焊接微缺陷磁光成像检测有限元分析

目的 研究铁磁材料焊接微缺陷的磁光成像规律.方法 运用漏磁检测原理和法拉第磁致旋光效应,建立微缺陷三维有限元模型,分析微缺陷磁光成像过程与磁场之间的关联,研究不同提离值、励磁电流、缺陷宽度、缺陷深度下的磁光成像,以及探索这些因素对磁光图像特征的影响.在此基础上,对最小宽度为0.05 mm的微缺陷进行磁光成像检测实验,并...     

贝叶斯算法BP神经网络缺陷量化研究

为克服传统BP神经网络中网络训练速度慢、量化精度低、数据过度拟合、容易陷入局部极小点等缺点,该文将贝叶斯算法引入BP神经网络用于基于漏磁检测的缺陷量化,有效地控制网络模型的复杂度,利用不同尺寸的缺陷特征量训练网络,从而实现对缺陷长度、宽度、深度的量化,节约网络的训练时间,提高量化精度。     

油气输运管道缺陷漏磁检测量化技术研究

通过实测得到油气输运管道缺陷漏磁(MFL)信号，分析了缺陷几何尺寸与信号特征量之间关系。采用特征提取和模式识别技术对缺陷进行量化分析。对缺陷的长、宽、深三个指标分别应用不同的特征量和相应的非线性方法进行定量识别。试验结果表明，缺陷长度、宽度和深度的预测准确度分别达到了100％、89％和77．8％。这里通过使用长宽比特征量描述方法，有效地提高了深度的估计精度，很好地解决了管道缺陷的量化识别问题。     

Characterization of Surface-Breaking Cracks Using One Tangential Component of Magnetic Leakage Field Measurements

We propose a procedure for full characterization of rectangular surface-breaking cracks based on measurements of only one tangential component of the magnetic field with the magnetic flux leakage (MFL) technique. The parameters of interest include orientation, length, and depth of the cracks. We assume that the length and the depth of the investigated cracks are much larger than the crack width, so that the variation of the MFL response with respect to the width is negligible. Our procedure employs fast direct methods that provide reliable estimation of the crack parameters in three separate consecutive steps. We propose denoising and correction techniques as well. We confirmed the accuracy of the methods by simulations based on the finite-element method (FEM) as well as by experimental MFL observations. A procedure is proposed for full characterization of rectangular surface breaking cracks based on measurements of only one tangential component of the magnetic field with the magnetic flux leakage (MFL) technique. The parameters of interest include orientation, length and depth of the cracks. We assume that the length and the depth of the investigated cracks are much larger than the crack width such that the variation of the MFL response with respect to the width is negligible. The proposed procedure employs fast direct methods which provide reliable estimation of the crack parameters in three separate consecutive steps. De-noising and correction techniques are proposed as well. The accuracy of the proposed estimation methods is examined via simulations based on the finite element method (FEM) as well as experimental MFL data.     

New findings in the field of non-destructive magnetic leakage field inspection

Non-destructive inspection of defects using the magnetic leakage flux method presents several experimental and theoritical difficulties. It is shown that the leakage field strength around a defect is linked linearly with the field strength inside it. Thus a prerequisite to a better understanding of the phenomenon is the determination of the magnetic field strength inside the defect as a function of defect geometry, magnetic properties of the material and applied field strength. Equations are presented which enable computation of this quantity. Discrepancies between measurement and calculations using the method of finite elements emphasize the need for accurate knowledge of the magnetic field strength inside the defect.     

Effect of stress concentration on magnetic flux leakage signals from blind-hole defects in stressed pipeline steel

Stress-dependent magnetic flux leakage (MFL) signals of the normal surface component (radial) MFL signal from blind-hole defects in pipeline steel were investigated. Three different stress rigs with uniaxial stress and field configurations were used. A double-peak feature in the MFL signal was defined quantitatively by a saddle amplitude, which was taken as the difference between the average of the double peaks and the corresponding saddle point. Results indicated that the saddle amplitude increased linearly with increasing tensile surface stress and decreased, or did not exist, for increasing compressive surface stress. Finite-element calculations indicated that stress concentration also increased with increasing defect depth. The measurements and analysis demonstrate that the stress-dependent saddle amplitude behavior in the radial MFL signal is associated with surface-stress concentrations near the blind-hole defects.     

