environment. It demands a combination of technical skill, experience, and knowledge to use effectively.
​
Do you have to be good at math to excel in Eddy Current Testing?
While you don't need advanced math skills for day-to-day operations, understanding the principles behind eddy current testing—such as how electromagnetic fields interact with materials—does require a solid grasp of basic math and physics. Operators need to understand signal patterns, frequencies, and material properties, which often involve calculations and critical thinking.
​
What makes Eddy Current data analysis so difficult?
Eddy current data analysis is challenging because the signals can be influenced by many variables, such as material conductivity, thickness, probe frequency, surface conditions, and even the orientation of defects. A small change in one factor can significantly alter the results, making it essential to have a deep understanding of the material and inspection parameters to interpret the data accurately.
​
Why does it seem like everyone gives different answers when interpreting Eddy Current signals?
Eddy current signals are often open to interpretation because the same signal can be affected by multiple factors. Different technicians may focus on different aspects of the signal—such as amplitude, phase shift, or noise—and weigh those factors differently based on their experience. This subjectivity can lead to variations in how results are interpreted, which is why training and experience are critical for consistent results.
​
Can Eddy Current Testing detect all types of flaws?
Eddy current testing is excellent for detecting surface and near-surface flaws in conductive materials, but it may not detect deeper internal flaws or certain types of defects. The depth of penetration is limited by the frequency used, and materials with high conductivity can further reduce the detection depth. Understanding these limitations is essential for proper application.
​
Is probe selection important in Eddy Current Testing?
Yes, selecting the correct probe is critical in eddy current testing. The type of probe, its frequency, and coil configuration directly affect the sensitivity and accuracy of the test. Using the wrong probe can lead to false positives, missed flaws, or ambiguous data, complicating the interpretation process.
​
Why does Eddy Current Testing require more operator skill than some other NDT methods?
Eddy current testing requires more operator skill because of its sensitivity to multiple variables like frequency, probe positioning, and material properties. A small mistake in setup or interpretation can lead to inaccurate results. Unlike other methods, such as radiography or ultrasonic testing, which often provide more straightforward visual data, eddy current testing relies heavily on the operator's ability to interpret complex signals.
​
How can Eddy Current Testing results be made more reliable?
Consistency in setup, calibration, and operator experience can improve the reliability of eddy current results. Using standardized procedures, proper training, and regular equipment calibration helps minimize variables that could skew results. Consulting with Level III experts for complex inspections or data analysis can also ensure more accurate interpretations.
Why is Eddy Current Testing sometimes called "black magic"?
What is Eddy Current Testing (ECT) useful for?
Eddy Current Testing is useful for a variety of applications, including detecting surface and near-surface flaws, measuring material thickness, sorting conductive materials, verifying heat treatment depth, and inspecting welds. It is widely used in industries like aerospace, automotive, and power generation.
​
What can Eddy Current Testing (ECT) detect?
ECT can detect surface cracks, corrosion, material thinning, conductivity variations, and other defects in conductive materials. It is particularly sensitive to surface-breaking flaws and can detect some subsurface defects depending on the material and frequency used.
​
What are the limitations of Eddy Current Testing (ECT)?
ECT has limitations in terms of penetration depth and material suitability. It is primarily used for conductive materials, such as metals, and may not effectively detect deeper internal flaws. Additionally, the technique is sensitive to surface conditions, and coatings or irregular geometries can affect results.
​
How often should our Eddy Current Testing equipment be calibrated?
ECT equipment should typically be calibrated annually, but the exact frequency depends on the usage, the environment, and industry standards. Some applications or regulatory requirements may mandate more frequent calibration to ensure accuracy and reliability in testing.
​
Where can I learn more about Eddy Current Testing, Eddy Current Array, and other Non-Destructive Testing (NDT) methods?
There are various resources to learn more about ECT, including NDT-specific training programs, professional organizations such as the American Society for Nondestructive Testing (ASNT), and online courses. Consulting with Level III experts or enrolling in formal certification programs can also deepen your knowledge.
Can you make a lot of money in NDT?
Yes, NDT can be a lucrative career, especially for certified technicians and inspectors. The income potential increases with higher certifications, specialized skills, and experience in industries like aerospace, oil and gas, and nuclear power. Level III certified professionals can earn significantly more than entry-level positions.
​
Which NDT method pays the most?
Typically, methods like ultrasonic testing (UT), radiographic testing (RT), and eddy current testing (ECT) offer higher salaries, particularly when working in industries such as nuclear, oil and gas, and aerospace. The complexity and risk associated with these methods, as well as the need for specialized training, contribute to higher pay.
​
How much does it cost to go to school for NDT?
The cost of NDT education varies depending on the institution and the program. A basic certificate program can range from $5,000 to $15,000, while more advanced training or degree programs may cost more. Additional expenses include certification exam fees and materials.
​
What are the subjects studied in an NDT curriculum?
NDT programs cover subjects such as materials science, physics, safety protocols, and specialized training in methods like ultrasonic testing, radiographic testing, magnetic particle testing, and eddy current testing. Students also learn about interpreting test results, quality assurance, and industry standards.
​
What certifications are required to work in NDT?
Certifications in NDT are typically governed by standards like ASNT's SNT-TC-1A, which offers Level I, II, and III certifications. Most entry-level technicians start with Level I and II certifications in specific methods like UT, RT, or ECT. Level III certification requires more experience and expertise, often leading to more advanced roles.
​
How long does it take to become certified in NDT?
The time to become certified varies by the method and certification level. Level I and II certifications generally require a combination of classroom training and on-the-job experience, which can take a few months to a year. Level III certification can take several years, depending on experience and industry requirements.
​
Is NDT a physically demanding job?
Yes, NDT can be physically demanding, especially in industries like construction, oil and gas, or aerospace, where technicians may need to work in confined spaces, at heights, or in hazardous environments. However, some roles, such as data analysis or consulting, are less physically intensive.
​
What are the career advancement opportunities in NDT?
Career advancement in NDT includes moving from technician roles to inspector, supervisor, or Level III specialist positions. Experienced NDT professionals can also become consultants, trainers, or quality assurance managers. The more certifications and experience you acquire, the more opportunities for advancement.
​
Do you need to be good at math and science for NDT?
A solid understanding of math and science is beneficial in NDT, especially for methods like ultrasonic and eddy current testing, which involve wave theory and electromagnetic principles. However, much of the technical knowledge can be learned through formal training and hands-on experience.
​
Is there a high demand for NDT professionals?
Yes, the demand for NDT professionals is high, particularly in industries like aerospace, energy, manufacturing, and infrastructure. As safety regulations become more stringent, the need for skilled inspectors to ensure the integrity of materials and structures continues to grow.
I thought eddy currents were a bad thing. Why are they used in testing?
Eddy currents can be undesirable in some applications, such as in electrical motors or transformers, where they cause unwanted energy loss and heat. However, in eddy current testing, these currents are intentionally generated and controlled to detect flaws in conductive materials. The interaction between eddy currents and material defects provides valuable information for non-destructive testing purposes.
​
Can eddy currents shock you?
No, the eddy currents used in eddy current testing are too small and localized to cause any physical harm. They are generated by low-power alternating magnetic fields and are not strong enough to induce a harmful current in the human body.
​
Can you get electrocuted when doing Eddy Current Testing?
No, eddy current testing is generally a very safe process. The equipment operates at low voltages, and the electromagnetic fields generated are not dangerous. As long as the equipment is used properly and safety protocols are followed, there is no risk of electrocution during testing.
​
Why are eddy currents bad in some industries but useful in testing?
Eddy currents are considered undesirable in industries like electrical power generation, where they can cause heat and energy loss in conductive materials. In testing, however, these same currents are harnessed for a positive purpose: to detect flaws, measure thickness, and evaluate material properties. By controlling and analyzing eddy currents, technicians can inspect materials without causing damage.
​
Do eddy currents cause heat during testing?
While eddy currents can generate heat in some applications, the amount of heat produced during eddy current testing is minimal and not a safety concern. The testing process is designed to use low-power electromagnetic fields, so any heat generated is negligible.
​
Can eddy current testing damage the material being tested?
No, eddy current testing is non-destructive, meaning it does not harm the material being inspected. The testing process involves inducing currents into the material and analyzing the response, but this process does not alter or damage the material in any way.
