Inspection Trends

Ultrasonic testing (UT) has long been essential for inspecting welds, particularly in industries where structural integrity is crucial. Recent advancements, such as full matrix capture (FMC) and total focusing method (TFM), have pushed the boundaries of weld inspection, offering improved capabilities over conventional phased array ultrasonic testing (PAUT).

FMC and TFM address limitations in traditional PAUT, particularly in detecting, sizing, and characterizing complex flaws. The latest development, Intermodal TFM (TFMi), enhances these methods by combining multiple propagation modes, offering even greater accuracy.

While PAUT remains the standard in many inspection codes, including AWS D1.1, Structural Welding Code — Steel and AWS D1.5, Bridge Welding Code, FMC and TFM provide valuable supplementary tools for enhancing inspection quality. Though not yet universally accepted, these techniques offer superior imaging and flaw characterization, making them indispensable where higher precision is required.

This article explores how FMC, TFM, and TFMi improve weld inspections, offering clearer imaging and more reliable flaw detection that position these techniques as key advancements in UT.

Full Matrix Capture

Full matrix capture (FMC) is a significant advancement in UT that captures raw data from all elements in an ultrasonic array. Unlike traditional PAUT, which focuses at a predetermined distance, FMC records every A-scan signal between each combination of transmitting and receiving pairs of elements, creating a comprehensive data set that can later be postprocessed into highly detailed images.

How FMC Works

During FMC acquisition, each element of the phased array probe transmits a pulse one at a time while all the other elements receive the returning signals. This process is repeated for each element in the array, so if you have a 64-element transducer, each element pulses once, and every other element receives the signals, generating a total of 4096 A-scans (64 × 64). These A-scans are stored and later processed to reconstruct detailed images of the weld.

This comprehensive data collection enables flexibility during postprocessing, allowing advanced imaging techniques like TFM to be applied. By capturing data from every possible transmit-receive combination, FMC gives inspectors a complete view of the weld, providing improved flaw detection and characterization compared to conventional PAUT methods, which rely on fewer data points — Fig. 1.

Total Focusing Method

TFM is a postprocessing technique that converts FMC data into fully focused images across the area of interest. Unlike PAUT, which focuses only on specific points using preset focal laws, TFM provides high-resolution images by calculating the behavior of the ultrasonic wave for each pixel in the inspection area — Fig. 2. However, TFM relies on a single propagation mode selected by the inspector before scanning. This mode could be direct, reflected, or mode-converted, concentrating on a single leg or skip of the ultrasonic beam’s travel path.

How Propagation Modes Work

Propagation modes describe ultrasonic waves’ paths as they travel through a material, including how they interact with defects and material boundaries. A direct wave travels through the material and echoes back from the flaw, while reflected waves bounce off surfaces like the backwall. Mode-converted waves switch from one wave type to another, such as from longitudinal (L) to transverse (T) or vice versa, when they encounter a defect or boundary, like the crown or root of a weld.

In TFM, the inspector selects a specific propagation mode before scanning, and that mode determines the wave path being analyzed. The mode’s name indicates the number of reflections and mode conversions. For example:

More complex modes exist, such as 5T, 6T, and 7T, as well as various longitudinal modes, but are less commonly used in weld inspections. These propagation modes provide critical insight into the location and nature of flaws by capturing how the ultrasonic waves behave as they travel through the material.

Advantages and Limitations of TFM

TFM offers clearer, more detailed images compared to sectorial scans, making it excellent for flaw characterization. However, the need to choose a single propagation mode limits its ability to detect all flaws. For example, selecting a direct wave may miss flaws that are better detected through mode-converted waves or vice versa. Since the chosen mode focuses on a specific part of the beam’s travel, defects like off-axis or transverse flaws can go undetected, reducing the overall effectiveness of TFM for complex inspections.

Intermodal TFM

TFMi, developed by Sonatest in collaboration with Holloway NDT, addresses the limitations of TFM by combining multiple propagation modes into a single scan view. Instead of relying on a single mode like TFM, TFMi integrates up to four modes, allowing inspectors to simultaneously visualize flaws from multiple perspectives — Fig. 3.

How TFMi Works

TFMi, available exclusively on the Sonatest Veo3, eliminates the need for the inspector to choose a single mode. TFMi provides a complete picture of the weld’s integrity by combining multiple propagation modes into a unified image. For example, a transverse crack that might appear acceptable with a single-mode TFM scan due to low amplitude or limited visibility could be rejected when seen with multiple modes combined in TFMi. This approach ensures critical defects are more easily identified, characterized, and sized, making TFMi a valuable advancement for comprehensive weld inspections.

FMC, TFM, and TFMi in the Context of PAUT and AWS Standards

PAUT in AWS D1.1 and D1.5

Phased array is widely accepted in industry standards, including AWS D1.1 and AWS D1.5, which govern the inspection of structural welds. These standards allow PAUT sectorial scans to assess weld acceptability by measuring the size and location of flaws. However, these scans primarily focus on the amplitude of indications and may struggle to provide the necessary characterization for more complex or subtle defects. This is where advanced techniques like FMC/TFM and TFMi can offer significant advantages.