铁磁性材料的力磁效应机理探讨与实验研究

基于sablik-jiles-atherton模型和材料的自发磁化规律,在理论上探讨了更为简洁的力磁关系模型,分析了铁磁性材料在外应力和地磁场作用下的漏磁测量机理,获得了试件表面漏磁场信号切向最大、法向过零的特点.结合试件的弹塑性分析,得到在出现应力集中的缺陷位置漏磁信号变化剧烈,即梯度突然变大.利用Q235A钢材为实例,测量了试件在不同应力状态下的漏磁信号,得出在弹性范围内漏磁信号随拉力增大而增大,实验结果与理论有较好的符合,该工作为深入探讨铁磁性材料的力磁关系奠定了一定的理论基础,为检测铁磁试件的潜在缺陷提供了实验依据.     

A Novel Non-destructive Testing Method by Measuring the Change Rate of Magnetic Flux Leakage

The application of magnetic sensors in the traditional magnetic flux leakage (MFL) technique has a significant influence on the detection results. The sensor is typically used to directly measure the amplitude of the magnetic leakage flux intensity as the detection signal. In view of noise effects on the detection result and the subsequent misinterpretation of defect signals, a new non-destructive testing method is proposed. The proposed method intends to measure the magnetic flux change rate using two sensors. A mathematical model is first established to present the principle of the change rate measurement. Based on the magnetic dipole theory, it is inferred that the new method is applicable and sensitive to the detection and location of defects. Moreover, this method is advantageous as it inhibits the interference of MFL noises such as the distension noise, background noise, and vibration noise. The model predictions are then verified by a series of simulations. Finally, an experimental platform is set up to practically detect the defect of a steel plate, and the results agree with the demonstrations and simulations.     

油气管道缺陷漏磁图像的小波压缩技术研究

针对漏磁信号的特点,提出了基于小波分析的缺陷漏磁图像压缩方案。该方案通过调整正交函数零极点位置来设计小波基函数,利用该函数对图像作小波变换,对获得的各级小波系数用更新的JPEG图像压缩表阈值量化,再用算术编码方案处理量化结果,从而得到压缩的图像。试验结果表明,当压缩比小于30%时,图像压缩引起的失真不会对缺陷分析产生影响。     

Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects

Simulated defects of different shapes and sizes were created in a section of API X70 steel line pipe and were investigated using a residual magnetic flux leakage (MFL) technique. The MFL patterns reflected the actual shape and size of the defects, although there was a slight shift in their position. The defect features were apparent even at high stresses of 220 MPa when the samples were magnetized at those particular stresses. However, unlike the active flux technique, the residual MFL needs a sensitive flux detector to detect the comparatively weaker flux signals.     

Determination of flaws located at different depth levels in the cross-section of steel rope

Signals received from the magnetic testing of steel rope vary depending on the location of the defect in the rope's cross-section. Measurements of the radial and tangential components of the leakage flux performed with a single Hall-effect probe have shown significant variations of the distribution of these components around the rope. The Hall-effect sensor, built in such a way that it measures the average value of the tangential component, indicates stronger and narrower signals from defects positioned closer to the rope's surface. This effect can be used to determine the depth of the flaw's location, which is especially useful in the testing of locked-coil ropes.     

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.     

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。     

A Space Mapping Methodology for Defect Characterization From Magnetic Flux Leakage Measurements

We present an inversion methodology for defect characterization using the data from magnetic flux leakage (MFL) measurements. We use a single tangential component of the leakage field as the MFL response. The inversion procedure employs the space mapping methodology. Space mapping is an efficient technique that shifts the optimization burden from a computationally expensive accurate (fine) model to a less accurate (coarse) but fast model. Here the fine model is a finite-element method (FEM) simulation, while the coarse model is based on analytical formulas. We achieve good estimation of the defect parameters using just a few FEM simulations, which leads to substantial savings in computational cost as compared to other optimization approaches. We examine the efficiency of the proposed inversion technique in estimating the shape parameters of rectangular and cylindrical defects in steel pipes. Our results show good agreement between the actual and estimated defect parameters.      