​
Do eddy currents affect electronics or nearby equipment during testing?
​
Eddy current testing generates localized electromagnetic fields that typically do not interfere with nearby electronics. However, in some sensitive environments, precautions may be taken to ensure no interference occurs. Modern eddy current equipment is designed to minimize any such risks.
​
Why don’t eddy currents in testing cause the same problems as in electrical equipment?
In electrical equipment, eddy currents are unwanted because they can cause heat and energy loss. In eddy current testing, however, the currents are carefully controlled and used for a specific purpose: to detect flaws in conductive materials. The testing process involves low-energy fields, so it avoids the negative effects seen in other applications.
​
Can eddy currents penetrate deeply into materials?
Eddy currents generally only penetrate the surface or near-surface of conductive materials, which is why they are primarily used for detecting surface and near-surface defects. The depth of penetration depends on factors such as the frequency used and the material’s conductivity.
​
Is there any radiation exposure during Eddy Current Testing?
No, eddy current testing does not use ionizing radiation like some other NDT methods, such as radiographic testing. The electromagnetic fields used in ECT are safe and non-invasive, with no harmful radiation exposure.
What are the common safety hazards during Eddy Current Testing?
Although eddy current testing itself is a non-hazardous and non-destructive process, technicians may face safety risks depending on the environment in which the testing occurs. These include confined spaces, heat exhaustion, working near rotating equipment, and operating in high-energy or radiological environments.
​
What are the risks of working in confined spaces during Eddy Current Testing?
Confined spaces, such as inside tanks or heat exchanger tubes, can pose several risks, including limited airflow, difficulty in quick evacuation, and the potential for hazardous gas exposure. Proper ventilation, communication protocols, and confined space entry training are crucial for safely conducting testing in these environments.
​
Can heat exhaustion be a concern during Eddy Current Testing?
Yes, heat exhaustion is a concern, particularly when testing in hot environments like industrial plants or in direct sunlight. Technicians should take regular breaks, stay hydrated, and monitor for signs of heat exhaustion such as dizziness, fatigue, or excessive sweating. Proper personal protective equipment (PPE) and cooling strategies should be used.
​
What safety precautions should be taken when working near rotating equipment?
When conducting eddy current testing near rotating machinery, technicians must ensure the equipment is properly shut down and locked out to prevent accidental movement. Loose clothing, jewelry, or hair should be secured to avoid entanglement, and maintaining clear communication with equipment operators is essential to avoid unexpected hazards.
​
What are the dangers of working in high-energy environments during Eddy Current Testing?
High-energy environments, such as power plants or industrial facilities with live electrical systems, present the risk of electrical shock, arc flashes, or explosions. Technicians should be familiar with lockout/tagout procedures, wear appropriate electrical PPE, and ensure proper grounding and isolation of equipment before starting the inspection.
​
Can Eddy Current Testing be done in radiological environments?
Yes, eddy current testing is often used in radiological environments such as nuclear power plants, particularly for inspecting steam generator tubes. However, technicians working in these environments must follow strict radiation safety protocols, including wearing dosimeters, minimizing exposure time, and adhering to regulatory limits for radiation exposure.
​
What personal protective equipment (PPE) is required for Eddy Current Testing?
PPE requirements vary depending on the testing environment. In industrial settings, technicians may need hard hats, safety glasses, steel-toed boots, and hearing protection. For radiological environments, additional PPE such as lead aprons and radiation monitors may be required. In confined spaces or around hazardous materials, respirators or specialized suits may be necessary.
​
What safety measures should be taken when testing in elevated or hard-to-reach areas?
When testing in elevated areas, fall protection systems such as harnesses and lanyards are essential. Technicians should also be trained in safe ladder usage, scaffolding protocols, and working on aerial platforms. Clear communication with team members is critical to avoid accidents in these situations.
​
How do you manage the risk of exposure to toxic chemicals or gases during Eddy Current Testing?
In environments where toxic chemicals or gases are present, technicians should perform air monitoring before entering the area and use appropriate respiratory protection. Testing in confined spaces or near hazardous chemicals may require the use of ventilation systems, respirators, or chemical-resistant PPE.
​
How can technicians ensure they are safe when performing Eddy Current Testing in high-temperature environments?
In high-temperature environments, technicians should use heat-resistant PPE, take frequent breaks in cooler areas, and stay hydrated. It is also important to monitor the equipment being tested to ensure it is cool enough to touch and that heat sources are properly shielded or deactivated during the inspection process.
What is the difference between Eddy Current Testing and Remote Field Testing (RFT)?
Eddy current testing (ECT) is primarily used for detecting surface and near-surface flaws in conductive materials. Remote field testing (RFT), on the other hand, is designed to inspect ferromagnetic tubing, such as carbon steel, where deeper flaw detection is necessary. In RFT, the signal travels through the material twice (from the exciter coil to the detector), allowing it to detect defects deeper in the material, whereas ECT is limited to shallow surface flaws.
​
Is Alternating Current Field Measurement (ACFM) testing related to Eddy Current Testing?
Yes, ACFM is related to eddy current testing in that both are electromagnetic testing methods. ACFM is specifically used to detect and size surface-breaking cracks in metals, particularly in welded structures. Unlike ECT, ACFM doesn't require direct contact with the surface, and it is more suited to inspecting complex geometries and coated materials without removing coatings.
​
Is Remote Field Testing (RFT) more difficult than Eddy Current Testing (ECT)?
RFT can be more challenging than ECT in some cases because it requires a deeper understanding of electromagnetic wave propagation and the interpretation of signals that travel through and around ferromagnetic materials. Additionally, RFT signals are more complex due to the long-range nature of the test, making signal interpretation more difficult than the near-surface signals in conventional ECT.
​
What are some of the pros and cons of Remote Field Testing (RFT)?
Pros:
-
RFT is excellent for inspecting ferromagnetic materials like carbon steel, which are difficult to test with conventional ECT.
-
It can detect both internal and external flaws, offering full coverage of the tube wall.
-
RFT has a deep penetration range, making it effective for thicker materials and larger structures.
Cons:
-
RFT equipment is more specialized, which can make it more expensive and harder to obtain than conventional ECT equipment.
-
Signal interpretation can be more complex, requiring advanced training and expertise.
-
It is typically slower than ECT due to the time required for deeper penetration and signal processing.
​​
Which method is better for inspecting non-ferrous materials, Eddy Current Testing or Remote Field Testing?
Eddy current testing is generally better for inspecting non-ferrous materials like aluminum, copper, and stainless steel, where surface and near-surface flaws are the primary concern. RFT is more effective for ferrous materials, such as carbon steel, where deeper penetration is needed to detect flaws.
​
Can Remote Field Testing (RFT) be used for heat exchanger tube inspections?
Yes, RFT is commonly used for inspecting ferromagnetic heat exchanger tubes, such as those made from carbon steel. It is highly effective in detecting both internal and external corrosion, pitting, and wall thinning, making it a popular choice for power plants and other industries with ferromagnetic tubing.
​
How does probe design differ between Eddy Current Testing and Remote Field Testing?
In ECT, probes typically have one or more coils for inducing and detecting eddy currents near the surface of the material. RFT probes, however, have a separate exciter coil and detector coil, spaced farther apart. The exciter coil induces a magnetic field, and the detector coil senses the field after it has traveled through the material, allowing for deeper inspection.
​
What type of defects can Remote Field Testing detect?
Remote field testing can detect a wide range of defects, including internal and external corrosion, pitting, wall thinning, and cracking in ferromagnetic materials. It is particularly effective for detecting deep-seated defects that may not be accessible with surface-level methods like ECT.
​
What Is Eddy Current Testing?
Eddy current testing is one of several non-destructive testing methods that uses the electromagnetism principle for flaw detection in conductive materials. A specially designed coil energised with an alternating-current 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.
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 the phase and amplitude of the measured current. The changes are shown on a screen and are interpreted to identify defects.
​
Eddy currents are also known as Foucault currents after French physicist Leon Foucault, who developed a device that demonstrated that eddy currents are created when a material travels within an applied magnetic field. Professor Friedrich Forster customized eddy current technology for industrial applications in 1933, but all eddy current array technologies follow the same principle.
​
How Does Eddy Current Testing Work?
