The article reviews some of the basic principle of FMC/TFM data acquisition and imaging processes and compares them to standard phased array techniques. An introduction to the most important American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code is also provided.
Introduction
In theory, the advanced ultrasonic testing technique Full Matrix Capture / Total Focusing Method (FMC/TFM) should offer improved inspection performance compared to Phased Array Ultrasonic Testing (PAUT) in terms of flaw detection, resolution, classification and some cases sensitivity. The pure data volume acquired by FMC provides the TMF imaging algorithm with significantly more data, or A-scans, from which accurate 2-D reconstruction of the region of interest. The FMC/TFM concept was considered for non-destructive testing about 2 decades ago [1] and compared to standard linear, focused, and steered PAUT. The linear 0 degree scan shown in Figure 1 pulses all the elements in the aperture at the same time and then sums up the received waveforms from each aperture to create a single A-scan. The process is repeated N – (a + 1) times where N is the size of the array, a is the aperture size and assuming step size of 1. This scan is referred to as B-scan, E-scan, and liner 0-degree scan. The pulsing sequence and resulting B-scan are shown in Figure 1 for an automated 2-D scan with the B-scan in the top right corner using an Olympus X3 PAUT platform.
Figure 1: Example pulsing sequence and resulting PAUT image for 0-degree B-scan.
The focusing point of a fixed aperture linear scan is static and set by the aperture length and testing frequency. Linear scan, 0-degree or at specific angles, are achieved by treating the individual elements of the aperture as separate emitters and applying time delays to the excitation signal sent to each emitter. Each element serves as a unique receiver to which an identical time delay is applied. The received time-delayed waveform for each element within the aperture are summed to create the aperture’s A-scan. The pulsing sequency and resulting focused B-scan are shown in Figure 2 for an automated 2-D scan with the B-scan on the right side of the display using an Olympus X3 PAUT platform.
Figure 2: Example pulsing sequence and resulting focused PAUT image for 0-degree B-scan.
An angled linear B-scan, for longitudinal or shear waves, applies time delays to the individual pulsing and receiving aperture elements and the latter are summed to create the B-scan. In this scenario a fixed angle aperture is dynamically swept across a large multi-element PAUT transducer. Each aperture generates a single A-scan from which the B-scan is created. The pulsing sequence and resulting fixed angle B-scan are shown in Figure 3 for an automated 2-D scan with the B-scan in the top right corner using an Olympus X3 PAUT platform.
Figure 3: Example pulsing sequence and resulting PAUT image for 0 degree B-scan.
Using a fixed aperture, the series of time delays is indexed through the aperture with each set creating a unique A-scan from which a sectoral B-scan, or S-scan, is generated. Each summed A-scan is referred to as a focal law, angle or beam. The focal laws are color mapped by intensity across the angle sweep plane.
Figure 4: Example pulsing sequence and resulting PAUT image for 45-70 S-scan.
FMC/TFM does not apply any time delays to the pulsed elements of the FMC aperture used. It simply pulses PAUT transducer element 1 and receives the waveforms generated will all elements, 1 to N. After element 1 is pulsed and elements 1 to N receive, the pulsed element is indexed and the receiving process is repeated. This data collection process results in an N x N matrix, hence the term FMC. TFM is a processing technique that first pixelates, grids, or discretizes, a defined region of interest (ROI). The intensity of each grid point is then synthesized by summing up each contribution of the N x N A-scans acquired during the FMC process. Each A-scan will be time-shifted prior to the synthetization based on the position of the grid point relative to pulsing and receiving elements and the selected wave mode, longitudinal or shear. The discretization process and TFM process is illustrated in Figure 5 using an Olympus X3 PAUT platform for a corner weld joint.
Figure 5: Example pulsing sequence and resulting FMC/TFM image.
Adopting FMC/TFM for Non-destructive Testing Applications
Adopting and implementing FMC/TFM requires code compliant equipment, procedures and NDT personnel. ASME BPVC.V-2021, Article 4 - Mandatory Appendix XI Full Matrix Capture [2] provides the written procedure requirements, personnel qualification, equipment, calibration, and examination minimum requirements. This documents also discusses procedure qualification and references ASME BPVC.V-2021, Article 4 - Mandatory Appendix IX Procedure Qualification Requirements for Flaw Sizing and Categorization ASME BPVC.V-2021 [3]. Personnel qualifications in [2] references ASME BPVC.V-2021, Article 1 – Mandatory Appendix II Supplemental Personnel Qualification Requirements for NDE Certification [5]. Adopting FMC/TFC requires a thorough understanding of these FMC/TFM ASME BPVC.V-2021 references and developing a procedure that addresses all the essential variable presented in Table XI-421.1-1: Requirements of an FMC Examination Procedure. Most NDT testing companies that are pursuing FMC/TMC service lines have previously developed PAUT procedures that address the essential variables in Table V-241 Requirements of Phased Array Linear Scanning Examination Procedures in Article 4 Mandatory Appendix V Phased Array E-scan and S-scan Linear Scanning Examination Techniques [5].
NDT personnel qualification is more rigorous for FMC/TFM candidates compared to conventional ultrasonic testing but is comparable to PAUT training and qualification requirements. SNT-TC-1A [6] recommends an additional 80 hours of classroom training for TFM/FMC candidate with ultrasonic level 2 training as a prerequisite. ASME BPVC Mandatory Appendix II Supplemental Personnel Qualification Requirements to NDE Certification [6] also cites a minimum of 80 hours in Table II -121-2 Additional Training and Experience Requirements for PAUT, TOFD, and FMC Ultrasonic Techniques. However, this standard also requires that supplemental specific hardware and software training shall be required for automated or semiautomated technique applications. This requirement is non-specific and is implemented by the testing company based on their target inspection market.
References
1. C. Holmes, B.W. Drinkwater, P.D. Wilcox, Post-processing of the full matrix of ultrasonic transmit–receive array data for non-destructive evaluation, NDT and E International 38 (8) (2005) 701–711.
2. ASME BPVC.V-2021, Article 4 - Mandatory Appendix XI Full Matrix Capture
3. ASME BPVC.V-2021, Article 4 - Mandatory Appendix IX Procedure Qualification Requirements for Flaw Sizing and Categorization
4. ASME BPVC.V-2021, Nonmandatory Appendix F Examination of Welds Using Full Matrix Capture
5. ASME BPVC.V-2021, Article 1 – Mandatory Appendix II Supplemental Personnel Qualification Requirements for NDE Certification