Improved FEM model for defect-shape construction from MFL signal by using genetic algorithm

In-line inspection of ferromagnetic gas or oil pipe lines having pipe wall defects is typically accomplished using magnetic flux leakage (MFL) technique. An efficient modelling and computational scheme for forward model, during the process of solving inverse problems in magnetostatic non-destructive evaluation using finite-element method is presented. The shape, size and place of defect are determined considering the nonlinearity of the pipe material using genetic algorithm as the optimisation technique. It is shown that the reduced model improves the FE computations significantly. The methodology for construction of defect shapes from particular MFL signals has been explained     

Modeling and Experimental Studies on 3D-Magnetic Flux Leakage Testing for Enhanced Flaw Detection in Carbon Steel Plates

ABSTRACT

For enhanced detection of flaws in engineering components using magnetic flux leakage (MFL) technique, measurement of the leakage magnetic field components along the three perpendicular directions is beneficial. This article presents the three dimensional-magnetic flux leakage (3D-MFL) modeling and experimental studies carried out on carbon steel plates. Magnetic dipole model has been used for the prediction of MFL signals and images. Sensitivity of the MFL signals peak amplitudes of tangential (H_X), circumferential (H_Y), and normal (H_Z) components with respect to flaw length, width, depth and lift-off have been studied. A 3D-GMR sensor has been used for simultaneous measurement of all the three components of leakage magnetic fields from surface flaws in 12 mm thick carbon steel plates. The experimental MFL images have been compared with the model predicted MFL images. The sensor has shown the capability to detect and image 0.9 mm deep surface flaws with a signal to noise ratio of 8 dB. Principal component analysis (PCA)-based image fusion has been performed for fusion of the 3D-MFL images to obtain a geometrical profile of the flaws. Study reveals that 3D-GMR enhances the capability for detection of flaws having irregular geometries.     

Flux Jump Behaviors and Mechanism of MgB<Subscript>2</Subscript> Synthesized by the Non-special Atmosphere Synthesis

MgB₂ samples were prepared from a stoichiometry mixture of Mg and B inside stainless steel tubes. The transition temperature of the specimens was 37.5 K with a sharp transition width within 1 K. From the magnetic hysteresis measurements, flux jump was observed up to 15 K. The flux jump is believed to begin at the fluxes pinned at the defects. An over-moving flux situation formed at the places where there were moving fluxes that had been pinned at the defect and steady state ones. The flux jumps depend not on the amount of impurities and second phases, but on their distribution, pinning strength and heat capacity.     

A transducer made up of fluxgate sensors for testing wire ropedefects

Based on analyzing the magnetic leakage feature of wire rope defects, the longitudinal resolved vector B_ot of the defect leakage field is taken for the object to be tested. In order to detect the weak magnetic leakage field, a fluxgate sensor of single core and single winding is developed. Using the fluxgate, a new kind of transducer for testing defects of wire rope has been designed. The structure and interface circuit of the transducer are studied. The experimental results of detecting the LF and LMA defects of the wire rope are presented     

CIRCUMFERENTIAL SHEAR HORIZONTAL WAVE AXIAL-CRACK SIZING IN PIPES

Circumferential shear horizontal (SH) waves are used for the inspection and sizing of axial cracks in pipelines. Experiments on two sample pipes having notches with different depths and lengths were carried out utilizing magnetostrictive sensor (MsS) technology for generating circumferential shear horizontal guided waves. A simplified two-dimensional model for crack sizing in pipes was studied through wave-reflection amplitude coefficients. The wave-reflection amplitudes are affected by both defect depths and lengths. To estimate the defect depth, which is critical, length compensation was taken for defects shorter than the beam width. An axial scan was carried out for the defect length estimations and the length compensation. Based on this axial scan, an approximate two-dimensional theory has been developed that has improved the defect depth estimation greatly. Two-dimensional boundary element modeling analysis and normal mode expansion technology are used to study defect sizing theoretically in a pipe-like structure by calculating reflection coefficients. The theoretical results agree with the experiments quite favorably.     

Core loss in amorphous cut cores with air gaps

The effects of air gaps on the high-frequency core loss in cut cores made of amorphous ribbons are discussed and methods to reduce the core loss in them are proposed. It has been found that the high-frequency core loss in amorphous cut cores with air gaps increases strikingly with increasing air-gap length. The increase in the core loss due to air gaps is remarkable in high-frequency and low-induction range. Measurement of leakage magnetic flux as well as analysis of the flux distribution and the eddy current in an amorphous core with the finite-element method suggests that the increase in the core loss due to air gaps observed in the amorphous cores can be attributed to the in-plane eddy current loss generated by the leakage flux perpendicular to ribbon surfaces. Suppression of the leakage flux normal to ribbon surfaces by using semicircular cores and reduction in width of ribbons of which cores are made decreases the high-frequency core loss in amorphous cut cores with air gaps