GPR signal de-noising by discrete wavelet transform

Ground penetrating radar (GPR) is a non-destructive investigation tool used for several applications related to civil infrastructures; including buried objects detection and structural condition evaluation. Although GPR can be effectively used to survey structures, signal analysis can be sometimes challenging. The GPR signals can be easily corrupted by noise because the GPR receiver has usually an ultra-wide bandwidth (UWB). The noise collected by the system can easily mask relatively weak reflections resulting from the inhomogeneities within the surveyed structure; especially when they are at a relatively deep location. This paper presents the use of discrete wavelet transform (DWT) to de-noise the GPR signals. Various mother wavelets were used in this study to de-noise experimental GPR signals collected from flexible pavements. The performance of wavelet de-noising was evaluated by computing the signal-to-noise ratio (SNR) and the normalized root-mean-square error (NRMSE) after de-noising. The study found that wavelet de-noising approach outperforms traditional frequency filters such as the elliptic filter. At the same level of decomposition, the Daubechies order 6 and Symlet order 6 outperform the Haar and Biorthogonal mother wavelets when de-noising GPR signals by soft thresholding.     

小波分析在超声检测信号处理中的应用

小波分析技术以其多分辨率的突出特点成为目前超声信号时频表达的最佳分析方法,被广泛应用于超声信号的降噪处理。分析了小波技术用于超声降噪的理论基础和基本过程,基于国内外小波降噪的应用研究成果,从母小波和分解层数的选择以及小波系数非线性处理方法两个方面论述了小波分析在超声检测信号处理中的应用。     

Continuous wavelet transform technique for fault signal diagnosis of internal combustion engines

A fault signal diagnosis technique for internal combustion engines that uses a continuous wavelet transform algorithm is presented in this paper. The use of mechanical vibration and acoustic emission signals for fault diagnosis in rotating machinery has grown significantly due to advances in the progress of digital signal processing algorithms and implementation techniques. The conventional diagnosis technology using acoustic and vibration signals already exists in the form of techniques applying the time and frequency domain of signals, and analyzing the difference of signals in the spectrum. Unfortunately, in some applications the performance is limited, such as when a smearing problem arises at various rates of engine revolution, or when the signals caused by a damaged element are buried in broadband background noise. In the present study, a continuous wavelet transform technique for the fault signal diagnosis is proposed. In the experimental work, the proposed continuous wavelet algorithm was used for fault signal diagnosis in an internal combustion engine and its cooling system. The experimental results indicated that the proposed continuous wavelet transform technique is effective in fault signal diagnosis for both experimental cases. Furthermore, a characteristic analysis and experimental comparison of the vibration signal and acoustic emission signal analysis with the proposed algorithm are also presented in this report.     

小波阈值函数在合成金刚石超声信号消噪中的应用

在对合成金刚石进行超声波探测的过程中,部分缺陷信号因噪声信号的混入而难以准确获取。本研究运用多分辨分析小波阈值的消噪方法,设计出带有改进效果的新函数并应用在金刚石缺陷信号处理领域。该函数方法可以减少硬阈值和软阈值函数各自的不足,例如在离散估计值和分解小波系数两者间存在的定值误差,具有表达式简单、计算方便等优点。通过实际缺陷噪声信号的测试表明:这种改进的阈值消噪函数与传统的硬、软阈值消噪函数相比较,可获得均方误差值减小和重构波形信噪比提高的优异消噪表现,能明显提高合成金刚石缺陷识别的准确率。本研究将超声波检测与小波变换技术相结合,并将其应用于合成金刚石的缺陷识别,对人造金刚石产业具有重要意义。     

Features of ultrasonic wave propagation to identify defects in composite materials modelled by finite element method

A two-dimensional plane strain finite element model with absorbing boundary condition has been developed to investigate the ultrasonic wave propagation in isotropic and orthotropic media. It is capable of simulating the experimental pulse echo technique to obtain A-scan data, when a short duration pulse is transmitted into a domain with or without a flaw. The flaw can either be a crack or an inclusion of different material such as a Teflon insert or a resin rich zone. After performing FFT on the A-scan data, frequency domain feature analysis is done. The study provides a guideline regarding the suitability of certain harmonics sensitive to certain types of flaw. The simulation shows other important artifacts of wave propagation such as mode conversion and scattering due to the presence of flaws.     

