Total Focusing Method Algorithm Research Articles

An optimized total focusing method based on delay-multiply-and-sum for nondestructive testing

Total focusing method (TFM) attracts much interest because of high image resolution and large inspection coverage. However, the synthetic focusing approach based on delay-and-sum beamforming employs only the defect information contained in the dataset while ignoring the spatial information of the array signals, leading to limited imaging performance mixed with artifacts and noise. In addition, the signal-to-noise ratio (SNR) suffers due to single-element emission of full matrix capture. This work combines a modified delay-multiply-and-sum (DMAS) beamforming approach with conventional synthetic focusing in the TFM algorithm, to achieve optimization of TFM imaging performance. DMAS-based TFM is able to take full advantage of the defect and spatial information in the array dataset, and to generate new frequency components for better image reconstruction. As demonstrated on a series of comparative simulation and experimental results, the imaging results of the optimized TFM provide a considerable improvement in SNR. Better lateral spatial resolution is also achieved due to the increased number of equivalent transducer elements and second harmonic component. Therefore, this work provides a quite promising alternative solution for the post-processing of ultrasonic phased array with improved imaging performance.

Lightweight and Amplitude-Free Ultrasonic Imaging Using Single-Bit Digitization and Instantaneous Phase Coherence.

In the field of ultrasonic nondestructive testing (NDT), the total focusing method (TFM) and its derivatives are now commercially available on portable devices and are getting more popular within the NDT community. However, its implementation requires the collection of a very large amount of data with the full matrix capture (FMC) as the worst case scenario. Analyzing all the data also requires significant processing power, and consequently, there is an interest in: 1) reducing the required storage capacity used by imaging algorithms, such as delay-and-sum (DAS) imaging and 2) allowing the transmission and postprocessing of inspection data remotely. In this study, a different implementation of the TFM algorithm is used based on the vector coherence factor (VCF) that is used as an image itself. This method, also generally known as phase coherence imaging, presents certain advantages, such as a better sensitivity to diffracting geometries, consistency of defect restitution among different views, and an amplitude-free behavior as only the instantaneous phase of the signal is considered. Some drawbacks of this method must also be mentioned, including the fact that it poorly reproduces planar reflectors and presents a lower signal-to-noise ratio (SNR) than amplitude-based methods. However, previous studies showed that it can be used as a reliable tool for crack-like defect sizing. Thus, a lightweight acquisition process is proposed through single-bit digitization of the signal, followed by a phase retrieval method based on the rising and falling edge locations, allowing to feed the phase coherence imaging algorithm. Simulated and experimental tests were first performed in this study on several side-drilled holes (SDHs) in a stainless steel block and then extended to an experimental study on angled notches in a 19.05-mm ( 3/4" )-thick steel sample plate through multiview imaging. Results obtained using the array performance indicator (API) and the contrast-to-noise ratio (CNR) as quantitative evaluation parameters showed that the proposed lightweight acquisition process, which relies on binary signals, allows a reduction of the data throughput of up to 47 times. This throughput reduction is achieved while still presenting very similar results to phase coherence imaging based on the instantaneous phase derived from the Hilbert transform of the full waveform. In an era of increasing wireless network speed and cloud computing, these results allow considering interesting perspectives for the reduction of inspection hardware costs and remote postprocessing.

Waveform correlation factor (WCF) weighted TFM imaging for Lamb wave phased array

Aimed at improving the imaging performance of the classical total focusing method (TFM), which is an effective post-processing technique for Lamb wave phased array based damage imaging, a waveform correlation factor (WCF) weighted TFM imaging algorithm is proposed. In the proposed algorithm, besides the signal amplitude used in the classical TFM, the waveform correlation coefficients between any two waveforms after dispersion compensation are also utilized for damage imaging. Waveform correlation coefficients will be large if the wave packets are caused by the same imaging point, or they will be small due to the change in waveform shape arising from dispersion. For each imaging point, dispersion compensation for all collected signals is respectively implemented so that the scattering waveforms can be extracted. Then the waveform correlation coefficients between any two scattering waveforms are calculated and weighted on the corresponding amplitude used in the TFM. By comparing the imaging results of the proposed algorithm with that of the classical TFM algorithm, it is found that the proposed algorithm can significantly reduce the background noise and thus improve the imaging performance.

Vector coherence imaging for enhancement of small omni-directional scatterers and suppression of geometric reflections

Ultrasonic imaging with full-matrix capture (FMC) datasets using the total focusing method (TFM) has been demonstrated to improve the detection, sizing and characterization of small defects in the area of Nondestructive Evaluation (NDE). Despite the general improvements in resolution and signal to noise ratio (SNR) offered by FMC/TFM, detection of small flaws remains challenging - particularly when flaws are in immediate proximity to the bounding surfaces of a test piece. In this paper, it is shown that by retaining only phase information during the application of the TFM algorithm, it is possible increase the image intensity of small omni-directional reflectors with respect to large specular reflectors (i.e. backwall/frontwall). The proposed algorithm, called Vector Coherence Imaging, is derived from the Vector Coherence Factor phase coherence metric and represents the amount of statistical (Shannon) information in favour of the presence of a coherent scatterer everywhere in the imaging domain. The VCI algorithm is shown to aid in detection of small defects located close to the boundaries of the specimen. Notably, VCI is seen to facilitate detection tip diffraction/scattering originating from surface connected fatigue cracks (less than 2 ultrasonic wavelengths in height) and early stage high temperature hydrogen attack (HTHA) damage (void nucleation and micro-fissuring).

