Time reversal and microwave techniques for solving inverse problem in Non-Destructive Evaluation

Non-Destructive Evaluation (NDE) methods are used to inspect materials without damaging their usefulness. The key problem in NDE is the inverse problem which involves reconstructing materials’ physical profiles, like inner discontinuity etc., using information in the measured NDE signal. Inverse problem solutions in NDE can be classified as model-based and system-based approach. In model-based approach, an accurate forward model is used in an iterative framework. This approach provides a resultant materials' physical profile that minimizes the error between the measured signal and a simulated signal. However, this approach requires repeated calculations of a numerical model in each iteration, making it computationally demanding. This paper presents a model-based method that also provides a direct approach to inversion using principles of time reversal. Time reversal focusing is based on the fact that when a wave solution is reversed in time and back-propagated the wave comes to focus at the source. Using a computational model, this paper applies principles of time reversal to microwave NDE data to solve the inverse problem of defect detection in dielectric materials. A two-dimensional finite difference time domain (FDTD) model, for simulating the propagation of forward and time reversed wave fields, is developed. A dielectric sample with artificial defects, illuminated by a Gaussian modulated pulse, is used in the simulations. The microwave measurements are recorded, time reversed and propagated using the FDTD model to highlight the scatterer/defect. Maxima in the energy image indicate locations of defects. Simulation results demonstrate the feasibility of the technique to detect defects in dielectric materials. The FDTD model is validated using experimental data.     

Modeling of transducer fields in inhomogeneous anisotropic materials using Gaussian beam superposition

This article describes a three-dimensional Gaussian beam model for calculating transducer-generated ultrasonic wave fields in inhomogeneous anisotropic media. The model is based on a formulation presented recently for homogeneous anisotropic materials. The inhomogeneity is modeled by dividing the material into several homogeneous layers, the approach accounting for the propagation through the various layers and the reflection/refraction processes at the interfaces. It thus calculates ultrasonic field patterns, including proper amplitude information and allows a quick evaluation of the sound fields. Application of the model in view of ultrasonic nondestructive testing of layered composite material and of austenitic steel components is addressed. In the latter case, examples are given for various weldment structures, wherein ultrasonic inspection using commercial angle beam transducers is of interest.     

Ultrasonic NDE of composite material structures using wavelet coefficients

A wavelet-based method is proposed to perform the analysis of NDE ultrasonic signals received during the inspection of reinforced composite materials. The non-homogenous nature of such materials induces a very high level of structural noise which greatly complicates the interpretation of the NDE signals. By combining the time domain and the classical Fourier analysis, the wavelet transform provides simultaneously spectral representation and temporal order of the signal decomposition components. To construct a C-scan image from the wavelet transform of the A-scan signals, we propose a selection process of the wavelet coefficients, followed by an interpretation procedure based on a windowing process in the time–frequency domain. The proposed NDE method is tested on cryogenic glass/epoxy hydrogen reservoir samples.     

Automatic defects classification — a contribution

The objective of this work is to provide a contribution to defect classification. More precisely, we try to prove that it is possible to identify and classify defects of different types using the pulse-echo technique. The classification process makes use of the time and frequency domain responses of the ultrasonic echo signals acquired from different specimens simulating defects with three different shapes (cylindrical, spherical and planar with rectangular cross-section) and sizes. Although the final goal is the characterisation of practical defects (for instance, voids, cracks, delaminations, and so on) appearing in composite materials during manufacturing and in service, we first use the already mentioned reflectors for simplicity reasons. In these experiments 66 reflectors are used with water as matrix material. The inclusion (reflector) materials are brass, copper, steel and polystyrene. From the time domain signals we extract three features, namely, pulse duration, pulse decay rate and peak-to-peak relative amplitude of the third cycle. From the spectra of the echoes we extract the frequency for maximum amplitude and the standard error estimate from the deconvolved spectrum responses.All experimental signals were obtained using only one normal incident ultrasonic transducer aligned to maximise the direct reflected signal. In spite of the fact that this kind of configuration does not provide complete information about the characteristics of the geometries being studied, all the extracted features proved to be important discriminating factors of the geometrical classes considered, as will be demonstrated by making use of a pattern recognition technique for classification.     

基于最小熵解卷积的汽车点焊质量超声评价

利用超声信号的时域信息,可以对焊点质量进行评价.白车身焊点超声检测的过程中,由于噪声信号的模糊作用,反映焊点质量的有用信息被掩盖,导致焊点质量评价结果不准确.超声探头接收到的超声信号可视为原始超声信号与噪声信号两种信号的叠加,基于超声信号的卷积模型及稀疏特性,可利用最小熵盲反卷积（MED）对超声信号进行解卷积处理,分离重叠的焊点超声信号,恢复信号反射系数,获取准确的回波个数.通过仿真与试验,结果表明,最小熵解卷积对焊点重叠超声信号分离的有效性.     

