基于ACFM检测技术的表面裂纹特征评价方法研究

交变电磁场检测技术(alternating current filed measurement, ACFM)是利用电磁感应原理,通过拾取缺陷处的磁场畸变信号,分析判断缺陷信息的一种电磁无损检测方法。ACFM检测技术在探头扫描方向与裂纹走向一致的情况下检测效果最佳。但在实际检测中,检测的金属表面通常都有油漆层保护,并不知道裂纹走向。因此,该文重点研究裂纹长度、裂纹深度、探头扫描方式、探头起落波动、探头偏离裂纹的水平距离及探头提离高度对ACFM检测信号的影响。实验结果表明:不同的扫描模式,检测信号具有不同的特征,可以从不同的模式特征对裂纹特征进行综合评价。该研究成果可为表面裂纹的检测与评价提供参考。     

ACFM电磁无损检测技术在汽车罐车定期检验中的应用

汽车罐车定期检验是移动式压力容器安全管理工作中的重要部分,对提高汽车罐车使用寿命和安全性具有重要作用。现行方法检验结果置信度较低且漏检率较高,无法达到预期检验效果。本文针对现行方法存在的不足和缺陷,提出在汽车罐车定期检验中应用ACFM电磁无损检测技术。通过对汽车罐车ACFM电磁进行无损扫描,采集罐车表面ACFM电磁信号,采用中值滤波技术对ACFM电磁信号进行滤波处理。根据麦克斯韦理论分析ACFM电磁信号,识别、检测汽车罐车表面缺陷长度与深度。试验证明,本文方法检验结果置信度超过96%,漏检率低于1%,ACFM电磁无损检测技术在汽车罐车定期检验方面具有良好的应用前景。     

基于ACFM焊缝体积型和簇群型缺陷检测仿真分析

交流电磁场检测技术(ACFM)对焊缝体积型缺陷和复杂缺陷检测识别需要完善和改进.本文利用ANSYS Maxwell建立了的电磁场仿真模型,采用有限元分析方法研究了长方体缺陷、椭圆形缺陷和圆柱体缺陷这3种不同的体积型缺陷形状对ACFM信号特征的影响,以及不同缺陷簇群的密集程度对检测结果的影响.通过对提取的Bx,Bz特征信号的影响进行分析与探讨,获得了缺陷形状及分布数量对信号曲线的影响规律,可以识别缺陷形状.缺陷簇群的密集分布会影响ACFM检测的准确度,识别不到缺陷的定量尺寸.     

具有抑制“提离”效应的旋转座标法

提离效应是在涡流检测中普遍存在的一种干扰因素。它对检测结果有很大的影响。为此，本文提出了旋转座标抑制提离效应的方法，并把单片机技术引入到相应的电路中去。实验结果表明．该法对提离效应的抑制是有效的。同时，智能化的提离抑制电路，大大降低了检测时人为因素的影响，提高了检测的准确率和重复精度，具有较大的实用价值。     

裂纹检测中的相位信息研究

裂纹涡流无损检测时,可以通过测量检测信号与激励信号的相位差来进行判断是否有裂纹.为实现牵引电机主极裂纹检测,对黄铜材料制成的牵引电机主极,在检测信号频率的变化、提离、边缘以及正常和裂纹等情况下,进行检测信号与激励信号的相位差变化测试实验、分析,得出了上述情况下的相位差变化范围,为牵引电机主极裂纹的动态检测提供了依据.     

涡流检测中提离干扰的抑制

提离造成的信号畸变是涡流检测信号的主要干扰之一,严重影响着对检测结果的判断。本文从检测方法、信号处理、探头设计等方面分析了抑制提离的关键所在,并从工程实现的角度对所提出各种方法进行了分析比较。     

高压电缆铝护套焊缝缺陷ACFM检测方法及检测系统的研究

针对在氩弧焊型高压电缆铝护套焊接过程中易出现表面漏焊、埋藏未焊透和焊穿等缺陷的问题,提出了焊缝缺陷的交流电磁场检测（alternating current field measurement, ACFM）方法。首先,利用COMSOL多物理场仿真软件建立高压电缆铝护套焊缝缺陷ACFM模型,研究U形磁芯上的励磁线圈在不同类型铝护套焊缝缺陷区域产生的感应电流的密度分布特点和和磁场信号特征;其次,设计了可获取缺陷长度和深度信息的正交式接收线圈,制作了带有缺陷的电缆铝护套焊缝试件及ACFM实验平台;最后,进行了不同类型铝护套焊缝缺陷的检测及结果分析。实验结果表明,ACFM方法能够有效用于3 mm厚的高压电缆铝护套焊缝表面漏焊和焊穿缺陷的检测,并且能够有效识别埋深为2 mm,长、宽、深分别为10,0.3,1 mm的埋藏未焊透缺陷。研究结果为高压电缆铝护套焊缝缺陷的识别和焊缝质量的评价提供了重要参考。     

