Is Phase Coherence Imaging Right for Your Application?

This blog post is provided by Evident Scientific, an ASNT Affiliate Partner.

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by Trevor Tartaglia

Phase coherence imaging (PCI) is a variant of the total focusing method (TFM) that detects defects using the signal’s phase information, disregarding the amplitude. Using only the phase portion of the signal offers advantages over conventional amplitude-based ultrasonic testing techniques—including phased array (PA) and TFM.

How PCI Works

PCI processes the same full matrix capture (FMC) data acquired for conventional TFM. It starts by normalizing the elementary A-scans and then it computes the phase coherence of the normalized A-scans for each point in the TFM frame. Because the resulting PCI image is based on the statistical variance of phase coherence, the level cannot exceed 100%. This means that signal saturation is impossible. The quantized A-scan’s phase information also remains more constant over time and distance, unlike the amplitude, which grows weaker the farther the beam travels after the near field (see Figure 1).

Moreover, tip diffractions from flaws have a much higher phase coherence than chaotic, incoherent background noise, so the contrast between them is accentuated in grainy and sound-attenuative materials, such as stainless steel.

The recent recognition by the industrial NDT industry of PCI as a method for improving data visualization provides, in applicable use cases for improving UT data, capabilities to help resolve some of the NDT industry’s most challenging use cases.

Could PCI improve the results for your application? To help you decide, here are some examples of damage mechanisms that PCI may show improvement over historical approaches.

High-Temperature Hydrogen Attack (HTHA)

PCI is especially sensitive to small defects and crack tips that typically have a highly coherent response yet weak amplitude. Because the PCI data is phase based, small defects are easy to distinguish in the final image. Furthermore, because amplitude is irrelevant with PCI, gain no longer needs to be adjusted. This means there is less risk that small defects located in proximity to large reflectors, such as the back wall, are lost in their strong echoes. 

In its early stages, high-temperature hydrogen attack (HTHA) damage can be difficult to detect using amplitude-based ultrasonic techniques. The orientation of HTHA defects, their size, and their proximity to the back wall are all contributing factors.

With PCI, however, the tip diffraction response from these small reflectors is highly coherent compared to large specular reflectors such as the back wall (see Figure 2). Since each tiny edge of the defect emits a diffraction signal, the direction and orientation can easily be seen, regardless of the orientation.

Wet Hydrogen Sulfide Damage

Another challenge when using conventional PA and TFM is determining whether hydrogen sulfide (H₂S) induced blisters have propagated to the surface—for example, the internal diameter of process piping. With amplitude-based techniques, it is possible to detect hydrogen blistering using a 0° inspection, but it’s difficult to determine the full extent of the damage. This is an inherent limitation, which is due to insufficient amplitude or the sound being unable to access the surface connection. Despite a weak signal amplitude, PCI can still evaluate the phase information, revealing this hidden but essential information (see Figure 3).

Stress Corrosion Cracking

Vertical defects are also an imaging challenge for conventional TFM, because the crack orientation results in a weak and inconsistent amplitude response. Self-tandem wave sets are sometimes successful, but the top and bottom of the defect are usually split between two groups. This makes it more difficult to characterize defects in applications such as sizing and analyzing stress corrosion cracking (SCC).

PCI, on the other hand, reliably detects vertical, irregular defects and they stand out clearly on the display. For a crack of any orientation, the tip diffractions from its sharp directional changes provide highly coherent phase responses, which generate “hot spots” in the PCI image. The tip diffractions enable you to easily identify the shape and direction of the crack, and you can use the hot spots to accurately size them.

When detecting SCC with PCI, fewer groups can be used to achieve high-quality images and using fewer groups improves the efficiency of your setup and data collection. Note that T-T and TT-TT pulse-echo propagation modes typically provide excellent PCI results for SCC detection.

Lack of Fusion

Signal saturation is a common issue when evaluating lack of fusion (LOF) in a weld using amplitude-based ultrasonic techniques, which can make sizing the LOF time consuming. As previously mentioned, the problem of signal saturation is eliminated with PCI. Additionally, sizing a LOF is faster and easier since you can use the tips diffractions as reference points, and you no longer need to adjust the gain or find the 6 dB drop like with amplitude-based techniques (see Figure 4 on the right).

Porosity

PCI is also a big help when trying to identify porosity, due to its high sensitivity to small defects. It’s so effective that you can clearly distinguish the individual pores (see Figure 4 on the left).

Cracks

Sizing cracks in welds is also eased when using PCI for the same reasons mentioned for SCC.

Conclusion

The intention of providing these examples is to show the types of applications where PCI can be advantageous compared with amplitude-based ultrasonic techniques. Though it’s a relatively new technique in the field of industrial NDT, PCI demonstrates enormous potential to solve many of the current challenges that inspectors are facing. If your application involves highly attenuative, noisy materials or especially small defects in difficult-to-detect locations or orientations, PCI may be the solution.

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Trevor Tartaglia is Product Applications Leader at Evident Scientific, trevor.tartaglia@olympus.com.