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.     

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.     

Flaw detection in metals by the ACPD technique: Theory and experiments

The Alternating Crack Potential Drop (ACPD) technique is mainly used to characterize surface cracks in metals. To-date, this technique has the following two limitations: it is limited to the so-called thin skin assumptions, and is applicable to open (visible) flaws. Saguy and Rittel recently proposed a methodology based on numerical simulations to overcome these limitations [Saguy H, Rittel D. Bridiging thin and thick skin solutions for alternating currents in crack conductors. Appl Phys Lett J 2005; 87: 84103–84103/3; Saguy H, Rittel D. Alternating current flow in internally flawed conductors: a tomographic analysis. Appl Phys Lett J 2006; 89: 94102–94102/3]. This paper presents experimental results, which support the proposed solutions and methodology to expand the universality of the ACPD technique as a key NDT tool.     

Modelling the transverse magnetic field generated by equipment in arc welding

The design of equipment for generating the transverse magnetic field in arc welding can be optimised by modelling the direct magnetic field produced by the device for generating (GD) the transverse magnetic field (TMF) by the electric field of the current flowing in flat models made of electrically conducting materials. The lines of force of the electric field in the flow of the current in the modelling medium correspond to the lines of force (induction lines) of the magnetic field generated by GD TMF. Using these modelling methods, it is shown that to obtain the maximum values of the transverse component of the induction of the magnetic field in the zone of the welding arc of the electrode droplet and liquid metal of the weld pool, the optimum design of GD TMF is the one in which the angle of inclination of the bars to the vertical is equal to 45° and the end surfaces have chamfers parallel to the plane of the welded sheets.     

交变磁场净化金属液时金属液紊流的形成及其控制

使用金属嫁液对交变磁场作用下的金属液紊流现象进行了直接观察,并对相应的成因进行了探讨,结果发现交变磁场中为磁场分布不均匀是引起紊流的根本原因。提出采用磁场分布不均度参数来衡量磁场不均匀性,该参数的提出为电磁净化器的设计和提高净化提供了理论依据。为了探讨控制紊流的方案,还采用了模拟体系,研究了不同截面形状的管型及不同磁感应分布不均度下颗粒的迁移规律,并分析了管型影响紊流形成的原因。     

Effect of power parameter and induction coil on magnetic field in cold crucible during continuous melting and directional solidification

Bottomless electromagnetic cold crucible is a new apparatus for continuous melting and directional solidification;however,improving its power efficiency and optimizing the configuration are important for experiment and production.In this study,a 3-D finite element (FE) method based on experimental verification was applied to calculate the magnetic flux density (Bz).The effects of the power parameters and the induction coil on the magnetic field distribution in the cold crucible were investigated.The results show that higher current intensity and lower frequency are beneficial to the increase of Bz at both the segment midpoint and the slit location.The induction coil with racetrack section can induce greater Bz,and a larger gap between the induction coil and the shield ring increases Bz.The mechanism for this effect is also discussed.     

电磁超声表面波换能器发射过程的精确建模与分析

针对具有曲折线圈结构的电磁超声表面波换能器模型精度较低问题,以铝合金板材表层缺陷检测为背景,在考虑交变磁场影响前提下,通过将有限元方法与解析法相结合,建立了一个精确描述电磁超声表面波换能器发射过程的三维模型。仿真分析表明,换能器在发射过程中产生的交变磁场可激发出两倍频率成分的表面波;该表面波分量能够有效提高换能器的缺陷敏感度;增加发射电流、降低提离距离均可显著增强该分量。试验验证了所建模型的有效性和精确性。     

埋弧焊磁控电弧焊缝跟踪系统中的横向磁场ANSYS模拟

针对埋弧焊磁控电弧焊缝跟踪系统中磁控电弧传感器产生的外加横向磁场,文中采用有限元分析方法建立了由磁控电弧传感器产生的横向磁场计算模型.应用ANSYS软件对横向磁场进行了模拟;对该横向磁场的纵截面分布、磁流密度矢量的分布规律进行了仿真;并对模拟结果的偏差进行了分析.实际测试结果说明了磁感应强度模拟值与实测值的一致性.针对励磁电流对焊接电弧运动状态的影响进行了实际焊接试验.结果表明,焊接电弧随励磁电流的增大而摆动,且励磁电流越大电弧摆动幅度也越大.     