Characterisation of metal loss defects from magnetic flux leakage signals with discrete wavelet transform

Metal loss defects in a buried pipeline are detected by magnetic flux leakage technique. Characterisation of the defects and sentencing according to the severity is extremely important for organised maintenance of pipelines. In this paper we identify the parameters that characterise a defect and the features of magnetic flux leakage signal (MFL) that are affected by those parameters. We show that analysis of the MFL signal using wavelet transform scores over any other method of its kind and exposes the incompleteness of the other analysis techniques that have appeared in the literature, to date. A number of experiments were performed on a rotating drum test rig having defects of different shapes and sizes. The results from these experiments are presented and discussed in detail. Wavelet transform decomposition and reconstruction techniques were applied for denoising the raw data. We test the efficacy of discrete wavelet transform for denoising MFL signal and present a complete scheme of characterisation of defects from denoised MFL signal. We discuss the issue of defect classification and suggest that characterisation to specified accuracy, amounts to designing a classifier that assigns a defect into known classes whose shapes and sizes are defined a priory.     

Improving the time resolution and signal noise ratio of ultrasonic testing of welds by the wavelet packet

In ultrasonic testing of welds, detection of small flaws is often difficult by the superimposed noise due to the grain structure of the material. The scattering of ultrasonic waves from grain boundaries can interfere and introduce disturbance in the received signal that can sometimes mask indications due to a small but potentially dangerous defect. However, to enhance the flaw characterization, methods based on ‘thresholding’ have given good results only when the signal to noise ratio is high, and since bandwidth of the reflected signal as well as its principal frequency is subject to wide variation, it is impossible to create an appropriate band pass filter. So linear filtering does not provide good results, because both, the structure noise and flaw signal concentrate energy in the same frequency band. Non-linear filtering can be used to reduce or suppress the noise from ultrasonic signals. One way out is to use the time frequency transforms, the method is based on the wavelet packet decomposition. The Debauchee function of order 8 [Daubauchee I. Orthogonal bases of capacity wavelets. Commun Pure Appl Math 1998;41] has been chosen as the analyzing function, and each measured ultrasonic signal is analyzed by a filter bank through only three levels of decomposition. This work demonstrates that the following analysis is very efficient with respect to signal recovery from noisy data. The experimental results have shown that the proposed method has excellent performances on SNR enhancements.     

NDE ultrasonic methods to characterise the porosity of mortar

Premature damage of mortar and concrete structures, due to environmental action, demands procedures to estimate durability of this type of components. Mortar or concrete composition (e.g. grain size, type and percentage of sand) may have some influence in the durability, but it is mainly related to porosity, which determines the interaction between aggressive agents and material. In this work, several IDE ultrasonic methods to estimate porosity of mortar are presented and evaluated. In these methods, porosity is related to (1) the material structural noise, (2) sound velocity and (3) ultrasonic attenuation. In all these methods, mortar is consider to be formed by only two phases: solid and pores.     

Morphological characterisation of engineered surfaces by wavelet transform

This work introduces a wavelet transform technique based on the detection of the multi-scale view of the components of a signal . The main advantages of wavelet transform over the existing signal processing techniques are its space-frequency localization and multi-scale analysis of roughness and waviness motifs. After an extensive review of the mathematical and signal processing fundamentals of the wavelet technique, numerical implementations are carried out to explore the potential applications of wavelet transform and its inverse transform multi-scale analysis of roughness and local morphological characterization by the detection of the roughness singularities and 21) motif size of an engineered surface.     