Towards an Alternative to Time of Flight Diffraction Using Instantaneous Phase Coherence Imaging for Characterization of Crack-Like Defects.

Time of flight diffraction (TOFD) is considered a reliable non-destructive testing method for the inspection of welds using a pair of single-element probes. On the other hand, ultrasonic phased array imaging has been continuously developed over the last couple of decades, and now features powerful algorithms, such as the total focusing method (TFM) and its multi-view approach to rendering detailed images of inspected parts. This article focuses on a different implementation of the TFM algorithm, relying on the coherent summation of the instantaneous signal phase. This approach presents a wide range of benefits, such as removing the need for calibration, and is highly sensitive to defect tips. This study compares the sizing and localization capabilities of the proposed method with the well-known TOFD. Both instantaneous phase algorithm and TOFD do not take advantage of the signal amplitude. Experimental tests were performed on a ¾″-thick steel sample with crack-like defects at different angles. Phase-based imaging techniques showed similar characterization capabilities as the standard TOFD method. However, the proposed method adds the benefit of generating an easy-to-interpret image that can help in localizing the defect. These results pave the way for a new characterization approach, especially in the field of automated ultrasonic testing (AUT).

Application of Ultrasonic Array Method for the Inspection of TC18 Addictive Manufacturing Titanium Alloy.

Ultrasonic arrays have been investigated for inspecting the quality of special materials. Unfortunately, non-destructive testing and evaluation (NDT&E) of internal defects in additive manufacturing (AM) materials are difficult due to the anisotropy and the coarse grain. To solve the problem, this paper brings forward research on the inspection of TC18 AM titanium alloy products using an ultrasonic array. Firstly, a three-dimensional acoustic field distribution of different ultrasonic array transducers is established to design an optimal detection solution for an AM titanium alloy. Then, a total focusing method (TFM) for the ultrasonic annular array transducer is proposed and its imaging method is analyzed. Besides, the relation between ultrasonic group velocities in a TC18 AM specimen with different propagating angles is measured using the full matrix capture (FMC) method. Based on the measurements, the anisotropy of the AM titanium alloy is discussed and the TFM algorithm of annular array is optimized as well. Finally, C-scan experiments are conducted on the specimen with a height of 55 mm using the linear ultrasonic array transducer of the conventional focusing method and the TFM of annular array transducer, respectively. The results show that the TFM of annular array has higher accuracy in quantifying the defects of flat bottom holes and transverse holes with a diameter of 0.8 mm. In addition, the detection results of different forming directions are analyzed and the 3D imaging of defects in the specimen is realized based on FMC data. The TFM of annular array is an innovative ultrasonic testing technology with high resolution for AM titanium alloy products.

The detection of flaws in austenitic welds using the decomposition of the time-reversal operator.

The non-destructive testing of austenitic welds using ultrasound plays an important role in the assessment of the structural integrity of safety critical structures. The internal microstructure of these welds is highly scattering and can lead to the obscuration of defects when investigated by traditional imaging algorithms. This paper proposes an alternative objective method for the detection of flaws embedded in austenitic welds based on the singular value decomposition of the time-frequency domain response matrices. The distribution of the singular values is examined in the cases where a flaw exists and where there is no flaw present. A lower threshold on the singular values, specific to austenitic welds, is derived which, when exceeded, indicates the presence of a flaw. The detection criterion is successfully implemented on both synthetic and experimental data. The datasets arising from welds containing a flaw are further interrogated using the decomposition of the time-reversal operator (DORT) method and the total focusing method (TFM), and it is shown that images constructed via the DORT algorithm typically exhibit a higher signal-to-noise ratio than those constructed by the TFM algorithm.

A multi-FPGA architecture-based real-time TFM ultrasound imaging

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.

Accurate depth measurement of small surface-breaking cracks using an ultrasonic array post-processing technique

In this paper, the half-skip configuration of the Total Focusing Method (TFM) is used to image and size surface-breaking cracks. The TFM is an ultrasonic array post-processing technique which is used to synthetically focus at every image point in a target region. This paper considers the case of inspecting for cracks which have initiated from the far surface of a parallel-sided sample using an array on the near surface. Typically, only direct ray paths between the array and image points are included in the TFM algorithm and therefore the image obtained for this case consists only of root and tip indications; no specular reflection from the crack faces is captured. The tip indication often has such a poor signal-to-noise ratio that reliable crack depth measurement is challenging. With the Half-Skip TFM, instead of using directly-scattered signals, the image is formed using ultrasonic ray paths corresponding to the ultrasound that has reflected off the back surface and has then undergone specular reflection from the crack face back to the array. The technique is applied to experimental and simulated array data and is shown to measure the depth of small cracks (depth <1mm) with greater reliability than methods which rely on tip diffraction.