基于模拟退火算法的超声回波参数估计

本文应用模拟退火算法来改进超声回波模拟问题。仿真结果表明，该算法不需计算梯度，与初始选择值无关，并可在较大范围内搜索。从而避免了应用高斯-牛顿迭代法模拟超声回波的缺陷，即解决了选择的初始值只有在真实值附近才能迭代出最优解的问题。     

强散射材料超声探伤的模糊检测

本文将模糊检测技术用于增强强散射材料的超声探伤信号，这里非缺陷散射体引起的结构噪声对缺陷回波干扰较大。我们在模糊检测中选择了互相关、相位差及分数维作为信号的特征量。实验结果表明，这种新方法较之常用的相关检测具有更好的性能。     

基于衍射波的孔类缺陷超声相控阵定量方法研究

针对常规超声检测方法对小孔型缺陷难以准确定量的问题,利用超声相控阵的声束偏转聚焦和成像技术,基于声波在体积缺陷处的衍射特性,提出采用一次反射回波和衍射回波来确定体积缺陷的直径尺寸。以横孔模拟孔型缺陷,利用动态光弹技术对声波入射横孔的传播规律进行研究,并采用超声相控阵对孔径为Φ1-Φ4mm不同埋深以及相同埋深下Φ0.8-Φ4.8mm的等差横孔进行了试验。研究结果表明利用本文提出的定量方法能快速有效的确定孔型缺陷直径尺寸,实际计算值与理论值误差较小,对相控阵的定量问题研究具有一定的参考价值。     

超声液位检测频率选择的理论计算和实验研究

非介入式超声液位检测中,中间层介质对反射回波强度有着显著的影响,为得到较强的检测回波信号,发射频率的选择是非常必要的。本文采用传输矩阵方法,对非介入式超声液位检测中声波在多层介质中的传播进行了研究,给出了反射回波声强与入射声波频率之间的关系,从理论上计算得到了最佳入射声波频率。实验结果与理论计算一致,由此为非介入式超声液位检测的频率选择提供了一有效的技术途径。     

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。      

利用人工神经网络实现缺陷类型识别

本文在对各向同性均匀固体中横穿孔，平底孔和裂和裂缝缺陷超声散射特性分析的基础上，分别用回波幅度谱和去郑积幅度谱作为特征量，利用人工神经网络对缺陷类型进行识别。     

超声频谱分析中应用自适应学习网络技术进行缺陷自动鉴别的研究

超声频谱分析是把频谱分析用于超声检验的技术，本文介绍了自行设计的超声频谱分析系统和研究开发的对频谱与缺陷作出相关分析的计算机程序，超声频谱分析系统中，研制了超声频谱探伤装置，以微机作为基本工具，利用步进采样技术对缺陷回波进行采样，通过软件进行快速傅里叶变换，获得频谱信号，然后，应用非线性自动增长的自适应学习网络技术，实现了对缺陷的自动鉴别，对有尖锐边缘与平滑边缘的两类人工缺陷所作的定性分析与横孔直径的定量分析都获得了较好的结果。     

多层结构中脱粘缺陷的超声检测方法

固体火箭发动机多层装药结构中的脱粘类型、位置和尺寸决定了其对整体安全性能构成的威胁程度。该文研究了多层结构中脱粘缺陷的超声检测方法,通过对不同脱粘缺陷超声脉冲回波的特征分析与统计,实现缺陷的定性、定位和定量。首先,采集含有多类脱粘缺陷的粘接结构的超声脉冲回波信号,分析信号中主能量波包所代表的声程,提取五种声程的波峰时刻和幅值作为特征值,组建已知脱粘类型训练样本并输入至BP神经网络,实现特征值域到类别域的非线性映射,即脱粘类型分类;其次,采用阈值法确定缺陷的界面位置;最后,提出分段线性插值-相关性定量法将待检测缺陷的定量结果缩小到±2 mm以内。该文利用COMSOL有限元仿真和实验操作验证了多层粘接结构中脱粘缺陷的定性、定位和定量方法的可行性和可靠性。     