一种GMA高速开关阀的结构设计与磁场仿真分析

本文设计了高速开关阀用GMA的结构形式,计算了GMM棒所需的长度和直径,并结合高速开关阀的工作特点设计了线圈尺寸。通过Ansoft软件对GMA的磁场分布做了详细分析,得出采用低磁导率的导磁块时GMM棒轴向磁场不均匀度最小,且导磁块与GMM棒相对磁导率相同或接近时磁场不均匀度改善可达到最优,气隙宽度对GMM棒磁场不均匀度影响较显著,且当气隙宽度为0.4mm时磁场不均匀度最小,线圈安匝数主要影响GMM棒上的磁场强度大小,对磁场不均匀度影响甚小。     

金属磁记忆在焊接缺陷检测领域的研究现状及发展

金属磁记忆检测技术是一种新型的无损检测方法,它具有能够对焊接结构中的微观缺陷进行早期诊断,防止突发性破坏事故发生的潜在优势,在焊接缺陷检测领域具有广阔的发展前景,被认为是迄今为止能够对材料的内部损伤进行早期诊断的惟一可行的无损检测方法.文中首先对金属磁记忆检测技术进行了简要的介绍,随后指出了该技术在焊接裂纹检测方面的独特优势.并与传统焊接缺陷检测方法进行了比较,然后就其在焊接检测领域的研究和应用进行了分析.最后指出了金属磁记忆检测技术在焊接检测领域的发展方向.     

磁场复合电解加工间隙磁场的有限元分析

为了研究永磁体不同设置对磁场复合电解加工间隙磁场分布的影响，建立了磁场复合电解加工间隙磁场分析的数学模型和有限元模型，采用ANSYS软件对其进行了分析。结果表明：当永磁体置于阳极下方且磁化方向垂直于阳极时，磁场对电解加工几乎无影响；而当永磁体置于阳极下方且磁化方向平行于阳极时，间隙中的磁场强度较为均匀且磁场方向与电场方向垂直，有利于提高电解加工的效率和型面的加工精度。分析结果与试验结果一致。     

稳恒磁场下Ni-W合金镀膜的制备与耐蚀性

研究了施加稳恒磁场(0~1 T)对电镀Ni-W合金的影响。测定不同方向、不同强度稳恒磁场下的电流效率、合金组成及镀层的耐蚀性,并用扫描电镜和X射线衍射对镀层的微观形貌及结构进行了观察和分析。对实验结果进行了分析讨论,找出稳恒磁场对这些性能指标的影响规律。结果表明:与不施加磁场相比,施加磁场后镀层的表面更均匀、细致、平整;镀层的含钨量上升:当B⊥J,B=1.0 T时,含钨量上升了约7%;当B∥J,B=1.0 T时,含钨量上升了约9%;镀膜的非晶化程度增强;镀膜的耐蚀性提高;但Ni-W合金电镀的电流效率降低。     

Experiment and simulation study of 3D magnetic field sensing for magnetic flux leakage defect characterisation

Magnetic flux leakage (MFL) testing is widely used to detect and characterise defects in pipelines, rail tracks and other structures. The measurement of the two field components perpendicular to the test surface and parallel to the applied field in MFL systems is well established. However, it is rarely effective when the shapes of the specimens and defects with respect to the applied field are arbitrary. In order to overcome the pitfalls of traditional MFL measurement, measurement of the three-dimensional (3D) magnetic field is proposed. The study is undertaken using extensive finite element analysis (FEA) focussing on the 3D distribution of magnetic fields for defect characterisation and employing a high sensitivity 3-axis magnetic field sensor in experimental study. Several MFL tests were undertaken on steel samples, including a section of rail track. The experimental and FEA test results show that data from not only the x- and z-axes but also y-axis can give comprehensive positional information about defects in terms of shape and orientation, being especially advantageous where the defect is aligned close to parallel to the applied field. The work concludes that 3D magnetic field sensing could be used to improve the defect characterisation capabilities of existing MFL systems, especially where defects have irregular geometries.     

The speed of flows in the pool in arc welding in a transverse magnetic field

Physical modelling of arc welding with a wire shows that the flow speed of liquid metal, directed into the tail part of the pool and induced by the constant transverse magnetic field, increases with the increase of current and the transverse component of induction of the field. Experimental results show that the distribution of the flows is determined by the distribution of the density component of the current spreading in the liquid metal of the weld pool. The results can be used in the determination of the optimum frequency of the transverse magnetic field for controlling the dimensions of the penetration zone of the metal and the process of solidification of the weld pool metal in arc surfacing and welding.     

