Advanced ultrasonic technique of Phase Coherence Imaging (PCI) as a complement to the Total Focusing Method (TFM)

Author: Ing. Carlos Álvarez, September 21, 2023

Introduction

Ultrasound tests

Ultrasound testing is Non-Destructive Testing (NDT) widely used in various industries to inspect and evaluate the integrity of materials and structures. One of the main problems of these tests is obtaining accurate and detailed images of the inspected objects. To address this challenge, researchers and engineers have developed advanced techniques over the years, such as the Total Focusing Method (TFM ).

However, even with TFM, there are certain limitations, such as when dealing with highly attenuative materials. In this article, we will explore the advanced ultrasonic technique known as Phase Coherence Imaging (PCI) and discuss how it can complement and enhance the capabilities of TFM ¹ .

The Total Focus Method (TFM)

The Total Focusing Method or as it is known by its acronym in English Total Focusing Method (TFM), is a recent technique to evaluate materials and structures in a non-destructive way. This method is based on the orientation and targeting methodology of conventional phased array ultrasound technology, but differs with targeting. It is applied to all parts of the area to be inspected and not just to a fixed depth.

This method uses advanced algorithms to improve the focusing and imaging capabilities of conventional phased ultrasonic testing (PAUT) systems. PAUT uses a series of transducers to emit and receive ultrasonic waves, allowing the beam to be steered and focused electronically.

How does the TFM work?

The Total Focus Method (TFM) is a technique integrated with the already existing Phase Array Ultrasound (PAUT), which significantly improves the detection and characterization of defects, as well as the probability of detection (POD) in critical industrial applications.

This technique uses an innovative approach where, in a first stage, each element of the transducer is activated individually and information is collected by all elements, this first part of the process is known as Full Matrix Capture (FMC). Subsequently, this information is processed by algorithms to dynamically adjust the focal point, this second part of the process is known as Total Focusing Method (TFM).

Using this technology, detailed images of the internal structure of the material are obtained, improving the POD and allowing the identification and precise characterization of defects such as cracks, inclusions and porosity.

Advantages of the TFM

1. Improved Resolution: TFM provides exceptional resolution by optimizing the focus point for each data point, resulting in clear, detailed images.
2. Improved signal quality: The technique reduces the impact of noise and interference, resulting in higher signal quality and better defect detection.
3. Versatility: TFM is versatile and can be used for a wide range of applications, from weld inspections to corrosion mapping.

While TFM represents a significant advance in ultrasonic testing, it has limitations, especially when dealing with highly attenuating materials. These limitations have led to the development and adoption of complementary techniques such as Phase Coherence Imaging (PCI).

Phase Coherence Imaging (PCI)

Phase Coherence Imaging, or (PCI) ² , is a relatively new ultrasonic testing technique that complements TFM and addresses some of its limitations. It is based on the concept of preserving and using the phase information of received ultrasonic signals. By doing so, you can provide valuable information about the structure being inspected.

The received wave signals (each defining an A-Scan) are in phase when they have the same frequency and the phase difference between them is constant. Coherence level refers to the measure of similarity or agreement between waveforms (phase) recorded in two different A-Scan data records.

In this way, the processing of the received signals through PCI is based exclusively on the phase information carried by the elementary A-scans in order to generate a TFM representation and is independent of the amplitude of the signals.

Omnidirectional sources, such as porosities, tip diffractions, and slag, tend to show high intensity as they are viewed with approximately the same phase by many Full Matrix Capture (FMC) emitter-receiver pairs. Reflectors whose phase has a strong directional dependence, such as front surface, back wall, delaminations, lack of fusion (LOF), etc., produce low value PCI. PCI facilitates the detection of volumetric defects (porosities and slag) and the identification of diffracted signals at the crack tip for better sizing.

The Phase Coherence Imaging technique is included in several instruments with the TFM option and in particular EddyFi offers it in three of its product line: Gecko, Mantiz and Topaz .

How does PCI work?

1. First, the acquired A scans are normalized. That is, the phase and amplitude differences of the A-Scans are adjusted or corrected to produce a reference A-Scan.
2. The phase distribution of each A-scan is then compared for each position in the TFM zone.
3. For a given position, the higher the level of coherence between A-scans, the stronger the signal response will be for that position (with a maximum of 100%).
4. Reflections and diffractions from defects result in a coherent response, compared to the incoherent response of signals acquired from high-frequency background noise. This makes defect identification very easy, especially for small defects in noisy or attenuating materials.

