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.     

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.     

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.     

金属材料小缺陷超声反射信号建模及识别

为了能从含噪声金属材料超声检测信号中有效识别出微小缺陷回波,建立了金属材料超声反射信号模型并提出了基于相关系数的微小缺陷回波识别方法。对含微小缺陷金属材料超声脉冲反射信号的成分进行分析,建立了基于散射声场与高斯回波理论的优化超声回波模型。设计了超声缺陷回波位置识别方法。该方法对超声脉冲反射信号去噪后,取探头发射脉冲信号为参考信号;然后与去噪后的信号逐段求解相关系数;最后对该相关系数序列进行阈值化处理,获得缺陷回波在超声回波信号中的位置。将利用上述优化超声回波模型生成的超声反射信号及其频谱与实验获得的金属材料超声反射信号及其频谱进行了对比,结果表明:两者的时频域特征具有一致性。当将阈值设定为相关系数序列最大值的60%时,能够有效从超声背散射信号中识别出金属材料微小缺陷回波。     

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 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.     

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 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.     

Inspection of rotor disks of HPT and LPT of TK-10-4 gas-compressor units by the ultrasonic flaw detection method

The special features of diagnosing disks of gas turbines of gas-compressor units by the ultrasonic flaw detection method are considered. The efficiency of detecting crack-type defects in disks of HPT and LPT rotors under the first tooth of fir-tree slots of the disks in the area of trapezoidal grooves for tightening plates is shown. The application of the ultrasonic flaw detection method substantially increases the reliability of crack detection.     

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.     

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.     

基于FastICA的超声A波信号降噪研究

为便于缺陷可视化和提高缺陷定量分析精度,必须降低超声检测过程中多种未知噪声源对超声A波信号的干扰。首先将超声A波信号进行相空间重构,得到一个相空间重构矩阵;然后进行基于FastICA算法的独立分量分析;最后从独立分量中提取出所需的超声信号。为了验证降噪效果,将该降噪方法与小波去噪方法进行对比。实验结果表明:该方法与小波去噪方法效果接近,极大地提高了超声信号的信噪比。同时,该方法与小波去噪方法相比具有自适应能力强、简单且易于实现等优点。     

Ultrasonic Nondestructive Testing Accurate Sizing and Locating Technique Based on Time-of-Flight-Diffraction Method

The ultrasonic time-of-flight-diffraction (TOFD) method is a widely used flaw sizing and locating method. A signal identification technique is used to improve the arrival time resolution of a TOFD signal and to size and locate flaws more accurately. The ultrasonic signal is decomposed into several intrinsic mode functions by empirical mode decomposition. Some modes are selected to reconstruct a new signal considering their frequencies and energy. The reconstructed signal has a better signal-to-noise ratio and enhanced flaw information. A Hilbert transform is conducted to get the envelope and exact arrival time of the signal. All vertical flaws can be detected correctly with average sizing and locating accuracy of 0.08 mm in the laboratory. The deeper the flaw is located, the higher the accuracy. The blind area of TOFD is reduced to 2.5 mm under the surface.     

Ultrasonic Flaw Echoes Detection Based on Generalized S-Transform

Micro defects usually lead to performance degradation of materials. Pulse-echo technique is an effective method of revealing micro defects in materials, and booming in which has been witnessed over the years. However, detection of flaw echoes submerged in intensive noise is challenging. In this paper, a method of flaw echoes detection based on generalized S-transform is proposed. The original signals are transformed to time-frequency domain by generalized S-transform in the first stage. Subsequently, the time-frequency spectrums are enhanced through image segmentation techniques, including global thresholding and dilation. Finally, clear flaw echoes are obtained by inverse generalized S-transform, contributing to further investigation of defects. Both the simulation and experimental results show effectiveness of the flaw echoes detection method.     

Matching-pursuit-based adaptive wavelet-packet atomic decomposition applied in ultrasonic inspection

A novel method of time-frequency analysis, wavelet-packet atomic decomposition based on the matching-pursuit (MP) algorithm, is proposed for improvement of ultrasonic flaw detection during ultrasonic NDT. The MP algorithm is used to decompose a nonlinear and nonstationary signal into given atoms in an overcomplete wavelet-packet dictionary. The wavelet-packet dictionary with Daubechies wavelet functions, which well match the observed ultrasonic flaw echoes, is selected. By means of adaptively selecting the optimum atoms to reconstruct a signal, it is possible to obtain spare approximations of the original signal with less complexity and to efficiently improve a signal corrupted by noise. A distinct SNR enhancement for an ultrasonic echo during flaw detection is verified by computer simulation and experimental results. The text was submitted by the authors in English.     

Internal defect detection using laser-generated longitudinal waves in ablation regime

Laser ultrasonics could be an attractive solution for the nondestructive testing of structures in harsh environments. Longitudinal waves generated in the ablation regime are especially well suited to internal defect detection because they provide a higher signal-to-noise ratio in comparison to ultrasonic waves generated under a thermo-elastic regime, while their propagation direction normal to the surface enables the simplified interpretation processing of received signals when the defect echoes are analyzed. The internal defect detection using laser-generated longitudinal waves in the ablation regime was investigated numerically and experimentally, and a numerical model to simulate the generation and propagation of ultrasonic waves in the ablation regime was developed. This model was based on the simulation of ultrasonic generation and propagation caused by the net reaction force that was directly converted from the laser intensity absorbed onto the surface. This model was also extended as a model for internal defect detection. A steel specimen containing artificial internal defects was fabricated and inspected by using the through-transmission (T-scan) and pulse-echo (B-scan) modes. Clear amplitude reduction was observed in the transmitted waves at the defect positions in T-scan images, while B-scan images clearly showed the defect echoes arriving at different times depending on the depth location of internal defects. These results demonstrate that longitudinal waves excited in the ablation regime can be effectively used for internal defect detection.     

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 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.     

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.