Numerical simulation for mold-filling of thin-walled aluminum alloy castings in traveling magnetic field

1 .IntrOdUction High一strength,light一weight and large一size alumimimalloy eastings are widely used in aviation,astronavigation,gUided missile and 50 on.With the develoPment of neweasting teehnology,it 15 imPortant     

Investigation of the process of electron beam welding in an external magnetic field

The process of electron beam welding of stainless steel in a strong magnetic field is investigated. It is shown to be possible to suppress the fluctuations of the penetration depth in the root part of the welded joint in electron beam welding of non-magnetic metals and alloys. The fluctuations are suppressed by restricting the penetration depth by rotating the electron beam. A permanent dipole magnet with a rapidly decreasing magnetic field is used to distort the trajectories of the electron beam and rotate it.     

Grain refinement of pure aluminum by direct current pulsed magnetic field and inoculation

The combined effects of direct current pulsed magnetic field (DC-PMF) and inoculation on pure aluminum were investigated, the grain refinement behavior of DC-PMF and inoculation was discussed. The experimental results indicate that the solidification micro structure of pure aluminum can be greatly refined under DC-PMF. Refinement of pure aluminum is attributed to electromagnetic undercooling and forced convection caused by DC-PMF. With single DC-PMF, the grain size in the equiaxed zone is uneven. However, under DC-PMF, by adding 0.05% (mass fraction) Al–5Ti–B, the grain size of the sample is smaller, and the size distribution is more uniform than that of single DC-PMF. Furthermore, under the combination of DC-PMF and inoculation, with the increase of output current, the grain size is further reduced. When the output current increases to 100 A, the average grain size can decrease to 113 μm.     

Effects of alternating magnetic field on the corrosion rate and corrosion products of copper

The effects of alternating magnetic field on the corrosion morphologies, corrosion rate, and corrosion products of copper in 3.5% NaCl solution, sea water, and magnetized sea water were investigated using electrochemical test, scanning electron microscopy/energy dispersive analysis system of X-ray (SEM/EDAX), and X-ray diffraction (XRD). The results show that the corrosion rate of copper in magnetized sea water is minimal. Moreover, the surface of the specimen in magnetized sea water is uniform and compact as compared with those in 3.5% NaCl solution and sea water. The corrosion products of copper in magnetized sea water are mainly Cu2O and CuCl2. However, the corrosion products in sea water are CuCl, Cu2Cl(OH)3, and FeCl3·6H2O. The electrochemical corrosion mechanisms of copper in the three media were also discussed.     

强磁场下时效处理对铸造镍基高温合金K52中碳化物的影响

针对时效处理铸造镍基高温合金K52的碳化物,研究了其在强磁场作用下的组织形貌与化学成分变化规律。结果表明:在不同磁场条件下时效处理后,合金中碳化物MC和M23C6的组织形貌与分布基本相同,强磁场的施加对碳化物的组织形貌与分布并未产生显著影响,但经强磁场条件下时效处理后,MC和M23C6中强碳化物形成元素W、Mo、Nb和Ti含量有所增加,而弱碳化物形成元素Cr和非碳化物形成元素Co、和Ni含量显著降低。分析指出强磁场的施加可能提高了强碳化物形成元素与C间的亲和力,增强了其形成碳化物的倾向;同时其可能减弱了弱碳化物形成元素和非碳化物形成元素与C间的亲和力,从而使其在碳化物中存在的倾向也进一步降低。     

强磁场下亚共晶Al-Cu合金初生相形核与生长行为(英文)

应用差热分析法研究了强磁场下Al-20.8%Cu(质量分数)亚共晶合金初生相形核与生长特性。差热分析曲线表明,初生相的形核温度随磁场强度的增大而降低,其生长速率则随磁场强度增大而增大。初生相枝晶由无磁场时无序生长转变为磁场下规则生长。研究表明,10T量级的磁场对Al晶体形核驱动力的影响可以忽略,初生相形核温度的降低主要归结为磁场下固液界面自由能的增加。枝晶形貌转变则源于磁场对熔体流动的抑制及铝晶体的磁各向异性。