地磁场方向对应力集中引起的磁场畸变的影响

金属零件在地磁场环境下,相对方向不断变化的地磁场对应力集中所引起的磁场畸变的影响至今缺乏一致的结论。通过模拟工件在地磁场环境中绕竖直方向旋转的试验,研究了地磁场等弱场的方向变化与应力集中导致的畸变磁场幅度之间的关系。试验结果显示,固定应力引起的磁场畸变与拉伸作用力方向上的磁场大小呈近似线性的正相关关系,说明在地磁场环境下,应力集中引起的畸变磁场的大小主要受与载荷方向平行的外磁场分量影响。     

磁致伸缩和磁记忆问题研究

从铁磁性材料的基本特性和最小自由能原理出发，探讨了应力和外磁场对磁性材料磁化状态的不同影响，讨论了磁致伸缩现象的规律和特点。用磁致伸缩方程组详细研究了磁记忆现象产生的条件和规律，分析了应力和地磁场在磁记忆现象中的不同作用。结论认为应力可改变材料的磁特性，地磁场则可改变材料的压磁性，两者共同作用产生净磁场，磁记忆现象不仅与应力和材料的磁特性有关，而且还与工件在空间的位置和方向有关。     

铁磁性物体在地磁场中的自发运动磁化

依据法拉第电磁感应定律和铁磁性物质在磁化一退磁过程中的能量差异，阐明了铁磁性物体在地磁场中的摆动、振动和周期性往复平动都会引起该物体的自发磁化。虽然每次磁化一退磁过程之后物体上的剩磁增量非常微小，但随着循环次数的增加该物体最终必然被强烈地自发磁化。从而推知：地磁场的矢量特性和纬度效应对金属磁记忆检测与诊断技术适用性和可靠性的影响都不大。     

脉冲磁场强度对GH99镍基合金力学和微动磨损性能的影响

为研究不同磁场强度对镍基合金力学性能和耐磨性能的影响规律,在脉冲强磁场设备上对GH99镍基合金试样进行脉冲磁处理。通过观察显微结构,分析了GH99镍基合金的磨损机理和强化机制。结果表明：外加脉冲磁场可改善材料位错分布,减小试样表面残余应力的分散性;在磁场强度为10 T时残余压应力达到最大值（-223.45 MPa）,且此时材料拉伸断口的特征主要表现为韧性断裂,脉冲磁场处理合金产生亚结构位错胞有助于发挥细晶强化作用;在0~15 T范围内,随磁场强度增大,材料表面显微硬度和耐磨性能呈现先增强后减弱的规律,在脉冲磁场作用下合金材料内部的位错发生增殖致使位错密度增大,产生类似加工硬化现象,但磁场强度过大会导致位错塞积从而造成晶胞点阵畸变严重,出现材料性能恶化。     

Research on magnetic testing method of stress distribution

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铁磁性物质在地磁场中的静置磁化和退磁

介绍了铁磁性物质静置于地磁场中产生的自发磁化和退磁的原因，主要有以下三点：一是磁时效—铁磁性物质的磁性和磁畴的空间排列都滞后于施加于其上的磁化场；二是地磁场强度大小和方向随着时间推移而发生的周期性变化；三是金属的时效—材料中机械应力随着时间流逝导致其磁化曲线和金相组织的改变以及延迟裂纹的出现等。     

Stress dependence of ferromagnetic hysteresis loops for two grades of steel

Residual stress is an essential input into calculations of the remaining life of operational plant. However, many life predictions are inherently conservative because measurements of the absolute stress are unavailable. The magnetic properties of material are sensitive to stress, and in this paper, the magnetic properties (at maximum magnetisation, at remnance, and at the coercive field) of two grades of steel (Durehete 1055 and AS1548-7-46OR) under uni-axial stress are presented. The magnetic parameters are more sensitive to compressive stress than to tensile stress. A mathematical model using the Jiles–Atherton theory for ferromagnetic hysteresis is used to explain the trends in the experimental data. The ability of magnetic techniques to measure stress is demonstrated.     

