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Ultrasonic imaging through interposed layer: (a) transducer wedge and (b) monitoring of multilayered structures (e.g. dry storage cask for spent nuclear fuel).
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An interposed coupling material between an ultrasonic transducer and the test medium can be present in various non-destructive inspections and structural health monitoring imaging applications. One example is the wedge medium often used to direct ultrasonic beams into the test material for optimal interaction with internal defects. Another example...
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Context 1
... other cases, the structure to be imaged has a multilayered geometry that requires accessing an internal layer from transducers placed on the outer layer. The use of a transducer wedge is a classic case of interposed medium (Figure 1(a)). The wedge is con- nected to the ultrasonic array and allows to direct the ultrasonic wave energy along a preferred direction to maximize the detection of reflections. ...
Context 2
... Ultrasonic imaging through an interposed layer is applicable to either portable transducer systems oper- ated in a manual or in a scanning mode, or to transducers rigidly connected to the test part for contin- uous monitoring. One example of the latter case is the monitoring of dry storage casks that house spent nuclear fuel 6 (Figure 1(b)). These structures are made of a steel-concrete multilayer that is highly inaccessible by routine inspections because of the harsh environ- ment and their often underground placement. ...
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Citations
... In many instances, because of the particular orientation of the reflectors or in cases of test pieces with particular geometries [26]- [31], it is customary to utilize a wedge interposed between the transducer array and the test medium to appropriately direct the ultrasonic waves so as to maximize the target reflections. Some applications of transducer wedges (or, equivalently, inclined transducers in fluid-filled wheels) in ultrasonic imaging are found in pipeline inspections and weld examinations [21], as well as detection of transverse defects in railroad tracks [32]- [36]. ...
... One final recent development in SAFT imaging consists of efforts to increase the transducer array gain without increasing its physical aperture by compounding multiple wave modes. This opportunity is particularly applicable to NDT imaging of bulk solids that can support both longitudinal (L-) wave and shear (S-) wave propagations [24], [32], or imaging of dispersive waveguides that can support multiple guided modes (e.g., Lamb modes in plates) [47]. ...
This paper discusses the application of sparse Synthetic Aperture Focusing Techniques (SAFT) for fast and accurate ultrasonic Non-Destructive Testing (NDT) imaging of solids in cases where a wedge is required between the transducer array and the test medium. A wedge is often used to appropriately direct the ultrasonic beams when testing for structural defects at particular orientations or when inspecting parts with particular geometries (e.g. waveguides). Both the Virtual Element and the Plane Wave modalities of sparse firing SAFT are examined for the wedge case that requires particular considerations in the beamforming algorithms for the wave refractions and mode conversions occurring at the wedge-medium interface. The method of wave mode compounding is also examined for this application in order to increase the array gain without increasing its physical aperture. Numerical simulations and experimental tests demonstrate the potential improvements in speed and accuracy obtainable by sparse SAFT adapted to wedge-transducer cases compared to a traditional Full Matrix Capture imaging mode. A practical implementation to the imaging of transverse defects in rail tracks is also presented.
... In CPWI, the beamforming operation needs to be repeated for each frame separately [18]. Thereafter, the averaging operation is performed before envelope detection in CCPWI, and after envelop detection in ICPWI [19], as shown in Fig 1. ...
Compound plane-wave imaging (CPWI) is a widely used and investigated imaging technique in medical ultrasound because it provides high quality ultrafast imaging for recent applications such as elastography. CPWI can be either coherent to provide high resolution and reduce sidelobe, or incoherent to provide high speckle homogeneity. To further improve imaging quality, coherence-based factors are used for weighting the output of ultrasound beamformers. This work studied the effects of the number of compounded frames and the step between these frames on the imaging quality produced by coherent and incoherent CPWI in the presence of the generalized coherence factor (GCF). The quality of the produced images of two different RF datasets was assessed in two different scenarios, in addition to conducting cyst phantom simulations. Results showed that the amount of image contrast improved by GCF increased, while the amount of resolution improved by GCF decreased, with the increase in step between frames. The same results were obtained in both types of CPWI. On the other hand, increasing the number of frames had almost no effect on the amounts of improvement provided by GCF. When CPWI is used in ultrafast imaging, it is important to monitor frame rates as well as imaging quality; these two factors are, respectively, inversely and directly proportional to the number of compounding frames. Therefore, the results of this research provide guidelines for accurate angle selection for CPWI so that a trade-off between imaging quality and frame rate is achieved.