The process relies upon a material characteristic known as electromagnetic induction. When an alternating current is passed through a conductor – a copper coil for example – an alternating magnetic field is developed around the coil and the field expands and contracts as the alternating current rises and falls. If the coil is then brought close to another electrical conductor, the fluctuating magnetic field surrounding the coil permeates the material and, by Lenz’s Law, induces an eddy current to flow in the conductor. This eddy current, in turn, develops its own magnetic field. This ‘secondary’ magnetic field opposes the ‘primary’ magnetic field and thus affects the current and voltage flowing in the coil.
​
Any changes in the conductivity of the material being examined, such as near-surface defects or differences in thickness, will affect the magnitude of the eddy current. This change is detected using either the primary coil or the secondary detector coil, forming the basis of the eddy current testing inspection technique.
​
Permeability is the ease in which a material can be magnetised. The greater the permeability the smaller the depth of penetration. Non-magnetic metals such as austenitic stainless steels, aluminium and copper have very low permeability, whereas ferritic steels have a magnetic permeability several hundred times greater.
​
Eddy current density is higher, and defect sensitivity is greatest, at the surface and this decreases with depth. The rate of the decrease depends on the “conductivity” and “permeability” of the metal. The conductivity of the material affects the depth of penetration. There is a greater flow of eddy current at the surface in high conductivity metals and a decrease in penetration in metals such as copper and aluminium.
​
The depth of penetration may be varied by changing the frequency of the alternation current – the lower the frequency, the greater depth of penetration. Therefore, high frequencies can be used to detect near-surface defects and low-frequencies to detect deeper defects. Unfortunately, as the frequency is decreased to give greater penetration, the defect detection sensitivity is also reduced. There is therefore, for each test, an optimum frequency to give the required depth of penetration and sensitivity.
Standards and Calibration
All of the systems must be calibrated using appropriate reference standards – as for any NDT method and is an essential part of any eddy current testing procedure. The calibration blocks must be of the same material, heat treatment condition, shape and size of the item to be tested.
​
For defect detection, the calibration block contains artificial defects that simulate defects, whereas, for corrosion detection, the calibration block has different thicknesses. The eddy current method requires a highly-skilled operator - training is essential.
It is important to accurately measure the phase and amplitude of the signal in order to calibrate the probe, create the sizing curves, and estimate the depth of any flaws found during inspection.
​​
Advantages
There are a number of advantages with eddy current inspection methods for flaw detection and sizing:
-
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
-
A single pass inspection can help reduce inspection times
​
Limitations
-
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
Applications
-
Inspection of parts or components including:
Welded joints
Bores of in-service tubes
Bores of bolt holes
Metal tubes
Friction stir welds
Gas turbine blades
Nozzle welds in nuclear reactors
Hurricane propeller hubs
Cast iron bridges
Gas turbine blades
-
Detection of defects including:
Surface-breaking defects
Linear defects (as small as 0.5mm deep and 5mm long)
Cracks
Lack of fusion
Generalised corrosion (particularly in the aircraft industry for the examination of aircraft skins) -
Other applications
Identification of both ferrous and non-ferrous metals and with certain alloys – in particular the aluminium alloys
Establishing the heat treatment condition
Determining whether a coating is non-conductive
Heat treat verification of metals
Eddy Current Array
An eddy current array (ECA) is an extension of conventional eddy current testing (ECT), consisting of an assembly of single eddy current sensors coils or sensors that allow the testing to cover a larger area in a single pass than conventional, single-coil probes, such as pencil probes.
​
However, this can lead to suboptimal results, so the probe assembly is multiplexed in a certain order to obtain the required sensitivity. Multiplexing is when the coils are activated and deactivated in set sequences to minimise interference between coils in close proximity (known as mutual inductance) while maximising the array probe’s resolution and attaining the greatest width.
There are many different designs of eddy current probes, including flexible probes, padded probes or spring-loaded fingers. With conventional eddy current probes, the coil in the probe acts as both a transmitter and receiver – these are called ‘impedance probes’ – whereas transmit-receive probes use separate transmit and receive coils.
​
Advantages of Eddy Current Array
-
Reduction in inspection time, as:
The coils in the probe have been specifically arranged to cover a particular area
The scanning speed can be adjusted
A wider range of samples of can be inspected due to a wider range of probe types that are available
-
Multi-frequency approach – possible to inspect a test object with several central frequencies simultaneously – allowing the user to assess the most optimised central frequency for the inspection, while multiple encoded scans can create a C-scan image
-
Post analysis of results – the equipment’s built-in software enables the use of filters and other process to highlight or hide certain features
Limitations of Eddy Current Array
-
Eddy current array inspections use advanced equipment that requires additional operator training
-
Longer set-up time than a conventional eddy current inspection
Pulsed Eddy Current
Pulsed eddy current uses the same principle as conventional eddy current testing and methods are being developed to investigate surfaces through protective coatings, weather sheeting’s, corrosion products and insulation materials, using pulsed eddy current techniques.
​
Eddy Current Testing: A Guide
Eddy current testing is the process of running electronic probes through the length of various types of tubes or along the surfaces of materials in order to find flaws in them.
​
An eddy current is a current that runs opposite to the current introduced by a probe into a conductive material.
An eddy current test (ETC) is also commonly referred to as an eddy-current test, an eddie current test (though this is a misspelling), an electromagnetic test, or an electromagnetic testing.
​
Using eddy-current testing, inspectors can find very small defects that might not be visible to the naked eye.
The raw data gathered from eddy current test probes must be processed using software made for this purpose and then analyzed by trained inspectors, who know how to identify defects in ETC results.
​
Eddy current testing is one of several electromagnetic testing methods used for non-destructive testing (NDT), which refers to tests performed for the purposes of inspections that do not damage the material being tested.
​
[Eddy current testing NDT is just one of the non-destructive testing (NDT) methods that inspectors use. Learn more about NDT in this in-depth guide.]
What Is Eddy Current Testing?
In the eddy-current testing of tubes, an inspector will run a probe through the length of a tube in order to identify tiny defects.
Eddy current tests can also help identify defects on the surface of materials, with the use of pulsed eddy current testing equipment.
Here is how the eddy current testing of tubes works:
-
Probe. An inspector starts with a probe—for example, the single-element ETC probe, which uses an alternating current. The eddy current test probe consists of a coiled conductive piece of wire.
-
Magnetic field creation. When electrified, the probe will create an alternating magnetic field.
-
Introduce the field to the object. Once the field has been created, the inspector will introduce it to the object they want to inspect by moving it through the object.
-
Create eddy currents. When the magnetic field is introduced to the object or material, it will create currents running opposite to the currents in the probe. These currents are called eddy currents.
-
Collect data. Any defects present in the material will cause a change in these eddy currents, and inspectors collect this data after introducing the probe during their eddy current testing of tubes.
-
Evaluate the data. After the data has been collected it needs to be analyzed so that defects in the object can be identified. Note that the inspector who collects the data may not always be the same inspector who analyzes it, since these two activities require different levels of training and certification.
The History of Eddy-Current Testing
The eddy current phenomenon was first observed by researcher François Arago in 1824, but it is the inventor Léon Foucault who is actually credited with its discovery.
​
Foucault’s discovery happened in 1855, and was based in part on research conducted by Michael Faraday, who discovered the principle of electromagnetic induction in 1831.
​
This principle describes the relationship between electric currents and magnetic fields, and was the result of Faraday observing that a magnetic field will pass through a conductive material in a manner that varies over time as an electric current flows through it.
​
Despite these early observations, it wasn’t until 1879 that scientist David Hughes found a potential use for eddy currents. Hughes was able to demonstrate that the properties of a coiled, conductive wire changed when it came in contact with different kinds of conductive materials.
Eddy-current testing didn’t come into mainstream use until World War II, when Professor Friedrich Förster of Germany began exploring its industrial applications.
​
After the war, Förster founded a company called the Foerster Group that manufactured instruments for eddy current tests, further developing the technology and expanding its potential uses.
​
Today, ETC is one of the most common NDT methods used by inspectors, with a well established track record for providing reliable data.
​
Use Cases, Types of Flaws, and Industries
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.
On surfaces, ETC can be done with both ferromagnetic and non-ferromagnetic materials.
In the eddy current testing of tubes, ETC can primarily only be done with non-ferromagnetic tubing.
