Eddy Current Techniques for Crack Detection in Metal Sheets and Tubing

Eddy current testing (ECT) for crack detection is one of the most effective nondestructive testing (NDT) approaches currently available for surface and near-surface flaw detection. The best ECT instruments have enough power to measure thin materials accurately and detect minuscule deviations with ease. However, to maximize the efficacy of ECT, it’s important to choose not only the right equipment, but the right technique.

Below, we’ll discuss how certain ECT techniques and best practices can detect more flaws quickly and accurately in metal sheets and tubing. 

Precise Metallic Weld Inspections

Small cracks within metallic welds can damage the integrity of the sheet and cause greater damage. A variety of factors can affect the sensitivity of an ECT probe, such as probe design, material, and surface finish. Using ECT equipment with simple, straightforward calibration features allows analysts to commence inspections without lengthy setup times or adjustments.

During the frequency adjustment process, choose higher frequencies to enhance sensitivity and resolution on the surface, or choose lower frequencies to foster deeper penetration depths. Adhere to the following core measures once the frequencies are fixed:

1. Use similar specimens as a test guide before implementing the actual test.
2. Adjust as necessary to ensure the probe finds an easy defect. Automated software can aid in the calibration process. 
3. Place the probe on the surface and slowly slide the probe across the metallic surface. (Ensure that the probe remains in contact with the surface, as probe deviation could interrupt the signals.)
4. Study the signal for any changes. Local changes indicate flaws.

In addition, the shape of the probe itself should match the design of the sheet weld. For this reason, surface array ECT tends to be among the most effective techniques for capturing additional defects. 

Comprehensive Inspections Using Surface Array

Surface array ECT can detect extremely minute flaws in greater detail; as such, utilizing equipment with eddy current array (ECA) functionality is imperative. ECA is an extension of ECT, fostering more comprehensive coverage, higher sensitivity rates, and faster inspection times.

In terms of metallic welds, ECA is ideal for detecting surface-breaking cracks that can compromise the integrity of the sheet itself. Instruments with a high signal-to-noise ratio (SNR) are key to ensuring a truly comprehensive inspection, as they minimize the amount of noise that can obscure flaw detection. In short, an enhanced SNR increases the probability of detection.

However, analysts must also consider adjusting the sample rate to get SNRs within a decent range.

Example: An analyst that configures 150 samples per second manages to find six notches within a metal weld. He later conducts the same test at a higher sample rate. With such a high noise level, the analyst found the same notches, but missed several smaller flaws that failed to read accordingly. 

Surface array technology is capable of detecting even the slightest loss of material or indentations within a welding sheet. ECA efficacy also extends to other unconventional shapes that are harder to test.

Tubing Techniques 

While surface array and tubing are different mediums, certain eddy current instruments offer both techniques in one system. A tubing/surface array instrument might be an ideal choice—or, analysts can choose other instruments that are solely reserved for heat exchanger tubing, such as a multiplex/simultaneous injection system. If choosing a specialized heat exchanger instrument, multiplex features improve the SNR in the following ways:

• Improving channel resolution
• Reducing noise levels
• Enhancing coil sensitivity

Multiplex capability also allows users to complete more inspections in less time, and when combined with simultaneous injection, it can test most magnetic alloys and carbon steel. The simultaneous injection function speeds up the testing process and supports remote field testing (RFT). RFT can be used to inspect ferromagnetic tubing components, such as ferritic stainless steel and carbon steel, and can gather sound data from a tested item. Further, it can read deviations such as corrosion and erosion, as well as minor cuts and wear—all of which can be hard to read without the right instrument or probe.  

Choosing the Right Probe for Circumferential Cracking

If talking about circumferential cracking, for example, it can be hard to detect if using the wrong probe type. Bobbin probes, in particular, are highly sensitive to ferromagnetic signals that stem from tube sheets or support plates from heat exchangers. These signals can block signals pointing to cracking indications, making it hard to distinguish between small-volume cracking and complex signals deriving from geometry shifts. Furthermore, the coils from a bobbin probe cannot read circumferential cracking since eddy currents from the probe run parallel to the cracking.

This can be a major problem, as cracking can cause dangerous leaking in nuclear or petrochemical plants. When a leak occurs in a heat exchanger, for instance, it can be all too easy to focus on the leak instead of the cracking that’s causing the leak.

That’s why a probe with combined bobbin and array features is the ideal choice, as it not only detects circumferential cracking but also characterizes and sizes the flaws accordingly. Analysts can get more coverage for each coil density, and just one row of array coils is sensitive enough to find circumferential cracking. In addition to detecting circumferential notches, bobbin/array probes allow analysts to measure and pinpoint axial cracking and accurately assess cracking close to tube sheets.

Unlike bobbin-only probes, bobbin/array probes allow analysts to determine the length and position of cracks as well as distinguish between complex geometries and any aberrations found during testing. 

Why Eddy Current for Crack Detection Cannot Be Compromised

When it comes to eddy current testing, analysts must consider the highest-quality instruments for the job. Cutting-edge instrumentation helps analysts find more flaws and defects that could cripple operations and cause safety hazards.

Eddy current for crack detection entails a variety of methods, but surface array is one of the best techniques for metal sheets or irregular shapes such as tubing. For heat exchanger tubing inspections, a probe that combines bobbin and array capabilities can offer additional flaw detection and shorter inspection times. ECT further highlights cracking anomalies thanks to evolving technologies, reducing cumbersome setup and calibration times to allow analysts to focus on what matters most—conducting thorough inspections, detecting flaws, and providing recommendations that will help companies prevent dangerous situations from developing in the first place.

Zetec is an experienced NDT provider that is proud to offer the latest in eddy current testing solutions. Our ECT and ECA equipment feature enhanced flaw detection in metal sheets and tubing and incomparable inspection efficiency. Contact us today for more information.