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.     

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.     

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.     

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.     

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.     

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.     

Superresolution in surveillance systems with an antenna array for synthesis of images of the Earth surface

A method that ensures superresolution in terms of angular coordinates in surveillance systems with a flat antenna array during synthesis of radio-location and radio-thermal-location images of the Earth surface with the help of an airborne station in the real beam mode is proposed. This method is based on reconstruction of reflection and emission fields.     

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.     

Determining the distance between antenna arrays and the speed of longitudinal waves in a test object when reconstructing the profile image of the bottom of a weld in ultrasonic testing

A method is considered for determining the distance between two wedges and the speed of longitudinal ultrasonic waves in a test object when measuring the profile of the surface of pipelines under the bead of a weld. The method makes it possible to reduce the error in determining the profile of the bottom of a pipeline in the course of testing by refining the distance between the wedges and the speed of sound using head-wave pulses. Results of numerical experiments are provided for the processing of echo signals calculated by the finite-difference method in the time domain. The efficiency of the proposed technique is demonstrated with some model experiments.     

Features of acoustic-emission signals during the initiation of a fatigue failure in a welded joint of an aluminum alloy of the Al-Cu-Mn system

The features of the generation of acoustic emissions (AEs) during the initiation of a fatigue failure in various zones of a welded joint of thermally hardened 2219-T6-grade aluminum alloy, which is produced using electron-beam welding, are considered. Metallographic studies showed that a welded joint of the alloy is structurally and mechanically inhomogeneous. This predominantly influences the initiation and development kinetics of fatigue cracks in the joint and the AE kinetics. It is shown that the area of a formed flaw is proportional to the sum of the amplitude of detected signals, and the transition from the initiation to the stable propagation of a failure is accompanied by an abrupt increase in AE activity.     

Numerical analysis of distortions by using an incorporated model for welding-heating-cutting processes of a welded lifting lug

A lifting lug is a widely used welded structure in the shipbuilding industry. Welded lugs with holes that are attached to heavy blocks are hooked onto and transported by a crane. The lugs are cut away after the heavy blocks are set up at the right position. In this study, a new thermal compensation method was proposed to improve the productivity of removing a lifting lug by band saw cutting. A lifting lug employs a welding process, a heat treatment process, and a cutting process, and thus a coupled 3D finite element model was developed to investigate the distortion and residual stress redistribution during the three processes. The results of the numerical and the experimental studies indicated that the thermal compensation method was effective in improving the productivity of removing the lifting lug. The numerical simulation ensured that the mechanism and influence of various local heating conditions were well illustrated and discussed. It is expected that the coupled finite element model developed in this study can solve practical problems of industrial production.

     

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.     

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.