Speckle reduction by energy time-frequency filtering

Structural noise is a very important limitation to the visibility of flaw echoes in ultrasonic testing and evaluation of highly scattering materials. In order to enhance the signal-to-noise ratio, different algorithms have been developed. One of these techniques is based on filtering the spectrum low band of the received echo to obtain a significant improvement of the defect visibility. Based on this idea, in this work a new time-frequency technique is presented. In this method, block-processing autoregressive techniques are used to estimate the instantaneous center frequency of the traveling wave. From this information, a time-frequency filter is designed tuned at half the estimated instantaneous center frequency. Experimental results and the comparison with the non-time-frequency filtering technique are also included, showing that the proposed method has an excellent performance on SNR enhancement.     

Study on the denoising method for the electromagnetic ultrasonic echoes from multiple interfaces

The electromagnetic ultrasound is used in the detection of interfaces of the adhesive multilayer structures to solve the unstable coupling problem in ultrasonic testing by traditional piezoelectric transducers. Based on the analysis of the transforming mechanism of electromag-netic ultrasound energy and the resultant dead zone from mutual inductance of the transducer, the wavelet filtering by soft-thresholding and adaptive noise canceling methods are used simul-taneously to the detected electromagnetic ultrasonic signals to overcome the drawbacks of the low signal to noise ratio （SNR） and the wide intrinsic dead zone of the transducer. Processed results in the interface detection of a three layered adhesive sample of steel and rubber materials demonstrate that the wavelet filtering enhances the SNR about 12dB while the adaptive noise canceling narrows the dead zone effectively.     

基于超声回波重组相位分析的颗粒粒径测量方法

将超声波作用于沉降的颗粒时,由于颗粒的移动,超声回波会出现相位差异。该文通过对测量杯中某一确定深度处的回波信号进行相位分析和重组,发现重组后信号的频率可以计算出粒径;并分别对两种不同粒径分布的聚甲基丙烯酸甲酯(PMMA)微球悬浮液进行了超声波信号采样重组和去噪的实验,实验结果经小波时频方法分析后,证实了颗粒粒径分布与重组信号频率构成的确存在很高的相关性。     

超声在固体火箭发动机无损检测中的应用

在固体火箭发动机的研制中，无损检测技术非常重要。本文介绍了一些主要的超声检测方法及其应用。脉冲多次反射法用于检测固体火箭发动机壳体与衬层之间的粘结质量。特性参数和扫频超声法以多层介质理论为基础，用于检测固体火箭发动机的深层粘结质量。     

Ultrasonic flaw detection using threshold modified S-transform

Interference noising originating from the ultrasonic testing defect signal seriously influences the accuracy of the signal extraction and defect location. Time–frequency analysis methods are mainly used to improve the defects detection resolution. In fact, the S-transform, a hybrid of the Short time Fourier transform (STFT) and wavelet transform (WT), has a time frequency resolution which is far from ideal. In this paper, a new modified S-transform based on thresholding technique, which offers a better time frequency resolution compared to the original S-transform is proposed. The improvement is achieved by the introduction of a new scaling rule for the Gaussian window used in S-transform. Simulation results are presented and show correct time frequency information of multiple Gaussian echoes under low signal-to-noise ratio (SNR) environment. In addition, experimental results demonstrate better and reliable detection of close echoes drowned in the noise.     

涂层厚度与粘接质量的电磁声谐振无损检测

为了解决电磁超声(EMAT)涂层质量检测中接收信号信噪比较低的问题,将电磁声谐振技术(EMAR)应用于涂层质量检测,有效提高了超声接收信号的信噪比。在EMAR仿真中,通过变步长离散仿真得出了谐振与非谐振状态下的时域波形和频谱图,验证了利用电磁声谐振进行涂层质量检测的可行性。以电磁超声谐振驻波作为检测信号,实验中谐振信号的幅值为未谐振反射回波的2.52倍,信噪比由26 dB(反射回波)提高到34 dB(谐振信号)。利用EMAR信号的谐振频率差进行涂层厚度检测,对0.2~0.8 mm的试件的检测误差在5%以内且涂层厚度越大,谐振频率越多,谐振频率差越小,检测精度越高;通过功率谱密度函数(PSD)表征不同粘接程度的试件,不同试件的PSD比值为61.17:4.15:1,区分度相对于峰峰值比值提升明显。      

High accuracy non-contact ultrasonic thickness gauging of aluminium sheet using electromagnetic acoustic transducers.