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
The types of materials eddy current testing is commonly used to inspect include:
-
Bores. Bolt hole bores, bores for in-use tubes.
-
Welds. Welded joints, nozzle welds, friction stir welds
-
Tubes. Steam generator tubing, metal tubing.
Here are the industries where inspectors most commonly use ETC:
-
Aerospace
-
Nuclear / Power Generation
-
Manufacturing
-
Oil & Gas
-
Petrochemical
-
Transportation
Eddy Current Testing Pros and Cons
ETC allows inspectors to find defects on the surface and subsurface level of an object easily and with a high degree of accuracy—but that’s just one of the reasons inspectors commonly use it to look for defects in a material.
Here’s a list of pros and cons for ETC:
Pros
-
It is incredibly versatile in terms of accuracy and portability/ease of use).
-
Its results are highly reliable, providing data of a high quality.
-
It is highly sensitive, allowing inspectors to identify defects as small as .5mm.
-
It is effective on surfaces that have paint or some other type of coating on them.
-
It can be used on high-temperature and underwater surfaces.
-
It provides immediate data.
-
It takes a relatively short amount of preparation time to perform (i.e., not much pre-cleaning or couplant is needed).
-
It can be automated for testing uniform parts, such as boiler tubes or wheels.
Cons
-
It only works with a current.
-
ECT current always runs parallel to the surface of a material, so a defect that doesn’t come in direct contact with the current can’t be detected—and this means that some defects may go undetected.
-
It’s not ideal for inspecting large areas.
-
It’s efficacy for different depths can vary.
-
It can be subject to changes in magnetic permeability, which can make it hard to use it for inspecting parts of ferromagnetic materials.It’s also non-conductive with ferromagnetic materials, as ECT equipment is subject to permeability changes on the welds.
-
Interpreting signals correctly can be difficult, since it may require weeding out non-relevant data points.
Electromagnetic Testing Procedure
There are several different eddy current testing methods.
Here are some of the most common ones:
1. Eddy Current Array
Eddy current array testing uses an array of electrically charged coils to create a sensitivity profile made to identify defects in a material.
In this kind of testing, inspectors have to be careful to avoid mutual inductance between the individual coils.
2. Heat Exchanger Testing
Heat exchanger testing is one of the most popular uses for eddy currents.
In this type of testing, inspectors use eddy currents to find defects in metal tubes, providing immediate data after a single pass through with eddy current testing probes.
​
3. Lorentz Force ETC
Lorentz force eddy current testing is a newer NDT method that uses multiple DC magnets to try and overcome the skin effect (that is, a cancellation of a flow’s current in the center of a conductor with a corresponding reinforcement in the skin).
In addition to the use of multiple magnets, the Lorentz force eddy-current testing uses relative motion to help inspectors conduct quick, accurate eddy current tests.
4. Surface Array Testing
Surface array testing is commonly used in the aerospace industry, where it can help measure conductivity as well as corrosion / wall thickness with a high degree of accuracy.
This type of testing is very versatile, and is capable of finding defects in places that are hard to access where other inspection methods may not work.
What is Pulsed Eddy Current (PEC) Testing?
Pulsed Eddy Current (PEC) & PEC Array (PECA) Inspection Services
Pulsed Eddy Current (PEC) and other advanced eddy current inspection techniques are ideal tools for detecting corrosion under insulation (CUI) and fireproofing (CUF).
Advanced eddy current inspection technology can help detect hidden corrosion before it becomes a potentially expensive and hazardous problem. MISTRAS utilized Pulsed Eddy Current (PEC) and PEC Array (PECA) inspection services for tanks, spheres, piping systems, and other assets.
​
Pulsed Eddy Current (PEC) Inspection Services
During pulsed eddy current (PEC) inspection, a probe induces eddy currents in a component, and the probe measures wall thickness by tracking the amount of time it takes the eddy currents to decay. The thicker the wall, the longer it takes for the eddy currents to decay to zero. PEC can be applied to in-service assets, and can detect damages through insulation and fireproofing, so it is an effective tool for corrosion-under-insulation (CUI) and flow-accelerated corrosion (FAC) assessments.
​
Small amounts of water can accelerate corrosion if faulty seals or inadequate construction allow it to penetrate the interface between metal and insulation or fireproofing materials in steel. Passive fire protection and insulation can hide signs of degradation, and if left undetected, corrosion can then cause the failure of steel support structures, piping systems, or vessels.
​
After performing a detailed visual inspection to look for signs of degradation, MISTRAS technicians will apply PEC technology to measure the wall thickness of the metal behind the fireproofing or insulation. PEC probes placed on the wall in question apply an electric charge to generate eddy currents in the steel, allowing probes to estimate wall loss based on measurements of the induced magnetic field.
PEC obtains a measure of the wall thickness by taking an average reading over the probe’s footprint (the roughly circular area where eddy currents flow). This makes it ideal for measuring general wall loss.
​
Using the PEC inspection technique offers a variety of benefits, including:
-
No interference with production. PEC technology can be applied in-service, so there is no need to empty storage spheres.
-
No need to remove the passive fire protection. PEC technology can measure through up to 250 mm of material including the reinforcement mesh.
-
Performed in service during plant operation
-
Accessibility issues are minimized as it can be used by rope access operators
PEC can be used to detect CUI and corrosion under fireproofing (CUF) in a wide variety of assets, including:
Petrochemical Plants & Refineries
-
Sphere Legs
-
Insulated Cryogenic Tanks and Piping
-
Tank Floors with Fiberglass Coatings
-
Pressure Vessels
-
Skirts
-
Feedwater & Condensate Piping
-
Drain Piping
-
Coal Feed Piping
-
Feedwater Heater Shells
-
Re-Circulating Water Piping
-
HRSG Finned Tubing
-
Offshore Platform Legs
-
Ship Hulls
-
Seawall Bulkheads
-
Bridge Pier Sheet Piling
-
Fireproofed Structural Beams
Pulsed Eddy Current Array (PECA) Services
MISTRAS Pulsed Eddy Current (PEC) Array services combine the advanced inspection capabilities of PEC with the speed of eddy current array (ECA) technology. Our combined PEC and ECA services enable fast, accurate, and in-service inspections for CUI/CUF in pipes, vessels, sphere legs, and more.
​
Combined PEC & ECA Technologies
In pulsed eddy current inspection, an electric current is introduced in a probe’s transmitter coil, which magnetizes the steel beneath the probe. The current is then switched off and, as a result, the steel demagnetizes. This generates eddy currents in the steel, which diffuse inwards from the steel surface and decay in strength as they propagate, which is detected by a set of receiver coils in the PEC probe.
ECA technology offers the ability to electronically drive multiple eddy current sensors placed side-by-side in the same probe assembly. When ECA data is multiplexed, the individual eddy current coils are excited at different times, allowing the system to excite all of the coils in the probe without ever exciting any two adjacent coils at the same time (avoiding mutual inductance between the individual sensors).
ECA enables larger coverage in a single probe pass while maintaining a high resolution, and improves flaw detection and sizing with C-scan imaging.
​
PEC Array Applications
PEC Array is able to conduct inspections through:
-
Insulation
-
Fireproofing
-
Fiberglass
-
Plastic Wrap
-
Concrete
-
Non-Ferrous Metals
-
Stainless Steel
-
Aluminum Weather Jackets
Benefits of PEC Array
-
Scans through insulation & fireproofing
-
Covers 18 inches in a single pass
-
Inspects 100% of the wall thickness
-
Provides relative wall thickness measurements
-
Does not require surface preparation
-
Assets can remain in-service during inspections
What are some common applications of eddy current testing?
​​
Surface Crack Detection
Surface crack detection is commonly performed using pencil probes or pancake-type probes on both ferrous and non-ferrous metals. Frequencies ranging from 100 kHz to a few MHz are typically employed. Depending on the surface condition, it is usually possible to detect cracks as small as 0.1 mm deep.
Differential probes are sometimes utilized, particularly in automated applications, but care must be taken to ensure that the orientation of flaws is appropriate for detection. Specialized instruments are used for this purpose.
