储罐角焊缝裂纹漏磁检测有限元分析

针对石油石化行业中的储罐角焊缝不易检测的特点,使用漏磁检测技术对储罐角焊缝裂纹进行检测。应用有限元软件建立了角焊缝裂纹缺陷的分析模型,得到了角焊缝裂纹缺陷漏磁场的空间分布,提取出了角焊缝裂纹缺陷的漏磁信号,分析了角焊缝裂纹缺陷影响漏磁信号的因素;在此基础上对储罐角焊缝裂纹缺陷进行了漏磁检测试验,对得到的漏磁信号进行分析,分析结果与有限元分析提取的漏磁信号结果相符,证明了所建立的检测系统的可行性与有效性。     

基于三维数值模拟的焊缝缺陷漏磁场分析

漏磁检测技术对钢板对接焊缝检测鲜有应用,根据铁磁性焊缝的特点,针对焊接结构中较敏感的裂纹缺陷,探讨应用漏磁法检测钢板对接焊缝裂纹缺陷。采用有限元数值模拟方法(FEM),建立了焊缝三维FEM模型;对比分析焊缝不同区域存在裂纹时漏磁场分布规律;建立不同焊缝余高包含相同尺寸裂纹的三维FEM模型,分析由于焊缝余高的存在对裂纹检出率的影响;研究在焊缝余高3 mm结构条件下,漏磁场磁感应强度分量峰值随裂纹深度的变化规律,得到漏磁场相关对比分析曲线。仿真结果表明:漏磁法适用于钢板对接焊缝缺陷的检测研究;焊缝余高越小,可获得更高的缺陷检出率;漏磁场磁感应强度分量峰值均随裂纹深度的增加呈递增趋势。     

NDE of a 3-D surface crack using magnetic field induced by DC current flow

A method is proposed for nondestructive evaluation of a 3-D surface crack by means of measurement of a magnetic field induced in the air by DC current flow in a material. A semielliptical crack is considered. The crack length is assumed to be known in advance and the crack depth is to be found. The current is applied between two close points on both sides of the crack and the change in magnetic: flux density in the air due to the crack is measured in non-contact with the surface of the material. An approximate equation relating the magnetic flux density to the depth and length of the crack is derived numerically and some experimental results show that the evaluated crack depth is in good agreement with the actual one.     

焊接裂纹磁场模拟及磁光成像检测

研究交变磁场下的焊缝裂纹磁化规律,针对碳钢（Q235）焊缝裂纹,进行交变电磁场励磁下焊缝磁光成像无损检测的仿真和试验. 基于交变励磁下焊缝励磁规律的仿真模型,研究裂纹间隙分别为0.03,0.05,0.07和0.1 mm的焊缝表面磁通密度（磁感应强度）模曲线分布差异. 钢板对接间隙分别限定为0.03,0.05,0.07和0.1 mm后,通过激光焊接钢板两端来模拟仿真试验中的裂纹. 进行磁光成像无损检测试验,并将试验结果与仿真结果进行对比. 结果表明,在仿真试验与工艺试验中,焊接裂纹感应磁场的变化规律基本相同,磁感应强度不会因采样起始点不同出现较大的差异.     

Composite magnetic flux leakage detection method for pipelines using alternating magnetic field excitation

Pipelines are an important transportation medium for petroleum and chemical products, but defects in the pipelines can present hidden dangers and affect the safe operation of the pipeline. The traditional pipeline magnetic flux leakage (MFL) scanning technique generally adopts the axial magnetization mode, which has increased the difficulty in detection and the possibility of missed detection of axial cracks. In this paper, a new composite MFL method using alternating magnetic field excitation is proposed for the detection of cracks in pipelines. The alternating magnetic field is first superimposed on the MFL magnetization field, which will form a parallel eddy current field perpendicular to the magnetization direction in the pipeline wall. The defects in the pipeline not only cause the flux leakage of the magnetization field, but also lead to the disturbance of the circumferential eddy current field. The disturbance signals can be picked up through a secondary induced magnetic field. Because the magnetic field and the eddy current field are orthogonal, the presented method can implement synchronous detection in two orthogonal directions to avoid missed detection caused by the crack orientation. A series of physical experiments are carried out in this paper. The results show that two orthogonal detection signals can be separated by a simple low pass filter. Therefore, with only one scan, the new detector can obtain the defect characteristics in the axial and circumferential directions to overcome the blind spot problem seen in traditional MFL detectors.     

