Total Focusing Method Research Articles

Layer-by-layer acoustic travel time approximation using ray theory for total focusing method imaging in carbon fiber reinforced polymer

This paper proposes a layer-by-layer acoustic travel time approximation method based on ray theory for total focusing method (TFM) imaging in carbon fiber reinforced polymer (CFRP) laminates. The method considers the anisotropy in every monolayer and heterogeneity of CFRP, which approximates the path of propagation as straight in the whole material. The application of this method to TFM imaging is called TravelTimeAppro-TFM. In comparison to isotropic-TFM and Dijkstra-TFM, the experimental results indicated that TravelTimeAppro-TFM outperforms isotropic-TFM in terms of imaging amplitude gain with a maximum gain of 4.67 dB. On the other hand, this approach reduces the computational work compared to Dijkstra-TFM. The proposed method demonstrates significant improvements in both focusing performance and the speed of calculation. This paper also investigates the effective angular range of the layer-by-layer acoustic travel time approximation method through experimental and finite element simulation analysis.

Phase coherence weighted ultrasound total focusing method towards the improved imaging of CFRP defects

Defect imaging in carbon fiber reinforced polymer (CFRP) combining the phase coherence imaging (PCI) and the total focusing method (TFM) is investigated in this study, in order to overcome the adverse influence on defect imaging from wave propagation anisotropy, inter-ply reflection, and high attenuation. Three kinds of coherence factors are proposed first to be combined with the wave velocity corrected TFM for defect imaging in CFRP. Then, the much stronger phase coherence (PC) from the signals scattered from the defect than those from the inter-ply is discovered experimentally. Finally, with the proposed PC-weighted-TFM, the noise of imaging from the inter-ply reflection is largely suppressed, resulting in largely improved signal-to-noise ratios of imaging with defects at different depths and close to each other. The findings demonstrate that the proposed PC-weighted-TFM is of great application potential in nondestructive testing of CFRP.

Total Focusing Method (TFM) and Phase Coherence Imaging (PCI) applied to various industrial cases

Les techniques d’imageries ultrasonores basées sur la Total Focusing Method (TFM) sont de plus en plus utilisées dans les applications industrielles. L’adoption de la méthodes a été rendue plus facile avec la publication de la méthode dans les codes ASME V, ISO 23864 et 23865. La TFM est une technique qui implique deux étapes : la première est l’acquisition appelée Full Matrix Capture (FMC) et la deuxième est la reconstruction elle-même (TFM). La TFM consiste à focaliser l’énergie acoustique à tous les pixels d’une région d’intérêt procurant une image de forte résolution. De nombreux développements ont été apportés à la technique pour améliorer notamment la productivité en utilisant des modes d’excitations tels que le Plane Wave Imaging (PWI) pour atteindre des vitesses de scan similaires à celles obtenues en multiéléments traditionnels. Alors que la TFM est basée sur l’extraction des amplitudes de la FMC, une autre technique appelée Phase Cohérence Imaging (PCI) extrait l’information de phase. Le PCI montre un fort contraste quand les éléments contribuent en phase. C’est le cas pour des défauts qui diffractent de l’énergie dans toutes les directions comme des petits défauts (HTHA, porosités…), les extrémités de fissures ou le grain des matériaux. A l’opposé, les échos spéculaires tendent à être minimisés puisque peu d’éléments contribuent en phase..