Application of the ultrasonic pulse-echo technique for quality control of the multi-layered plastic materials

The main aim of the ultrasonic pulse-echo technique application for quality control is to characterize the internal structure of the object under investigation. The advantage of such technique is a possibility to perform non-contact and one-side access measurements, and to investigate the internal structure of multi-layered materials as well. The presented novel application of the ultrasonic pulse-echo technique for characterization of the multi-layered plastic materials covers the complete attenuation measurement in the frequency domain and is based on ill-posed Tikhonov regularization task for each layer separately. The law of the frequency-dependent attenuation has been estimated from the inverse transfer function approximation in the frequency domain. Phase velocity dispersion curves have been estimated in two ways: from the experimental signal phase spectra and from the causal Kramers–Kronig relations. The developed method enables to predict waveforms of the reflected signals from the interfaces of the individual layers in on-line mode.According to this approach, the step-by-step iterative analysis has been performed for each layer using the information about the previous layers. During each step, the acoustic properties of an individual layer, such as density, absorption, ultrasound velocity and phase velocity dispersion, have been recovered using numerical optimization. Optimization has been performed comparing the real ultrasonic signal, reflected by multi-layered object, with the simulated response of the model. The comparison of the predicted waveforms with the experimental ones has shown a good correspondence.     

Estimation of porosity content of composite materials by applying discrete wavelet transform to ultrasonic backscattered signal

As the use of composite materials in the aerospace industry increases, the development of advanced nondestructive evaluation (NDE) techniques for composite materials is in demand. Ultrasonic quantitative NDE technique for composite materials may provide good information on manufacturing quality, material strength and perhaps useful lifetime. It is well known that the effects of porosity in composite laminates on ultrasonic attenuation and velocity can be used in gauging the porosity content in composites, but back surface echoes may be absent or unusable due to complex geometry and bonding effects. In such cases the backscattered signals may be processed to extract porosity information. Measuring the porosity content in composite material by ultrasonic backscattering signal is a significant challenging problem in NDE of composite material. Backscattering signals are random and sensitive to volume fraction of pore and thickness of ply in composite material. Therefore the backscattering signal has various frequency bands and hence a signal decomposition method is required to analyze the ultrasonic backscattering signals. In this study, the discrete wavelet transform (DWT) using a MATLAB decomposition algorithm was applied to ultrasonic backscattered signals acquired in various porous composite laminates containing a porosity content that ranges from 0.01 to 11.90%. The ultrasonic backscattered signals were decomposed into two parts: the high frequency components called “Details” and the low frequency components called “Approximation”. And then, the correlation analysis was performed between the porosity content and the peak amplitude and magnitude of peak frequency of the decomposed signal. Overall, the correlation was reasonably good. As a conclusion, the DWT technique showed good benefits for analyzing the porosity content in composites using ultrasonic backscattered signal from composite materials.     

基于激光超声的铝合金搅拌摩擦焊典型缺陷检测及分析

利用不同搅拌摩擦焊(FSW)工艺参数获得2219铝合金薄板根部未焊合、隧道孔两种典型缺陷,对比X射线检测结果,采用激光超声对这两种典型FSW缺陷进行检测. 从时域上对采样信号峰值点及幅值进行分析,并进一步运用PSD-welch功率谱法进行频域分析,提取频域不同频段的能量,结合时域最大幅值作为特征,利用支持向量机建模并训练,识别率达到85%. 结果表明,基于激光超声检测的时、频域特征能够有效对铝合金FSW两种典型缺陷进行分类识别,从而为铝合金FSW缺陷检测提供一种技术途径.     

