Eddy Current Array or Phased Array?

If you’ve worked with me in the past few years you already know that I recommend eddy current array testing on gear teeth as my #1 choice and suggest phased array as a second option among the advanced methods of inspection. I want to take the time to write in plain language my reasons for this. I also want to help de-shroud some of the common misunderstandings surrounding phased array and eddy current array and help manage expectations with both of these NDT methods in regards to inspecting large diameter gears found in mining and other heavy industries.

Let’s start off with some introductions:

The name “Phased Array” is actually short for "Phased Array Ultrasonic Testing" (PAUT). It utilizes ultrasonic wave propagation inside the gear tooth to detect flaws. Phased array probes are made up of many small transducers that send and receive ultrasonic pulses with time delays calculated in such a way as to allow for electronic beam formation, steering and focusing. All ultrasonic data is traditionally viewed on an “A-scan” display which shows the amplitude and time delay of ultrasonic pulses. Phased array data is basically just a visualization of multiple A-scans merged together (but I’ll get to more on that later).

Key points of PAUT:

• Low surface sensitivity
• Scans full volume of tooth (outside of near-field zone)
• Scan data not typically saved (screenshots only)
• Highly operator dependent
• Moderate scanning speed
• Cleaning required (contact method: probe must be touching the gear tooth)

Eddy Current Array (ECA) uses electromagnetic induction to detect surface discontinuities on the gear teeth. ECA probes contain multiple eddy current coils that form magnetic fields. These magnetic fields induce current into the gear tooth surface. The current flows in tight concentric paths at and close to the surface of the tooth. Any surface flaws cause the currents to change pathways through the tooth and therefore alter the magnetic field being created and sensed by the coils. Changes sensed by the coils are displayed traditionally on an impedance plane, however, with multiplexed data, a 2-D or 3-D colour display is used to visualize the impedance changes that represent surface flaws. Using calibration standards, defect shape, orientation and depth can be approximated using the scan data.

Key points of ECA:

• Low depth of penetration (can only estimate defect depth up to ~10mm)
• Highly sensitive to surface flaws of any orientation
• All raw scan data is recorded
• Less operator dependent
• High scanning speed
• No cleaning required (no-contact method: probe doesn't need to touch the gear tooth)

I want to point out that confusing the two methods is actually very common for non NDT personnel due the the vagueness of the word “phased array” and the fact that “eddy current array” also includes the word “array”...in fact, this has lead to enough confusion over the years that clients have started asking for “Phased Array Eddy Current” (which doesn’t exist). The worst part is that that some NDT companies offer “Phased Array Eddy Current” as a service...even though in reality they are performing the eddy current array service only or the phased array service only. Hint: If your NDT provider is selling you "phased array eddy current" it might be because they don't know what they are talking about.

This leads me to a side-side note; recently something called “Pulsed Eddy Current Array” has become a thing, but don’t be confused, it still has nothing to do with phased array ultrasonic testing and is actually an advanced eddy current application designed for scanning pipe through insulation. Again, it has nothing to do with gears. What is relevant is that the technology is complex and not everyone has the time to do all the research required to make sense of all these NDT methods...especially if they keep inventing new ones. Right now, eddy current array and phased array and other methods such as ACFM are becoming more advanced at accelerating rates. You can bet that in the next ten years there will be new contenders emerging in the NDT world, but for now I am focused on the two most commonly available and affordable advanced methods; ECA and PAUT.

So getting back to comparing the two methods; it’s important to understand that both are very good options but each method has different strengths and weaknesses for gear inspections. Instead of listing the extensive pros. and cons. of each method or creating a ridiculous matrix and having you determine which is better, I’m going to make the comparison using real-world factors that are relevant specifically for gear teeth.

Firstly, gear failure is typically initiated by surface conditions. And even if a sub-surface defect turns into a crack, it always reaches one of the tooth surfaces before it can break through the entire tooth for a complete failure. This is what makes eddy current array my first choice. It’s highly sensitive to surface flaws while being a “no-contact” method. No-contact, in this case means that no gear cleaning is required. Combine that with the “J-probe” design made specifically for gear teeth, and advanced data imaging software you have the highest probability of detection available within a reasonable price range. Phased array on the other hand is a “contact” method and therefore requires at least some cleaning and has poor surface sensitivity due to several ultrasonic testing principles such as the probe near-field (a “blind spot” at the start of the ultrasonic beam).

