Total Focusing Phased Array

Phased array ultrasonic total focusing method (TFM) imaging has attracted much attention due to its high detection image resolution and large dynamic range. However, due to the small emission energy of its single element, the echo signal is weak and susceptible to noise interference, resulting in a relatively low signal-to-noise ratio (SNR). Especially when detecting high sound attenuation carbon fiber reinforced polymer (CFRP) composites, the signal amplitude can decrease with depth to a level lower than the electronic noise introduced by the signal acquisition system. To solve this problem, a new imaging algorithm was proposed that combines the Golay code excitation using linear frequency modulation (LFM) signal as the carrier with the TFM algorithm (LFM-Golay TFM). Meanwhile, in order to further overcome the disadvantage of the LFM-Golay code encoding excitation method requiring two excitations, an AB code encoding excitation conversion factor was introduced, two sets of echo data were obtained through processing single excitation, TFM imaging based on LFM-Golay code single excitation (IMP-LFM-Golay TFM) is realized. Simulation and experimental studies were conducted to compare the TFM imaging performance of single-pulse excitation (Pulse TFM), Barker coding excitation (Barker TFM), LFM excitation (LFM TFM), and improved LFM-Golay coding excitation. The results show that IMP-LFM-Golay TFM has the best SNR, axial resolution, and depth resolution.

Ultrasonic arrays have been widely employed in industrial applications because of their ability to perform quick inspections for detection and characterization of flaws. The total focusing method (TFM) is an advanced ultrasonic imaging technique that utilizes the full matrix capture (FMC) data of the phased array elements to generate a high-resolution image by focusing on every image pixel in both transmission and reception. In this work, the time of flight framework involved for the TFM post-processing is illustrated for an angular wedge configuration. Further, the TFM algorithm is extended for imaging using multiple direct and half-skip wave modes by taking into account the wave mode conversions in the specimen. The implications of the mode conversions on the TFM images are analyzed, and the challenges posed on the TFM, specific to weld inspection, are explained in detail. In this work, experimental studies are performed on weld specimens with five common weld defects using the TFM algorithm. FMC data is acquired from the ultrasonic instrument and is post-processed to get the TFM images. The TFM results of weld defects show high resolution, better defect detectability, and easy visualization of defects compared to phased array sector scan images.KeywordsUltrasonic arraysFull matrix captureTotal focusing methodAngular wedgeWeld inspection

Real-time imaging, using ultrasound techniques, is a complex task in non-destructive evaluation. In this context, fast and precise control systems require design of specialized parallel architectures. Total focusing method (TFM) for ultrasound imaging has many advantages in terms of flexibility and accuracy in comparison to traditional imaging techniques. However, one major drawback is the high number of data acquisitions and computing requirements for this imaging technique. Due to those constraints, the TFM algorithm was earlier classified in the field of post-processing tasks. This paper describes a multi-FPGA architecture for real-time TFM imaging using the full matrix capture (FMC). In the acquisition process, data are acquired using a phased array and processed with synthetic focusing techniques such as the TFM algorithm. The FMC-TFM architecture consists of a set of interconnected FPGAs integrated on an embedded system. Initially, this imaging system was dedicated to data acquisition using a phased array. The algorithm was reviewed and partitioned to parallelize processing tasks on FPGAs. The architecture was entirely described using VHDL language, synthesized and implemented on a V5FX70T Xilinx FPGA for the control and high-level processing tasks and four V5SX95T Xilinx FPGAs for the acquisition and low-level processing tasks. The designed architecture performs real-time FMC-TFM imaging with a good characterization of defects.

Damage localization in aluminum plate with compact rectangular phased piezoelectric transducer array

Advanced ultrasonic techniques are now widely adopted in industry, though they present a risk of operators misunderstanding key features and misinterpreting results. The new phased array post-processing techniques, such as the total focusing method (TFM) and phase coherent imaging (PCI), have been implemented in the absence of agreed standards and consistent training. As human factors play a crucial role in the reliability of non-destructive testing (NDT), the effectiveness of these advanced techniques depends heavily on the proficiency of the inspector. The development of new equipment and techniques must be accompanied by appropriate training to mitigate the risk of human error. An assessment of inspectors’ perceptions regarding the reliability and implementation of phased array ultrasonic testing (PAUT) in weld inspection, with a specific focus on TFM and PCI, was conducted. The primary objective was to evaluate the industry’s proficiency, challenges and constraints in applying these advanced methods. Surveying Level 2 inspectors predominantly in the oil and gas sector, 76% of respondents have utilised TFM/PCI within the industry. However, 74% expressed little to no confidence in their ability to use these methods effectively. A significant concern highlighted was the inadequate training available, with over 75% of respondents indicating they had received little to no formal instruction on these techniques. This lack of comprehensive training underscores the importance of addressing human factors to enhance NDT reliability. Respondents also expressed a strong need for more detailed guidelines and standardisation across original equipment manufacturers (OEMs), specifically regarding pre-system set-up, including defining the region of interest (ROI) and selecting the correct wave set. Limited understanding of relevant codes and standards results in a shortage of qualified written instructions and structured procedures, while the high cost of training remains a major concern.

