Things to Know and Factors to Consider
As a company that does a significant amount of work with specialty metals, Metal Cutting Corporation often employs eddy current testing (ECT) to inspect materials for defects such as cracks or voids. This method utilizes electromagnetic induction to detect and characterize flaws in the surface or sub-surface of conductive materials, including metals. In addition to detecting flaws, the eddy current testing procedure can be used to measure thickness and conductivity.
Below are 5 interesting things to know about eddy current testing.
Eddy Current Testing Is Nondestructive Testing
Eddy current testing is an important method of nondestructive testing (NDT) — one of the techniques used in science and industry for performing inspections and taking measurements to ensure that:
- Structural components and systems perform reliably, safely, and cost-effectively
- The testing itself is done in a way that does not damage parts or materials, and does not affect their future use and function
There are, in fact, a variety of NDT techniques, with new ones being developed all the time. The most basic method is a visual examination, which may mean simply looking at a part for visible surface imperfections or using computer-controlled optical systems to detect and measure the features of a component.
Some of the technologies used in NDT are familiar because they are also used in medicine, such as radiography (RT), which uses gamma-radiation or X-rays to look for defects or see internal features. Another example is ultrasonic testing (UT), using high-frequency sound waves to detect imperfections or changes in material properties.
Magnetic particle testing (MT) uses a magnetic field in a ferromagnetic material and a dusting of iron particles to produce a visible indicator of surface defects. Leak testing (LT) finds leaks in pressurized parts by using various methods ranging from electronic listening devices to pressure gages, to simple soap-bubble tests.
Another method is acoustic emission testing (AE), which finds imperfections by detecting bursts of acoustic energy. We often encounter helium leak testing, which uses the second lightest element to find a leak path, with or without the use of penetrant testing (PT), which uses visible or fluorescent dye.
In eddy current testing — the NDT technique we focus on here at Metal Cutting — electrical currents (eddy currents) are generated in a conductive material by exposing it to an expanding and collapsing magnetic field. The strength of these eddy currents can be measured; defects or changes in the material cause interruptions in the flow of the currents, alerting us to problems in the material or part being tested.
It Is Critically Important in Daily Life
While not everyone has heard of eddy current testing and NDT, these methods touch all of our lives, perhaps even on a daily basis. That’s because these techniques are used in a wide range of industries — not least of all, in those where component failure could cause devastating damage and loss.
For example, eddy current testing is used to inspect tubing and other structures for applications such as oil and gas pipelines, nuclear reactors, chemical manufacturing, and municipal water systems. Portable eddy current testing equipment is used for on-site inspections in the field, such as looking for cracks in bridges and in airplane components from wings to landing gear. That makes ECT, as well as other methods of nondestructive testing, vitally important to public safety — playing a role in helping to prevent catastrophic events like pipeline breaks, bridge collapses, and plane crashes.
Even in a world of small parts, such as the metal components we produce here at Metal Cutting, eddy current testing has an impact on safety, in less visible but still critically important ways. For instance, we use this method to inspect glass to metal seals in parts for night vision goggles that are ultimately used by military personnel, who may need them long after manufacture and far from home.
There Are Different Probes for Different Modes
Eddy current testing equipment includes test probes, which are available in a variety of shapes, sizes, and configurations. These probes also have different modes of operation, depending on how the test coils are wired and how they interface with the test sample.
For example, an absolute measurement probe uses a single coil to generate eddy currents, detect changes in the current field, and provide a reading from a single point on the test sample. A differential probe uses two coils to provide a basic of comparison for detecting flaws, even in materials that may have inconsistencies; when one coil is over a defect and the other is over good material, a differential signal is produced. There are also reflection and hybrid probe modes.
An alternating current (AC) is passed through the coil or coils to create an expanding and collapsing magnetic field in and around the coil(s). When the probe is positioned next to a conductive material — the test sample — this changing magnetic field is what generates the eddy currents within the sample. Through the interaction of the coil’s magnetic field and the eddy currents, we can observe and measure changes in frequency, amplitude, sensitivity, impedance, and other characteristics that indicate the presence of a crack, void, or other defects in the test sample.
Many Factors Affect the Eddy Current Testing
In addition to settings such as frequency, amplitude, sensitivity, and so on, which make up the “recipe” for eddy current testing, there are other factors to consider — things that can affect the flow of eddy currents, including the properties of the material or part being tested. Some are beneficial while others may require making adjustments to the settings or using other techniques to compensate for the effects.
Obviously, the electrical conductivity of the material being tested — or what we can think of as the ease with which electron flows in the material — has an effect on the flow of eddy currents it produces, as does magnetic permeability. While the measurement of permeability can be useful in sorting materials, this property can pose problems. For instance, the so-called “noise” created by changes in permeability when testing ferrous materials makes it difficult to use eddy current testing on carbon steel welds. However, issues may be overcome by using magnetic saturation, multi-frequency inspection, or differential coil arrangements.
Speaking of noise — actual room noise is a physical, ambient factor that can have an impact on eddy current testing. However, noise can often be filtered out, to produce a clearer signal. When a test sample is a part with edge or sharp changes in geometry, there can be what is called an “edge effect” on the eddy currents; placing and balancing the probe near the edge and scanning at that distance can avoid this effect. Similarly, a sample with a complex geometry could create false signals, caused by changes in geometry rather than a defect in the material itself.
Another important consideration is the coil fill factor, which is used to establish how much space an inspected tube or rod should take up inside the inspection coil. By determining the correct allowance between the coil and the test sample, you can make sure the sample will be able to move freely during scanning while also making sure the coil is close enough to the sample to generate eddy currents and perform the inspection correctly.
The frequency of AC passing through an eddy current testing coil affects the depth of penetration of the eddy current field in a test material; with increasing depth into the material, there is decreasing intensity of the eddy current flow. The depth of a crack cannot be measured accurately by using eddy current testing, and the method also will not detect flaws such as laminations, which run parallel to the flow of eddy currents. However, cracks, lack of weld fusions, and other planar discontinuities that are perpendicular to the flow of eddy currents will be detected.
Metal Cutting Is Skilled in ECT
Here at Metal Cutting, we frequently use the eddy current testing procedure to inspect tungsten and molybdenum rods and other metal parts for potential issues such as cracking, pitting, and fractures. We also utilize ECT to look for surface flaws in the round rod, flat ribbon, and capillary tubing used in glass to metal seals. (You can read more about that in our blog Problems with the Glass to Metal Seal in Electronics.)
Whether we have purchased a material on behalf of a customer or the customer has provided the material for us to process, we speak with the vendor or customer to find out what settings they use on their own eddy current testing equipment. This enables us to create our mutual, shared recipe for successful ECT, adjusting the settings as needed, using either absolute or differential measurements, and choosing from an array of coil sizes and tooling options. For passing rods through an ECT coil, we also pay close attention to the fill factor and use a bushing to position the rod so it is centered within but never touching the coil.
Additionally, we often seek out a reference sample as a basis for comparison — especially when we are inspecting for internal defects, which cannot be seen. A reference sample allows us to check whether we are likely to find the defects we will be looking for by using our established ECT settings. Using a sample with a known defect, we can adjust our equipment settings as needed to find that specific, verified defect.
It can be difficult to find a good reference sample. After all, you don’t want to cut open a sample to verify an internal defect and thus destroy the sample for any future ECT use. However, we can use a sample our vendor or customer believes has a sub-surface defect based on their testing and corroborated by previous failed ECT inspection. For external flaws, we can work with a vendor or customer to attempt to create a specific surface defect on a part, and then both of us can use that as our reference sample.