The possibility of using the maximum-entropy method in ultrasonic nondestructive testing for increasing the resolution of echo signals

The problem of deconvoluting echo signals via regularization according to the maximum entropy method followed by the reconstruction of 2D images via the Fourier transformation synthetic aperture focusing technique (FT-SAFT) is considered. Numerical and experimental simulations have been performed. The possibility of improving the ray resolution of echo signals and reducing the speckle-noise level is demonstrated. The results obtained are compared to the results of constructing an AR model of the spectrum of echo signals.     

Reconstruction of flaw images by the C-SAFT method from echo signals measured by an antenna array in the triple-scanning mode

The application of antenna matrices (AMs) for obtaining flaw images during automated nondestructive ultrasonic testing is considered. The conventional technique of using an AM as a phased AM (PAM) has a number of drawbacks. One of them is related to the small number of AM elements and consists in a low frontal resolution of the thus-formed images. Scanning with an AM operating in the double scanning mode, viz., the so-called triple-scanning mode, allows coherent summation of partial images for each position and obtaining a resulting image with a frontal resolution that cannot be attained with a PAM. In order to test the serviceability of the proposed algorithm in the CIVA program, echo signals reflected from a crack model with a height of 6 mm and a length of 30 mm in a tested object, which simulated a welded joint of a pipeline with a conventional diameter of 800 mm (дy800), were calculated. The results of the reconstruction of images of model objects in the form of a drilled side hole with a diameter of 6 mm at a depth of 15 mm in a CO-2 specimen and a fatigue crack in a welded joint of a д y800 pipeline are presented. To reduce the influence of reverberation noise, a B-type median template was subtracted from echo signals. The flaw images that were reconstructed in numerical and model experiments demonstrate the efficiency of the proposed algorithm.     

Application of an ultrasonic antenna array for registering echo signals by the double-scanning method for obtaining flaw images

The prospects for applying the double-scanning mode in ultrasonic nondestructive testing for recording echo signals are considered. A variant of implementing the double-scanning mode using a phased antenna array is proposed. Algorithms for reconstructing flaw images by the method of projection in the spectral space and the combined SAFT (C-SAFT) method are presented. It is shown that, to obtain high-quality flaw images, the ultrasound propagation velocity in a tested object must be known to within a high accuracy (at least 0.5%). The results of numerical and model experiments in which images were obtained by these methods are presented.     

Instantaneous frequency estimation used for the classification of echo signals from different reflectors

A comparative analysis of the frequency parameters of echo signals from artificial reflectors of different shapes and from a natural spill-type flaw has been performed. The use of the instantaneous frequencies of ultrasonic signals that correspond to certain instants inside a pulse was suggested as an informative parameter for determining the flaw type. Instantaneous frequency is estimated based on the algorithm of the continuous wavelet transform, which increases the noise immunity of the method. It is shown that for the algorithm to be practically implemented it is appropriate to present the results in the form of dimensionless parameters, namely, normalized frequency deviations determined between the pulse center, edge, and tail. Their joint application makes it possible, in particular, to reliably distinguish echo signals that are reflected at junction flaws that rise to the surface of a test object (notches, dihedral angles, and spills of the weld joints), flat specimen surfaces, and local flaws, such as cylindrical side through holes and flat bottom drills.     

Restoring the image of reflectors using the C-SAFT method during multiple reflection of echo signals from the boundaries of a cylindrical inspection object

A modified method of combined SAFT (C-SAFT) for restoring the images of reflectors is considered; it allows the multiple reflection of a pulse from the boundaries of the wall of a cylindrical inspection object. To verify the efficiency of the proposed algorithm, images of a crack were restored from echo signals that were calculated using the CIVA software package, which is intended for modeling the propagation and scattering of ultrasonic pulses. It was shown in a model experiment that the consideration of changes in the pulse phase during reflection from the inspection-object boundaries at various incident angles of an S-wave in the image-restoration algorithm increases the frontal resolution by more than two times. The consideration of five reflections from the inspection-object boundaries made it possible to obtain images of reflectors by the M-C-SAFT method using many acoustic schemes. The images allow one to determine the type of defects, as well as their dimensions and location in the depth of the wall of a pipeline that is 720 mm in diameter.     

The use of an ultrasonic flaw detector with a phased antenna array for the recording of echo signals to reconstruct the images of reflectors by the method of projection in the spectral space

A method of automated ultrasonic testing based on the use of a PAA flaw detector for the recording of echo signals followed by the reconstruction of the image of reflectors by applying the 3D variant of the method of projection in the spectral space (3D-PSS) is considered. Customized setting of the PAA flaw detector makes it possible to simulate the operation of many single-element transducers with different angles of incidence. This approach allows one to obtain the images of reflectors with a high resolution and a high signal-to-noise ratio at a depth of more than 100 mm. A procedure for the calibration of an antenna array on a prism operating in the mode of emulation of several piezoelectric transducers with different angles of incidence is discussed. The results of a testing a fragment of the support billet for the ITER diverter when obtaining the images by the classical PAA method and with the use of 3D processing, which demonstrate the efficiency of the developed testing technology, are presented.     

