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All Your Flaws Revealed—TFM Imaging You Can Trust

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When examining a part for flaws, it’s easy for inspectors to fall prey to self-doubt. Making the wrong call can have major impacts in terms of time and money. A flaw characterization that overestimates the severity could mean undertaking a costly excavation or repairs without cause. When the severity is underestimated, it could result in a catastrophic failure. The pressure to be right weighs heavily on their shoulders.

Pushing the Envelope of TFM Processing

The OmniScan™ X3 flaw detector’s sharp, high-resolution images give inspectors the imaging data they need to feel more confident in their calls. Thanks to the X3 flaw detector’s full matrix capture (FMC) and total focusing method (TFM) technology, you can see flaws clearly and more accurately represented in the part. The instrument’s powerful TFM processing is pushed even further through an advanced feature called the “envelope.”

When the envelope is turned on, the OmniScan X3 software’s TFM algorithms extract the real as well as the theoretical components of the signal and combine them before performing the envelope detection. This process helps ensure no data is lost, clears out noise and artifacts, and fine-tunes the image. In the resulting image, flaws are more focused, and their shapes and sizes are easier to characterize.

Envelope OFF—reconstruction artifacts are visible on the flaw signals.

Envelope OFF—reconstruction artifacts are visible on the flaw signals.

Envelope ON—the TFM image reconstruction results are clearer and sharper, the amplitude is increased, and no data is lost.

Envelope ON—the TFM image reconstruction results are clearer and sharper, the amplitude is increased, and no data is lost.

If the TFM Envelope Image Is So Much Better, Why Turn It Off?

Once you see just how much the envelope improves the OmniScan X3 image, you may wonder why an inspector would opt not to use it. There are two main reasons, one more easily solvable than the other.

The first reason is performance related. The processing power required to achieve the high image quality offered by the envelope is a drain on the instrument’s pulse repetition frequency (PRF), or the speed at which it fires and receives ultrasonic signals. The acquisition rate is decreased, slowing down the instrument’s scanning speed.

But there is a work-around. With a couple minor adjustments to the TFM grid resolution and the points per wavelength (“pts/λL” for longitudinal waves and “pts/λT” for transverse waves) settings, the acquisition rate can be increased so it’s even faster than before.

Keep the Envelope Image without Sacrificing Your Acquisition Rate

Because the envelope processing is so powerful, the grid resolution is less consequential than with standard TFM, so even when it is reduced (less fine), the image doesn’t suffer. When the grid resolution is lowered, the number of points per wavelength (the pts/λ) decreases accordingly. Since less processing power is required for the resolution, the acquisition rate speeds back up, by more than double in some cases.

Envelope ON—the TFM image reconstruction results are clearer and sharper, the amplitude is increased, and no data is lost.

Envelope ON with a coarse grid resolution setting—this TFM grid has 2.9 pts/λL. The result is an increased PRF (or Acq. Rate), but no significant distortion of the image.

Usually in ultrasonic testing (UT), the higher the number of points per wavelength, the better the resolution and the better the image, but the performance of the TFM envelope shatters this notion. Even after reducing the resolution and the pts/λ, the high image quality offered by the envelope is maintained.

The TFM Envelope’s Influence on Amplitude Fidelity and Grid Resolution

The points per lambda (pts/λ) reading is an important factor in preserving an acceptable amplitude fidelity value. Inspection codes, such as the ASME’s new TFM appendix, require that the amplitude fidelity remains steady at around two decibels (dB) or lower.

Without the envelope, a ratio of around 7 pts/λ reliably preserves the two-decibel amplitude fidelity variation, which results in a fine grid resolution. When the envelope is on though, the safe ratio to ensure the minimal amplitude fidelity required for detection purposes can drop as low as 2 pts/λ. However, a setting of around 3 pts/λ is usually required to preserve the image quality associated with the envelope.

A coarser grid resolution can be used when the envelope is on because both real and theoretical signal components are included in the processing algorithms. Learn more about how this works in the application note Using the Amplitude Fidelity Reading for Total Focusing Method (TFM) Grid Resolution Definition in the OmniScan X3 Flaw Detector.

Confidence in the TFM Envelope Comes with Experience

The simple adjustment to the grid resolution makes it possible to use the envelope and still achieve a high acquisition rate. But there is still one hurdle to overcome, which is the second reason why some inspectors may choose not to use the envelope: TFM is a newer technology they may be unfamiliar with, and the envelope is an even newer innovation. It may take some experimentation of their own to finally be convinced of the envelope’s capability.

It’s important to remember that no information is lost. On the contrary, the signal response from flaws is reinforced due to the envelope’s reconstructive effects.

TFM Envelope Consistently Provides Clearer Images of Flaws

In our own experimental testing, time and again, the envelope enhances the TFM image so that flaws are much crisper and clearer. There’s a notable advantage for applications involving flaws that are usually too small to see and difficult to characterize with standard phased array, such as high-temperature hydrogen attack (HTHA).

Envelope ON—the TFM image reconstruction results are clearer and sharper, the amplitude is increased, and no data is lost.

Example of TFM image of early stage HTHA acquired by the OmniScan X3 flaw detector with the envelope processing turned on

HTHA, also known as hot hydrogen attack, is an insidious corrosion that can develop gradually in steels that are exposed to hydrogen at high temperatures, such as refinery or petrochemical tanks or pipes. Using TFM imaging, inspectors can confirm their suspicions that HTHA is present in its early stages so measures can be taken to avoid failure.

Like with Most Revolutionary Technologies, Seeing Is Believing

When inspectors are given the opportunity to see the envelope in action on real-life samples, we think the empirical evidence will be undeniable. Like with most claims of extraordinary improvements, people are skeptical until they see it with their own eyes.

Contact your local Olympus representative to get more details on the envelope feature and our innovative TFM solutions. Or, check out to find out more or fill out the Request a Demo form.

Related Content

Top 3 FMC/TFM Enhancements of the New OmniScan X3 Flaw Detector Explained

White paper: TFM Amplitude Fidelity Reading

FAQs about TFM

Senior Product Manager, OmniScan Products

Tommy has worked for Olympus (formerly R/D Tech) for over 19 years. Prior to his current position as Senior Product Manager for the OmniScan product line, he worked as Product Manager for other in-service portable NDT product lines, including OmniScan ECA, MS-5800, NORTEC, and BondMaster. Throughout his career, Tommy has contributed to the development of probes and applications, worked to improve existing products, and has performed numerous trainings.

November 7, 2019
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