裂纹漏磁场的数值模拟

漏磁检测技术广泛应用于储罐底板扫查、管道内壁缺陷检测中。文章以裂纹漏磁场为研究对象,以麦克斯韦方程组为理论基础,以数值模拟为手段,建立裂纹漏磁场三维静态数值模拟模型,用数值模拟和试验方法研究裂纹深度、宽度、裂纹倾斜角度以及裂纹间距等参数对裂纹漏磁场的影响,得到裂纹参数与裂纹漏磁场幅值之间的关系。结果表明：裂纹倾斜角度对裂纹漏磁场幅值影响显著,因此在工程实际检测中,要从不同方向进行漏磁扫描,以防止漏检;当两条裂纹间距〈5mm时,裂纹漏磁场将产生叠加。数值模拟结果与试验数据较为一致,表明所用数值方法的有效性。文章所得结论对裂纹漏磁检测工程实践有重要的拳者意义。     

Magnetoelastic interactions in a cracked ferromagnetic body

The stress–strain state associated with a moving crack in a ferromagnetic material is investigated. The model considers a soft magnetic ferroelastic body and incorporates a realistic (nonlinear) susceptibility. The moving crack propagates in a direction perpendicular to the magnetic field. A closed-form solution yields the magnetoelastic stresses and a stress intensity factor. An applied magnetic field makes the stress intensity factor depend upon the crack velocity. The nonlinear susceptibility produces a completely different magnetoelastic stress state than a constant susceptibility, and the stress intensity is highly sensitive to material properties. The stresses around the crack are largely insensitive to the external magnetic field and crack speed except at critical combinations for which the stresses are singular. Some combinations of magnetic field and crack velocity cause the stress components ahead of the crack tip to change sign.     

On the suitability of induction coils for crack detection and sizing in metals by the surface magnetic field measurement technique

The surface magnetic field measurement (SMFM) technique has proved to be an accurate means for crack detection and sizing cracks in ferrous metals. The technique involves the use of two U-shaped current-carrying wires of sufficiently high frequency while measuring the discontinuity in the resultant magnetic field at the crack edge with an appropriate magnetic field sensor. In this work, we describe a mathematical algorithm to obtain the crack signal from the output of an induction coil used in a SMFM probe. We also discuss the measurement errors due to the coil size and shape. To reduce the measurement errors, we present an algorithm in which the crack signal is recovered by appropriate deconvolution of the coil output signal and its spatial transfer function. The algorithm is then used to recover crack signals for various coil shapes and sizes. The study of the results demonstrates the effectiveness of the algorithm in the case of large coils.     

Thin-skin analysis technique for interaction of arbitrary-shape inducer field with long cracks in ferromagnetic metals

In using the AC field measurement (ACFM) technique for non-destructive evaluation (NDE) of metals, a current-carrying wire structure is used to induce eddy current within a thin layer of the metal and a magnetic field sensor to measure the field perturbations in the vicinity of the metal. The sensitivity of ACFM crack detection and sizing relies on an appropriate design of the wire structure geometry together with a dully placement of the sensor. This paper presents an analytical modeling technique for evaluating the electromagnetic field interaction of an ACFM probe with a long uniform crack in a ferromagnetic metallic slab. The probe in the proposed model can have an arbitrary-shape wire inducer with no restrictions on its relative sensor position. The technique is accurate and very efficient computationally. It first uses the two-dimensional Fourier transform to obtain the field distribution at the metal surface. The Laplacian field distribution above the metal is then determined by satisfying the so-obtained boundary condition at air–metal interface. To demonstrate the accuracy of the model, we consider the special case of a rhombic wire inducer. The comparison of our results with those obtained using the conventional algorithm in the literature validates the accuracy of the model introduced in this paper. To show the generality of the model, we also present theoretical and experimental results associated with a solenoid inducer with a three-dimensional geometry for which no analytical solution is available in the literature. The theoretical prediction of crack signal supported by experimental results is used to develop a model-based method for inverting crack signal into crack depth.     