基于磁导率检测技术的传感器设计研究

磁导率检测技术是一种依据磁导率的变化检测铁磁试件应力集中状态和疲劳损伤程度的无损检测方法,可实现对构件残余寿命的有效评价。依据磁导率检测原理,针对棒状待检构件设计研制一高灵敏检测传感器,考察检测线圈绕线直径、绕线线圈匝数、激励电压对最佳检测频率和检测分辨率的影响。研究发现,最佳激励频率随线径的增加而减少,信号分辨率随线径的增加而增加;绕线匝数与激励最佳频率无关,但与信号分辨率有关;激励电压对最佳频率的范围无影响,但对信号分辨力有影响。该研究结论对研制高灵敏度的磁导率检测仪具有重要意义。     

基于U型探头的ACFM和AC—MFL法的机理辨析

交变磁场测量技术(ACFM)和交流漏磁检测技术(AC-MFL)在工程实践中有较多应用,都采用了U型磁轭式探头,两种检测方法在探头和信号处理上有诸多相同点,但检测机理有较大差异,造成在研究和应用上的混淆.针对此问题,基于有限元仿真的方法,分析了两种方法在信号检测特征的差异,提出了一种可有效区分的方法,为剖析其检测原理提供理论依据.采用AC-MFL法的检测参数,对U型磁轭式探头进行了仿真,利用上述方法进行评定,结果表明:该探头的检测结果表现为ACFM法的信号特征,而非AC-MFL法.     

牵引电机主极裂纹涡流检测及其信号的小波分析研究

本文针对电力机车牵引电机主极表面包有绝缘层、常规裂纹检测方法难以适用的问题,阐述了透射式涡流检测方法检测电机主极裂纹的原理以及克服提离效应、边缘效应的方法.论文还阐述了运用小波分析方法把检测信号分解,将其频带能量作为不同状态时信号的特征量,对有裂纹、无裂纹、边缘、提离等不同状态的检测信号进行识别.     

基于交变磁场测量技术的裂纹缺陷定量检测仿真分析与实验研究

裂纹缺陷的定量评估是无损检测的一项重要研究内容,本文采用交变磁场测量技术对平板裂纹缺陷进行了检测.在分析交变磁场测量技术原理的基础上,首先在大型电磁仿真软件AN SY S中建立了交变磁场测量模型,包括三组不同参数的裂纹缺陷和检测线圈相互垂直的两种传感器模型,然后用这两种传感器分别对不同缺陷进行了检测,研究了裂纹长度、深度和宽度变化对水平方向和垂直方向检测电压的影响规律,提取出了对长度和深度进行定量的特征量.最后,采用实验的方法对仿真结果进行了验证,实验结果的规律与仿真结论相一致,证明了仿真结果的正确性.从而为裂纹缺陷在实际中的检测和定量提供了借鉴.     

Analytical solutions to eddy current in carbon fiber-reinforced composites induced by line current

This paper presents analytical solutions to eddy current distribution in carbon fiber-reinforced plastics induced by line current. Derived solutions show that eddy current distribution in unidirectional carbon fiber composite is dependent only on electrical conductivity in the drive current direction when the drive current is directed in the fiber direction or the transverse direction. Moreover, according to the derived analytical solution, skin depth of eddy current depends not only on frequency of drive current but on lift-off and width of the drive current unlike general expression of skin depth. Finally, we discuss improvement of sensitivity of eddy current conductivity sensor using analytical solutions. It is found that analytical solutions can clearly divide effects of magnetic field from drive current and that from eddy current. Sensitivity of eddy current sensor can be improved by 10 times placing pickup coil at region where the effect of magnetic field from eddy current is large.     

Analytic Critical Flow Method (ACFM): A Reliability Allocation Method Based on Analytic Hierarchy Process

RAMS is an acronym for reliability, availability, maintainability and safety. These four properties concern the application of important methodologies to design and manage complex systems. In the present research, starting from the analysis of several literature reliability allocation techniques, a reliability allocation method has been implemented called analytic critical flow method (ACFM). Critical flow method is a reliability allocation method for series-parallel configurations, based on failure analysis of each unit of the system. The new approach is based on critical flow method, whose results are matched with the analytic hierarchy process multicriteria method. The result is a dynamic model that combines the advantages of the allocation method and the multicriteria approach. The need to develop the ACFM is the outcome of a careful analysis of the current military and commercial approaches. In particular, no literature method takes into account to assign a different level of significance (weight) to the different units of the system, simultaneously to the considered factors. The proposed approach has been applied and compared with other traditional methods on an aerospace prototype (series-parallel configuration), where the reliability allocation process is rigorous. The results demonstrate the effectiveness of the new approach and its ability to overcome the criticalities highlighted in literature.     