Aluminium sheet thickness has been calculated from ultrasonic data obtained using a send-receive, radially polarised electromagnetic acoustic transducer (EMAT). Sheets in the thickness range between 0.1 and 0.5 mm have been measured using this non-contact approach at a stand-off of up to 1.5 mm. Normal incidence shear waves generated and detected in the sheet and the resultant waveforms have been processed using transit time measurements and Fourier analysis. Two broad band EMAT systems have been used to perform the measurements with centre frequencies of approximately 5 MHz and frequency content up to 10 and 20 MHz respectively. The most accurate measurements of thickness on thin sheets have been made using Fourier analysis and have yielded measurements accurate to within 0.2% (or 0.4 microm) for 280 microm thick aluminium sheets. Discrete shear wave echoes can be observed for sheets down to a thickness of 250 microm using the higher frequency EMAT system. However temporal measurements of these signals yield lower accuracy results when compared to the Fourier analysis method which is capable of sub-micron accuracy.     

Nonlinear elastodynamics in micro-inhomogeneous solids observed by head-wave based dynamic acoustoelastic testing

Dynamic acoustoelastic testing provides a more complete insight into the acoustic nonlinearity exhibited by micro-inhomogeneous media like granular and cracked materials. This method consists of measuring time of flight and energy modulations of pulsed ultrasonic waves induced by a low-frequency standing wave. Here pulsed ultrasonic head waves were employed to assess elastic and dissipative nonlinearities in a region near the surface of a solid. Synchronization of the ultrasound pulse sequence with the low-frequency excitation provided instantaneous variations in the elastic modulus and the attenuation as functions of the instantaneous low-frequency strain. Weak quadratic elastic nonlinearity and no dissipative nonlinearity were detected in duralumin. In limestone, distinction between tensile and compressive behaviors revealed an asymmetry in the acoustic nonlinearity and hysteresis in both the elastic modulus and the attenuation variations. Measured nonlinear acoustical parameters are in good agreement with values obtained by different techniques. Reversible acoustically induced conditioning modified the acoustic nonlinearity both quantitatively and qualitatively. It reduced tension-compression asymmetry, suggesting a nonequilibrium modification of the sources of acoustic nonlinearity. Additionally to the metrology of the acoustic nonlinearity, head wave based dynamic acoustoelastic testing may be a useful tool to monitor changes in the microstructure or the accumulation of damage in solids.     

Modulated angle beam ultrasonic spectroscopy for evaluation of imperfect interfaces and adhesive bonds

An experimental method incorporating high frequency pulsed angle beam ultrasonic measurements modulated by low frequency vibrations of a bonded structure is described. This method uses parametric/nonlinear mixing between high and low frequencies to characterize adhesive degradation. It is demonstrated that good quality (undamaged) bonds exhibit little dependence of ultrasonic signature on the overlay of low frequency vibration loads; however, environmentally degraded or imperfect bonds exhibit strong modulation of the resonance frequency of the ultrasonic signal reflected from the bond. The results are interpreted using a model for normal and oblique wave interaction with two nonlinear interfaces separated by an adhesive layer under quasi-static stress modulation.     

水平连铸圆钢超声检测结果的信号处理

水平连铸圆钢为多晶种的不均匀粗晶材料，当采用超声波对其进行检测时，将产生较强的组织散射噪声，从而使得对缺陷回波的判断与评价无法进行本文采用分离频谱的方法对原始检测信号进行处理，再用截止地对缺陷回波进行恢复运算。实验结果本文所采用的方法能较为有效地消除原始检测信号中的组织噪声，从而提高超声检测的信噪比。     

基于双高斯衰减模型的超声回波处理方法

信号降噪与特征提取是超声检测数据处理的关键技术.基于超声信号有特定结构而噪声和超声信号的结构无关,本文提出一种旨在解决强噪声背景下超声回波的参数估计和降噪问题的方法.该方法将超声回波的参数估计和降噪问题转换为函数优化问题,首先根据工程经验建立超声信号的双高斯衰减数学模型,然后根据观测回波和建立的超声信号模型确定目标函数,接着选择人工蜂群算法对目标函数进行优化从而得到参数的最优估计值,最后由估计出的参数根据建立的超声信号数学模型重构出无噪的超声估计信号.通过仿真和实验表明本文方法可以准确估计出信噪比大于-10 dB的含噪超声回波中的无噪信号,且效果优于基于自适应阈值的小波降噪方法和经验模态分解方法;此外相比常用的指数模型和高斯模型,本文提出的双高斯衰减超声信号模型与实测超声信号更接近,其均方误差为9.4×10~(-5),波形相似系数为0.98.     

Acoustic response from adherent targeted contrast agents

In ultrasonic molecular imaging, encapsulated micron-sized gas bubbles are tethered to a blood vessel wall by targeting ligands. A challenging problem is to detect the echoes from adherent microbubbles and distinguish them from echoes from nonadherent agents and tissue. Echoes from adherent contrast agents are observed to include a high amplitude at the fundamental frequency, and significantly different spectral shape compared with free agents (p <0.0003). Mechanisms for the observed acoustical difference and potential techniques to utilize these differences for molecular imaging are proposed.     

