Wednesday, August 02, 2017 / Perth Australia / By Niekie Jooste
In this edition of "The WelderDestiny Compass":
A few decades back when, I got into the welding game, Eddy Current Testing (ECT) was a bit of an oddity. Not only was it far from a mainstream non-destructive examination (NDE) method, but it was considered rather unreliable as a general weld inspection tool.
Don't get me wrong. There were particular jobs that ECT appeared to be the best option out of a bad bunch. Typical of such an application, was the pitting corrosion detection in heat exchanger tubes. On the welding front, it made an appearance every now and again, but the equipment was considered very specialized, and the results were considered rather patchy at best.
Today it is considered a reliable method for surface and near surface crack detection in welds, and is used quite frequently. For crack detection through paint coatings, ECT is today considered the standard method.
Today we look at eddy current testing, and consider why this NDE method has great potential to become one of the mainstream methods in an automated future within which cheap artificial intelligence (AI) is a dominant technology.
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Now let's get stuck into this week’s topics...
We will not go too much into the basics of eddy current testing. There are many websites that cover this very well. A short primer is however in order, as it will help us understand why ECT has such a promising future in an automated environment.
An eddy current probe is essentially just a coil through which an alternating current is passed. This alternating current generates a magnetic field around the coil. This magnetic field in turn generates an alternating electric current within the metal. In particular, the electric current that is generated is in the form of a "circular" current. These small circles of electric current are called "eddy currents".
Eddy currents in turn generate their own magnetic fields. The ECT probe can detect these resultant magnetic fields generated by the eddy currents. In a totally homogenous material, a very constant "feedback" magnetic field will be measured by the ECT probe.
When there are fluctuations in the magnetic fields created by the eddy currents, the ECT probe detects this, and shows the results on a screen. In essence, any change in the magnetic properties of the material being tested will result in fluctuations in the "feedback" magnetic fields. Typically a crack like defect would be a strong driver for changing the feedback magnetic field, so it can be detected.
The initial patchy performance of ECT was largely due to the fact that there are many drivers for a change in the feedback magnetic fields. Initially this was a curse, but actually this is going to be one of the big drivers for its usefulness going forward.
The breakthroughs for ECT came when the developers of the technology were able to devise more accurate mathematical models for what effect different material features have on the feedback magnetic fields. As these more accurate models were developed, more accurate interpretation of the output signals could be made.
So, in summary, the eddy current test is able to do the following:
The depth sizing of cracks is achieved by varying the frequency of the alternating current to the coil. The higher the frequency the shallower the depth of eddy current generation. The lower the frequency, the greater the depth of eddy current generation. By analysing the feedback magnetic fields at different frequencies, the crack depth can be estimated.
Different phases within a metal results in different magnetic responses of the material. The metallurgical structure of a metal can therefore be inferred by analysing the magnetic response of the base metal.
As mentioned earlier, the big advance in eddy current testing is primarily due to the improvements in the mathematical models that the developers of the ECT technology have achieved. They can now differentiate between different "signature" magnetic responses to decide if the signal is telling us something about changes in the structure of the base metal, or signal the presence of discontinuities.
Another big advance has been in the development of eddy current array (ECA) testing. An eddy current array is essentially just a whole lot of these little coils placed next to each other in a single "probe". Such an ECA can have 32 or even 64 coils. With such an eddy current array, a rather wide surface can be tested in a single pass. Say 25 - 50mm wide.
These eddy current arrays are made in many different shapes, to be able to be used on many different surface profiles. In fact, there are even flexible array probes that can be shaped "on the fly".
Using the traditional "manual" ECT display, a technician would find it rather difficult to keep track of, and interpret such a lot of data. Luckily, the eddy current array testing is accompanied by rather clever computers, that can interpret the signals and provide a rather easy to use and understand output on the display. Typically the output is displayed as a "plan" view of the material, showing where there are discontinuities such as cracks. For the initiated, this is also termed a "C Scan" view of the data.
The "normal" ECT probes are rather sensitive to any "lift-off". In other words, it really wants to stay in close contact with the material it is testing. Fluctuations in the distance will result in fluctuations in the output signal. To get around this problem, to some extent, a variant on the conventional eddy current testing has been developed. It is called pulsed eddy current testing. (P-ECT) Pulsed eddy current testing is now used to do in-service inspections of metallic structures that have lagging attached.
In issue 2 of The WelderDestiny Compass, we discussed machine learning vision systems. In issue 7 we discussed the potential of augmented reality in welding. In issue 9 we discussed the potential of LASER ultrasonics in welding. If you don't recall the discussions, I urge you to read those issues again.
Well, with ECT, and particularly eddy current array testing, we have another technology that can be integrated with artificial intelligence and augmented reality. After completing a weld, and allowing it to cool down enough, the weld could be scanned with an ECA probe, and the output overlaid on a real-time view of the welded surface as seen through the augmented reality welding helmet of the welder.
Not only will the test give us feedback on the presence of surface breaking, or near surface flaws, but it can also give us information about the metallurgical structure of the weld and heat affected zone. In applications where excessive hardness could be a problem, such a test can provide some really useful information.
Not only will the ECT information be really useful for the quality control aspects of that job, but it will be some additional information that can be "pooled" on our welding data platform for integration with data from other jobs, providing a great deal of value along the way.
I can't wait to see how this technology develops and matures in a future with artificial intelligence.
Yours in welding
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