Obtaining flaw images by the SAFT method taking the variable velocity of sound in a test object into account

A modification of the SAFT method for obtaining flaw images in test objects containing three regions with different velocities of sound (SV) is proposed. Complex composite welded joints and repair welds are classified as objects in which the SV in a welded joint may differ from the velocity in a parent metal by >5%; therefore, a high-quality image of flaws can be obtained by taking different SVs into account. To solve this problem, a method for obtaining a test object with three regions with different SVs is proposed. The delays of propagating ultrasonic pulses were calculated using the Fermat principle. The results of reconstructing flaw images in a 300 welded joint from echo signals obtained as a result of numerical simulation by the finite-element method are presented. The images obtained by the SAFT method without taking different SVs into account are displaced from their true position, thus they do not allow determination of their coordinates and location. Consideration of different SVs allows one to obtain unshifted reflections of flaw images and, hence, evaluate the types and dimensions of flaws more accurately.     

Coherent Reconstruction of Flaw Images during Detection of Echo Signals in a Separate Mode

The possibility of using a scheme for separate detection of echo signals for obtaining high-quality flaw images is substantiated. A conventional algorithm for obtaining images by the projection method in the spectral space and an algorithm for obtaining images that is not limited by the approximation of a plane incident wave are considered. Coherent storage of images reconstructed from echo signals, which were detected in a separate scheme but at different positions of the radiator (transmitter), reduces the speckle-noise level and increases the resolution of flaw images. Positive and negative features of these algorithms are analyzed. Numerical and model experiments have demonstrated the feasibilities of different schemes of detecting echo signals for obtaining flaw images.     

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.     

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.     

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.     

Comparison of systems for ultrasonic nondestructive testing using antenna arrays or phased antenna arrays

This paper considers the features of forming the images of reflectors using phased antenna array technology and the images obtained by the C-SAFT method by echo signals measured in by double-scanning mode. It is shown that in some cases the images obtained by the phased antenna array technology are less informative, while the images obtained by the C-SAFT method have a higher frontal resolution over the entire area of image restoration; the partial images restored at different positions of the antenna array can be coherently combined to obtain a high and homogeneous resolution throughout the entire volume of thickwalled articles and increasing the signal/noise ratio. For the sake of brevity, the registration of echo signals in the double-scanning mode and restoration of the images of reflectors by the C-SAFT method will be called digital focusing by an antenna array (DFA). The ability to restore the partial images of reflectors by once measured echo signals according to many acoustic schemes with their consequent combination into one high-quality image should provide reliable automation of the process of recognition and dimensioning of scatterers. Another advantage of the images obtained by the C-SAFT method with three-dimensional focusing is the possibility of restoring images in the same coordinate system when using antenna arrays on prisms of different configurations. This facilitates the joint analysis of the images. Speeds of forming the images by the technology of phased-antenna arrays and the images obtained by the C-SAFT method are comparable. If the testing technique is based on the use of nonlinear effects, then in this case PA flaw detectors have a distinct advantage over DFA flaw detectors. However, within the framework of linear acoustics, PA flaw detectors have no fundamental advantages over DFA flaw detectors. It is fairer to say that PA flaw detectors have drawbacks. This article shows images that illustrate the features of the images that were obtained by PA and DFA flaw detectors.     

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.     

Imaging of flaws with allowance for multiple reflections of ultrasonic pulses from plane-parallel boundaries of a tested object

The possibility of applying coherent methods to imaging of flaws in objects with plane-parallel boundaries during ultrasonic inspection is considered. A version of the SAFT method for obtaining flaw images on the basis of one or several echo signals measured in the transceiver mode and the correlation imaging method are considered. A variant of using the maximum-entropy method for obtaining flaw images from measured echo signals is proposed. A substantial advantage of the maximum-entropy method over the SAFT and correlation methods is demonstrated. The results of numerical experiments in which images were obtained with the use of the aforementioned methods are presented. It is shown that the obtainment of high-quality flaw images requires knowledge of the velocity of ultrasound propagation and the object thickness with a low error (no worse than 0.5%). For this purpose, additional measurements must be performed, e.g., in the dual-probe operating mode, which will allow determination of the velocity of sound and the thickness of the tested object. The results of using these methods for obtaining images in model experiments are presented     