Advantages of PCI

1. Live 2D imaging using signal phase information: This allows identification of misoriented or very small defects, such as high temperature hydrogen attack (HTHA). On the other hand, compared to the TOFD technique, which is based in part on the phase shift of the signal, PCI inspection does not require additional movement in the index direction to observe the actual location of the fault at through the thickness of the material.
2. Impossibility of signal saturation: This is not a problem for PCI since it is based on the coherence and not the amplitude of the signals.
3. No gain manipulation required: For the same reason that it is based on the coherence and not the amplitude of the signals, it is not necessary to adjust the gain to a known reflector (the gain control is locked on the instrument).
4. More consistent results and easier sizing: The results are repeatable between successive inspections and/or different inspectors, which makes it ideal for monitoring cracks created and detected during service (Fitness For Service).
5. Fewer groups required to cover the same area: PCI will provide good results even if the return amplitude is low, since coherence can be evaluated even if the amplitude is weak. But more importantly, the position of a fault within the TFM zone will have little effect on signal coherence. On the other hand, spike diffractions can often be lost in the background noise, but PCI makes these diffractions stand out. All of these factors together translate into fewer groups required for the same coverage area.
6. Improved contrast: PCI can detect subtle variations in material properties, improving contrast and detection of defects, especially those located near the surface.
7. Reduced sensitivity to attenuation: Unlike traditional ultrasonic techniques, PCI is less affected by signal attenuation, making it suitable for highly attenuating materials.

A disadvantage of PCI is the need to use a full FMC to obtain a good SNR. This usually has an impact on productivity, because it considerably reduces the scanning speed (1). One way to counterbalance this limitation is to implement a Plane Wave Imaging (PWI) function to improve scanning speed for TFM inspections and available on equipment from the EddyFi product line: Gecko, Mantiz and Topaz .

Complementing TFM with PCI

While TFM and PCI are powerful techniques on their own, their true potential is realized when used together. The combination of these two advanced ultrasonic methods (See Image) can result in a comprehensive inspection strategy that overcomes many of the limitations associated with the individual techniques.

Both techniques complement each other in the following aspects:

1. Improved image quality: By integrating TFM and PCI, it is possible to obtain highly detailed and precise images of complex structures.
2. Improved defect characterization: When TFM and PCI work in synergy, defect characterization becomes more robust. TFM’s ability to provide an accurate location of the defect is complemented by PCI’s ability to reveal the nature and extent of the defect.
3. Greater reliability in challenging environments: In scenarios involving highly attenuating materials, the TFM alone may struggle to provide reliable results. PCI’s reduced sensitivity to attenuation makes it an ideal complement to TFM. Together, they offer a reliable solution for inspections in challenging environments.
4. Versatility in NDT applications: The combination of TFM and PCI is versatile and can be applied in various industries and NDT applications. From aerospace components to oil and gas pipelines, this dual-technique approach adapts to different inspection needs, making it a valuable asset for inspectors and engineers.

Conclusions

The integration of phase coherence imaging (PCI) ² with the total focus method (TFM) represents a significant advance in the field of ultrasonic testing. While TFM excels at improving resolution and reducing noise, PCI complements it by offering improved defect characterization and the ability to inspect highly attenuating materials. Together, TFM and PCI provide a comprehensive solution for non-destructive testing in various industries.

As technology continues to evolve, the integration of TFM and PCI is poised to become standard practice in ultrasonic testing, allowing engineers and inspectors to achieve more accurate and reliable results, while ensuring the integrity of structures and critical materials. This dynamic duo of ultrasonic techniques will play a critical role in the future of nondestructive testing.

Bibliographic references

1. FREDÉDÉRIC REVERDY. The Pros and Cons of Phase Coherence Imaging (PCI); Consulted on September 18, 2023; https://blog.eddyfi.com/en/the-pros-and-cons-of-phase-coherence-imaging-pci
2. EVIDENT. Phase Coherence Imaging (PCI). Better practices; Consulted on September 19, 2023