基于多模态集成卷积神经网络的数控机床齿轮箱故障诊断

针对数控机床齿轮箱在实际工作环境中负载多变且噪声干扰大、传统神经网络难以充分提取信号中的故障特征等问题,提出一种多模态集成卷积神经网络（MECNN）用于数控机床齿轮箱故障诊断。该方法将多模态融合技术与多个卷积神经网络结合,利用快速傅里叶变换方法将时域信号转换成频域信号;利用时域信号和频域信号对2个卷积神经网络进行训练,使模型能够分别从时域和频域2个角度提取特征,再将浅层特征融合;最后,将融合后的特征输入到卷积神经网络中进行故障特征的深度挖掘,并进行故障诊断。使用东南大学的齿轮箱数据集进行验证,设计了2种特征融合的方法并进行了对比。实验结果表明：在噪声下,MECNN模型用于故障诊断的准确性和鲁棒性均优于单一的时域CNN和频域CNN。     

基于小波变换的声发射模拟信号特性分析

小波变换作为一种新的信号处理方法,可对任一时刻、任一频段的声发射信号进行时频分析.通过基于Gaussian方程的Gabor小波及其Fourier变换,研究了Q235B钢板中三种声发射模拟信号的特点及三种信号的区分和识别.试验表明,基于小波变换的信号处理方法对于研究实际声发射信号的特点、声发射源的特征和定位以及传感器选择有一定的帮助.     

基于空化效应的功率超声珩磨再生型颤振研究

功率超声珩磨加工过程中的颤振影响因素很多,不仅包含物理磨削过程和机械振动环节,还包含了高频振动和超声波传动系统,是一个非常复杂的复合振动系统。通过建立珩磨颤振动力学模型,重点研究珩磨中空化泡溃灭产生辐射声压对再生型颤振的影响,结果表明:决定功率超声珩磨再生型颤振的主要原因为磨削厚度;空化泡溃灭产生的辐射声压会加剧系统颤振的频率,但对系统颤振的时域图变化趋势和振幅的大小基本没有影响。     

基于声发射和振动信号的振动钻削钻头故障诊断试验研究

针对钻削过程中钻头状态监测问题,基于声发射采集系统和振动采集系统设计超声轴向振动钻削钻头故障监测装置,分别应用完整钻头和故障钻头进行45钢板的超声振动钻削对比试验,采集不同钻头状态的AE和振动信号,通过时域分析、频域分析和小波分解,分析故障钻头对AE和振动信号的影响。试验结果表明：通过AE和振动信号判别钻头状态,判别结果与实际一致,能够实现钻头的故障诊断。     

New matching pursuit-based algorithm for SNR improvement in ultrasonic NDT

In this paper a fast and efficient matching pursuit-based algorithm is proposed for SNR improvement in ultrasonic NDT of highly scattering materials. The proposed algorithm utilizes time-shifted Morlet functions as dictionary elements because they are well matched with the ultrasonic pulse echoes obtained from the transducer used in the experiments. The proposed algorithm is fast enough to be used in the signal processing stage of real time inspection systems. Computer simulation has been performed to verify the SNR improvement for diverse ultrasonic waves embodied in high-level synthetic grain noise. This improvement is also experimentally verified using ultrasonic traces acquired from a carbon fibre reinforced plastic material. Numerical results show meaningful SNR improvements for low input SNR ratios (below 0 dB).     

基于随机森林算法的汽车轴承故障检测

针对传统机器学习算法受输入变量限制、且易出现过学习或欠学习,提出不受输入变量限制且存在大量数据缺失时有很好保持精确性的随机森林算法对汽车轴承故障进行检测。对采集到样本数据进行滤波处理,抑制信号中噪声;利用随机森林算法对采集到的时域信号进行分类标识,确定包含故障信息的信号序列;再将信号转换到频域,利用随机森林算法对频域内信号进行检测,确定出故障频率;最后采集试验数据对所提及算法进行验证,结果表明:相比于传统的机器学习算法,随机森林算法响应速度快,且准确率高。     

Effect of alternating ultrasonic frequency electric signal on the distribution of pore in aluminum alloy weld

The application of ultrasonic vibration in the welding process can effectively suppress the pore defects of the weld. The different ultrasonic application methods own different effects on the weld pore. In this paper, the ultrasonic frequency vibration in the weld pool is excited by the coupling of the ultrasonic frequency electrical signal and the welding electrical signal. The influence of ultrasonic excitation voltage and excitation frequency on porosity, number, location, and size of pore in weld was investigated. The results show that the variation of ultrasonic excitation voltage and frequency has a direct influence on the pore distribution. The pore defects can be decreased by the coupling of ultrasonic frequency electrical signal with reasonable parameters or increased by the coupling of ultrasonic frequency electrical signal with unreasonable parameters. The ultrasonic excitation frequency is fixed at 30 kHz and the ultrasonic excitation voltage is changed.The porosity of the weld is close to that of the weld without ultrasonic action when the ultrasonic excitation voltage is 25 V and 75 V. The porosity of the weld is significantly lower than that of the weld without ultrasonic action when the ultrasonic excitation voltage is 50 V and100 V. The ultrasonic excitation voltage is fixed at 100 V and the ultrasonic excitation frequency is changed. The porosity of the weld is the largest and exceeds that of the weld without ultrasonic action when the ultrasonic excitation frequency is 20 kHz. The porosity of the weld is lower than that of the weld without ultrasonic action when the ultrasonic excitation frequency is 25, 30, 35, and 40 kHz. The pores were mainly concentrated on the upper part of the weld and the number of pores of small size increased significantly after the coupling of an ultrasonic frequency electrical signal, indicating that ultrasonic promoted the rise and escape of bubbles in the weld pool.     