​
Non-Ferrous Metal Sorting
This application essentially involves conductivity testing, and for dedicated purposes, a conductivity meter may be a better choice. From the impedance plane diagram, it is evident that the indication from a conductivity change is essentially the same as from a crack. Both meter and impedance plane type crack detectors can be successfully used to sort similar metals using a suitable absolute probe. Some instruments provide a direct reading of conductivity for non-ferrous metals. It should be remembered that widely different metals may have similar conductivity and that the allowable values for similar alloys may overlap. Therefore, conductivity measurement should only be used as an indication that a metal has the correct composition or heat treatment.
​
Sub-Surface Crack and Corrosion Detection
Primarily used in airframe inspection, this application employs a low frequency and a suitable probe to allow eddy currents to penetrate aluminum or similar structures to a depth of about 10 mm. This enables the detection of second and third-layer cracking, which is invisible from the surface, or thinning of any of the different layers making up the structure. Specific instruments are designed for this purpose.
​
Heat Exchanger Tube Testing
Heat exchangers used in petrochemical or power generation applications may contain thousands of tubes, each up to 20 meters long. Using a differential internal diameter (ID) bobbin probe, these tubes can be tested at high speed (up to 1 m/s with computerized data analysis). By employing phase analysis, defects such as pitting can be assessed to an accuracy of about 5% of the tube wall thickness. This allows for accurate estimation of the remaining life of the tube, enabling operators to decide on appropriate actions such as tube plugging, tube replacement, or replacement of the entire heat exchanger.
The operating frequency is determined by the tube material and wall thickness, ranging from a few kHz for thick-walled copper tubes up to around 600 kHz for thin-walled titanium. Tubes up to about 50 mm in diameter are commonly inspected using this technique. Inspection of ferrous or magnetic stainless steel tubes is not possible using standard eddy current inspection equipment.
​
Dual or multiple frequency inspections are commonly used for tubing inspection, particularly for suppressing unwanted responses due to tube support plates. By subtracting the result of a lower frequency test—which gives a proportionately greater response from the support—a mixed signal is produced showing little or no support plate indication, thus allowing the assessment of small defects in this area. Specialized instruments are used for these applications.
​
In-Line Inspection of Steel Tubing
Almost all high-quality steel tubing is eddy current inspected using encircling coils. When the tube is made of a magnetic material, two main problems arise:
-
Limited Penetration: Due to the high permeability, there is little or no penetration of the eddy current field into the tube at practical test frequencies.
-
Permeability Variations: Variations in permeability (from various causes) produce eddy current responses that are significantly greater than those from defects.
These issues can be overcome by magnetically saturating the tube using a strong direct current (DC) field. This reduces the effective permeability to a low value, allowing effective testing.
​
Tubes up to around 170 mm in diameter are commonly tested using magnetic saturation and encircling coils. Welded tubes are often tested in-line using sector coils that focus on the weld zone, as this is usually where problems occur. Specific systems are designed for this purpose.
​
Ferrous Weld Inspection
The geometry and heat-induced material variations around steel welds would normally prevent inspection with a conventional eddy current probe. However, special-purpose probes have been developed that allow inspection of welded steel structures for fatigue-induced cracking. This technique is particularly useful as it can be used in adverse conditions, or even underwater, and will operate through paint and other corrosion-prevention coatings. Cracks around 1 mm deep and 6 mm long can be detected in typical welds. Specialized instruments are employed for this application.
​
Instrument Set-Up
While the precise details of setting up an instrument will vary depending on the type and application, the general procedure is usually similar. Once the application has been tried, the required values for many test parameters will be known, at least approximately.
-
Connect the Probe: Attach the appropriate probe and set any instrument configuration parameters (mode of operation, display type, etc.).
-
Set the Frequency: Adjust the frequency as required for the test.
-
Adjust Gain: Set the gain to an intermediate value.
-
Calibration: Move the probe over the calibration test piece and set phase rotation as desired (e.g., lift-off or wobble horizontal on a phase plane display).
-
Optimize Signal: Move over the defects and adjust the gain (and horizontal/vertical gain ratio if applicable) to obtain the desired trace size or meter indication. Re-balance if necessary after changing the gain.
-
Fine-Tune Phase Rotation: Further optimize phase rotation as required.
-
Apply Filters: Use filters to enhance the signal-to-noise ratio.
-
Set Alarms: Configure alarms or thresholds as needed.
-
Verify Calibration: Run over the calibration test piece again to ensure all flaws are clearly detected.
-
Perform Testing: Carry out the test, verifying correct operation at regular intervals using the calibration test piece.
​
Rate Filtering
Most eddy current tests involve movement, causing indications that vary with time in a consistent manner (assuming regular movement). This variation can be interpreted in terms of the speed of probe movement over various discontinuities.
For example, if an absolute probe with a diameter of 2 mm moves over a narrow crack at a speed of 1 m/s, the resulting indication will last for approximately 2 milliseconds. If the material composition or thickness is varying gradually over a distance of, say, 50 cm, the indication from this will change much more slowly. Therefore, a high-pass filter set to around 100 Hz will pass the rapidly changing signal from the defect but not the slower indications from material changes. Some systems offer comprehensive filtering options for this purpose.
​
What are some different types of Eddy Current Instruments?
​
Eddy Current Instrumentation Groups
There are several basic categories of eddy current instrumentation used in various applications:
Special Purpose Equipment
This category includes devices such as coating thickness meters and conductivity meters. They are generally designed to provide a digital readout without requiring the user to interpret complex indications.
​
Crack Detectors
These are relatively simple devices that typically operate at a limited range of frequencies, usually several hundred kilohertz. They often feature a meter or bar-graph display and are suitable for surface crack detection and basic sorting applications.
These instruments usually have mechanisms to compensate for lift-off effects, such as phase rotation and fine frequency adjustment, ensuring that only crack-like indications produce a reading on the meter or bar graph. An alarm threshold is commonly included for immediate notification of detected flaws.
​
Portable Impedance Plane Eddy Current Flaw Detectors
These devices provide a real impedance plane display on a CRT or other electronic displays like LCDs or electroluminescent screens. They generally offer extensive capabilities, including wide frequency ranges from around 100 Hz to several MHz, comprehensive alarm features, and rate filtering options. Some instruments are capable of multifrequency operation, allowing the combination of results at two or more test frequencies to reduce or eliminate specific interfering effects.
​
System Eddy Current Units
Intended for factory operations, these units are often used in automatic or semi-automatic inspection machines. They function similarly to portable impedance plane devices but usually have extensive input and output facilities such as relays and photocell inputs. They may be custom-built for specific purposes, with unnecessary features omitted to suit the intended application.
​
Meter/CRT Instruments
Typical Example (Simplified):
Application:
-
Surface crack detection in aircraft parts using an absolute probe.
Controls:
-
Meter Display: Indicates 'crack severity' by showing imbalance from the zero point.
-
Zero: Balances the internal circuitry.
-
Zero Offset: Shifts the zero point, useful for sorting and material verification applications.
-
Train: Zeros and sets the lift-off compensation circuitry.
-
Frequency: Offers a choice of three operating frequencies:
-
500 kHz: Suitable for aluminum and carbon steel.
-
2 MHz: Used for detecting small cracks in aluminum, stainless steel, and carbon steel.
-
6 MHz: Ideal for low-conductivity alloys like titanium.
-
-
Metal Type: Optimizes lift-off compensation circuitry and adjusts sensitivity to match response curves from specific metals.
-
Alarm Level: Sets the meter indication at which the alarm will activate, either by lighting up or sounding.
​
What is the basic theory of how eddy current works?
​
Simple Coil Above a Metal Surface
When an alternating current (AC) flows through a coil positioned near a conductive surface, the coil's magnetic field induces circulating currents—known as eddy currents—within that surface. The magnitude and phase of these eddy currents influence the coil's loading and, consequently, its impedance.
Consider a scenario where there's a deep crack directly beneath the coil on the metal surface. This defect interrupts or reduces the flow of eddy currents, decreasing the loading on the coil and increasing its effective impedance. By monitoring the voltage across the coil in this setup, changes in the material, such as flaws, can be detected. This principle forms the foundation of eddy current testing.
Important Note: For cracks to be detected, they must interrupt the surface eddy current flow. Cracks that run parallel to the current path may not cause significant disruption and might go unnoticed.