Determination of the reflector type from an image reconstructed using echo signals measured with ultrasonic antenna arrays

The application of the digital image focusing (DFA) method to the determination of the types of detected reflectors is considered. For this purpose two antenna arrays (AAs) are used, which are placed on opposite sides of the reflector and using which echo signals are recorded in three acoustic channels in the double-scanning mode. The first and second acoustic channels transmit and receive echo signals using the first and second AA, respectively, and the third channel is tuned so that the first AA transmits pulses and the second AA receives echo signals. Using signals in each channel, many partial images can be reconstructed in a common coordinate system according to different acoustic schemes with allowance for both multiple reflections from irregular boundaries of a tested object and effects of transformation of the wave types. Combining partial images makes it possible to obtain a high-quality image, in which the entire boundary of the reflector is seen and using which an attempt to automate the procedure of evaluating the reflector size and determining its type was made. Such an approach allows one to reduce the subjective influence of an operator on the testing results. It is shown that in order to increase the image resolution, the spectrum of echo signals can be extrapolated by the spectrum splitting method jointly with the construction of an AR model of their spectrum. The results of model experiments that confirm the possibility of determining the reflector type are presented.     

Interaction of pulse ultrasonic signals with reflectors of different types

The results of experimental studies of the processes of interaction of pulse ultrasonic signals with artificial reflectors of different types are presented. To estimate nonstationary frequency characteristics of echo signals, it is proposed to use an algorithm for determining the instantaneous frequency that is based on the use of a wavelet transform characterized by increased noise immunity. It is shown that the instantaneous frequency of the reflected pulse signal may substantially differ from the rated frequency of a piezoelectric transducer for different types of reflectors. The frequency difference has a minimum value at a minimum curvature of the reflected-wave front. The obtained results must be considered during implementation of conventional methods of ultrasonic nondestructive testing because disregarding the effect of frequency deviation from the rated value may lead to a violation of the requirements of the acting standards for the tolerance for the operating frequency of a piezoelectric transducer. The difference of the instantaneous frequency from the rated frequency depends on the reflector type and can be used as an informative indicator of a flaw’s shape.     

Reconstruction of images of reflectors using the correlation method at an arbitrary number of reflections of an ultrasonic pulse from the boundary of a test object that consists of regions with different acoustic properties

An algorithm for reconstructing images of reflectors from echo signals that propagate in a test object, which consists of several regions with different acoustic properties, is considered. The ray trajectories are calculated using the method of direct construction of the family of rays that escape from the point where a transmitter is located but not the Fermat variational principle. After the family of rays is constructed, their belonging to a certain acoustic scheme can be analyzed and the approximation of the calculated delays on the spatial mesh of the image reconstruction region (IRR) can be performed. This will allow the reconstruction of both the pulse travel time from the transmitter to any point in the IRR and back to the receiver and the attenuation of the pulse amplitude caused by the divergence, reflection, and refraction effects at the boundaries of the regions that constitute the test object. Numerical and model experiments show the working capacity of the proposed algorithm for reconstructing reflector images.     

On the calculation of characteristics of echo signals of transverse and longitudinal waves from reflectors with flat surfaces

On the basis of an earlier developed model on scattering of transverse waves at a crack in the form of a strip, exact and approximate (estimated) scattering indicatrices for transverse and longitudinal elastic waves are studied, taking into account the influence of their transformation at the flat free surface of a reflector into other wave types on the change in the echo-signal amplitude. The limits of applicability of the estimated indicatrices for mirror and back reflections are numerically studied. The exact results for longitudinal waves are compared to those obtained by the Kirchhoff method.     

Some features of echo signals of longitudinal waves from flat-bottom reflectors in the far-field zone of a normal transceiver

Features of reflection of longitudinal elastic waves from flat rectangular, elliptic, and segmental reflectors (flaws) placed in the far-field zone of a normal transceiver are investigated for continuous (quasi-monochromatic) and pulsed transmission-reception modes. It is shown that, as the dimensions of the reflector increase, the echo-signal amplitude oscillates and approaches a value determined by the limiting shape of this type of reflector. It has been established that, for the reflector models considered, the character of an echo signal is determined by the value of the generalized parameter depending on the distance to the reflectors, their characteristic dimensions, and the wave number of the acoustic wave.     

False echo signals during ultrasonic testing of a steam generator

One important special case, namely, the ultrasonic testing of the welded joint of a steam generator was considered. The signal from a flaw is distinguished against a background of signals from technical reflectors.     