Evaluation of Surface Cracks Using Magnetic Flux Leakage Testing

The magnetic field distribution characteristics of surface cracks with various widths are discussed based on finite element (FEM) results.The crack depth was 0.20 mm ,the width range was from 0.02 to 1.00mm .The results showed that crack width and lift-off( the distance between surface and sensor )will influence singals.Discussed in this paper is the influence of various lift-off parameters on the peak to peak values of the normal component in magnetic flux leakage testing .The effects can be applied to evaluate surface breaking cracks of different widths and depths.An idea is presented to smooth narrow, sharp crack tips using alternating current (AC) field magnetization.     

Simulation and Analysis of 3-D Magnetic Flux Leakage

In this paper, we present simulation results and analysis of 3-D magnetic flux leakage (MFL) signals due to the occurrence of a surface-breaking defect in a ferromagnetic specimen. The simulations and analysis are based on a magnetic dipole-based analytical model, presented in a previous paper. We exploit the tractability of the model and its amenability to simulation to analyze properties of the model as well as of the MFL fields it predicts, such as scale-invariance, effect of lift-off and defect shape, the utility of the tangential MFL component, and the sensitivity of MFL fields to parameters. The simulations and analysis show that the tangential MFL component is indeed a potentially critical part of MFL testing. It is also shown that the MFL field of a defect varies drastically with lift-off. We also exploit the model to develop a lift-off compensation technique which enables the prediction of the size of the defect for a range of lift-off values.      

Multiphysics Modeling and Analysis of the Photoinductive Imaging Effect for Crack Detection

Numerical multiphysics modeling of the photoinductive imaging (PI) effect was performed with a 2-D transient to characterize corner cracks at the edge of a specimen with a bolt hole. We present how the finite-element method (FEM) can be utilized to model the PI effect and observe the influence of critical factors on the coil probe impedance for a rectangular crack in the Ti-6Al-4V specimen. As anticipated, the proposed model can show that the PI method has a higher spatial resolution in the defect in 2-D models compared to the conventional eddy current testing method. The FEM simulation results for 0.25-, 0.50-, and 0.75-mm rectangular notches are shown and discussed. The effects of coil current frequency, laser-point temperature, and lift-off distance on the PI signal are also examined and analyzed. We demonstrate that the PI effect is a novel sensing method for characterizing the geometric shape of cracks and that the enhanced output signals of the coil probe can also be obtained given an appropriate quantity of factors.      

Neural network approach for determination of fatigue crack depth profile in a metal, using alternating current field measurement data

A neural-network-based technique is described to determine the depth profile of a fatigue crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The main feature of this technique is that it requires only the measurements along the crack opening. The network uses the multilayer perceptron structure for which the training database is established by systematically producing semi-elliptical multi-hump cracks with narrow openings and random lengths and depth profiles. A fast pseudo-analytic ACFM probe output simulator is also used to produce network input data around each crack for a specified inducer. To demonstrate the accuracy of the proposed inversion technique, the simulated results of cracks with both common and complex geometries are studied. The comparison of the actual and reconstructed depth profiles substantiates the technique introduced here. To further validate the technique, the experimental results associated with several fatigue cracks of complex geometries are presented.     

反射式光纤电流传感器频率特性计算和测试

建立反射式光纤电流互感器R-FOCT(Reflecting Fiber-optic Current Transducer)的教学模型,得到描写模型的差分方程,推导出与之对应的传递函数,仿真系统的频率特性.根据电流传感器的工作特点,提出应用锁定放大器SR830测试R-FOCT频率特性的方法.比较测试数据和仿真结果,结果表明,系统的带宽计算和测量方法是可信的,得到了系统带宽大于6kHz的结论.     

Analysis of an open-ended coaxial probe with lift-off fornondestructive testing

The open-ended coaxial probe with lift-off is studied using a full-wave analysis, and an uncertainty analysis is presented. The field equations for the following terminations are worked out: (1) the sample extends to ∞ in the positive axial direction, (2) the sample is backed by a well-characterized material, and (3) the sample is backed by a short-circuit termination. The equations are valid for both dielectric and magnetic materials. The model allows the study of the open-ended coaxial probe as a nondestructive testing tool. The analysis allows a study of the effects of air gaps on probe measurements. The reflection coefficient and phase are studied as a function of lift-off, coaxial line size, permittivity, permeability, and frequency. Numerical results indicate that the probe is very sensitive to lift-off. For medium to high permittivity values and electrically small probes, gaps on the order of fractions of a millimeter strongly influence the reflection coefficient. In order for the field to penetrate through the air gap, larger size coaxial line or higher frequencies need to be used. A comparison of the theory to experiment is presented. The results are in close agreement. A differential uncertainty analysis is also included