Automated IBEX crawler for PAUT inspection for in-service ferromagnetic assets

In order to guarantee the safety of the gas production plants, Air Liquide conducts NDT inspections regularly closely following the assets’ maintenance plans. One high criticality asset is the Pressure Swing Adsorber (PSA), found in the last stages of the hydrogen production process, where product purification takes place. These adsorbers are pressure vessels and need to be inspected for hydrogen enhanced fatigue cracks. Conventional inspection would require stopping site operation, building scaffolding for the totality of the height, which depending on design can be up to 15 meters tall and 6 meters in diameter, grinding of the inspection surface to remove the paint, conducting the inspection manually by 2 operators, and repainting once inspections are concluded. Commonly, inspection plans for these high-criticality assets outline a high inspection frequency, resulting in elevated costs associated with logistics, personnel and time and monetary resources required to plan and execute each inspection. This necessity is what drove Air Liquide R&D and INTACT to create a partnership and develop an automated crawler that can be deployed on in-service ferromagnetic assets for Phased Array Ultrasonic Testing (PAUT) inspection, using Total Focusing Method (TFM) with INTACT proprietary imaging and software. The robot, named IBEX Crawler, is deployed at the lowest part of the vessel and controlled safely at the ground level by a single operator. It is able to change directions to accommodate the inspection of longitudinal and circular welds. It includes an encoding system that enables operators to exactly locate any indications present. The crawler’s payload includes an electronic 128 elements PAUT board controlled by fiber optics, laser tracking and machine vision camera. The inspection results are analyzed by 3D reconstruction and managed by mechanical integrity assessments proprietary technology, calculating the stability of the found indications and using fracture mechanics and Fitness-For-Service (FFS) to estimate the residual lifetime of the indications and in consequence, the interval between inspections to develop a more accurate maintenance plan. Today INTACT’s IBEX crawlers travel worldwide between Air Liquide sites to ensure the integrity of the assets.

A receiver-optimized total focusing method for detectability enhancement of small defects in coarse grained materials

The identification and localization of small defects are among the main objectives of ultrasonic non-destructive testing (NDT). In particular, the inspection of polycrystalline materials remains a challenge due to ultrasonic attenuation and backscatter noise caused by grain scattering. An adaptive ultrasonic array imaging algorithm is proposed in this paper, which is capable of enhancing the signal-to-noise ratio (SNR) of small subwavelength defects. The proposed approach adopts an iterative optimization procedure based on sequential backward selection, and is termed receiver optimized total focusing method (ROTFM). ROTFM is formulated in a baseline subtraction (BS) setting, and it is shown to achieve higher defect SNRs compared to TFM results obtained by directly applying the BS method to measured array data. More importantly, the performance of the proposed ROTFM is robust to time and spatial misalignments, and hence, it alleviates the need for complex compensation schemes when performing imaging computations. Compared to conventional TFM, ROTFM achieved an average SNR improvement of 3.41 dB and 6.05 dB in experiments for four test defects (cracks and holes of sizes 0.5 mm and 1 mm) at the frequencies of 5 MHz and 7 MHz, respectively.

Sectorial Plane Wave Imaging for Weld Inspection Studies

Advanced ultrasonic array–based imaging techniques that rely on A-scan data acquired through individual parallel reception of array elements and subsequent post-processing have shown promising performance by generating high-resolution images. An advanced array-based imaging algorithm called sectorial plane wave imaging (SPWI) has been recently proposed with multimode imaging and rapid inspection capabilities. In this work, the efficiency of the SPWI algorithm is demonstrated and compared to the total focusing method (TFM) and plane wave imaging (PWI) algorithms in detecting different types of common weld defects: lack of sidewall fusion, slag, porosity, toe crack, centerline crack, and root crack. The full matrix capture data acquired over each weld defect using a 32-element linear array on an angular wedge configuration is converted to the PWI matrix data to perform SPWI. The corner and indirect mode SPWI inspection results are able to detect all the weld defects with image quality comparable to that of TFM and PWI, but at a higher processing speed due to the slight compromise over reception focusing, making SPWI a viable advanced ultrasonic array–based imaging technique for inspecting welds.

In situ detection of lithium-ion batteries by ultrasonic technologies

Overcharging is one of the most frequent and dangerous hazards in lithium-ion batteries, which not only increases the risk of battery failure but also causes thermal runaway and catastrophic outcomes. In this work, we combine the A-scan and 2D/3D Total Focusing Method (TFM) ultrasonic detecting technologies to in situ monitor and image the battery's abnormal behavior under overcharging. The ultrasound wave behavior during the charge/discharge process at various current densities and cut-off voltages is demonstrated first to obtain a general comprehension of its relations to the lithiation/de-lithiation, volume changes, concentration polarization, and temperature elevation process. Moreover, the early-warning characteristic of ultrasonic detecting has been proven in overcharging. The onset and location of side reactions can be precisely and facilely detected by ultrasonic detecting technologies as early as 102% SoC after the overcharging begins, and various overcharged states can be differentiated with an accuracy of 0.4% SoC. The astonishing precision of ultrasonic detecting technologies allows a timely remedy to be applied to batteries, thus elongating batteries’ lifespan. In conclusion, the ultrasonic detecting technique is excellent at not only accurately detecting the battery state but also acting as an alarm for latent danger.

