Flaw-to-grain echo enhancement by split-spectrum processing

A split-spectrum processing technique for an ultrasonic flaw detection system has been developed which improves the flaw-to-grain echo ratio in large-grained materials. The enhancement is achieved by partitioning a wide-band received spectrum to obtain frequency shifted bands, which are then processed to suppress the grain echoes with respect to the flaw echo, using a novel signal minimization algorithm. Experimental data for titanium and stainless steel samples are presented which show superior flaw detection capabilities for the minimization algorithm with respect to frequency averaging techniques.     

Ultrasonic flaw detection using radial basis function networks (RBFNs)

Ultrasonic flaw detection has been studied many times in the literature. Schemes based on thresholding after a previous matched filter use to be the best solution, but results obtained with this method are only satisfactory when scattering and attenuation are not considered. In this paper, we propose an alternative solution to thresholding detection method. We deal with the usage of different flaw detection methods comparing them with the proposed one. The experiment tries to determinate whether a given ultrasonic signal contains a flaw echo or not. Starting with a set of 24,000 patterns with 750 samples each one, two subsets are defined for the experiments. The first one, the training set, is used to obtain the detection parameters of the different methods, and the second one is used to test the performance of them. The proposed method is based on radial basis functions networks, one of the most powerful neural network techniques. This signal processing technique tries to find the optimal decision criterion. Comparing this method with thresholding based ones, an improvement over 25-30% is obtained, depending on the probability of false alarm. So our new method is a good alternative to flaw detection problem.     

Quasi pulse-echo ultrasonic technique for flaw classification

One of the important flaw characterization tasks in the field of ultrasonic non-destructive evaluation (NDE) is to provide flaw type information by analysing the flaw responses acquired during an inspection. Here a new quasi pulse-echo ultrasonic classification technique is presented which utilizes the time separation and amplitude difference of mode-converted diffracted signals to distinguish between smooth and sharp-edged flaw geometries. Experiments with cylindrical cavities, surface-breaking fatigue cracks and slag inclusions have been used to test the practicality of this approach. All results of these tests show good consistency in the separation of smooth and sharp-edged flaws, provided that the signal-to-noise ratio is sufficient. Furthermore, the scattering feature used for classification in this method is also verified by detailed elastodynamic scattering calculations.     

Signal-to-noise ratio enhancement based on wavelet filtering in ultrasonic testing

In ultrasonic non-destructive testing of materials with a coarse-grained structure the scattering from the grains causes backscattering noise, which masks flaw echoes in the measured signal. Several filtering methods have been proposed for improving the signal-to-noise ratio. In this paper we present a comparative study of methods based on the wavelet transform. Experiments with stationary, discrete and wavelet packet de-noising are evaluated by means of signal-to-noise ratio enhancement. Measured and simulated ultrasonic signals are used to verify the proposed de-noising methods. For comparison, we use signal-to-noise ratio enhancement related to fault echo amplitudes and filtering efficiency specific for ultrasonic signals. The best results in our setup were achieved with the wavelet packet de-noising method.     

强冲击载荷下含杂质的纯铝中微孔洞长大的动力学行为

采用LS-DYNA瞬态动力学有限元程序,对平板撞击加载下含初始杂质的纯铝样品中微孔洞的成核与长大进行了数值模拟。结果表明:微孔洞首先在杂质与基体的边界处成核,随后在局部严重塑性变形驱动下快速线性增长;微孔洞半径的增长速率与冲击加载强度两者之间近似成线性关系;材料屈服强度和初始杂质的大小对微孔洞相对的增长速率有明显的影响;当微孔洞长大阈值取屈服强度的3.5倍时,数值仿真结果与理论分析结果基本一致,这有助于进一步认识孔洞长大的动力学行为。     

Cryptanalysis and improvement on a block cryptosystem based on iteration a chaotic map

Recently, a novel block encryption system has been proposed as an improved version of the chaotic cryptographic method based on iterating a chaotic map. In this Letter, a flaw of this cryptosystem is pointed out and a chosen plaintext attack is presented. Furthermore, a remedial improvement is suggested, which avoids the flaw while keeping all the merits of the original cryptosystem.     

A note on rotor-bearing stability

In a recent paper by the author in this Journal, one of several ideas put forth was an unconventional approach to study rotor-bearing stability. The approach would appear to be considerably more accurate than conventional approaches and overall significantly enhance study of the subject. However, the experiment that was proposed to implement the approach has since been found to contain a fundamental flaw. This short note has been written primarily to point out that flaw and to propose an experiment in which the approach previously presented is correctly implemented.     

