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Technical Insight: Full Matrix Capture

Full matrix capture (FMC) is a phased array ultrasonic testing (PAUT) data collection technique capable of capturing every possible transmit-receive combination from a transducer. The data, once acquired, is processed in real time using an optimised version of the total focusing method (TFM) algorithm to generate images. These high resolution, fully focused images show even small flaws in the sub-surface of a component in real time. As a process, it increases the reliability of ultrasonic inspections, reducing costs and improving safety for industry.

TWI has conducted a number of projects over the years to investigate, advance and promote FMC for industry. This includes core research projects for the benefit of our Industrial Membership base, joint industry programme projects which allow interested parties to join as sponsors in return for exclusive access to the outcomes and the chance to steer the direction of the work, public funded projects that benefit wider industry, and projects undertaken exclusively on behalf of individual Industrial Member companies.

Although some of this work is necessarily confidential, we can provide some example details of FMC projects we have completed in the past…

Core Research Projects

- Ultrasonic Full Matrix Capture to Improve Characterisation

As the FMC technique was developed it was ready for use by industry for applications such as mainstream ultrasonic weld inspection and long-range ultrasonic examination. Although it was known that the technique offered higher resolution and increased sensitivity compared to conventional phased array (PA) techniques, it was felt that a better assessment of a flaw’s morphology could be achieved by determining the shape and character of any flaws. Improved flaw sizing would facilitate more accurate fitness for service assessments, leading to enhanced levels of safety and reduced costs for industry. This 2010 project developed algorithms to implement the FMC concept and demonstrated the method by detecting and sizing a set of flaws designed to illustrate the benefits of the FMC implementation, in comparison with conventional focused/steered array implementation.

- Full Matrix Capture Ultrasonic Inspection with Sparse Arrays

This 2012 core research project (CRP) worked to develop the methods for the acquisition of FMC ultrasonic array data, specifically aiming to improve the speed of inspection whilst maintaining a high-quality image. This included the use of novel sparse array designs to improve image frame rates and the development of an efficient algorithm that allowed for inspection through layered material using minimal processing power. This project also marked the first time that methods to calibrate FMC image sensitivity were suggested, with calibration methods being adapted and developed from other existing phased array technology. At the time of this and the previous project, FMC was still a relatively new technique, with the fundamental principles only having been developed within academic institutions since 2005. The developments from these early TWI CRPs meant that FMC was ready to be developed for commercial use.

- Multimode Virtual Source Aperture Imaging for Non-Destructive Testing

Virtual source aperture (VSA) is a supplementary technique for FMC that was developed at TWI to improve imaging rates by combining phased array-like transmission with FMC reception, improving the A-scan signal to noise ratio (SNR) and reducing the number of transmissions required. This project worked to develop a VSA multi-mode imaging algorithm and find a computationally efficient approach to solving Fermat’s principle of least time for multiple refractive boundaries. Single mode signals were typically preferred to weaker mode-converted signals, however it can be beneficial to analyse them as shown in figures 1a and 1b, which demonstrate how a multimode sound path is more likely to return to the transducer to be detected. This project created an approach that reduced the path calculation by a factor of two, thereby increasing frame rates for inspection. The effect of this approach for improving flaw characterisation and sizing accuracy can be seen in figures 2a and 2b.

Joint Industry Projects

Where our CRPs helped develop FMC for the benefit of multiple industries, our joint industry projects (JIPs) allowed our experts to focus on particular challenges on behalf of specific industries.

- Advanced Ultrasonic Imaging Techniques for Aerospace Components

This project worked for the benefit of the aerospace industry by assessing FMC-based ultrasonic testing techniques developed at the University of Bristol. This allowed us to provide objective technical information on the potential industry use of the processing techniques and their likely benefit to industry (Figures 3-4).

- Full Matrix Capture Ultrasonic Inspection of Girth Welds in CS Pipe and CRA Clad Pipe

Completed in 2016, this JIP addressed the issue of accurate sizing positioning and characterisation of flaws in carbon steel (CS) and corrosion resistant alloy (CRA)-clad pipeline girth welds for the oil and gas industry. By combining FMC data acquisition with the total focusing method (TFM) image reconstruction algorithm it was hoped to overcome several shortcomings associated with the existing pipeline girth weld inspection techniques and procedures. This project included the development of an optimised ultrasonic FMC+TFM inspection setup and procedure, the creation of girth weld pipe samples, the validation of the inspection performance, and the development of a best practice guide for FMC+TFM inspection of girth welds.

- Evaluation of Ultrasonic Array Inspection Techniques for Coarse Grain Metallic Structures

Although ultrasonic testing techniques including FMC have proven to be reliable and efficient for non-destructive testing in industrial applications, there are still challenges around their use for coarse-grained metallic structures. The coarse-grain structure presents significant challenges due to signal attenuation, scattering, and reduced resolution. To address this, TWI created a JIP to compare different advanced ultrasonic array inspection techniques for detecting and sizing flaws in coarse-grained metallic structures to provide guidance on the suitability of these techniques, underpinned with experimental evidence on a range of industry applications of NDT in demanding industry sectors, such as power generation, nuclear, defence, and oil and gas.