Total Focusing Method (TFM) and Phase Coherence Imaging (PCI) applied to various industrial cases

Imaging techniques based on ultrasonic Total Focusing Method (TFM1) are more and more applied for industrial applications. Adoption of the technique has been made easier with the publication of the revised ASME V and ISO 23864 and 23865 standards. TFM is an inspection technique that involves two steps: the first one is the data acquisition process called Full Matrix Capture (FMC) and the second one is the data reconstruction itself called TFM. TFM is able to focus the energy at every pixel within a region of interest providing a high-resolution image. Efforts have been made to improve the productivity of the technique using various excitation modes such as Plane Wave Imaging (PWI) to reach scanning speeds comparable to standard phased-array inspections while maintaining code compliance. While TFM is based on the extraction of the amplitudes of the FMC matrix, another technique called Phase Coherence Imaging (PCI2) uses the phase information from those signals. The PCI image shows high values when all the elements contribute in phase. This is the case for small diffractors that tend to send energy in all directions such as small cracks (HTHA, porosities…), tips from cracks and grain noise. At the opposite, specular echoes tend to be minimized since few elements contribute to the echo. In this paper, we’ll show the advantages of both techniques on various industrial cases.

The effect of ultrasound wave path estimation to defect characterization capability in half-skip total focusing method

The total focusing method (TFM) is a post-processing imaging technique applied on full matrix capture (FMC) ultrasonic inspection (UT) dataset. In TFM the ultrasonic wave energy is synthetically focused on every pixel in the image region of interest (ROI). In terms of half-skip TFM (HSTFM), wave mode conversion happens when the wave rebounds at interface, such as specimen backwall. This paper aims to propose and evaluate a method that involves Snell’s law to address accurate estimation of distanceof-flight (DOF) of wave propagation when wave mode conversion appears in HSTFM. This HSTFM algorithm is applied to both experimental and simulated FMC dataset that inspects a surface-breaking notch for notch image reconstruction. Comparisons between images with and without considering Snell’s law in wave mode conversion show visible difference that could lead to misinterpretations in characterizing the defect. The sensitivity of TFM to varying defect features such as defect tilt angle is also studied using simulated FMC datasets.

Inspection for non-planar shaped welded joints of pipes using FMC ultrasonic technique

In applying ultrasonic testing (UT) to pipes with welded joints, one of the challenges is about the access limitation of UT probes caused by non-planar weld reinforcement profiles. One way to avoid this problem is to remove weld reinforcement for ensuring the contactability of a probe on welded parts. However, the removing process for flatten weld reinforcement needs extra day per joint and might damage pipes surface. To overcome this issue, we have been developing a proprietary UT method that enables inspection of non-planar welded joints using the Full Matrix Capture (FMC) and the Total Focusing Method (TFM). A specially designed wedge is used between a probe and a welded part in order to extract the weld surface shape from the recorded data. In the TFM, multilayer propagation paths are analyzed with considering refractions at interfaces between the layer and the weld surface. The analysis is based on the Fermat's principle of least time, whereas this is generally known to be a time-consuming process. With the aim of performing real-time scanning in inspection to pipes on weld reinforcement, the shape of the wedge is optimized for welded joints to reduce the calculation time of the multilayer analysis. In this presentation, we will report the developed FMC/TFM technique and, furthermore, the validation results using pipe specimens with weld reinforcement.