HHT在粗晶材料超声检测中的应用

超声检测粗晶材料时,结构噪声会严重降低检测信号的信噪比,造成缺陷反射波很难分辨出来。为提高检测信号的信噪比,增加粗晶材料超声检测的可靠性,本研究采用希尔伯特-黄变换(HHT)对检测信号进行分析处理。用超声检测系统对材料进行检测,采集粗晶材料测试数据;通过经验模态分解获得组成信号的本征模态函数,并经过希尔伯特变换得到不同模特对应的边际谱;分析信号的时频信息,去除噪声信号,提高了信噪比,使缺陷反射更加明显。实验结果表明:HHT能够有效去除无效的结构噪声,提高信噪比,缺陷反射更加突出。     

电子束焊接工件的传导电流特性

讨论了电子束焊接过程中电子失去动能的能量转换和电子定向流动的电流传导这两种物理过程，提出并定义了工件传导电流概念，设计了专用的多通道电流信号采集系统，从复杂的电子束焊接动态过程中剥离并定量检测出包涵焊接信息的工件传导电流的动态行为。全面系统地研究了工件传导电流的时域、频域、排序特性、传导比等特性；发现并证明了传导比可定量地描述焊接动态过程的临界穿透状态；利用研究获得的传导比均值与输入束流的函数关系建立了可描述电子束焊接过程的动态特性判据。     

One-dimensional time-domain finite-element modelling of nonlinear wave propagation for non-destructive evaluation

This one-dimensional time-domain finite-element model achieves accurate quantitative modelling of ultrasonic wave propagation in multi-layered structures. First, a sinusoidal wave toneburst is sent into a single layer of material exhibiting inherent material nonlinearity characterised by the nonlinear parameter β and thick enough for the toneburst received in through transmission to be resolved. The signal processing protocol that yields the theoretically correct quantitative value of β involves measuring the received toneburst for several propagation distances as well as the use of scaling factors taking into account the fast Fourier transform implementation, input signal windowing and material damping. Using that model configuration, model parameters (element size, time step, frequency step, input pressure, etc.) are then optimised and chosen quantitatively to generate accurate results. Finally, these model parameters are used for cases of interest where the configuration is not such that the exact β value can be obtained – e.g. thinner sample, pulse-echo etc. but where confidence in the results remains. This quantitative model that can be used for multi-layered structures provides a tangible resource useful to NDE engineers: a new prediction tool expected to enable them to choose the experimental set-up, driving frequency and post-processing method that would optimise kissing bond detection capability.     

超声频域合成孔径聚焦技术在主轴缺陷检测技术中的应用

以钢制主轴为研究对象，将线性超声阵列探头置于主轴端面采集超声阵列信号。基于相位迁移（PSM）成像算法对采集到的超声阵列信号进行傅里叶变换，并通过角谱运算对频域内声场进行重建，最后通过反傅里叶变换即可实现整个成像区域的聚焦。成像结果表明:将主轴中宽度0.5 mm、深度1 mm表面切槽半波高横向水平宽度范围由23~29 mm缩小到19~22 mm，分辨率可提高的范围至少为17%~24%。此算法计算效率高，充分满足实时成像要求。     

Numerical calculation of ultrasonic propagation with anisotropy

A computer simulation technique for three-dimensional ultrasonic propagation was utilized for the visualization and investigation of ultrasonic propagation in various materials such as anisotropic solids and combined materials. The calculation technique used here is based on a finite-difference method, but uses an improved nodal calculation method following fundamental consideration of the elastic wave equations. The improvement enabled unified treatment of calculation nodes at the inside and the boundary of the solid, and offered simplicity for calculation at the boundary and applicability for combined and anisotropic materials with limitation on the applicable type of anisotropic stiffness matrix. Here, we present the applications of the calculation technique for the two-dimensional and three-dimensional ultrasonic propagation relating to nondestructive material evaluation using ultrasound, and show the usefulness and applicability for the complex ultrasonic phenomena observed in ultrasonic measurements.