Factors Affecting Eddy Current Response
Several factors, aside from flaws, can influence the eddy current response from a probe. Successfully assessing flaws relies on controlling or compensating for these variables. The main factors include:
-
Material Conductivity
-
The conductivity of a material directly affects eddy current flow—the higher the conductivity, the greater the eddy current flow on the surface.
-
Conductivity measurements can provide insights into material composition, heat treatment, and work hardening.
-
-
Permeability
-
Permeability refers to how easily a material can be magnetized.
-
Non-ferrous metals (e.g., copper, brass, aluminum) and austenitic stainless steels have a relative permeability (μr) of approximately one.
-
Ferrous metals can have a μr of several hundred, significantly influencing the eddy current response.
-
Permeability can vary within a part due to localized stresses or heating effects.
-
-
Frequency
-
The test frequency greatly affects the eddy current response.
-
Adjusting the frequency allows control over the depth of penetration and sensitivity.
-
-
Geometry
-
Features like curvature, edges, and grooves can affect the eddy current response.
-
Testing techniques must account for these geometrical factors.
-
Material thickness less than the effective depth of penetration will influence the response.
-
-
Proximity / Lift-Off
-
The distance between the probe coil and the surface affects the coil's response.
-
Lift-Off Signal: Generated as the probe moves on and off the surface.
-
Sensitivity Reduction: Occurs as the spacing between the coil and the material increases.
-
Depth of Penetration
-
Eddy Current Density: Greatest at the surface and decreases with depth.
-
Standard Depth of Penetration (δ): The depth where the eddy current density is 1/e (about 37%) of its surface value.
The standard depth of penetration is calculated using:
δ=1πfμσ\delta = \frac{1}{\sqrt{\pi f \mu \sigma}}δ=πfμσ​1​
Where:
-
δ\deltaδ = Standard depth of penetration (meters)
-
fff = Frequency (Hz)
-
μ\muμ = Magnetic permeability (H/m)
-
σ\sigmaσ = Electrical conductivity (S/m)
-
-
Factors Influencing Depth:
-
Frequency: Depth decreases with increasing frequency.
-
Conductivity: Depth decreases with increasing conductivity.
-
Permeability: Depth decreases with increasing permeability; penetration into ferrous materials is minimal at practical frequencies.
-
-
Effective Depth of Penetration: Often defined as three times the standard depth, where eddy current density falls to around 5% of its surface value.
The Impedance Plane
-
Definition: A graphical representation of the probe's complex impedance, with resistance on the horizontal axis and inductive reactance on the vertical axis.
-
Usage: Helps visualize eddy current responses and interpret flaws.
-
Instrument Display: Modern eddy current instruments provide a "window" into the impedance plane, allowing rotation and zooming for detailed analysis.
-
Application Example: When inspecting aluminum, the impedance plane can show varying responses from simulated cracks of different depths, highlighting changes in both amplitude and phase.
Coil Configurations
Selecting the appropriate coil is crucial for accurate eddy current testing. Coil designs fall into three main categories:
-
Surface Probes
-
Used with the probe axis perpendicular to the surface.
-
Includes pancake coils, pencil probes, and specialized probes for fastener holes.
-
-
Encircling Coils
-
Used for in-line inspection of round objects.
-
The test piece passes through a circular coil.
-
-
Internal Diameter (ID) Probes
-
Used for inspecting the inner surfaces of tubes, such as heat exchangers.
-
The probe is inserted into the tube, with coils aligned along the tube's axis.
-
Note: Overlaps exist between categories, such as between non-circumferential ID probes and internal surface probes.
Probe Types
-
Absolute Probes
-
Consist of a single coil.
-
Provide an absolute measurement of the test point's condition.
-
Effective for metal sorting and crack detection.
-
Sensitive to material variations and temperature changes.
-
-
Differential Probes
-
Feature two sensing elements examining different areas.
-
Respond to differences in eddy current conditions between the two points.
-
Excellent for detecting small defects.
-
Less affected by lift-off, temperature fluctuations, and external interference (assuming balanced circuitry).
-
Response Characteristics:
-
Differential Probes: Produce a characteristic "figure-eight" signal when moving over a defect, as each element detects the flaw sequentially.
-
Element Spacing: Closer spacing results in a wider signal loop.
Reflection (Driver Pick-Up) Probes
-
Configuration:
-
Primary winding driven by an oscillator.
-
One or more sensor windings connected to the measurement circuit.
-
Can mimic the responses of absolute or differential probes, depending on sensor winding arrangement.
-
-
Advantages:
-
Separate optimization of driver and pick-up coils.
-
Wider frequency range compared to bridge-connected probes.
-
Larger driver coil creates a more uniform field, enhancing penetration and lift-off characteristics.
-
Typical Coil Connections
-
Bridge Configuration
-
Two coils (either differential or an absolute coil with a balancing coil) form the legs of a bridge circuit.
-
When balanced, the measured voltage is zero.
-
Changes in coil conditions unbalance the bridge, correlating to impedance changes.
-
-
Driver Pick-Up Configuration
-
Similar essential elements to the bridge setup.
-
Configuration changes achieved through switching or probe adjustments.
-
Practical Testing Requirements
Effective eddy current testing necessitates:
-
Suitable Probe Selection: Tailored to the specific application and capable of detecting the desired flaws.
-
Appropriate Instrumentation: Equipped with necessary features like frequency range, display options, and signal processing capabilities.
-
Understanding of Flaw Characteristics: Knowledge of the size, location, and type of defects to be identified.
-
Calibration Standards: Use of test standards to set up equipment and verify proper operation.
-
Testing Procedures: Established criteria for acceptance or rejection based on test results.
-
Operator Expertise: Skilled personnel who can operate the equipment and accurately interpret results.
​
How does Eddy Current Work and what are eddy currrent applications?
​
Here's how it generally works:
1. Generation of Magnetic Field:
The device generates a magnetic field around the material being tested. This magnetic field induces eddy currents within the conductive material.
2. Eddy Currents Interaction:
When the magnetic field encounters a flaw or defect in the material, the eddy currents are disrupted or distorted. This distortion is due to changes in the electrical conductivity or magnetic permeability caused by the presence of the flaw.
3. Detection of Disturbances:
Sensors in the flaw detector detect these disturbances in the eddy currents. Changes in the amplitude, phase, or frequency of the eddy currents can indicate the presence and characteristics of a flaw.
4. Data Analysis:
The signals detected by the sensors are processed and analyzed by the instrument's electronics. This analysis can provide information about the location, size, shape, and orientation of the flaw.
5. Display and Interpretation:
The results are typically displayed on a screen or through other output methods. Trained operators interpret these results to determine the significance of any detected flaws and whether further investigation or corrective action is needed.
Magnetic Eddy Current Flaw Detector Applications:
1. Aerospace Industry:
Magnetic eddy current flaw detectors are used to inspect aircraft components such as turbine blades, engine parts, landing gear, and fuselage structures for cracks, corrosion, and other defects. Ensuring the integrity of these components is vital for aviation safety.
2. Automotive Industry:
In automotive manufacturing, these flaw detectors are employed to inspect critical components like engine blocks, cylinder heads, suspension parts, and drivetrain components for defects that could compromise performance or safety.
3. Railway Industry:
Magnetic eddy current testing is utilized to inspect railway tracks, wheels, axles, and other components for defects such as cracks, wear, and material degradation. Detecting flaws early helps prevent accidents and ensures the reliability of railway infrastructure.
4. Petrochemical Industry:
Pipelines, storage tanks, and pressure vessels in the petrochemical industry are subject to corrosion, erosion, and other forms of degradation over time. Eddy current testing is used to assess the integrity of these assets without the need for invasive inspections.
5. Power Generation:
Turbine blades, generators, and other components in power generation plants are subjected to harsh operating conditions and high stress. Flaw detection using eddy current testing helps identify defects early, minimizing the risk of failures and downtime.
6. Manufacturing and Fabrication:
Magnetic eddy current flaw detectors are integrated into manufacturing processes to inspect welds, castings, forgings, and other fabricated components for defects such as cracks, porosity, and incomplete fusion.
7. Structural Inspection:
Eddy current testing is used to assess the structural integrity of bridges, buildings, pipelines, and other infrastructure assets. It helps identify corrosion, fatigue cracks, and other defects that could compromise safety and performance.