Quality Control Using Ultrasonic Phased Array Inspection of Components Produced by Directed Energy Deposition in Ti6Al4V Alloy

Additive manufacturing (AM) is rapidly becoming a viable manufacturing method for industrial metallic parts. This is related to the benefits achieved, such as reducing material wastage, an increase in design freedom and weight savings. But, to guarantee the highquality of the components produced, the non-destructive methods of inspection need to be optimised or developed. In this study, a Ti6Al4V 3D coupon was produced by the Laser Wire Directed Energy Deposition process (w-DED-LB), in which artificial holes of different diameters simulating defects were machined. To validate the quality control method after production by means of Ultrasonic Phased Array Techniques, the component was inspected using the Gekko equipment from Eddyfi Technologies, through the methodologies of the phased-array conventional mode and the Total Focusing Method (TFM). The probe used was a 5L64-64X7-NW1-P-5-OM, with a 5.0 MHz frequency system with 64 elements, with dimensions of length 64 mm, width 7 mm, and pitch of 1.0 mm. The results obtained showed that the TFM mode provides better detection of the defects, resolution, and measurement capabilities when compared with the conventional phased array mode.

Total Focusing (TFM) for the Ultrasonic Testing (UT) of drawn arc stud welding

This article describes the results obtained applying ultrasonic testing (UT) with phased arrays (PAUT) using full matrix capture (FMC) and total focusing method (TFM) for the inspection of drawn arc stud welding. Weld studs are usually directly applied on the top of a metal slab using a dedicated welding gun. This procedure involves heating the end tip of the stud using a strong electric arc created by a positive electric current. This welding procedure shall comply to various normative requirements, but on the other side there is not a clear reference to any applicable non-destructive method (NDT) to control the quality of the weld seam. Shear connectors are normally used in concrete/steel composite structures to allow the combined action of the concrete slab and the steel profile thereby increasing both the stiffness and strength of the structure. Flaws in this connection would result in severe bending and cracking of the entire structure. This article proposes an advanced UT method to inspect the weld between shear connections and metal slab using FMC. Data acquired with FMC are processed by TFM algorithms improving image resolution, signal to noise performances and allowing 3D data imaging. The combined use of PAUT searching units and signal processing with high resolution measurement shows a great potential in industrial NDT applied to civil engineering, resulting in a more effective, reliable, and faster inspection methodology than many other methods, such as radiographic testing (RT).

Merged Mode TFM with Mode Conversion Artifact Suppression

The objective of the paper is to compare the merits of the multiplication and max value schemes for producing a total focusing method (TFM) representation that combines multiple acoustic modes. We have chosen to compound phase coherence imaging (PCI) images because it eliminates the difficulties associated with handling vastly different acoustic sensitivities across different propagation modes. We also introduce an apodization-based algorithm that can filter out unwanted mode-conversion artifacts that are caused by similarities in time of flights. Experimental data were gathered on an electric resistance welding (ERW) hook crack and a natural bottom-surface-connected vertical crack. Experimental results indicate the multiplication scheme suppresses defect echoes that can only be imaged by a subset of acoustic modes. This can potentially lead to false negatives or underestimation of the extent of the defect. In contrast, the max value scheme retains all indications that appear in a subset of acoustic modes. For the vertical crack configuration imaged by shear wave acoustic modes, the max value scheme retains all mode-conversion artifacts and therefore complicates the defect analysis. The apodization-based filtering method significantly suppressed mode-conversion artifacts without affecting the imaging of the vertical crack. The combination of PCI with max value merging and apodization-based mode-conversion filtering is a promising methodology for merged mode TFM in shear wave weld inspections.

Stage 1 Round-Robin Report: On the Use of Total Focusing Method for Sizing Flaws in Assessing Probability of Detection

Reliability of non-destructive (NDT) procedures is usually assessed using probability of detection (POD). However, the reliability of the POD is rarely questioned. A POD usually produces a curve that relates the size of a defect to the likelihood that it will be detected by the NDT procedure. PODs are often used in conjunction with fracture mechanics that can relate the severity of a flaw to the service life of a component (Fitness for Service or FFS). To this end, it is the flaw vertical extent relative to the component thickness that is important. Closely related to the concern for flaw height is how close the flaw is relative to the surface of the component. Proximity to a surface can increase the stresses raised by the flaw and make it more critical to the fitness for service. Use of full matrix capture (FMC) techniques in ultrasonic testing, in conjunction with the postprocessing use of Total Focussing Method (TFM), has shown good potential to provide accurate flaw sizing. This paper examines the results of a round-robin trial using CIVA simulated data that were configured to assess flaw height and ligament in simulated welds. Results suggest that FMC with TFM post-processing can provide sizing estimates as good or better than manufacture estimates and can be used as the actual measured values for POD assessments.

