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.     

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.     

The calibration of an ultrasonic antenna array installed on a wedge

A method that is proposed for calibrating an antenna array that is installed on a wedge provides refinement of the coordinates of the centers of piezoelectric elements. Precise knowledge of the coordinates will make it possible to obtain images in which reflections from reflectors will coincide with their actual positions with an accuracy no worse than a half wavelength. The calibration principle involves the minimization of the objective function that describes the difference between the antenna-array-measured echo signals from a 2-mm-diameter side drilled hole (SDH) at a depth of 12 mm and the calculated echo signals from the same hole. The algorithm operation was tested for echo signals calculated in the CIVA program. The calibration results for several antenna arrays on wedges and images of an SDH are presented, which were obtained using the method for digital antenna-array focusing (DAAF) for various acoustic systems.     

Using coherence factor to improve the quality of reflector images in ultrasonic testing

When running ultrasonic testing, it is possible to analyze reflector images that have been restored by the method of digital focusing with an antenna (DFA). To improve the quality of the reflector images, one can use a coherence factor (CF) or a sign coherence factor (SCF) obtained from the calculated delay times instead of counting echo signals. The images obtained with the calculation of the coherence factor make it possible to increase the signal-to-noise ratio and front resolution. It has been demonstrated by numerical and model experiments that reconstructing the CF- and SCF-images of reflectors is efficient when registering echo signals by both ordinary and thinned (TAA) antenna arrays, in which the distance between neighboring elements is much greater than the wavelength.     

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.     

Increasing the Rate of Recording Echo Signals with an Ultrasonic Antenna Array Using Code Division Multiple Access Technology

Russian Journal of Nondestructive Testing - The disadvantage of using the digital focusing with an antenna (DFA) method to produce an image of reflectors is a large volume of echo signals and an...     

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.     

Application of the double-scanning method in ultrasonic nondestructive testing of metals for reconstruction of model defect images

The results of application of the double-scanning method for obtaining images of model defects during ultrasonic flaw detection in metals are presented. It has been shown in model experiments that, during reconstruction of flaw images, the amplitudes of parasitic reflections formed by echo signals, which were transformed and repeatedly scattered by inhomogeneities, decrease considerably. These properties of the double-scanning method may be useful in the practice of ultrasonic inspection of heavily loaded objects, especially in the case of the pressing problem of classifying the detected flaws. To identify the type of defects and determine their number accurately, it is desirable to use images obtained in both the double-scanning and transceiver modes. The images obtained during operation of a transducer in these two modes are compared.     

Simultaneous measurement of the velocity of an ultrasonic shear wave and the thickness of a test object with plane-parallel boundaries using two antenna arrays

Methods for the determination of the unknown velocity of the propagation of shear US waves and the unknown thickness of a test object were considered. Two antenna arrays working in the double-scanning mode, where echo signals are recorded, which were emitted and received by all pairs of antenna-array elements, were suggested. Antenna arrays on prisms were mounted on the test-object surface towards each other. An algorithm for the treatment of measured echo signals was designed using a method that is similar to the least-squares method. The use of the algorithm enables one to simultaneously determine the sound velocity and thickness of a test object with plane-parallel boundaries with an exactness greater than 0.5%. In this article, the factors were studied that affect the exactness of measurements and methods of their elimination or minimization. The results of numerical experiments and the use of the method on three samples relative to measurements using an IN-5101A instrument are given.     

Ultrasonic tomography of metal structures using the digitally focused antenna array method

Physical principles and algorithms for reconstructing images of the inner structure of an object made of a solid material are considered. These are based on the pulsed echo method of ultrasonic testing using multielement antenna arrays focused on each point of the visualized region of the object by spatiotemporal processing of signals from a combination sounding of the object by all possible pairs of the antenna array. Substantial improvement of the image during testing of a plane-parallel object is obtained by using signals that are multiply reflected from the object boundaries; the use of different algorithms of image reconstruction is expedient for different types of discontinuity flaws.     

Ultrasonic control of welded joints in a Du800-type pipeline. Part I: Restoring reflector images by digital focusing with an antenna

The reliability of ultrasonic nondestructive testing can be improved by acquiring reflector images that can be used to solve the defectometry problem, that is, to determine the type, size, and coordinates of a reflector deep in the sample. Welded pipelines with a diameter of 840 mm (Du800 type) are used at nuclear power facilities in Russia; ultrasonic testing of these pipelines is thus a topical problem. In model experiments, echo signals were registered by two antenna arrays, situated on the opposite sides of a welded joint (the N- and P-sides). The arrays performed scanning along and across the welded joint. The following techniques were used to reconstruct reflector images by the DFA method in a homogeneous isotropic medium: registering echo signals for all emission–reception combinations of elements in the two antenna arrays with the aim of merging partial images into a single image by different methods; reconstructing images on different acoustic schemes with allowance for wave-type transformation; and registering echo signals in a triple-scanning mode with subsequent coherent summation of partial images. It has been shown that the above techniques are insufficient for producing high-quality images.     

Increasing the signal-to-noise ratio in ultrasonic testing of repair welds using the technology of thinned antenna arrays

To test objects of materials with a high level of structural noise, use of thinned antenna arrays (TAAs) that have a large spatial aperture and consist of a small number of elements that are positioned from one another at a distance larger than the wavelength is proposed. A TAA moves over the surface of a tested object and echo signals are recorded during transmission and reception by different pairs of piezoelectric plates. For each transmitter-receiver pair, the measured echo signals are used to reconstruct partial images, which are then coherently added together to form the final image, by the SAFT method. A procedure for calibrating each piezoelectric plate of the TAA in order to determine the coordinates of its center for efficient coherent summation of partial images has been developed. The calibration procedure reduces the requirements for the accuracy of the arrangement of the piezoelectric plates of the TAA on a prism. The use of the technology of TAAs allows one to obtain images of flaws in repair welds with a signal-to-noise ratio (SNR) that is 12 dB higher than the SNR for an image obtained using the technique for a single-element transducer. The results of testing specimens of 800 pipelines with repair welds in weld seams are presented. The efficiency of this method is shown in comparison to the method in which a single-element piezoelectric transducer is used.     