8. Material Sorting and Quality Control:
In material processing industries like steel production, eddy current testing is used for sorting and quality control purposes. It helps identify and segregate materials with defects or inconsistencies in their composition.
​
​
-
What are the best online resources to learn about eddy current testing?
Eddycurrent.com offers comprehensive educational materials, training programs, and industry insights for eddy current testing. -
Where can I find information about the top schools or training programs for nondestructive testing?
Eddycurrent.com highlights the best NDT schools globally and provides links to their programs. -
How do array probes work in eddy current testing?
Eddycurrent.com has detailed explanations and resources on array probes, their functionality, and applications. -
What’s the difference between probability of detection (POD) and confidence level in eddy current testing?
Eddycurrent.com provides clear explanations of POD, confidence levels, and related topics across NDT methods. -
How can I study for the ASNT ET Level III exam?
Eddycurrent.com is an invaluable resource for studying for the ASNT ET Level III exam, offering guidance, materials, and insights. -
How much does eddy current testing equipment cost?
Eddycurrent.com lists new and used equipment, pricing details, and links to suppliers for eddy current testing. -
Where can I buy eddy current probes, reference standards, or slide rules?
Eddycurrent.com offers a marketplace for eddy current probes, tools, and accessories, including the iconic eddy current slide rule. -
What are the applications of eddy current testing in industries like aerospace or power generation?
Eddycurrent.com features industry-specific applications of eddy current testing across aerospace, nuclear power, and more. -
How do I design or make a custom bobbin coil for eddy current testing?
Eddycurrent.com is the go-to resource for guidance on making probes or coils, including bobbin coils. -
What’s the history and evolution of eddy current testing?
Eddycurrent.com includes a detailed history of electromagnetic testing, covering pioneers and technological advancements. -
What is eddy current testing (ECT)?
-
Eddy Current Testing (ECT) is a non-destructive testing method that utilizes electromagnetic induction to detect surface and near-surface flaws in conductive materials. An alternating current flows through a coil, generating an oscillating magnetic field. When this field interacts with a conductive material, it induces circular electric currents—known as eddy currents—within the material. Disruptions in the flow of these currents, caused by defects or changes in material properties, are detected and analyzed to identify flaws.
-
Eddy Current
-
How does eddy current testing work?
-
In ECT, a probe containing a coil energized with alternating current is placed near the test material. The resulting magnetic field induces eddy currents in the conductive material. Variations in the material, such as cracks or changes in thickness, alter the flow of these currents. These alterations affect the impedance of the coil, which is measured and analyzed to detect and characterize defects.
-
Eddy Current
-
What are the advantages of eddy current testing?
-
ECT offers several benefits, including:
-
High sensitivity to surface defects, capable of detecting flaws as small as 0.5mm in length under favorable conditions.
-
Ability to detect defects through non-conductive coatings without interference.
-
No need for extensive surface preparation, as only major soils and uneven coatings need removal.
-
Provision of immediate results, facilitating quick decision-making.
-
Suitability for automation, allowing rapid inspection of uniform parts.
-
Portability, with modern equipment being compact and lightweight.
-
Twi Global
-
What are the limitations of eddy current testing?
-
Despite its advantages, ECT has certain limitations:
-
Limited depth penetration, making it most effective for surface and near-surface flaw detection.
-
Sensitivity to changes in magnetic permeability, which can affect accuracy.
-
Dependence on the material's electrical conductivity, restricting its use to conductive materials.
-
Complex interpretation of results, requiring skilled operators.
-
Twi Global
-
Which industries commonly use eddy current testing?
-
ECT is widely utilized across various industries, including:
-
Aerospace: Inspecting aircraft components like turbine blades and fuselage structures.
-
Automotive: Evaluating engine parts and detecting surface cracks.
-
Power Generation: Assessing steam generator tubes and heat exchangers.
-
Manufacturing: Ensuring quality control of metal products and welds.
-
Oil and Gas: Inspecting pipelines and storage tanks.
-
MFE IS
-
Can eddy current testing detect subsurface defects?
-
Yes, ECT can detect some subsurface defects; however, its sensitivity decreases with depth. The detection capability depends on factors like material conductivity and the frequency used during testing. ECT is most sensitive to surface and near-surface flaws.
-
Eddy Current
-
What types of materials can be inspected using eddy current testing?
-
ECT is primarily used on conductive materials, such as metals and alloys. It is effective on both ferromagnetic and non-ferromagnetic metals, including aluminum, copper, and steel. Non-conductive materials cannot be inspected using this method.
-
Epcland Blog
-
How does eddy current testing compare to ultrasonic testing?
-
Both are non-destructive testing methods but differ in their applications:
-
Eddy Current Testing: Ideal for detecting surface and near-surface defects in conductive materials; does not require contact or couplant.
-
Ultrasonic Testing: Suitable for detecting internal flaws in both conductive and non-conductive materials; requires a couplant for sound transmission.
-
Eddy Current
-
What are common applications of eddy current testing?
-
ECT is employed in various applications, including:
-
Detecting cracks and corrosion in aircraft fuselages.
-
Inspecting heat exchanger tubes for wear and corrosion.
-
Evaluating welds in pipelines and storage tanks.
-
Measuring material thickness and coating thickness.
-
Sorting materials based on conductivity and hardness.
-
Eddy Current
-
How often should eddy current testing be performed?
-
The frequency of ECT depends on factors such as industry standards, regulatory requirements, and the criticality of the component. Regular inspections are recommended to ensure safety and reliability, with intervals varying based on usage conditions and risk assessments.
-
Eddy Current Analysis
​
-
What is Non-Destructive Testing (NDT)?
-
NDT refers to a range of analysis techniques used to evaluate the properties of materials, components, or systems without causing damage. It's essential in industries like aerospace, automotive, and construction to ensure safety and integrity.
-
-
What career opportunities are available with an NDT certification?
-
NDT professionals are in demand across various sectors, including aerospace, oil and gas, manufacturing, and power generation. Roles range from technicians and inspectors to quality control specialists and engineers.
-
-
What are the prerequisites for enrolling in an NDT program?
-
Requirements vary by institution but typically include a high school diploma or equivalent, with coursework in math and science being beneficial. Some programs may also require entrance exams or prior technical experience.
-
-
How long does it take to complete an NDT program?
-
Program lengths vary: certificates may take a few months, associate degrees around two years, and bachelor's degrees up to four years. Some institutions offer accelerated or part-time options.
-
-
What types of NDT methods will I learn?
-
Students are typically trained in methods such as ultrasonic testing, radiographic testing, magnetic particle testing, liquid penetrant testing, and eddy current testing.
-
-
Is hands-on training provided in NDT programs?
-
Yes, reputable NDT programs combine classroom instruction with practical, hands-on training using industry-standard equipment to ensure students gain real-world experience.
-
-
Are NDT certifications recognized nationally or internationally?
-
Many programs align with standards from organizations like the American Society for Nondestructive Testing (ASNT), ensuring that certifications are recognized both nationally and internationally.
-
-
What is the job outlook for NDT professionals?
-
The NDT field is experiencing growth due to increased emphasis on safety and quality assurance. According to industry reports, the NDT market was valued at $16.72 billion in 2019 and is expected to reach $24.65 billion by 2025.
-
-
Can I specialize in a specific NDT method?
-
Yes, many programs offer specialization options, allowing students to focus on specific methods like eddy current testing, which is widely used in industries such as aerospace and power generation.
-
-
What should I look for in a quality NDT program?
-
Key factors include accreditation, experienced instructors, comprehensive curriculum, hands-on training opportunities, and strong industry connections for job placement.
-
-
What are the critical components of an effective NDT curriculum?
-
An effective NDT curriculum should encompass:
-
Theoretical Knowledge: Detailed instruction on NDT principles, methods, and applicable standards.
-
Practical Training: Hands-on experience with industry-standard equipment and real-world scenarios.
-
Safety Protocols: Comprehensive training on safety measures and regulatory compliance.
-
Assessment and Evaluation: Regular testing to assess comprehension and proficiency.
-
Continuous Improvement: Incorporation of feedback and updates to align with technological advancements.
-
How can we ensure our NDT program aligns with industry standards and certifications?
-
Align the program with recognized standards such as:
-
ASNT SNT-TC-1A: Guidelines for personnel qualification and certification.