超声检测中人工神经网络对缺陷定量评价

分析了均匀各向同性介质的超声检测中,耦合因素和仪器因素造成的脉冲回波幅度的差异,提出了误差校正方法。以校正后的缺陷回波和底面反射波的峰－峰值为特征量,利用人工神经网络进行缺陷类型识别和大小评价。为模拟自然缺陷的二基本要素——光滑曲面和带棱边的平面,用有机玻璃制作了代表性的横穿孔和平底孔缺陷样品共１８个．对１８个缺陷样品的缺陷回波和底面反射波的峰－峰值测量了四次,并进行的校正．用人工神经网络对这组缺陷样品进行的处理结果表明：（１）设定的缺陷类型全部准确识别。（２）估计缺陷大小与标称孔径吻合较好。最后,对测量误差和缺陷大小估计误差进行了分析。     

超声脉冲窄带激励的水泥环第二界面探测方法*

如何评价水泥环的第二界面固井质量一直是声波测井领域的世界性难题。本文利用有限元方法研究了基于超声脉冲反射法的水泥环第二界面固井质量探测方法。数值模拟了不同声源脉冲宽度及不同声阻抗地层对第二界面超声反射回波的影响。研究结果表明采用窄带的激励信号源可有效的提高水泥环第二界面反射波回波幅度,有望解决水泥环第二界面固井质量评价问题。制作了水泥环第二界面具有胶结缺陷的实验室样品及大型固井质量刻度井,在此基础上开展了基于窄带超声脉冲激励的实验室及刻度井群实验研究。实验结果表明满足水泥环第一界固井质量良好条件,采用窄带的超声脉冲回波技术可有效评价水泥环第二界面固井质量。     

The ultrasonic echo signal analysis of welding flaw andthe selection of optimum feature subset

The ultrasonic echoes of real welding flaws are analyzed for featureextraction of signal processing. Evaluation of ultrasonic echo featureand selection of optimum feature subset are discussed. Preliminaryexperimental results show that, the method of feature selectionexpounded in this paper is relatively effective, and it could be usedfor the recognition and classification of flaws.     

A new technique for the identification of ultrasonic flaw signals using deconvolution

The identification of ultrasonic flaw signals is a difficult task in the angle beam ultrasonic testing of welded joints due to the presence of non-relevant signals from the geometric reflectors such as weld-roots and counter-bores. This paper describes a new approach to identify ultrasonic flaw signals in such a problematic situation. A similarity function is defined as the deconvolution of a target signal by a reference signal. The similarity functions for the same type of flaws/references are symmetric bandlimited impulse-like patterns with larger amplitudes while those for different types of flaws/references are asymmetrical broad patterns with relatively smaller amplitudes. Therefore, ultrasonic signals could be identified by the pattern of the similarity function. In the initial experiments, the proposed technique showed great potential for identifying ultrasonic flaw signals in the inspection of weld joints.     

Location of multiple proximate flaws using perpendicular NDT ultrasonic arrays

An improved method for ultrasonic location of multiple flaws using perpendicular ultrasonic arrays, working in near-field conditions, is presented. The method uses the pulse-echo technique and is based on the processing of the information acquired by both the elemental emitting transducers and their adjacent ones. This type of processing improves the performance of a previous method that sometimes presents limitations for the detection of multiples flaws. This new method is applied to a set of ultrasonic traces, obtained with an ultrasonic prototype specifically designed for testing this method, showing the improvement obtained in imaging results.     

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.     

Noise reduction in ultrasonic NDT using undecimated wavelet transforms

Translation-invariant wavelet processing is applied to grain noise reduction in ultrasonic non-destructive testing of materials. In particular, the undecimated wavelet transform (UWT), which is essentially a discrete wavelet transform (DWT) that avoids decimation, is used. Two different UWT processors have been specifically developed for that purpose, based on two UWT implementation schemes: the "à trous" algorithm and the cycle-spinning scheme. The performance of these two UWT processors is compared with that of a classical DWT processor, by using synthetic grain noise registers and experimental pulse-echo NDT traces. The synthetic ultrasonic traces have been generated by an own-developed frequency-domain model that includes frequency dependence in both material attenuation and scattering. The experimental ultrasonic traces have been obtained by inspecting a piece of carbon-fiber reinforced plastic composite in which we have mechanized artificial flaws. Decomposition level-dependent thresholds, which are suitable for correlated noise, are specifically determined in all cases. Soft thresholding, Daubechies db6 mother wavelet and the three well-known threshold selection rules, Universal, Minimax and SURE, are applied to the different decomposition levels. The performance of the different de-noising procedures for single echo detection has been comparatively evaluated in terms of signal-to-noise ratio enhancement.     

