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.     

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.     

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     

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.     

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.     

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.     

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.     

SAFT imaging of flaws in the rail base blade with consideration of multiple reflections of an ultrasonic pulse from the boundaries of a test object

The possibility of obtaining images of type-69 flaws in blades of the railroad rail base is considered. For this purpose, a modification of the SAFT algorithm is proposed, in which, during the calculation of echo-signal delays, multiple reflections of an ultrasonic pulse from the rail boundaries are taken into account. The Fermat principle is used to calculate the delay times. Typical acoustic schemes, each of which can be used to obtain partial flaw images, are determined. Combining the partial images in a final image allows the detection of type-69 flaws. It is shown that the proposed algorithm can be applied for obtaining type-69 images in P65 railroad rails.     

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.     

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.     

Determination of the profile of the surface of a test object during automated nondestructive ultrasonic testing in the immersion mode and reconstruction of flaw images using the SAFT method

The problem of using an elastic organosilicon polymer (??aquapolymer??) as an immersion medium for providing a stable acoustic contact between a test object and a piezoprobe in the process of automated ultrasonic testing of objects with rough surfaces is considered. The use of an ??aquapolymer?? allows one to decrease the consumption of water during ultrasonic testing. Flaw images were reconstructed using a modification of the SAFT method, which takes the profile of a test object into account; this allows improvement of the image quality. An algorithm for obtaining information on the profile of the surface of a test object and taking this profile into account during reconstruction of flaw images with the SAFT method is proposed. Model experiments yielded flaw images with the refraction of beams on a rough surface taken into account.     

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.     

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.     

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.     

Application of the TANDEM scheme for reconstructing flaw images by the SAFT method

The question of applying the TANDEM scheme for reconstructing flaw images by the SAFT method is considered. When the reflection of rays from a bottom is taken into account, it becomes possible to reconstruct the image of a boundary of a vertically oriented planar flaw. Formulas are proposed for calculating the optimal parameters of the scanning scheme for data collection according to the TANDEM scheme and the resolution is estimated. It is shown that the depth resolution for images obtained taking into account the reflection from the bottom is much worse than that for a direct beam. To increase the resolution, it is necessary to use coherent summation of images obtained for the same test object at different bases. This is confirmed by model experiments.     

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.     

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.     

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.