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.     

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.     

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.     

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     

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 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.     

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.     

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.     

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.     

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.     

Noise suppression and flaw detection of ultrasonic signals via empirical mode decomposition

In ultrasonic nondestructive testing, the precise detection of flaw echoes buried in backscattering noise caused by highly scattering materials is a problem of great importance. In this paper, a new signal decomposition method for analyzing nonstationary or nonlinear data, empirical mode decomposition, is proposed to deal with ultrasonic signals. A new denoising technique that combines empirical mode decomposition and filtering simultaneously in the time domain and frequency domain is designed to suppress noise and enhance flaw signals. Synthetic and experimental signals are denoised with this EMD-based filtering technique. Simulated results are presented and analyzed, showing that the proposed technique has an excellent performance even when the signal-to-noise ratio is very low (−23 dB). The improvement in flaw detection was experimentally verified on a pipeline sample with artificial flaws. The text was submitted by the authors in English.     

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.     

钢球表面缺陷的自动检测与识别

在计算机图像处理理论与模式识别理论的基础上,提出钢球表面缺陷评价方法,给出二窗法确定阈值、噪声去除和轮廓跟踪方法;再从处理后图像中提取能表征缺陷性质的几何特征和纹理特征,以实现计算机对缺陷图像的理解和自动识别。     

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.     

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.     

缺陷尺寸模糊表征方法

准确测定结构材料中缺陷的尺寸是断裂力学手损伤容限设计在工程中应用的前提。但是,工程实际中无损检测得到的缺陷尺寸与真实缺陷尺寸之间相差较大,有时还将缺陷尺寸测成“０”毫米,即造成漏检。并且不同人员对同一缺陷进行检测时,得到的缺陷尺寸之间也存在较大差异。当根据不同人员检测出的缺陷尺寸结构进行断裂力学分析时,将会得出不同的结果,没有一致性,甚至出现根据甲的检测尺寸得出安全的结论,而根据乙的检测尺寸又得出     

奥氏体不锈钢涡流阵列C扫成像检测技术的仿真研究与应用

为了解探头设计参数对涡流阵列C扫成像效果的影响和涡流阵列C扫成像检测技术对奥氏体不锈钢材料典型缺陷定性能力,采用CIVA软件仿真方法建立奥氏体不锈钢试样上平底孔、刻槽试样的涡流阵列C扫模型,研究涡流阵列探头的设计参数如探头工作模式、线圈外径和阵列排数对涡流阵列C扫成像的影响,并通过试验分析了裂纹、圆形缺陷等典型表面开口缺陷的涡流阵列C扫成像特征。结果表明,对于涡流阵列探头,绝对发射-接收式工作模式对缺陷方向敏感,不利于涡流阵列C扫成像,而绝对桥式工作模式对缺陷不敏感,有利于涡流阵列C扫成像;涡流阵列探头的线圈外径越小,阵列排数越多,涡流阵列C扫成像的纵向分辨力越小,更有利于涡流阵列C扫成像;奥氏体不锈钢均匀表面对涡流阵列C扫成像技术干扰少,涡流阵列C扫成像技术能够反映表面开口缺陷形状特征,可在一定程度上对表面开口缺陷进行定性。     

大口径精密光学元件表面疵病检测系统研究

根据大口径精密光学元件表面疵病检测的工程特点,提出了一种基于优化模糊相似度算法的案例推理疵病识别法,解决了疵病图像获取中的摄像机平面移动定位、自动调焦和图像拼接等自动检测技术问题,研制了基于机器视觉的大口径精密光学元件表面疵病检测系统.实验结果表明,以上算法和技术是正确的,达到了理想的效果,可推广应用到其他材质的精密表面缺陷检测中.