Ultrasonic defect detection of high-density polyethylene pipe materials using FIR filtering and block-wise singular value decomposition

Condition monitoring of high-density polyethylene (HDPE) pipes used for fluid and gas transfer is important for the safety of energy conservation and the environment. Ultrasonic phased array imaging methods provide a solution to detect and assess defects in HDPE pipes. However, ultrasonic bulk waves propagating in these viscoelastic media are strongly attenuated, resulting in reduced signal amplitude. In this study, a linear-phase Finite Impulse Response (FIR) filter is used to remove unwanted frequency components from the measured ultrasonic signals to improve the signal-to-noise ratio before applying the imaging algorithm of the total focusing method (TFM). Building upon this, a block-wise singular value decomposition (SVD) technique, which can adaptively determine the singular value cutoff threshold based on each block in the whole TFM image, is used to enhance the obtained TFM image quality. The performance of the combination of FIR filtering and block-wise SVD technique is validated by experimental data of HDPE pipe materials. Results demonstrate that the proposed approach generates good images to provide the detection and characterization of side-drilled holes in HDPE pipe materials.

Studies of Angular Resolution for Acoustic Arc Arrays.

Currently, phased arrays are increasingly used in ultrasonic nondestructive testing. One of the most important parameters of ultrasonic nondestructive testing with the application of phased arrays is the angular resolution. This paper presents the results of studies of the angular resolution of concave and convex acoustic arrays in ultrasonic testing with the application of the total focusing method. Computer modeling of concave and convex acoustic arrays consisting of 16, 32 and 64 elements with distances between elements of 0.5 and 1 mm and arc radii of 30 and 60 mm have been performed. The results obtained by computer modeling were confirmed via in situ experiments.

Automated defect recognition (ADR) for monitoring industrial components using neural networks with phased array ultrasonic images

In this paper, we propose a framework to automate the process of defect characterizing for industrial structural component health monitoring by implementing automatic defect recognition (ADR) system. The ADR system consists of a convolutional neural network (CNN) and an edge detection algorithm medial axis transform (MAT). The CNN learns the defect feature space from the training dataset to detect and classify the defect. The MAT algorithm is used upon post-validation of the ADR, and the predicted feature’s edges are extracted to size them. The ADR is trained using the simulation-assisted finite element (FE) simulation datasets consisting of side drilled holes (SDH) and crack defects images. The training datasets are generated by introducing virtual array source aperture (VASA), which is a full matrix capture (FMC) scanning strategy by activating the group of elements in an active aperture with predefined focal laws to form a focused beam at a virtual source in the material. The VASA technique uses multiple virtual sources and active aperture positions in a given transducer, which are determined using the Poisson point process. The ultrasound beam is excited in sequence on each virtual source, and the reflected wave is recoded using all the transducers in the array to create FMC A-scans signals. The total focusing method (TFM) technique is a postprocessing algorithm implemented on the FMC signal to generate an image. A large quantity of training datasets is created for each defect by modeling various FE models with varying defect morphology. To create nearly close to experimental images, the experimental noise is introduced in the simulated images. The three separate ADR systems are trained with individual defects class and combined defects. The effectiveness of the trained ADR system is validated by conducting experiments on the plates with laboratory-made SDH and crack defects, the casting components, and weldments with unknown defect types and sizes. The mAP of ADR training is 82%, and the F1-score on testing image classification is 89%. The ADR system could detect and size the smallest defect is 0.219 mm, which is λ L /5.