- UT in lieu of RT: Guidance and Validation of Through Cap Inspection of Welds with Intact Caps

Although significant benefits with regards to costs and workplace health and safety can be achieved by switching from radiographic to ultrasonic inspection, ultrasonic techniques such as FMC have not been fully adopted in lieu of radiographic inspection due to limitations for complex joint configurations as a result of the requirement to remove weld caps to achieve full coverage. Radiographic inspection avoids this additional machining and preparation cost. To address this, TWI created a JIP to develop ultrasonic techniques and procedures to use ultrasonic testing for the inspection of complex configurations with the weld caps in place. This work included a focus on factors such as welding process, joint configuration and weld cap profile and how they impact the ability to perform weld scanning with the cap intact.

Public Funded Projects

Our experts have lent their expertise to a wide range of public-funded projects, working alongside other organisations to achieve aims for the benefit of different industries. This includes one example project that aimed to apply FMC to the inspection of railway crossings…

- Ultrasonic Synthetic Aperture Focusing Technique for Inspection of Railway Crossings (Frogs)

The European Commission-funded SAFTInspect project addressed the NDT inspection of rail infrastructure at safety critical locations such as crossings that employ high manganese, high strength steel components. Restricted access limited the range of techniques available for in-service use and those capable of inspection from the top surface. This project proposed phased array ultrasonic testing FMC in combination with the synthetic aperture focusing technique (SAFT) post processing method to reduce the time required at the asset as well as removing the need for additional interpretation by highly skilled operators.

Other Projects and TWI Crystal

Because TWI has decades of experience in a broad range of technologies and capabilities, working across - and for the benefit of - all industry sectors, we have been able to bring together different areas of specialisation to provide innovative new solutions for industry.

This includes the development of TWI’s CrystalTM software, which was created to make ultrasonic inspection of industrial structures using FMC, virtual source aperture (VSA) and plane wave imaging (PWI) easy, as demonstrated with a project to assess pipeline girth welds…

- Fast Fully Focussed Ultrasonic Inspection of Girth Welds

Millions of miles of pipeline rely on pipeline girth weld inspection to maintain their integrity and ensure the continuous flow of resources from production sites to users. Ultrasonic testing has played a vital role in this for decades, but advances in algorithms to interpret data have simplified the inspection process and allowed users to view the data in ways not considered before, improving the way in which flaws can be detected and sized, while also increasing the variety of flaw types that can be found. Real-time total focussing method (TFM) based algorithms, such as FMC, VSA and PWI, process the ultrasonic data to synthesise a PA focal spot at every pixel in an image, giving a fully focussed, easily interpreted image. The CrystalTM software provided TFM-based, fully focussed ultrasonic imaging capable of many different imaging views that are specifically designed for a certain type of flaw, in a certain part of the weld, and at a certain orientation. The software allowed for a coordinated inspection with two transducers, each displaying a half skip, full skip and self-tandem channels, along with a half skip and full skip pitch-catch channel. These imaging views can be combined into a single image by using known reflectors for calibration. Each view was designed to highlight a specific flaw type and orientation. By combining the imaging views, the Crystal software provides the highest probability of detecting and sizing a wide variety of flaws that can occur within a girth weld (Figures 5-7). You can find out more about TWI CrystalTM here.

These are just some examples of FMC utilisation at TWI over the years, to find out more about full matrix capture at TWI, please see here:

https://www.twi-global.com/what-we-do/services-and-support/asset-management/non-destructive-testing/ndt-techniques/full-matrix-capture

Figure 1a. Depiction of sound paths for multimode
Figure 1a. Depiction of sound paths for multimode
Figure 1b. Depiction of sound paths for single mode inspection
Figure 1b. Depiction of sound paths for single mode inspection
Figure 2a. VSA image of a surface breaking flaw using self-tandem using single mode. Here only the lower part of the flaw can be detected
Figure 2a. VSA image of a surface breaking flaw using self-tandem using single mode. Here only the lower part of the flaw can be detected
Figure 2b. VSA image of a surface breaking flaw using the self-tandem multimode method. Here the base of the flaw can be distinguished
Figure 2b. VSA image of a surface breaking flaw using the self-tandem multimode method. Here the base of the flaw can be distinguished
Figure 3. Aluminium sample with fatigue crack used in the experiments. All dimensions in millimetres
Figure 3. Aluminium sample with fatigue crack used in the experiments. All dimensions in millimetres
Figure 4. Image of the fatigue crack using (a) FMC, (b) the nonlinear technique
Figure 4. Image of the fatigue crack using (a) FMC, (b) the nonlinear technique
Figure 5. Full matrix capture inspection of a pipeline girth weld sample using TWI’s Crystal testing software. Half skip, full skip and self-tandem modes considered, along with a pitch catch channel
Figure 5. Full matrix capture inspection of a pipeline girth weld sample using TWI’s Crystal testing software. Half skip, full skip and self-tandem modes considered, along with a pitch catch channel
Figure 6. Image depicting the half skip and full skip views (top), the self-tandem views (middle), and the pitch catch half skip and full skip channels (bottom)
Figure 6. Image depicting the half skip and full skip views (top), the self-tandem views (middle), and the pitch catch half skip and full skip channels (bottom)
Figure 7. Full Matrix Capture inspection of a pipeline girth weld sample using TWI’s Crystal testing software
Figure 7. Full Matrix Capture inspection of a pipeline girth weld sample using TWI’s Crystal testing software
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