Discretized tensor-based model of total focusing method: A sparse regularization approach for enhanced ultrasonic phased array imaging

Imaging using the total focusing method (TFM) is a popular tool, which is becoming a standard, for nondestructive testing and evaluation. From full matrix capture data, it consists in focusing at each point of a defined reconstruction zone. It is generally more efficient than conventional phased array focusing, which only focuses at a few points. Despite its good image quality, TFM suffers from a lack of resolution and contrast, in particular in the case of close defects or in scattering materials. The TFM algorithm can be formalized as a linear operation on the data. The contribution of this paper is then to include a sparsity-inducing penalization to the TFM procedure. The reconstructed image is therefore supposed to contain only a few non zero values, corresponding to flaws or geometry. The final image is obtained by minimizing a penalized least-squares criterion within an iterative procedure. A first example uses data acquired from two close side drilled holes (SDH) in an aluminum block. The proposed algorithm shows good echo separation whereas the reference method suffers from overlapping, demonstrating resolution improvement. A second example comes from a stainless steel specimen, with high scattering noise level. The proposed method reduces the scattering noise and improves the contrast.

Additive manufacturing (AM) enables the fabrication of complex components with high geometric freedom, but it can introduce near-surface flaws due to rapid solidification, resulting in porosity and lack of fusion. In addition, localized melting and steep thermal gradients favor the formation of micro-cracks. Conventional ultrasonic techniques have shortcomings in detecting such flaws because of front-wall interference, further affected by surface roughness and anisotropy. This study evaluates the effectiveness of the Total Focusing Method (TFM), an advanced ultrasonic imaging technique implemented in Full Matrix Capture (FMC), for near-surface flaw detection in Laser Powder Bed Fusion (LPBF) AM components. To assess TFM performance, subsurface side-drilled holes (SDHs) in AM Ti-5Al-5V-5Mo-3Cr (Ti-5553) material were used as the reference reflectors and compared with Phased Array Ultrasonic Testing (PAUT) under identical conditions. Results showed that TFM achieved higher spatial resolution and more reliable detection of shallow flaws, successfully detecting features as shallow as 0.40 ± 0.05 mm below the surface, whereas PAUT was limited to greater depths. These findings demonstrate TFM as a reliable non-destructive evaluation method for shallow flaws in AM parts, while contributing one of the first systematic comparative datasets of PAUT and TFM for shallow SDHs in LPBF titanium alloys.

The marriage of welding and Non-Destructive Testing (NDT) processes at the point of manufacture has enabled the detection and correction of defects during the welding process. This has demonstrated clear financial and production benefits by reducing weld rework and ensuring schedule certainty, however this is yet to be demonstrated for use with narrow-groove welding practises. Narrow-groove welds are notoriously difficult to inspect using traditional Phased Array Ultrasonic Testing (PAUT) techniques due to large thicknesses and the vertical nature of Lack-of-Sidewall Fusion (LOSWF) defects. This is further complicated by the presence of partially-filled weld geometries during in-process inspection, which cause geometric reflections which can mask or falsely indicate the presence of a defect. A solution to this is proposed in this work, by adapting a dual-tandem phased array imaging system for the imaging of LOSWF defects in a partial weld geometry. This considers a two array system utilising a phased array probe on each weld side, coupled with an advanced dual-aperture Full Matrix Capture (FMC) acquisition technique. Advanced multi-mode image processing algorithms such as the Total Focusing Method (TFM) and Phase Coherence Imaging (PCI), with adaptive delay law calculation, have shown high sensitivity to LOSWF defects in a mock partial weld geometry. Additionally, an adaptive Probe Centre Spacing (PCS) technique is defined for in-process inspection based on amplitude and phase coherence sensitivity in partial weld geometries, with the effects of partial weld reflections analysed and discussed. These results have demonstrated the effectiveness of a dual-tandem phased array approach to imagine LOSWF defects during the in-process inspection of narrow-gap welds.

The application of total focusing method (TFM) for imaging of Carbon fiber reinforced plastics (CFRP) with ultrasonic phased array is particularly challenging due to the layered structure and anisotropy of CFRP. To improve the TFM imaging performance, the precise knowledge of complex ultrasonic paths in anisotropic CFRP laminates should be obtained for accurate calculation of time of flights (TOFs), which makes the inspection process complicated and time-consuming. In this work, an adaptive TFM is developed for full-matrix imaging of CFRP laminates using ultrasonic phased array. Based on the acquired full matrix data, the elastic constants, including C 11, C 22, C 55, and C 13, can be directly derived using TOFs of A-scan signals. Based on the measured elastic constants, group velocity profiles of quasi-pressure (QP) wave were then calculated for different stacking sequences, from which the TOFs were corrected. To verify the effectiveness of proposed method, simulations and experiments are demonstrated on the CFRP laminates with different stacking sequences. As a representative example, the delamination defects are showcased. Compared with isotropic TFM, the SNR of proposed method is increased by 10.2-18.0 dB. The simulated and experimental results agreed well with each other, indicating that the feasibility of proposed method in full matrix imaging of anisotropic CFRPs.