Optimized ultrasonic total focusing imaging of diverse and multiple defects in crossply CFRP: Floquet wave theory, numerical simulation, and experimental validation

Carbon fiber reinforced polymers (CFRPs) with large thickness (≥20 mm) are being extensively used for aerospace structures, and are prone to the manufacturing and in-service defects due to the complexity and difficulty of manufacturing. This study addresses an intensive interrogation of multi-parameter optimization towards a high-quality ultrasonic total focusing method (TFM) imaging for diverse and multiple defects in crossply CFRP. Firstly, material parameters of unidirectional laminate are inversely constructed based on Christoffel equation by inputting the experimentally measured wave speed, based on which the wave speed and homogenization region for the crossply CFRP are obtained with the Floquet wave theory. Then, the wave propagation at crossply CFRP with preset side drilled holes (SDHs) at depths of 4 mm and 12 mm is modelled, from which the full matrix data are extracted for TFM imaging using multiple parameter optimization, including wave velocity correction, propagation restriction angle, and wave central frequency. With the proposed signal-to-noise ratio as the criterion of TFM imaging quality, the combined parameters for optimized imaging are sorted out, which are strongly related to the feature of propagating and non-propagating waves calculated with the Floquet wave homogenization theory. A further experimental validation is performed to image multiple SDHs and delamination with size over 2 mm and depths covering 2.5 mm to 17.5 mm. This systematic analysis of parameter optimization for TFM imaging in thick CFRP sets up the foundation for the TFM towards a real engineering application of CFRP detection.

Compressive Sensing Strategy on Sparse Array to Accelerate Ultrasonic TFM Imaging

Phased array ultrasonic testing (PAUT) based on full matrix capture (FMC) has recently been gaining popularity in the scientific and nondestructive testing communities. FMC is a versatile acquisition method that collects all the transmitter-receiver combinations from a given array. Furthermore, when postprocessing FMC data using the total focusing method (TFM), high-resolution images are achieved for defect characterization. Today, the combination of FMC and TFM is becoming more widely available in commercial ultrasonic phased array controllers. However, executing the FMC-TFM method is data-intensive, as the amount of data collected and processed is proportional to the square of the number of elements of the probe. This shortcoming may be overcome using a sparsely populated array in transmission followed by an efficient compression using compressive sensing (CS) approaches. The method can therefore lead to a massive reduction of data and hardware requirements and ultimately accelerate TFM imaging. In the present work, a CS methodology was applied to experimental data measured from samples containing artificial flaws. The results demonstrated that the proposed CS method allowed a reduction of up to 80% in the volume of data while achieving adequate FMC data recovery. Such results indicate the possibility of recovering experimental FMC signals using sampling rates under the Nyquist theorem limit. The TFM images obtained from the FMC, CS-FMC, and sparse CS approaches were compared in terms of contrast-to-noise ratio (CNR). It was seen that the CS-FMC combination produced images comparable to those acquitted using the FMC. Implementation of sparse arrays improved CS reconstruction times by up to 11 folds and reduced the firing events by approximately 90%. Moreover, image formation was accelerated by 6.6 times at the cost of only minor image quality degradation relative to the FMC.

Damage tracking in laboratory reinforced concrete bridge columns under reverse-cyclic loading using fusion-based imaging

ABSTRACT Fusion-based imaging using ground-penetrating radar (GPR) and ultrasonic echo array (UEA) was employed to track damage progression in the columns of two full-scale reinforced concrete (RC) bridge column-footing subassembly laboratory specimens. The specimens had different lap-splice detailing and were subjected to reverse-cyclic lateral loading simulating a subduction zone earthquake. GPR and UEA scans were performed on the east and west faces of the columns at select ductility levels. Reconstructed images were obtained using the extended total focusing method (XTFM) and fused using a wavelet-based technique. Composite images of each column's interior were created by merging the images from both sides. A quantitative analysis based on the structural similarity (SSIM) index accurately captured damage progression. A backwall analysis using the amplitude of the backwall reflector was also performed. Changes as early as in the first measurement (μ = 0.5 displacement ductility level) could be detected. Damage variation along the column height was observed, consistent with greater damage at the base. The proposed analyses distinguished the structural behavior differences between the two specimens. In summary, the SSIM metric provides a valuable tool for detecting changes, while the backwall analysis offers simple yet informative insights into damage progression and distribution in full-scale RC members.