Importance of Flaw Characterization

While sectorial scans are effective for initial detection, weld inspectors often face situations where further flaw characterization is necessary. In AWS codes, some types of flaws may be accepted or rejected depending on their amplitude, but others require more detailed evaluation. TFM and TFMi provide the additional clarity needed to differentiate between various flaw types, helping to avoid unnecessary weld repairs or ensuring that critical defects are not overlooked. This enhanced characterization is crucial, as small differences in flaw type can significantly impact weld integrity.

Accurate Sizing with TFM and TFMi

Another key benefit of TFM and TFMi is their ability to provide more accurate flaw sizing compared to PAUT sectorial scans. Since TFM creates a fully focused image at every point in the inspection volume, it allows for precise measurements of flaw dimensions. Accurate sizing is especially important for making informed decisions about weld repairs, as improperly sized flaws can lead to unnecessary rework or missed defects that could compromise the weld’s performance.

Detection of Off-Axis and Transverse Flaws

Off-axis or transverse flaws are particularly challenging for sectorial scans to detect because of their orientation relative to the probe. Although time-of-flight diffraction (TOFD) is commonly used to detect these defects, TFM and TFMi offer an alternative that uses the same transducers as PAUT rather than requiring a subsequent TOFD scan with a separate set of specialized probes. By incorporating multiple wave modes, TFMi is more effective at discovering these flaws, which might otherwise go unnoticed, making it a valuable supplement to standard sectorial scans.

Practical Application: Integrating FMC, TFM, and TFMi in Weld Inspections

Workflow Integration

FMC, TFM, and TFMi can seamlessly integrate into existing inspection workflows alongside traditional PAUT. A common approach involves starting with a PAUT sectorial scan to detect and locate indications. Once a potential defect is detected, TFM or TFMi is used for enhanced flaw characterization and accurate sizing. This method combines the speed of PAUT with the detailed imaging capabilities of TFM and TFMi, offering inspectors more comprehensive insights without compromising efficiency.

Case Examples

In a typical scenario, a sectorial scan detects a small indication in a weld. Due to the flaw’s low amplitude and small size, it is deemed acceptable under AWS standards. However, when a supplementary FMC/TFM scan is performed, the inspector suspects the defect may be more serious. Unfortunately, because the defect appears differently in the two TFM propagation modes, it is still difficult to accurately size or characterize.

At this point, a TFMi scan is employed, combining the two modes into a single view. This reveals the flaw’s accurate size and shape, clearly visualizing a transverse crack that is now recognized as a rejectable defect. Thanks to the precise imaging provided by TFMi, the inspector can confidently recommend repair, demonstrating the critical role of these advanced techniques in ensuring weld integrity — Fig. 4.

In another instance, a sectorial PAUT scan detects an indication, and based on its amplitude, it is deemed acceptable, provided it is not a crack. The sectorial and A-scan readings disguise the flaw as a slag inclusion, an acceptable defect. However, when performing a TFMi scan, the geometrical detail clearly shows a planar crack-like flaw. With TFMi’s superior imaging, the flaw can now be accurately identified as a crack, leading to its rejection and necessary repair. This highlights how TFMi’s precise imaging can prevent serious defects from being overlooked, ensuring better weld integrity — Fig. 5.

Challenges and Considerations

Regulatory and Acceptance Issues

While FMC, TFM, and TFMi offer significant advancements in weld inspection, one of the main challenges is that these techniques are not yet universally accepted in formal inspection standards. Codes like AWS D1.1 and AWS D1.5 currently allow the use of PAUT sectorial scans for weld acceptability, but the adoption of FMC and TFM remains limited. This lack of universal acceptance can make it challenging to use these techniques in regulated environments where adherence to established standards is mandatory.

Training and Equipment

Another challenge is the need for specialized training to effectively interpret the data produced by FMC, TFM, and TFMi. These advanced techniques provide more detailed information than traditional PAUT, but inspectors may find it difficult to extract meaningful insights from the scans without proper training. Moreover, advanced equipment capable of capturing and processing FMC data is required, which could represent a significant investment for inspection teams.

Despite these challenges, the benefits of integrating FMC, TFM, and TFMi into weld inspection workflows are clear. As more inspectors become trained in these techniques and the technology becomes more widespread, it’s likely that regulatory acceptance will follow, making these tools an even more integral part of the inspection process.

Conclusion

As industries recognize the benefits of FMC, TFM, and TFMi for improving weld inspections, these advanced techniques are gaining momentum. Their ability to offer superior flaw characterization, accurate sizing, and better detection of off-axis defects makes them invaluable tools, especially in scenarios where conventional PAUT may fall short. While current standards, such as AWS D1.1 and AWS D1.5, primarily rely on sectorial scans, the advantages of these methods are sparking interest in their broader adoption. As technology continues to advance, these techniques are becoming more accessible, thanks to improvements in data processing, equipment, and ease of use.

Despite the challenges of regulatory acceptance and the need for specialized training, the future of ultrasonic testing is clear. FMC, TFM, and TFMi provide critical supplemental insights for weld inspections, ensuring more comprehensive evaluations. As more inspectors adopt these tools, they will likely become integral components of routine workflows, driving higher standards for flaw detection and weld integrity.

Works Consulted

AWS. 2020. D1.1, Structural Welding Code — Steel.

Sonatest. 2020. Ultrasonic Inspection of Welds E-Book.

The American Society of Mechanical Engineers (ASME). 2023. PTB - Full Matrix Capture Training Manual.

WILL HAWORTH (will.haworth@sonatest.com) is application engineer, Sonatest, Spokane, Wash.