Interaction of a scanning laser-generated ultrasonic line source with a surface-breaking flaw

The scanning laser source (SLS) technique has been proposed recently as an effective way to investigate small surface-breaking cracks. By monitoring the amplitude and frequency changes of the ultrasound generated as the SLS scans over a defect, the SLS technique has provided enhanced signal-to-noise performance compared to the traditional pitch-catch or pulse-echo ultrasonic methods. In previous work, either a point source or a short line source was used for generation of ultrasound. The resulting Rayleigh wave was typically bipolar in nature. In this paper, a scanning laser line source (SLLS) technique using a true thermoelastic line source (which leads to generation of monopolar surface waves) is demonstrated experimentally and through numerical simulation. Experiments are performed using a line-focused Nd:YAG laser and interferometric detection. For the numerical simulation, a hybrid model combining a mass-spring lattice method (MSLM) and a finite difference method (FDM) is used. As the SLLS is scanned over a surface-breaking flaw, it is shown both experimentally and numerically that the monopolar Rayleigh wave becomes bipolar, dramatically indicating the presence of the flaw.     

Flaw characterization based on diffraction of ultrasonic waves

A method of flaw characterization based on time domain analysis of the diffracted sound field from a flaw when irradiated by short ultrasonic pulses is described. The echo formation mechanism developed by Freedman has been utilized to predict the return pulse pattern. Experimental results that are in good agreement with theoretical predictions are presented. Using a microcomputer-based scanning and testing system the feasibility of reproducing planar, straight-edged flaw shapes has been successfully demonstrated.     

Standardisation of electrostatic test methods and electrostatic discharge prevention measures for the world market

With the development of world markets and large organisations working with sites in many continents, the need has grown for international standards in electrostatic test methods and Electrostatic discharge (ESD) prevention measures. Progress in world standards developed by the International Electrotechnical Commission (IEC) Technical Committee 101 (Electrostatics) has so far been mainly driven by the needs of the electronics industry, but has also laid down basic materials test methods, test methods for specific applications, and requirements for protection of electronic devices from electrostatic damage. Much of the work done in this area has its roots in test methods developed by the ESD Association in the US. More recently, work on test methods for electrostatic ignition hazards avoidance, for example in Intermediate bulk containers (IBCs), has commenced. Meanwhile the European Committee for Electrotechnical Standardization (CENELEC) has been active issuing a code of practice for avoidance of electrostatic hazards, as well as various test methods. Much of this is driven by the need for standards to address compliance with European Directives. This paper examines some areas in which new test methods are needed or are currently being developed by IEC TC101 for the world market.     

Signal-to-noise ratio enhancement based on the whitening transformation of colored structural noise

In the ultrasonic testing and evaluation of highly scattering materials (i.e. non-homogeneous media such as composites, layered and clad materials) structural noise is an important limitation to the visibility of flaw echoes. This noise cannot be reduced by conventional linear filtering or by time-averaging techniques. In order to enhance the defect-to-background noise ratio (SNR), many different algorithms have been developed over the years. This work analyzes three new strategies for SNR enhancement based on the whitening transformation of the colored structural noise. By using this transformation, the small spectral differences between noise and flaw echoes are exploited, thereby allowing an improvement in the visibility of the flaw.     

Ultrasonic flaw detection in NDE of highly scattering materials using wavelet and Wigner-Ville transform processing

In ultrasonic non-destructive evaluation of highly scattering materials the backscattering noise may attain peak values greater than the searched flaw pulse and the mean value of noise spectrum is very similar to the searched echo spectrum. Several specific methods have been proposed for the reduction of this type of noise, but the comparison of the performance of different methods is still an open problem. In this paper, we make a comparison among some methods based on simultaneous representations in time and frequency/scale domains of the ultrasonic traces. Synthetic and experimental traces are de-noised using a discrete wavelet processor with decomposition level-dependent threshold selection and a method that combines Wigner-Ville transform and filtering in the time-frequency domain. The results are comparatively evaluated in terms of signal to noise ratio and probability of detection.     

基于机器视觉的电机换向器质量在线检测系统开发

电机换向器质量检测是换向器生产线中的一个重要工序,但其仍采用人工方式,存在检测效率低、漏检率高等问题。针对此问题,本文运用图像处理和机器视觉技术,开发换向器质量在线视觉检测系统。该系统针对生产过程工艺多变,造成形状检测标准不一致问题,提出自适应学习模板方法;在轴孔孔径检测,提出基于Freeman链码改进的孔径快速检测算法;在端面缺陷中,提出基于改进视觉注意力模型的端面缺陷检测方法。实验结果表明,系统检测精度达到99.80%,漏检率为0%,F-measure值为99.89%。该系统能够快速有效检测换向器存在的外观质量问题,可满足换向器在线质量检测需求。     

Computerized signal processing for detecting cracks under installed fasteners

This paper describes the development work on a computerized system designed to detect cracks by rotating a shear wave transducer around installed fasteners or bolt holes. Reflected ultrasonic signals from the cracks are digitized in 2° intervals, processed, and displayed in either polar or rectangular coordinates by the computer. Symmetrical noise resulting from an imperfect fit between the bolt and the hole is suppressed by a statistical digital rejection procedure which increases the signal to noise ratio. Corner slots of .051 × .051 and .025 cm and fatigue cracks under installed fasteners have been detected by this computerized system. Two promising methods to compute flaw sizes are also described in this paper.     