Electrochemical noise analysis of localized corrosion by wavelet transform

Electrochemical noise (EN) was measured during pitting and stress corrosion tests in NaCl solutions for type 403 stainless steel tempered at 390 °C for 2 h. The EN data were analyzed mainly by wavelet transform in terms of the energy contribution of smooth crystal and detail crystals in three frequency regions. In the early stage of the pitting tests, general corrosion occurs, and contribution of smooth crystal to the total energy was greater than that of low frequency detail crystal, but less with the development of meta-stable pitting. A similar trend was observed in the early stage of stress corrosion cracking (SCC) tests. The contribution of low frequency detail crystals to the total energy decreased as stable pits were formed, and it was less than the energy contribution of smooth crystal when crack propagation occurred in the later stage of SCC. In addition, the contribution of energy of high frequency detail crystal is shown to be indicative of the occurrence of plastic deformation. These results demonstrate that the onset of detailed processes in pitting and SCC can be sensitively detected by wavelet analysis in terms of the energy contribution of smooth crystal and low and high frequency detail crystals, but not with that based on the change in position of peak in energy contribution of detail crystals.     

Interpretation of electrochemical noise by wavelet transform and fractal analysis

Electrochemical noise(EN) was measured during pitting and stress corrosion tests in 3.5 % NaCl solutions for type 403 stainless steel tempered at 520 °C and 610 °C. The energy contributions of smooth crystal and detail crystals in three frequency divisions were analyzed after wavelet transform and compared with the change in the Hurst parameter based on the rescaled range analysis. The contribution of smooth crystal to the total energy is greater than that of low-frequency detail crystal in general corrosion and stable pit formation, but it is less in both meta-stable pitting and growth of stable pits. Crack growth rate can be estimated qualitatively by the evaluation of the energy contributions of smooth and detail crystals, and correlation was found between the change in the Hurst parameter and that in the energy contribution of smooth crystal or one of detail crystals in three frequency divisions.     

Multiscale characterizations of painted surface appearance by continuous wavelet transform

The appearance of paint is an important factor in the overall product quality of engineered surfaces, especially in the automotive industry. This paper introduces a multiscale characterization technique of surface topography from roughness to waviness. It uses continuous wavelet transform to correlate the characteristic scale of the surface texture to the paint's appearance. The results show good correlation between the multiscale roughness spectrum and standard distinctness of image (DOI) measurement. After an extensive review of the mathematical and numerical implementations, the multiscale attenuation function of the painting process (MAF) is defined to study the effect of the successive layers of paint coat and the painting orientation modes (horizontal, oblique and vertical).     

纳米CeO2p/Zn-4.5%Al复合材料的高能超声制备及其力学性能

设计一套高能超声搅拌装置,并在氩气氛保护下,通过对纳米CeO2进行预处理及控制工艺参数,用该装置制备了CeO2质量分数分别为1%～6%的纳米CeO2p/Zn-4.5%Al复合材料。用SEM观察颗粒的分散情况,测试材料的室温拉伸性能。结果表明：所设计的装置结构合理、超声作用效果佳;纳米CeO2可以单粒分散入基体合金中;随着CeO2质量分数的增加,复合材料的抗拉强度和弹性模量明显高于基体合金,伸长率则降低;复合材料的断裂机制为脆性断裂。     

A contribution to the applicability of complex wavelet analysis of ultrasonic signals

Ultrasonic signal is always composed of a series of transient feedback impulsive waves, which are created during the propagation of ultrasonic energy when it meets the discontinuities within the material, such as inclusions, voids, micro-cracks, the surface of the object being inspected and so on. Complex wavelet based envelope detection method was designed to extract the impulsive features contained in the signal. The complex wavelet based correlation analysis was further proposed for distinguishing the ‘real impulsive waveform’ that indicates the object being searched from a number of unrecognized disturbing waveforms also presented in the signal. The effectiveness of both methods was proved by practical experiments on a common tube sample. The experiments proved that, by using the proposed methods, the impulsive waveform that really indicates the end surface of the tube could be distinguished successfully. With the aid of the analysed results and the calibration equation, the relative distance between the position where the transducer was placed and the end surface of the tube could be identified correctly. It is believed that the proposed methods will be widely applied to the analysis of ultrasonic signals.     