Closed cracks are difficult to detect using conventional ultrasonic testing because most incident ultrasound passes completely through these cracks. Nonlinear ultrasound inspection using sub-harmonic frequencies a promising method for detecting closed cracks. To implement this method, a sub-harmonic phased array (PA) is proposed to visualize the length of closed cracks in solids. A sub-harmonic PA generally consists of a single transmitter and an array receiver, which detects sub-harmonic waves generated from closed cracks. The PA images are obtained using the total focusing method (TFM), which (with a transmitter and receiving array) employs a full matrix in the observation region to achieve fine image resolution. In particular, the receiving signals are measured using a laser Doppler vibrometer (LDV) to collect PA images for both fundamental and sub-harmonic frequencies. Oblique incidence, which is used to boost sub-harmonic generation, inevitably produces various surface waves that contaminate the signals measured in the receiving transducer. Surface wave interference often degrades PA images severely, and it becomes difficult to read the closed crack's position from the images. Various methods to prevent or eliminate this interference are possible. In particular, enhancing images with signal processing could be a highly cost-effective method. Because periodic patterns distributed in a PA image are the most frequent interference induced by surface waves, spatial frequency filtering is applicable for removing these waves. Experiments clearly demonstrate that the spatial frequency filter improves PA images.

This paper presents the results of a project aimed at evaluating the performance of ultrasonic techniques for detecting flaws in Francis turbine runners. This work is the first phase of a more ambitious program aimed at improving the reliability of inspection of critical areas in turbine runners. Francis runners may be utilized to supply power during peak periods, which means that they experience additional load stress associated with start and stop sequences. Inspection during manufacturing is then of paramount importance to remove as much as feasible all flaw initiation sites before the heat treatment. This phase one objective is to collect initial data on a simplified mock-up and then to compare the experimental ultrasonic data with the results of simulations performed by CIVA, a computer simulation package. The area of interest is the region with the highest stress between the blade and the web. A welded T-joint coupon made of UNS S41500 was manufactured to represent this high-stress area. During the FCAW welding process, ceramic beads were embedded in the weld to create discontinuities whose size is in the critical range to initiate a crack. Inspection of the material was carried out by various nondestructive testing (NDT) methods namely conventional pulse-echo, phased array, total focusing method (TFM). With these results, detection rates were obtained in order to compare the effectiveness of each method.

In Chap. 12, we described the ways in which images can be formed with phased arrays, including a two-dimensional (2-D) far field imaging measurement model. More general imaging measurement models will be developed in this chapter for forming images in 3-D with 2-D arrays and for forming images of 2-D scatterers with linear arrays. As found in Chap. 12 for the 2-D case, these imaging measurement models can be directly related to SAFT imaging and the total focusing method (TFM).

Due to the dissimilar properties of the cladding layer and carbon steel substrate, which lead to inaccurate inspections, nondestructive testing (NDT) needs to be verified with a reference specimen. This research presented the preparation of stainless-clad steel reference for NDT with imaging inspections such as profile radiography (PRT), ultrasonic phased array (PA), and full matrix capture with the total focusing method (FMC/TFM). The effect of cladding thickness and complex defects on the inspection results was evaluated. In the experiment, the reference specimen was prepared by welding stainless steel on carbon steel and drilling holes to simulate defects as cluster, aligned, and interlayer characteristics. The PRT exhibited asymmetry in two-dimensional images with varying cladding thicknesses, which could be measured by image density analysis but could not identify complex defects. The PA provided images with good accuracy for measuring cladding thickness, but it was limited in detecting some complex defects due to signal reduction. The FMC/TFM demonstrated the best image for thickness measurement and identification of complex defects because its individual element probe could reduce signal intensity loss. In addition, a possible application of FMC/TFM in the butt weld inspection was also demonstrated.

The article deals with the internal defects identification and characterization in butt weld joints by non-destructive ultrasonic Phased Array and X-ray technique. Basics of ultrasonic and X-ray testing are described in the theoretical part of manuscript. Phased Array and X-ray technique are volume nondestructive methods that can detect internal defects without breaking of construction. Ultrasonic Phased array and X-ray test procedures and test results obtained in non-destructive testing of butt weld are shown in experimental part. Ultrasonic record, X-ray record and weld macrostructure are given for each identified weld deffect. Advantages and disadvantages as well as comparison of ultrasonic and X-ray testing resulting from experimental measurements are described in the end of this article.