Secondly, these gears are made to be turning...so that they can make money. Having to scan your gear for cracks is costing you thousands of dollars per hour in production losses alone. So clearly, time is a factor. Even if you can align your inspections with other work there are always tie ups and delays. Once again, eddy current array takes a lead. Thanks to that “J-probe” design I mentioned earlier, ECA scans the addendum, dedendum and root of the tooth in a single pass. This means that the entire tooth surface can be scanned in two passes (one on the active flank and one on the non-active flank, in fact the root gets scanned twice with this procedure). Phased array is slower than eddy current array mainly because there is more surface preparation required to properly couple the probe with the tooth. But there’s another reason why phased array gets us into trouble…

Phased array instruments can be set up in several scan modes; the one that works best for gear teeth is called “Sectoral Scanning” or “S-scan” for short. S-scans allow us to sweep ultrasonic energy in the tooth through a range of angles. The draw back of this method is that the operator has to make multiple passes from different sides of the tooth in order to have 100% confidence that they aren’t missing anything. This is especially true for finding root cracks. Much of the analysis with phased array inspections gets prolonged by having to interrogate indications in order to avoid making false calls. Unlike eddy current array’s ASTM E2905 / E2905M standard for mill and kiln gears, there is no official inspection standard for performing phased array on gear teeth. It is up to the technician to decide how many passes they need to make for each tooth before proceeding to the next one...which opens the inspection up to a higher degree of human error...and possible inconsistency of inspection quality from tooth to tooth.

The third major factor in making the right choice is the condition of the gear teeth. Knowing the general condition of the gear teeth prior to hiring an inspection company is probably not what you would expect me to add in this list, but hear me out. Eddy current array is very sensitive to surface defects and phased array is very sensitive to internal defects. So if your gear teeth are “severely pitted” you might not want to use eddy current array simply because it’s probably not going to tell you anything you don’t already know. In this case, phased array might be a better choice given it’s insensitivity to surface defects, allowing the focus of the inspection to be on finding deeper flaws. Alternatively, you may have a gear that is in pristine condition or only has minor or moderate surface flaws, in this case ECA is the better choice because of its ability to render so much more information about the surface such as pitting patterns, wear patterns, indications of gear alignment issues and lubrication issues as well as the detection of surface breaking cracks.

The fourth factor I consider is the stage at which the gear is in its life. This is different from the tooth condition. When a gear is forged by a manufacturer, they inspect the blank for flaws before the teeth are even cut. This inspection is done with phased array or conventional ultrasonic testing. Eventually the gear sections must be shipped to a construction site and installed on a grinding mill or some other equipment. At this stage, just before installing the gear / putting the gear into service, I recommend Phased Array and a surface method such as ECA or MPI (Magnetic Particle Inspection) be done as a baseline inspection of the gear teeth. At this stage it is also a good opportunity to ensure that the gear teeth are properly numbered on every 5th or 10th tooth. After the gear is put into service it should be inspected with eddy current array without gear cleaning every 12 or 24 months until crack indications are detected. If crack indications are found on gear teeth, (edit: the following comment is only based on what I have seen done in the field. Always consult with an application specialist in tooth repair!) the crack should be tapped or grinded out of the tooth and “chased” using MPI. A weld material can then be used to fill the grinded area in. It is also a good idea to increase inspection frequency if cracks are being found. It is suggested by major insurers and risk analysis engineers that an inspection be performed every 6 months for gears with known cracking.

At the end of the day, reliability engineers have to look at the information provided by an inspection report and make decisions about their gears and the landslide winner in customer satisfaction in my experience is ECA. The reason for this divide, I believe, is due to the fact that the ECA inspection always includes a visual reference of indications observed in scans and because the report includes a complete list of inspection results for each scan. Also, in some ways, the use of ECA is more of a tool to narrow the focus of the visual inspection of the gear teeth rather than completely replace it. I never write the word “crack” in a report unless I visually observed a crack with my own eyes, and that is where phased array often falls short. Due to phased array’s tendency to detect indications that cannot be visually confirmed, the report data remains frustratingly inconclusive, or at least intangible regardless of the operator’s level of confidence which can make a reliability engineer's job more difficult.

I hope that reading this has been informative and helpful. If you have questions about gear inspection or NDT in general please feel free to ask me. I know a lot of phased array nerds will read this and scoff at my criticism but please be reminded that this article is SPECIFIC to gear teeth inspection which is not at all the same beast as weld inspection (where phased array dominates the field).

Cheers,

Richard Malenfant - Owner / Field Services Manager

Images taken from several websites including wikipedia, olympus-ims, eddyfi, google image search, I claim no ownership of the image content.