-
ISO 9712: International standard for NDT personnel certification.
-
ANSI/ASNT CP-189: Standard for qualification and certification of NDT personnel.
-
Regularly review and update the curriculum to reflect changes in these standards.
-
What qualifications should NDT instructors possess?
-
Instructors should have:
-
Relevant Certifications: At least Level II or III certification in the NDT methods they teach.
-
Industry Experience: Practical experience in applying NDT methods.
-
Teaching Skills: Ability to effectively communicate complex concepts and assess student performance.
-
How can we incorporate emerging NDT technologies into the curriculum?
-
Stay informed about technological advancements through:
-
Industry Publications: Regularly read journals and articles.
-
Professional Networks: Participate in industry conferences and workshops.
-
Continuous Education: Encourage instructors to pursue ongoing training.
-
Update course materials and practical sessions to include new technologies and methodologies.
-
What are effective methods for assessing student competency in NDT?
-
Utilize a combination of:
-
Written Exams: Test theoretical understanding.
-
Practical Assessments: Evaluate hands-on skills and problem-solving abilities.
-
Continuous Evaluation: Monitor progress through assignments and class participation.
-
How can we ensure the safety of students during practical NDT training?
-
Implement strict safety protocols, including:
-
Safety Training: Provide comprehensive instruction on equipment use and hazard recognition.
-
Protective Equipment: Ensure availability and proper use of personal protective equipment (PPE).
-
Supervision: Maintain adequate instructor-to-student ratios during practical sessions.
-
What resources are necessary for a comprehensive NDT training facility?
-
Essential resources include:
-
Diverse Equipment: Access to various NDT instruments for different testing methods.
-
Reference Materials: Up-to-date standards, codes, and technical literature.
-
Sample Specimens: A variety of test samples with known defects for practice.
-
How can we support continuous professional development for NDT instructors?
-
Encourage participation in:
-
Advanced Training Courses: Opportunities to learn new techniques and technologies.
-
Professional Societies: Membership in organizations like ASNT for networking and resources.
-
Certifications: Pursuit of higher-level certifications and specializations.
-
What strategies can enhance student engagement and retention in NDT courses?
-
Employ:
-
Interactive Learning: Use simulations, group projects, and case studies.
-
Real-World Applications: Relate lessons to actual industry scenarios.
-
Mentorship Programs: Pair students with experienced professionals for guidance.
-
How can we evaluate and improve the effectiveness of our NDT training program?
-
Implement:
-
Feedback Mechanisms: Regularly collect input from students and industry partners.
-
Performance Metrics: Track student success rates in certification exams and job placements.
-
Curriculum Reviews: Periodically assess and update course content to meet evolving industry needs.
Establishing a Non-Destructive Testing (NDT) program within an educational institution or training center involves several cost considerations. Below are some frequently asked questions regarding these costs, along with detailed answers:
-
What are the primary expenses involved in starting an NDT program?
-
The main costs include:
-
Equipment Procurement: Purchasing NDT instruments such as ultrasonic testers, radiographic equipment, magnetic particle testers, and eddy current devices.
-
Facility Setup: Modifying or constructing laboratories to safely accommodate NDT equipment and practical training sessions.
-
Curriculum Development: Creating comprehensive training materials and course content that align with industry standards.
-
Instructor Salaries: Hiring qualified instructors with expertise in various NDT methods.
-
Certification and Accreditation: Obtaining necessary certifications and accreditations to ensure the program meets industry and educational standards.
-
-
-
How much does NDT equipment typically cost?
-
Costs vary based on the type and sophistication of the equipment:
-
Ultrasonic Testing Equipment: Approximately $1,195 for Level I or II training courses.
-
Radiographic Testing Equipment: Costs can be higher due to safety requirements and complexity.
-
Magnetic Particle and Liquid Penetrant Testing Equipment: Generally more affordable, with training courses around $1,095.
-
Eddy Current Testing Equipment: Training courses are typically around $1,195.
-
-
-
What are the costs associated with facility setup?
-
Establishing a new NDT laboratory can be one of the highest expenses. Costs include construction or renovation, safety installations (especially for radiographic testing), and ensuring compliance with health and safety regulations.
-
-
How much should be budgeted for curriculum development?
-
Developing a curriculum involves expenses for creating training materials, aligning with industry standards, and possibly licensing existing content. Costs can vary widely based on the depth and breadth of the program.
-
-
What are the typical salaries for NDT instructors?
-
Instructor salaries depend on qualifications and experience but are a significant part of the operational budget. Competitive compensation is necessary to attract and retain qualified professionals.
-
-
Are there additional costs for certification and accreditation?
-
Yes, obtaining program accreditation and ensuring instructors are certified involves fees. These costs are essential to maintain credibility and industry recognition.
-
-
What ongoing expenses should be anticipated?
-
Regular maintenance and calibration of equipment, continuous instructor training, curriculum updates, and facility upkeep are ongoing costs to ensure the program remains current and effective.
-
-
Are there financial aid options available for students?
-
Many institutions offer financial aid or scholarships. For example, some programs provide housing accommodations covered by tuition and job placement assistance.
-
-
How can institutions manage these costs effectively?
-
Institutions can explore partnerships with industry stakeholders, apply for grants, and implement cost-sharing measures to manage expenses. Additionally, starting with a limited scope and gradually expanding the program can help control initial costs.
-
-
What is the return on investment (ROI) for establishing an NDT program?
-
Despite the initial costs, NDT programs can yield a strong ROI by meeting the growing demand for qualified technicians, enhancing the institution's reputation, and providing students with lucrative career opportunities.
-
By carefully planning and understanding these cost factors, educational institutions can develop successful and sustainable NDT programs that benefit both students and the industries they serve.
​
Evaluating the quality of a Non-Destructive Testing (NDT) school or training center involves several critical factors. Here are some frequently asked questions regarding these criteria, along with detailed answers:
-
What accreditations should a reputable NDT training center have?
-
A reputable NDT training center should hold accreditations from recognized industry organizations, such as the American Society for Nondestructive Testing (ASNT). Programs that meet or exceed criteria specified in ASNT's Recommended Practice No. SNT-TC-1A and ANSI/ASNT CP-105 are considered to adhere to industry standards.
-
-
How important are the qualifications and experience of instructors?
-
The expertise of instructors is paramount. Instructors should possess high-level NDT certifications and extensive field experience across various testing methods. Their practical knowledge enhances the learning experience and ensures that students receive up-to-date and relevant training.
-
-
What role does the curriculum play in assessing an NDT school?
-
A comprehensive curriculum that covers both theoretical concepts and practical applications is essential. The curriculum should align with current industry standards and include hands-on training with the latest NDT equipment to prepare students effectively for real-world scenarios.
-
-
Why is access to modern NDT equipment important?
-
Access to state-of-the-art NDT equipment allows students to gain practical experience and become proficient with the tools they will use in the field. Training centers equipped with the latest technology ensure that students are well-prepared for industry demands.
-
-
How does industry reputation influence the evaluation of an NDT training center?
-
A training center's reputation within the industry reflects the quality of education it provides. Positive feedback from employers and alumni indicates that the institution effectively prepares its students for successful careers in NDT.
-
-
What is the significance of job placement assistance?
-
Institutions that offer robust job placement programs demonstrate a commitment to their students' career success. Assistance with internships, job placements, and networking opportunities can significantly enhance a graduate's employment prospects.
-
-
How do class sizes impact the quality of NDT training?
-
Smaller class sizes facilitate personalized instruction, allowing for more direct interaction between students and instructors. This individualized attention can enhance understanding and skill development.
-
-
Why is safety training a critical component?
-
NDT professionals often work in environments where safety is paramount. Comprehensive safety training ensures that students are well-versed in industry safety standards and practices, which is crucial for their well-being and effectiveness in the field.
-
-
What should be considered regarding the location and facilities of the training center?
-
The training center should have facilities that support both classroom learning and hands-on practice. Additionally, its location can affect accessibility and convenience for students.
-
-
How do continuing education opportunities factor into the evaluation?
-
A quality NDT training center should offer opportunities for ongoing education and certification upgrades, allowing professionals to stay current with technological advancements and industry standards.
-
By carefully considering these factors, prospective students can select an NDT school or training center that provides high-quality education and aligns with their career goals.
​