... Effective detection of TDs is critical for the prevention of rail accidents and derailments. Industrial and research communities have extensively investigated NDE methods based on various physical phenomena, including but not limited to ultrasonic bulk waves, ultrasonic guided waves, acoustic emission, eddy currents, magnetic flux leakage, magnetic induction, and radiography (4)(5)(6)(7)(8)(9)(10)(11)(12). The railroad industry has widely adopted ultrasonic bulk waves along with roller search unit (RSU) for rail defect detection. ...
Rail defects, especially transverse defects (TDs), can pose risks to safe and efficient railroad operations. Effective rail defect detection is critical for the prevention of broken rail-induced accidents and derailments. In this study, a deep autoencoder (DAE) rail defect detection framework is developed to process ultrasonic A-scan data collected by a roller search unit and to identify the presence of TDs in rail samples. An autoencoder is a semi-supervised learning algorithm that identifies observations in a dataset that significantly deviate from the remaining observations and can be used for rail defect detection. Ultrasonic A-scan signals collected from both pristine and damaged rail segments are analyzed, where the pristine dataset is used to train a DAE model. To improve the accuracy and sensitivity of defect detection, we optimize the architecture and hyperparameters of the DAE model. Moreover, we evaluate the performance of two features extracted from the DAE model through receiver operating characteristic curves and confusion matrix. The DAE features outperformed conventional knowledge-driven features in the accuracy and robustness of defect detection, especially with the presence of noise.
Ultrasonic Synthetic Aperture Focus Techniques (SAFTs) using less than the total number of available array elements to transmit (“sparse” transmissions) have been recently used in both medical imaging and industrial NDT imaging to increase test speed and simplify multiplexer hardware. The challenge of sparse arrays is to obtain a reasonable image quality given the reduced transmitter-receiver combinations available to the beamforming process. This paper proposes a “ultrasparse” SAFT method that employs a minimum number of transmitter elements (from one to four elements only) to obtain an entire Full-Matrix Capture (FMC) set of waveforms. Specifically, a “virtual” FMC is obtained from normalized cross-power spectra between each array element pair in an implementation of “passive” ultrasonic sensing. In order to maintain high image quality without sacrificing imaging speed (e.g. applying a minimal initial time delay and keeping a short time recording window), several key steps have to be taken in this “passive” imaging mode, specifically (a) the use of carefully-designed segment-averaged normalized cross-power spectrum for robust passive reconstruction of the ultrasonic Impulse Response Function (
IRF
) between two receivers, (b) the use of both the causal and acausal portions of the passively-reconstructed
IRF
s, and (c) the compounding of multiple wave modes in the beamforming process. These steps also ensure the elimination of the near-field blind zone hence potentially enabling near-field imaging. The paper first reviews the theory of passive
IRF
reconstruction between two receivers, comparing time-averaged cross-correlation vs. segment-averaged normalized cross-power spectrum, and then demonstrates the application to ultrasparse SAFT FMC imaging of drilled holes in an aluminum block using a linear transducer array where only one to four elements are used in transmission.
This paper presents an experimental prototype developed for rail flaw imaging. This capability can help obtain quantitative information on detected flaws during manual flaw verification. Ultrasonic synthetic aperture focus (SAF) imaging has advantages over phased-array imaging for both speed and accuracy. The prototype developed is hosted in a portable and battery-powered carry-on size case. The probe is a linear ultrasonic array mounted on a wedge and with a position encoder to build 3D point clouds from 2D beamformed images. The prototype includes several advances over the basic SAF technique, including sparse subarray firing that allows fast imaging speeds (e.g., 25 Hz) without sacrificing image accuracy. Validation results are presented from scans performed on rail sections from the FRA rail defect library, which contains natural transverse defects and artificial end-drilled hole defects. The tests showed good accuracy in defect size and shape, as compared to the available ground truth information, for defects located away from the railhead corners. Additional developments are required to properly cover the head corners, and especially in the case of heavily worn rails.
