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.     

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 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.     

Sensitivity of a flaw detector during ultrasonic testing by chord-type transducers

During testing of girth-welded joints by chord-type transducers, it is suggested that the sensitivity of a flaw detector additionally be tuned by using a corner reflector (notch); the corresponding structure of transducers and specimens is presented. Additional tuning by a notch makes it possible to detect and estimate root discontinuities (on the bottom surface of a welded joint) and simultaneously to exclude the interfering echo pulse caused by a sag (the lower reinforcing bead) of the welded joint.     

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.     

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.     

用线性补偿算法和CCD响应补偿来提高频谱OCT图像质量

在频谱光学相干析成像(频谱OCT)系统中普遍使用的CCD采样的数据是波长的函数。CCD对不同波长的信号具有不同的响应,而且图像重建进行的傅里叶逆变换所需要的数据是波数的函数,忽略这两个问题对频谱OCT成像质量的影响比较大。基于以上分析,提出了响应度补偿法和波长数据-波数数据转换线性补偿法,并对样品进行了实验验证。实验结果表明,该方法能够将OCT图像的信噪比(SNR) 提高约30%,明显改善了图像质量。     

Dithering method for improving the surface quality of a microstructure in projection microstereolithography

In layered manufacturing, geometrical gap occurs between the computer-aided design (CAD) model and the fabricated, due to the stair-stepping effect. It will deteriorate the surface quality of the microstructure. By reducing the stair steps, the surface quality of a layered manufactured microstructure can be improved. In this research, dithering method is introduced to improve the surface quality of a microstructure in projection microstereolithography. The dithering method can represent grayscale effect with monochrome image, so it is applicable to the digital micromirror device for ultraviolet, which allows monochrome image as an input format. By adjusting the beam intensity using the dithered image, it is possible to control the cure depth of the resin within a layer. In a cross-section, the region to be dithered is obtained by the difference of the cross-sections between the current layer and the previous layer. It is called a compensation area (CA) in this research. The gray level to be applied is determined considering the CAD model and the curing characteristics of the resin. The dithered CA is then combined with the current cross-section. The parabola-shaped microstructures, using dithered and non-dithered cross-sectional images, have been fabricated and compared. The dithering method has shown good performance for improving surface quality and reducing the fabrication time.     

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.     

超声检测机械手控制系统的设计

为实现超声检测的自动化,研制了一种关节机械手控制系统.该机械手由传感检测系统、伺服系统、总线智能节点和PC机等组成,借助单片机、数字控制、现场总线、计算机、机器人等技术,利用CAN现场总线进行通讯,通过控制实现自动控制并实时获取坐标信息,有利于缺陷的定位分析,其结构简单,性能价格比高,可应用于其他场合,具有广泛的适应性.     

The possibility of using the maximum-entropy method in ultrasonic nondestructive testing for increasing the resolution of echo signals

The problem of deconvoluting echo signals via regularization according to the maximum entropy method followed by the reconstruction of 2D images via the Fourier transformation synthetic aperture focusing technique (FT-SAFT) is considered. Numerical and experimental simulations have been performed. The possibility of improving the ray resolution of echo signals and reducing the speckle-noise level is demonstrated. The results obtained are compared to the results of constructing an AR model of the spectrum of echo signals.     

Application of increase spatial sampling for improving the resolution of images obtained from photodetector arrays

A possibility of increasing the resolution of photodetector arrays by doubling the spatial sampling frequency and subsequent processing of the samples obtained is considered.     

Experimental study of sensitivity of electromagnetic-acoustic transducers for flaw detection by nondestructive echo testing using bulk shear waves

Results of experimental studies of sensitivity of electromagnetic-acoustic (EMA) transducers in the echo-testing mode are given. Flaws equivalent to reflectors shaped as disks and side surfaces of cylinders with diameters of 1 mm and more have been detected with a high probability in samples of rails, round and square blanks, and metal sheets. We have concluded that nondestructive echo testing of metals using EMA transducers can be performed with a sensitivity close to that of piezoelectric transducers.     