基于漏磁检测的螺栓孔裂纹定量检测方法

建立了飞机大梁螺栓孔内裂纹漏磁场模型,并通过现代频谱分析,导出了漏磁信号与裂纹的几何尺寸间的定量关系公式,实现了螺栓孔理解纹的定量检测。     

基于三维霍尔传感器的钢轨缺陷漏磁检测系统

为检测钢轨顶面的剥离、裂纹等缺陷,设计基于三维霍尔传感器的钢轨缺陷漏磁检测系统.采用钕铁硼环形永磁体对钢轨检测区域进行垂直于钢轨方向的励磁,并将检测车的车轮轮轴同时作为励磁回路的磁扼,由ARM单片机采集三维霍尔传感器检测的缺陷漏磁场数据.在钢轨顶面人工制作一系列横向矩形凹坑缺陷样本,检测结果表明:钢轨缺陷三维漏磁检测系...     

镍铜合金棒材裂纹的弱磁检测

针对现有无损检测技术难以满足镍铜合金棒材裂纹检测需求的现状,提出一种地磁场环境下的弱磁无损检测方法。首先理论分析镍铜合金棒材的弱磁检测原理和缺陷处磁异常分布特征,其次对含自然缺陷的镍铜棒材进行弱磁检测。采用包络分析法对信号进行处理,并通过金相显微镜、扫描电镜分析,验证弱磁检测镍铜合金棒材的可行性。结果表明:通过对棒材表面磁异常分布特征分析,并结合磁梯度信号的阈值处理和包络分析法处理的结果,实现了镍铜合金棒材内部裂纹缺陷的有效检出,并且裂纹检测精度达到了微米级。     

漏磁二维检测的试验研究

传统的漏磁检测是经磁化后测量试样表面漏磁场的垂直分量再对拾取的信号量进行分析研究的方法,然而这种仅通过单一垂直分量特性来判定缺陷的方法易出现漏检、误判。针对该方法的不足,本研究采用了一种在测量漏磁场垂直分量的同时又提取并分析水平分量特性的二维检测方法,考虑到水平分量易受干扰信号影响,运用了矢量合成法提取水平分量,通过联合利用漏磁场的垂直分量存在过零点和水平分量具有最大值特性的方法来判断缺陷,并选取多种标准与自然试样进行试验研究。试验结果表明：采用漏磁二维检测可有效实施铁磁构件的缺陷检测,提高检测可靠性,可望有很好的实际工程应用前景。     

A simulation of magneto-optical eddy current imaging

A magnetic or an electrical field applied to a metallic specimen will be distributed according to the properties of the crack in the specimen. Researchers have used the finite element method (FEM) to simulate the distribution of an alternating magnetic field around a crack on a paramagnetic material. However, the simulation using this method becomes complex and time-consuming depending on the degree of accuracy desired. In this paper, we suggest a new, easier method, using dipole model (DM). The simulated results from this new method are verified by comparing them to the experimental results obtained by using Magneto-Optical/Eddy Current Imager system.     