Fiber-optic acoustic sensor for nondestructive evaluation

An experimental study was conducted to investigate the applicability of fiber-optic acoustic sensors to detect internal flaws in polymeric materials. A polarimetric fiber-optic sensor embedded in a plexiglass model received the acoustic signals generated by an ultrasonic transducer. It is shown that proper control of the polarization and phase of the optical beam is required to obtain meaningful results from the amplitude of the fiber-optic sensor signal. The sensor has shown promising results in determining acoustical properties of plexiglass and locating internal defects. The attractive feature of this sensing scheme is that the optical fibers are not modified prior to embedding. Therefore, they preserve their mechanical properties which makes the embedding process much easier.     

Application of adaptive time-frequency decomposition in ultrasonic NDE of highly-scattering materials

In the paper, adaptive time-frequency decomposition by basis pursuit (BP) is utilized to improve ultrasonic flaw detection in highly-scattering materials as an alternative to the Wavelet Transform technique. The detection of ultrasonic pulses using the BP is described. Computer simulation was performed to verify the signal detection improvements for an ultrasonic wave embodied in white noise, and numerical results show good detection even for signal-noise ratio (SNR) of -18 dB. The improvement in detection is experimentally verified using cast steel samples with artificial flaws.     

Laser ultrasonic propagation imaging method in the frequency domain based on wavelet transformation

A wavelet-transformed ultrasonic propagation imaging method capable of ultrasonic propagation imaging in the frequency domain was developed and applied as a new structural damage or flaw visualization algorithm. Since the wavelet-transformed ultrasonic propagation imaging method has strong frequency selectivity, it can visualize the propagation of ultrasonic waves of a specific frequency (for example, to isolate ultrasonic mode of interest and a damage-related ultrasonic wave). The strong frequency selectivity of the wavelet-transformed ultrasonic propagation imaging method was demonstrated, isolating only the zeroth-order asymmetrical mode of the fundamental Lamb wave modes in an anisotropic carbon fiber-reinforced plastic plate with a thickness of 5 mm. The wavelet-transformed ultrasonic propagation imaging method can also convert a complex time domain multiple wavefield into a simple frequency domain single wavefield. This feature enables easy interpretation of the results, and facilitates the precise evaluation of the location and size of structural damage or flaws. We demonstrated this capability by detecting a disbond in a sandwich structure made of Al-alloy skins and a foam core. A disbond with a diameter of 20 mm, which is representative of a common manufacturing flaw, was successfully detected, localized, and evaluated. Since a method to determine the allowable maximum pulse repetition frequency depending on target materials and structures was found by investigating the residual wave caused from the previous laser impinging, our laser ultrasonic system can scan rapidly the target with an optimal pulse repetition rate. In addition, the proposed wavelet-transformed ultrasonic propagation imaging method can visualize damage or flaw without the need for reference data from the intact state of the structure. Hence, we propose the wavelet-transformed ultrasonic propagation imaging approach for automatic inspection of in-service engineering structures, or in-process quality inspection in manufacturing.     

Diffuse ultrasonic backscatter at normal incidence through a curved interface

Diffuse ultrasonic backscatter techniques are useful for probing heterogeneous materials to extract microstructural parameters and detect flaws which cannot be detected by conventional ultrasonic techniques. Such experiments, usually done using a modified pulse-echo technique, utilize the spatial variance of the signals as a primary measure of microstructure. Quantitative ultrasonic scattering models include components of both transducer beams as well as microstructural scattering information. Of particular interest for interpretation of many experiments is the propagation through a liquid-solid interface. Here, a recent single-scattering model is expanded to include components needed for comparison with experiments. In particular, the Wigner distribution of the displacement profile is derived to model the beam pattern of an ultrasonic transducer through a curved liquid-solid interface. A simple Gaussian beam is used to model the transducer beam pattern. This expression is then used in conjunction with an appropriate scattering operator to complete the derivation. The theory developed is then compared with experimental results for a fine-grained steel using both a planar and a cylindrical interface. These results are anticipated to impact ultrasonic nondestructive evaluation and characterization of heterogeneous media with arbitrary curvatures.