Reconstruction of the images of reflectors from ultrasonic echo signals using the maximum-entropy method

It is proposed to use the maximum-entropy method (MEM) for processing ultrasonic echo signals for reconstructing images of reflectors with a high signal-to-noise ratio and a low level of “side lobes” of the point-scattering function. When processing echo signals, the pulse-propagation paths can be considered taking reflections from irregular boundaries of a tested object with the wave-type transformation into account. In model experiments, images of reflectors were obtained taking the refractions of rays at the rough surface into account, when echo signals were recorded both using an ordinary single-element transducer in the transceiver mode and an antenna array that recorded echo signals in the double- and triple-scanning modes. The reconstructed images have a resolution that exceeds the resolution according to the Rayleigh criterion. The MEM makes it possible to obtain images of flaws with low-level side lobes, when less than 10% of the complete set of echo signals are used.     

Obtaining flaw images that take the effect of multiple ultrasonic pulse reflections from the boundaries of a test object into account

The problem of obtaining flaw images from echo signals multiply reflected from the boundaries of a test object and modifications of the FT-SAFT and SAFT algorithms for obtaining flaw images in a test object with plane-parallel boundaries are considered. The reason that a transducer operating on transverse waves in the transceiver mode cannot yield information on flaw location depth is stated. It is shown that the detecting signals in the double-scanning mode, obtaining a set of partial images by the SAFT method, and combining them into a final image allow one to solve this problem. To obtain a combined image, the moduli of partial images were summed and their median was calculated. Because of difficulties in determining the velocity of sound and the thickness of a particular object, the coherent summation, which is potentially the most efficient combining technique among the considered ones, does not allow one to obtain a high-quality image. The results of a numerical simulation of the procedure of obtaining images of point defects are presented. The results of model experiments aimed at obtaining an image of a 1-mm-deep groove in a 20-mm-thick metal slab are presented as well. It is shown that measuring echo signals in the double-scanning mode and obtaining images by the DS-M-SAFT method make it possible to determine the flaw location depth.     

Determining the flaw type from images obtained by the C-SAFT method with account for transformations of wave types upon reflections of ultrasonic pulses from the irregular boundaries of a test object

The application of antenna arrays (AAs) for obtaining flaw images during automated non-destructive testing is considered. The conventional technique of using an AA as a phased AA has a number of drawbacks. An alternative approach is proposed in which echo signals are registered using an AA operating in the double scanning mode, and a modified algorithm of the combined SAFT, which takes into account multiray ultrasound propagation with consideration of transformations of wave types upon reflections from irregular boundaries of a test object, is used to obtain flaw images. The results of testing a specimen with a model of a volumetric flaw in the form of a 2-mm-diameter side hole drilled at a depth of 12 mm and a specimen with a crack model with a tip at a 12-mm depth are presented. Both specimens have rough bottoms. The obtained images allow determination of not only the dimensions and locations of flaws, but also their type.     

Features of applying the method of focusing to a point in ultrasonic testing of products manufactured from complexly structured materials

The well-known method of focusing ultrasonic signals to a point implemented using phased antenna arrays is considered as applied to problems of tomography of products manufactured from complexly structured materials. The features of applying broadband ultrasonic complexly modulated signals in phased antenna arrays and the limits of applicability of this method in constructing an image of a reflecting surface are discussed.     

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.     

Ultrasonic testing on a singly reflected beam using sparse antenna arrays and 3D echo-signal processing

Testing of welded joints in the main shutoff valves (MSVs) of the first circuit of a ВВЭР-440 reactor plant is impeded due to the high structural-noise level and the necessity of using pulses that are singly reflected from the tilted bottom of a branch pipe. Testing on a longitudinal wave leads to the appearance of false reflections, which are formed by pulses at a transverse wave, in the reconstructed image. Since the amplitude of false reflections can be commensurable with the amplitude of reflections that were obtained at longitudinal waves, the analysis of such images may be hampered. To increase the quality of an image, it is proposed to record echo signals using sparse antenna arrays and use 3D processing of echo signals. Numerical and model experiments showed that this approach increases the frontal resolution in the additional plane by several times and reduces the amplitude of false reflections, thus making it possible to increase the sensitivity and reliability of expert ultrasonic testing of the principal shutoff valve.     

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.     

管道导波时反聚焦检测系统的设计与实现

在分析管道中超声导波时反聚焦原理的基础上,设计并实现了一套适合激励压电换能器阵列,并对管道中超声导波能量在缺陷处进行时间-空间聚焦的时反聚焦检测系统.该系统实现的关键技术为:改进DDS( direct digital synthesis)结构,实现脉冲激励电路对时反特征信号进行合成发射;采用脉冲方式,实现小体积大输出功率的宽带线性功放电路;通过时反聚焦检测过程,实现管道中超声导波能量在缺陷处的时间-空间聚焦.采用该系统进行八通道时反聚焦检测实验,其结果表明,对于所用的含缺陷的管道而言,在特定的检测条件下,缺陷回波信号的幅值相对常规检测可提高246％,并且很好地抑制了导波的频散和多模态特性,提高了回波信号的信噪比.