大功率单脉冲超声信号发生器的设计

为了提高探伤深度,实现混凝土大坝等散射强介质的超声检测,本文提出了一种成本低,小体积,结构简单的大功率单脉冲超声信号发生器,并给出了原理电路设计。所述装置利用控制电容充放电经脉冲变压器产生瞬时高压脉冲的新思路,克服了过去由于信号幅度限制功率上不去的困难。经实验验证该装置与现有的单脉冲信号发生器相比,探伤深度得到了很大提高,可用于混凝土结构构建的超声探伤。     

Acoustic wave generation by microwaves and applications to nondestructive evaluation

Although acoustic wave generation by electromagnetic waves has been widely studied in the case of laser-generated ultrasounds, the literature on acoustic wave generation by thermal effects due to electromagnetic microwaves is very sparse. Several mechanisms have been suggested to explain the phenomenon of microwave generation, i.e. radiation pressure, electrostriction or thermal expansion. Now it is known that the main cause is the thermal expansion due to the microwave absorption. This paper will review the recent advances in the theory and experiments that introduce a new way to generate ultrasonic waves without contact for the purpose of nondestructive evaluation and control. The unidirectional theory based on Maxwell's equations, heat equation and thermoviscoelasticity predicts the generation of acoustic waves at interfaces and inside stratified materials. Acoustic waves are generated by a pulsed electromagnetic wave or a burst at a chosen frequency such that materials can be excited with a broad or narrow frequency range. Experiments show the generation of acoustic waves in water, viscoelastic polymers and composite materials shaped as rod and plates. From the computed and measured accelerations at interfaces, the viscoelastic and electromagnetic properties of materials such as polymers and composites can be evaluated (NDE). Preliminary examples of non-destructive testing applications are presented.     

超声探头的响应特性及暂态回波模型

建立检测系统的数学模型,可以更好地理解超声检测的物理本质。分析了超声波从产生、介质中传播、缺陷耦合以及接收的全过程,将缺陷回波表示为探头响应函数与缺陷响应的时域卷积。利用空间脉冲响应和基尔霍夫近似建立了超声波与平面型缺陷的耦合模型,用大平面试块底面回波和大平面响应进行反卷积求得了探头的响应函数,并详细分析了探头在不同偏置位置时不同大小缺陷响应的特点,发现缺陷回波由直达回波和边缘回波组成,直达回波和边缘回波极性相反,且直达回波的幅值远远大于边缘回波。      

Transient elastodynamic model for beam defect interaction: application to non-destructive testing

Modeling tools have been developed at the French Atomic Energy Commission (CEA) for the simulation of ultrasonic non-destructive testing inspections. In this paper the model for the prediction of echoes arising from defects within a piece (Mephisto) is presented and some examples are given and compared with experimental results. The model for computing wave defect interaction is based on Kirchhoff's approximation, and uses the principle of reciprocity and a mode-by-mode (between the transducer and the defect) calculation of the echoes. It accounts for possible mode conversions. These approximations and other approximations for the radiated field incident on the defect allow us to obtain a formulation of the echo received at the transducer, which is able to be computed rapidly.     

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.     

Quicker,simpler hardness testing using ultrasonics

After a brief review of conventional hardness testing techniques the ultrasonic micro-hardness tester, or ultrasonic contact impedance technique, is described, also briefly. An alternative ultrasonic hardness tester is then described which consists of a modification of the Brinell indenter, enabling instantaneous automatic readout using ultrasonics (‘instantaneous’ measuring beginning as soon as the elastic deformation has taken place). It can also follow the deformation as it takes place enabling some conclusions to be drawn on the building up of work hardening.     

一种压电超声波物料输送器*

利用压电陶瓷元件具有良好声电转换特性和输出特性,本文提出了一种新型结构的压电超声波物料输送器。设计了超声波物料输送器的结构并分析了其工作原理,采用纵扭复合型压电换能器为驱动源,对压电换能器进行了结构设计和参数计算,并对其进行了性能测试。由此研制出了压电超声波输送器的样机并进行了试验测试,结果表明:当频率为17.9 kHz–18.6 kHz时,输送器具有输送物料能力。驱动负载为1.5 kg时系统的谐振频率为18.3 kHz,输送速度最快,达到64个/min。随着电源输出功率的增大输送物料速度呈线性增加。该输送器的稳定性好,噪声低。与压电片式和电磁式输送器噪声进行对比,超声波输送器的工作噪声仅为31 dB,略小于压电片式,远小于电磁式。     

Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection

A pulsed Nd:YAG laser with an approximately Gaussian beam shape is directed onto the surface of an aluminium sheet at an energy density below which damage by laser ablation occurs, generating Lamb waves in the sheet. The laser beam is raster scanned across the surface of the sample. The Lamb waves travel radially outwards from the generation point and are detected some distance away by an electromagnetic acoustic transducer with sensitivity to in-plane displacements of the sheet. A number of static EMATs are located around the edges of the sheet, some distance from the generation point. The presence of a crack-like defect on the sheet can be detected by either a sudden change in the ultrasonic waveform or by an enhancement in the frequency content of the waveform when the laser beam illuminates directly onto the crack.