Using coherence factor to improve the quality of reflector images in ultrasonic testing

When running ultrasonic testing, it is possible to analyze reflector images that have been restored by the method of digital focusing with an antenna (DFA). To improve the quality of the reflector images, one can use a coherence factor (CF) or a sign coherence factor (SCF) obtained from the calculated delay times instead of counting echo signals. The images obtained with the calculation of the coherence factor make it possible to increase the signal-to-noise ratio and front resolution. It has been demonstrated by numerical and model experiments that reconstructing the CF- and SCF-images of reflectors is efficient when registering echo signals by both ordinary and thinned (TAA) antenna arrays, in which the distance between neighboring elements is much greater than the wavelength.     

Adaptive instruments for ultrasonic nondestructive testing of large objects with complex structures

An ultrasonic multifunctional adaptive measuring complex, which is built on the basis of a personal computer and algorithmically generates various simple and complexly modulated probing signals, is described. This system implements various radio-engineering algorithms for processing echo signals (storage, optimal filtering, synchronous detection, etc.) and provides adaptation of the signal parameters to the characteristics of inspected objects. Using the example of a frequency-modulated signal, it is shown that an adaptive measuring complex can increase the testing reliability and provide high sensitivity and high resolution owing to both the adaptive adjustment of the parameters of a probing signal and the compensation for distortions in the received signal in the electroacoustic path.     

Sparsity enhancement for blind deconvolution of ultrasonic signals in nondestructive testing application

The received signal in ultrasonic pulse-echo inspection can be modeled as a convolution between an impulse response and the reflection sequence, which is the impulse characteristic of the inspected object. Deconvolution aims at approximately inverting this process to improve the time resolution so that the overlap between echoes from closely spaced reflectors becomes small. This paper presents a modified minimum entropy blind deconvolution algorithm for deconvolving ultrasonic signals. Enhancement of the resolution is achieved by using the presented method. In addition, the presented approach will, in many cases, lead to a faster computation. A nonlinear function is the key point to the efficiency of the modified blind deconvolution algorithm, which is used to increase the sparsity of the iteration output and to decrease the influence of the added noise by replacing each iteration output by output of the nonlinear function. Simulations showed the efficiency of the modification as compared with minimum entropy deconvolution when deconvolving synthetic ultrasonic signals. Experimental results using real ultrasonic data evaluated further that the exact solution consistently yields good performance. The thickness of a thin steel sample can be calculated by the modified blind deconvolution filter with a reasonable accuracy.     

Data processing and representation in the MFL method for nondestructive testing

A method for the retrieval of measurement data, as well as their processing and graphical representation in a nondestructive testing, which is based on the recording of magnetic flux leakage, is briefly described. The requirements for the corresponding measuring system are described. Examples from the field of pipeline inspection and steel rope inspection and production boring well are given.     

Processing of active thermal nondestructive testing results by the method of wavelet analysis

Wavelet transforms can be used for active thermal nondestructive testing for the identification of flaws that are located at different depths and produce Gaussian-like signals characterized by a time shift on a heated surface. The results of the application of scalar and complex wavelet analysis for increasing the signal/noise ratio in the active thermal inspection of flaws in composite materials and thermal-barrier coatings, as well as also for revealing the corrosion in metals, are described. Parent Morlét, Paul, and Mexican-hat wavelets were used.     

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.     

Modal analysis of the piezoelectric generation of ultrasonic guided waves for nondestructive testing of cylindrical structures

This paper presents a method based on a modal analysis, which allows efficient quantitative determination of such characteristics of the surface load of an ultrasonic probe as the mode-selection selectivity, the frequency characteristic, and the transient characteristic. The use of this method is illustrated by simulation of generation of axially symmetric and antisymmetric ultrasonic waves in pipes with a piezoelectric array transducer. The text was submitted by the authors in English.