漏磁检测技术在圆钢裂纹检测中的应用

针对表面裂纹缺陷占圆钢质量异议的比例超过了80%,裂纹缺陷成为制约特钢产品质量的关键问题。通过漏磁检测可实现对圆钢的100%检测,最大程度地消除了试样圆钢与被检圆钢的检测结果差异,极大地降低了产品的成本。由于漏磁检测过程可监控、检测结果可统计、产品质量可追溯,因此对及时发现产品缺陷,快速改善上游工序生产状态,阻止质量事故扩大化起到了至关重要的作用。     

小径管周向裂纹脉冲涡流检测仿真和试验研究

针对小径管周向裂纹缺陷,通过有限元仿真及试验,研究了利用磁导体环形激励脉冲涡流检测技术检测小径管周向裂纹缺陷的问题。仿真给出了管道在有缺陷和无缺陷状态下磁场分布、涡流分布以及接收线圈的电压值。从仿真结果可以观察出,周向裂纹端头处的磁场分布以及涡流分布会发生明显变化,产生沿管壁法向的磁场,检测线圈位于裂纹端头处正上方时检测灵敏度最高。实际检测结果与仿真结果一致,表明磁导体环形激励轴向涡流对小径管周向缺陷具有显著的检测效果。     

2D surface defect images applying Tikhonov regularized inversion and ECT

This paper presents an inversion problem solution to obtain the geometrical profile of defects in a 2D surface view using an eddy current method with sinusoidal excitation. The inversion model deals with the perturbation of the eddy currents when a spatially uniform excitation magnetic field distribution is imposed in the vicinity of a metal surface. An adequate experimental setup was used to measure the magnetic field around a crack region. A planar excitation coil was used to generate the uniform magnetic field distribution around the points under measurement, and a giant magneto-resistance (GMR) was used as a sensing element. Tikhonov regularization was applied to the inversion problem algorithm. Five different geometric defects machined in an aluminum plate were tested experimentally using the inversion algorithm. Finite element simulations are provided to validate the 2D eddy current density images obtained by the regularized inversion process.     

Electromagnetic-thermal NDT in thin conducting plates

The effectiveness of an electromagnetic-thermal method for non-destructive testing of conductive materials is investigated numerically in thin conductive plates. The detection region is related to the diameter of the exciting coil while the detection period is affected from the crack length and its distance from the coil. Both, the detection range and detection period depend on the crack orientation with respect to the heat flow, the magnetic flux density and the heating time. By proper selection of the coil positions, during a second inspection it is possible to elongate the detection period for the ambiguous cracks and make clear their shape.     

Modeling of the yoke-magnetization in MFL-testing by finite elements

The yoke-magnetization is very popular in magnetic particle testing of welds. The detectability of a flaw by using this method largely depends on the magnetic flux density passing through a specimen to be examined or on the intensity of magnetic field acting in/on the specimen. In Japan inspectors have to check and confirm the appropriate magnetizing situation of the specimen by using an A-type standard test specimen specified in the standard JIS G 0565-1992. The development of indications by magnetic particles on the standard specimen is influenced by the air gap between the standard specimen and the specimen surface to be examined. Since the height and breadth of an artificial flaw in the standard specimen also influence the leakage of the magnetic flux density from the flaw, the information about the magnetizing situation is complex.In this paper we first identify influences of some factors on the magnetic leakage flux density from an artificial flaw in the standard specimen by using FEM modeling. Since the check with the standard specimen gives not a unique information to the magnetization state we investigate the technique, in which intensity of magnetic field acting on the specimen surface is used to characterize the magnetization. A finite element approach is applied to model the magnetization situation. The effectiveness of the modeling is confirmed by an experiment.     

基于三维有限元的平行钢丝拉索断丝漏磁场仿真

随着桥梁检测技术的发展,漏磁检测技术开始应用于在役拉索检测。由于拉索自身结构复杂,拉索漏磁检测具有大直径和大提离的特点,导致背景磁场较强。利用有限元方法建立三维漏磁检测模型,通过将存在断丝缺陷的磁场信号与无缺陷的磁场信号求差,获得了真实漏磁场信号;研究了断丝断口宽度、埋藏深度等参数对检测信号的影响,给出了漏磁场随上述参数的变化曲线,从而为拉索漏磁检测信号的解释提供依据,为断丝量化分析奠定了基础。