Application of 3D coherent processing in ultrasonic testing

A 3D variant of the coherent method of projection in the spectral space (PSS) for obtaining images of reflectors on the basis of measured echo signals is considered. Its application allows recon-struction of reflector images with a high frontal resolution in both the main plane of an ultrasonic transducer and an additional plane. The application of the PSS method is especially efficient when a reflector is far from the receiving aperture. In this case, it is possible to increase the signal-to-noise ratio by more than 12 dB and a tenfold increase in the frontal resolution can sometimes be attained as compared to the 2D layer-by-layer variant of the PSS method. Examples of the application of this method during reconstruction of reflector images in a specimen of the wall of a chemical reactor, in the case of multiple reflections from the walls of a welded T-branch with an overlaid plate (WTP), and when testing large-diameter studs and supports of the divertor, which is manufactured as part of the work on the thermonuclear reactor (ITER) project, are presented. The efficiency of applying the 3D variant of the PSS method for testing at large depths is shown.     

Use of Homomorphic Filtering for Upgrading Flaw Imaging in the Expert Examination of Welded Joints of Atomic Power Plant Pipelines

The possibility of using homomorphic filtration of ultrasonic echo signals, as a preliminary data processing method, for upgrading the flaw images obtained during coherent echo-signal processing is considered. Homomorphic filtration makes it possible to attenuate the effect of multiplicative noises appearing in the data recording, such as variation of the acoustic contact or the rough surface of the tested object. Obtaining of the images consists of the following stages: calculation of holograms on the basis of measured echo signals, taking the logarithm of the calculated holograms, determination of the hologram spectrum, projection operation in the spectral space, and the Fourier transform. The results of the practical use of homomorphic filtration for expertise of flaw dimensions in austenite welded joints of 325 × 15-mm stainless-steel pipelines are given.     

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.     

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.     

Sensitivity of a flaw detector during ultrasonic testing by chord-type transducers

During testing of girth-welded joints by chord-type transducers, it is suggested that the sensitivity of a flaw detector additionally be tuned by using a corner reflector (notch); the corresponding structure of transducers and specimens is presented. Additional tuning by a notch makes it possible to detect and estimate root discontinuities (on the bottom surface of a welded joint) and simultaneously to exclude the interfering echo pulse caused by a sag (the lower reinforcing bead) of the welded joint.     

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.     

Determining the Coordinates of Reflectors in a Plane Perpendicular to Welded Joint Using Echo Signals Measured by Transducers in the TOFD Scheme

Russian Journal of Nondestructive Testing - The TOFD method, widely used in ultrasonic flaw detection, makes it possible to distinguish a crack from a volume reflector by the phase of echo signals...     

APPLICATION OF SINTAP TO THE SAFETY ASSESSMENT OF X65 PIPELINE STEEL

0 INTRODUCTIONFlaw assessment concepts based on fracture mechanics arebeing increasingly used in industrial regulations and standards. Aconsiderable number of guidelines and procedures are availablewhich are partly based on each other but also exhibit sig…     

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.     

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.     

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.     

Determining the Speed of Longitudinal Waves in an Isotropic Homogeneous Welded Joint Using Echo Signals Measured by Two Antenna Arrays

A technique is proposed for measuring the speed of longitudinal ultrasonic waves in a homogeneous welded joint, based on comparing measured and computed echo signals reflected from the bottom of a test object when using two antenna arrays mounted on prisms and operating in the double scanning mode. The effect of errors in setting the values of such parameters as the distance between the antenna arrays, test-object thickness, and others on the accuracy of calculating the wave velocity in the weld has been analyzed. Results of numerical and model experiments on calculating the wave velocity in the welded joint are presented. In a model experiment, the technique has made it possible to measure the speed of longitudinal waves in the weld model with an error of less than 0.7%. The method can be used to find the initial approximation in a nonlinear inverse problem of tomographic inspection of welded joints in the wave approximation.