CNTs含量对CNTs/Al5083复合材料力学性能的影响

利用高能球磨和冷压烧结工艺制备出碳纳米管（CNTs）增强Al5083复合材料,并对球磨过程中CNTs的演变及成型后复合材料的力学性能和形貌进行研究。结果表明,在球磨过程中,通过机械力的作用下带动钢球将CNTs切断,长径比变小,并均匀地分散在Al基体中;在CNTs含量为2wt%下,复合材料抗拉强度和屈服强度分别达到294和239 MPa,硬度达到95 HV5,复合材料的力学性能最好。通过观察复合材料的断口,随着碳纳米管含量的增加,复合材料的断口形貌从韧性断裂向脆性断裂转变。     

Application of signal processing techniques to ultrasonic testing of plates by S0 Lamb wave mode

In recent years, much attention has been paid to the use of Lamb waves for structural health monitoring. This choice is prompted by the high speed of Lamb wave inspection, although their dispersive nature can complicate the interpretation of results, especially when dealing with closely-spaced reflectors. In this paper, the objective is to improve the time resolution and signal-to-noise ratio of signals obtained from inspection of plates by the S₀ Lamb mode. The signal processing scheme used is based on deconvolution of the measured signal by Wiener filtering, followed by autoregressive spectral extrapolation. The deconvolution technique is applied to signals obtained from finite element models and also to experimentally measured signals; both sets of data are based on plates with various types of notch discontinuities. Using this technique, the separation distance between adjacent notches was estimated with high accuracy in both simulated and experimental ultrasonic signals.     

基于小波变换的MAG快速成形焊缝截面建模

提出一种基于小波变换的焊缝截面形态建模方法。基于小波变换模极大原理提取了典型焊缝截面轮廓,分别采用三次样条、带约束的三次样条以及B样条曲线方法对提取的焊缝轮廓进行插值,并基于最小二乘法对插值后的焊缝边缘轮廓进行了数据拟合,得到了焊缝轮廓数学模型。结果表明,采用小波变换提取焊缝截面轮廓是可行的,带约束的三次样条曲线过渡平滑、无突变点,在该试验条件下得到的焊缝截面轮廓数学模型为正弦曲线。     

Cf/SiC复合材料与Ti合金的Ag-Cu-Ti-TiC复合钎焊

以Ag-Cu-Ti-TiC复合钎料为中间层,在适当的工艺参数下真空钎焊Cf/SiC复合材料与Ti合金.利用SEM、EDS和XRD分析接头的微观组织结构,利用剪切实验检测接头的力学性能.结果表明:钎焊时,借助液态钎料,复合钎料中的Ti与Cf/SiC复合材料反应,在Cf/SiC复合材料与连接层界面形成Ti-Si-C、Ti-Si和少量TiC化合物的混合反应层;复合钎料中的Cu与Ti合金中的Ti发生互扩散,在连接层与Ti合金界面形成不同成分的Cu-Ti化合物过渡层;钎焊后,形成TiC颗粒强化的致密复合连接层,TiC的加入降低了接头的残余热应力,Cf/SiC/Ag-Cu-Ti-TiC/TC4接头的剪切强度明显高于Cf/SiC/Ag-Cu-Ti/TC4接头的.     

Behavior of porosity and copper oxidation in W/Cu composite produced by plasma spray

A spherical tungsten (W)/copper (Cu) composite powder was plasma-sprayed onto a boron-nitride-coated graphite substrate to produce heat sink material for the electronic packaging by atmospheric plasma spray using different input powers. At the surface of the deposited layer, Cu became oxidized to cuprite (Cu₂O). The degree of oxidation at the surface layer did not vary significantly with input power. Very little Cu₂O was detected at the inner layers after grinding off the 70 μm from the surface. The input powers had a strong effect on the porosity in the deposit layers. The porosity in the deposit layers at 25 kW was very small, about 3 vol%. The microstructures of W/Cu composite were discussed.     

Study of the cold compaction of composite powders by the discrete element method

We present numerical simulations of the cold isostatic and close die compaction of mixtures of soft and hard powders. The simulations use the discrete element method with periodic boundary conditions on packing of 4000 spherical monosize particles. The two mechanisms that are generally put forward to explain the retarding effect of hard particles on the compaction have been analysed. First, we have studied in which condition the hard particles form a cluster that hinders the homogeneous distribution of the load. Friction between particles is shown to affect significantly the formation of such a cluster. Second, the additional deformation that soft particles must undergo in the presence of hard non-deformable particles has been evaluated. Related to this last issue, the importance of including hardening effects in the constitutive equation of the soft phase is demonstrated.