High-speed digital Golay code flaw detection system

A new, entirely digital, correlation flaw detection system is described which maintains optimal correlation detection over a much wider range of input signal-to-noise ratio conditions and operating speeds than previous single transducer correlation flaw detection systems. This new system uses special pseudo-random codes called complementary Golay codes to overcome the problem of self-noise which results from incomplete pulse-compression in the correlation process. Results are presented which show that the Golay code system produces better performance in the presence of grains and large reflecting surfaces, while offering much higher operating speed than previous single transducer correlation flaw detection systems.     

Detecting small flaws near the interface in pulse-echo

Flaw detection near the interface surface is a common problem in many pulse-echo NDT applications due to interference with the interface echo, orders of magnitude above the flaw echoes. Several digital signal processing techniques like deconvolution, Hilbert transform and cepstrum analysis have been proposed to improve axial resolution. However, they require strict linearity, which takes a large portion of the system dynamic range just to fit the interface echo, thus reducing the dynamic range available for flaw detection. This work presents a new alternative based on the time-domain phase analysis of the received signals. Differently from conventional approaches, it works quite well with saturated signals resulting when a high gain is applied to detect small flaws. These can be detected in a range of a fraction of one wavelength from the interface surface, even using narrow-band transducers, as it has been experimentally verified. The method can be easily hardware implemented for real-time processing.     

Classification of flaw severity using pattern recognition for guided wave-based structural health monitoring

In this paper, the authors present a formal classification routine to characterize flaw severity in an aircraft-grade aluminum plate using Lamb waves. A rounded rectangle flat-bottom hole is incrementally introduced into the plate, and at each depth multi-mode Lamb wave signals are collected to study the changes in received signal due to mode conversion and scattering from the flaw. Lamb wave tomography reconstructions are used to locate and size the flaw at each depth, however information about the severity of the flaw is obscured when the flaw becomes severe enough that scattering effects dominate. The dynamic wavelet fingerprint is then used to extract features from the raw Lamb wave signals, and supervised pattern classification techniques are used to identify flaw severity with up to 80.7% accuracy for a training set and up to 51.7% accuracy on a series of validation data sets extracted from independent plate samples.     

An improved automated ultrasonic NDE system by wavelet and neuron networks

Despite of the widespread and increasing use of digitized signals, the ultrasonic testing community has not realized yet the full potential of the electronic processing. The performance of an ultrasonic flaw detection method is evaluated by the success of distinguishing the flaw echoes from those scattered by microstructures. So, de-noising of ultrasonic signals is extremely important as to correctly identify smaller defects, because the probability of detection usually decreases as the defect size decreases, while the probability of false call does increase. In this paper, the wavelet transform has been successfully experimented to suppress noise and to enhance flaw location from ultrasonic signal, with a good defect localization. The obtained result is then directed to an automatic Artificial Neuronal Networks classification and learning algorithm of defects from A-scan data. Since there is some uncertainty connected with the testing technique, the system needs a numerical modelling. So, knowing the technical characteristics of the transducer, we can preview which are the defects that experimental inspection should find. Indeed, the system performs simulation of the ultrasonic wave propagation in the material, and gives a very helpful tool to get information and physical phenomena understanding, which can help to a suitable prediction of the service life of the component.     

A practical ultrasound axicon for non-destructive testing

A practical water-coupled ultrasound axicon has been developed, which is realized by a combination of an annular PZT and a conical plexiglass wedge bonded to it. The acoustic beam from the PZT is refracted by the wedge and focused over a certain range in the axial direction. Two kinds of axicon are prepared for flaw examination of metals or ceramics. Suppression of the surface echo and sidelobes is discussed. Some examples which show the abilities of the axicons are also presented.     

Accurate measurement of pressure

This paper briefly describes the fundamental principles of the instruments used for accurate measurement of hydrostatic pressure and in particular the use of piston gauges as primary pressure standards. Different methods for the calibration of secondary standards have been discussed and in particular, emphasis has been given to the calibration of secondary piston gauges against the primary standards by the cross-float method along with the evaluation of uncertainties attached to different correction factors associated with the measurement of pressure from these gauges. The importance of secondary pressure standards in the region 0.1 GPa to several GPa has also been defined.