Two approaches to the solution of problems of ultrasonic flaw metering: Analysis of a high-quality image of reflectors and correlation analysis of measured echo signals

The problem of ultrasonic flaw metering consists in the obtainment of information on the reflector type, its dimensions, and the coordinates of its location. As the accuracy of determining the parameters of a discontinuity increases, the results of ultrasonic testing become more reliable. A highquality image of reflectors, which is reconstructed using the C-SAFT method with consideration for multiple reflections of pulses from the boundaries of a test object that makes it possible to determine the types and dimensions of reflectors, can be considered as one of the tools of flaw metering. However, the C-SAFT method disregards the features of reflections from discontinuities with nonuniform scattering indicatrices. As a development of flaw-metering methods, it is proposed to determine the parameters of a discontinuity from the results of comparison of the measured echo signals and estimated echo signals, which are calculated as functions of the discontinuity parameters. The results of using the proposed methods of ultrasonic flaw metering in numerical and model experiments, which demonstrate the working capacity of the proposed approach, are presented.     

Calculation of amplitudes of echo signals for a normal transceiver and reflectors in the form of flat-bottom holes of different shapes

The acoustic path for a noncircular reflector is calculated on the basis of the method for determining the echo-signal amplitude from a reflector in the form of a flat-bottom hole using the Kirchhoff approximation in the far-field zone of a normal transceiver. Examples of calculations of the directivity characteristics for reflectors in the form of an ellipse, triangle, and rhomb are presented. In limiting cases of circular and square reflectors, the results of numerical calculations of the directivity characteristics coincide with the data obtained from known analytic expressions.     

Noise cancellation of ultrasonic NDE signals using blind source separation

This paper proposes a new denoising method for ultrasonic NDE (nondestructive evaluation) signals using blind separation (BSS) technology. The proposed denoising method consists of four steps. First, a reconstructed phase space (RPS) is constructed from observed ultrasonic NDE signals. The information about the underlying sources (e.g., ultrasonic signal, noise, etc.) acting on this system is contained in this RPS. Second, independent component analysis (ICA) is performed on the RPS to recover all sources underlying the RPS. Next, the ultrasonic signal component is selected by a decision criterion related to the denoising application and, finally, is reconstructed to obtain the denoised ultrasonic signal. To validate the proposed method, it has been applied to the experimental ultrasonic NDE signals of the test sample and is compared with the wavelet denoising method in SNR (signal-to-noise ratio) enhancement. The experimental results show that the SNR of the ultrasonic NDE signals can be enhanced greatly using the proposed denoising method and the proposed method has almost the same denoising performance as the wavelet denoising method in SNR enhancement. A trait of the proposed denoising method is the ability to denoise ultrasonic NDE signals by separating the ultrasonic signal and noise using blind source separation technology. The text was submitted by the authors in English.     

Calculation of echo signals from a flat-bottom hole for normal probes

On the basis of the earlier developed method for determining the echo-signal amplitude from a flat-bottom-hole flaw via the use of the Kirchhoff approximation for constructing DGS diagrams, signals are calculated at an arbitrary arrangement of identical normal circular transducers on the tested surface parallel to the surface of the hole. Expressions for the pressure of a reflected longitudinal wave on the receiving transducer are obtained in a scalar approximation and in a general case. Examples of numerical estimates of the echo-signal amplitudes are presented.     

Comparative classification of flaws using ultrasonic-tomography methods and evaluation of the instantaneous frequency of echo signals

The results of the visualization of artificial reflectors of different shapes and a natural flaw of the faulty-fusion type in the bottom run are presented. The results of the evaluation of the instantaneous frequency, which is proposed as an additional informative index for shape classification of flaws, are presented for the same reflectors. A comparative analysis of the data that were obtained by these two methods was performed. The measurement results are represented in the form of B-scans for phased arrays, which provide a clear idea of the spatial location and configuration of reflectors, and in the form of diagrams of the dependences of the normalized frequency deviations for single dual transducers.     

Evaluation of echo signals from dihedral angles in specimens with cylindrical surfaces

Formulas for an acoustic channel are obtained for an angle probe and a reflector in the form of dihedral angle, which is formed by the end and cylindrical surfaces of a specimen—a round cylinder. It has been shown that, when the wave path in a prism is much shorter than the cylinder’s radius, the decrease that occurs in the amplitude of the echo-signal from the dihedral angle (without additional repeated reflections) owing to a wave-front divergence is the same as that observed during the reflection from a plane surface. The calculated and experimental data are compared.     

Investigation of the technique of ultrasonic thickness control using the EMA method

Experimental studies of the electromagnetic-acoustic (EMA) method for controlling the thickness of sheets, metalware, equipment, and pipelines were performed. New methods for the measurement of ultrasonic thickness that provide thickness control without cleaning the contact surface of a tested object and without using contact greases, as is required when using conventional contact ultrasonic transducers, are shown. On the basis of these studies and the experience of ultrasonic thickness gauging (USTG) by the contact and EMA methods, a USTG technology that uses the EMA method has been developed.