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.     

Application of a Double Scanning Method in Ultrasonic Nondestructive Testing to Improve the Quality of Flaw Images

An algorithm for obtaining flaw images by a double scanning method for application in ultrasonic nondestructive testing, in which a radiator and a receiver move independently along parallel straight lines, is considered. A formula for reconstruction of flaw images is presented. The advantages and drawbacks of the double scanning method are discussed in comparison with the method of projection in the spectral space (PSS), which is also used for a coherent reconstruction of flaw images. The efficiency of the double scanning technique in suppressing the phantom images produced by transformed and rescattered pulses is shown in numerical and model experiments. The immunity of this method to the distortions introduced by an irregular measurement surface and the higher noise immunity compared to the PSS method are also demonstrated.     

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 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.     

Application of linear interpolation for improving the quality of flaw images obtained by the spectral projection method during ultrasonic nondestructive testing

The possibility of applying the linear interpolation of a 2D spatial spectrum during reconstruction of flaw images by the Fourier transform synthetic aperture focusing technique (FT-SAFT) from the data of ultrasonic nondestructive testing is considered. Different interpolation algorithms are analyzed. The results of numerical and model experiments in which images were obtained with and without interpolation are presented. The efficiency of applying the algorithm proposed for improving the quality of images obtained by the FT-SAFT method is shown.     

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.     

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.     

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.     

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.     

Shape reconstruction of three-dimensional flaw from backscattering data

Three dimensional Born and Kirchhoff inverse scattering methods are modified to convenient forms for a cylindrical specimen that includes three dimensional defect. One aluminum cylinder with flaw model is prepared and ultrasonic measurements are carried out. The measurement area in the modified methods is restricted in the plane perpendicular to the axis of cylindrical specimen. That is to say that the methods are modified to convenient form to use measured waveforms in the x₁–x₂ plane. The measured wave data are fed into the inversion method and cross-sectional images are obtained. Then, three dimensional shape reconstruction of flaw model in aluminum specimen is performed by piling up the cross-sectional images.     

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.     

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.     

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     

基于漏磁检测的缺陷信号分析

为了应用漏磁检测技术检测管道缺陷,需要对缺陷信号进行分析。在漏磁检测原理的基础上,运用三种磁偶极子模型来描述各种表面缺陷。分析了缺陷参数对漏磁信号的影响。     

A nonlinear effect-based ultrasonic flaw detector for detecting cracks

The operation of an ultrasonic flaw detector using a nonlinear modulation method of crack detection that is suitable for effectively testing cracks in wafers is considered. It differs from analogues in the technique of modulation of elastic characteristics of flaws, namely, a sequence of phase-inverted pulses of Lamb waves. The results of testing of an ultrasonic flaw detector intended for detecting model flaws (single cracks and cavities) in wafers are presented.     

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.     

Estimating the probability of flaw detection by modeling processes of image generation and interpretation

A method for determining the probability of flaw detection by modeling optical images generated by X-ray nondestructive testing methods is described. Processes of formation of intrinsic noises in transducers have been studied. Normal distribution laws have been substantiated for the optical density in radiographic evaluation and the image brightness in fluoroscopic testing. Based on the image recognition theory, application of convolution in X-ray image interpretation has been justified and the accompanying change in the signal-to-noise ratio has been demonstrated. Dependence of flaw-detection probability on the shape and dimensions of test articles has been studied.     

移动式油管纵横缺陷检测系统的研制

根据磁性无损检测基本原理，对油管纵横向缺陷检测实现方法进行了研究，研制出移动式油管纵横缺陷检测系统。该系统在油管局部实施轴向和周向磁化，油管由电机主轴带动旋转，检测探头紧贴油管直线运动，从而实现对腐蚀坑、腐蚀孔等横向缺陷和杆状损伤、劈缝等纵向缺陷的综合检测。     

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.     

Structural features of crack formation during corrosion cracking

Stress corrosion cracking (SCC) causes damage to main gas pipelines due to formation and development of cracks on the outer surface of a tube. Prevention of SCC crack development and fracture of a gas pipeline is an important problem of diagnostics that can be solved in various ways. The most effective approach involves systematic use of in-tube nondestructive testing and local testing in shafts, owing to which the position of a flaw and its linear size are determined. The hazard of the flaw (residual life of a tube with cracks) is found and a decision is made about the necessity and methods of flaw repair, such as cutting out the tube, grinding the flaw, and reinsulating the tube surface, and the date of repeated nondestructive testing is set. These measures ensure timely discovery of potentially hazardous flaws and make it possible to take the necessary steps for preventing development of SCC.