Calculation of magnetic leakage field from a surface defect in a linear ferromagnetic material: an analytical approach

A novel analytical approach for calculating the magnetic leakage field from surface defects is proposed and demonstrated for the case of a linear ferromagnetic material. The novelty of the theory is that it relates the distribution of induced magnetic charges to the surface shape. An excellent agreement between the analytical and numerical results is shown. The functional relations between different magnetic field components are discussed.     

典型铁磁构件磁记忆效应的试验研究

研究了静拉伸试验时铁磁构件的金属磁记忆效应。以Q235钢为研究对象,于不同加载阶段对试件的表面漏磁场进行了实时测量。结果表明,应力集中对构件的磁特性有显著影响,在应力引起的小范围塑性变形区域存在有效的磁记忆信号,采用磁记忆方法对承载铁磁构件由应力集中造成的早期损伤进行检测是可行的。     

强磁场屏蔽有限元设计

在国际热核聚变实验堆计划中，为降低磁场对电磁阀的影响，必须将电磁阀周边磁场从2000Gs降低到50Gs以内。本文通过有限元分析软件计算分析和实验测量相结合对2000Gs的强磁场的屏蔽进行了设计。计算和实验结果表明，高饱和磁感应强度材料对强磁场屏蔽效果较好，可有效降低材料厚度。磁场方向对磁屏蔽效果有着明显影响，当磁场方向与磁屏蔽罩轴向垂直时，屏蔽效果最好。     

射流压力对淹没水射流冲击与空蚀效果的影响

目的探究淹没水射流对铝合金及铜合金材料的冲击与空蚀作用。方法为获得关键射流工况参数对射流冲击和冲蚀效果的影响,对射流压力、冲击时间和靶距的作用进行了研究。在较低压力条件下对材料表面的残余应力进行测量与分析;在较高压力时对材料表面的破坏形貌进行观测。结果残余应力随射流压力的增加而增加,当冲击时间为20 min,射流压力为40 MPa时,残余应力达到最大值91 MPa;在空蚀作用初期,试样表面出现尺寸较小、呈离散分布的空蚀坑,其为离散空泡冲蚀所致。随着空蚀作用的增强,材料表面的空蚀坑增大,同时出现塑性变形,当冲击压力为300 MPa,靶距为7cm,冲击时间为15 min时,达到最显著冲击效果。结论当压力较小时,淹没水射流可以强化金属表面;当压力较大时,淹没水射流可以使金属表面产生空蚀坑和塑性变形。尽管淹没水射流具有连续的射流速度分布,但材料表面的破坏更多地取决于空化现象。     

基于磁畴聚合模型的磁记忆效应机理研究

研究了铁磁构件在受载时产生磁记忆效应的机理。结合试件的断裂机制和位错理论，分析了试件组织的微观变化。建立畴壁聚合模型，探讨了构件形成的有取向的宏观磁化。在应力集中区域或微观损伤区域建立带磁偶极子模型，计算试件表面的泄漏磁场。结果表明，畴壁聚合模型可以解释应力集中区域或微观损伤区域的磁场变化。     

基于有限元的铁磁物质电阻点焊磁场分布分析

根据电阻点焊机构电流回路特点,利用ANSYS软件建立了铁磁性物质电阻点焊时磁场计算的有限元模型,计算了点焊过程中电流密度分布电场分布与磁感应强度分布,并采集了点焊过程中工件周围的磁感应强度,与模拟结果相比,验证模型的可靠性.结果表明,电流密度、电场强度与磁感应强度均在工件内达到最大.在铁磁性工件内部最大磁感应强度达到7 T,说明铁磁物质的磁化磁场在熔核形成、生长过程中的作用不应忽视.验证试验的测量值与计算值吻合良好,表明文中建立的有限元模型是可靠的.     

Magnetic anisotropy of Co/Cu/Co nanocrystalline films upon magnetic annealing

The magnetic anisotropy of Co/Cu/Co films with the thickness of the copper spacer corresponding to the antiferromagnetic and ferromagnetic indirect exchange coupling between Co layers has been studied. The films deposited on naturally oxidized (111) Si single crystals were produced by magnetron sputtering. The films were annealed at 240°C. Such an annealing virtually neither changes the grain size nor leads to the mixing of layers; i.e., no disturbance of the coupling type between the cobalt layers takes place. Changes in the surface and induced magnetic anisotropy and in the effective energy of indirect exchange coupling have been studied upon annealing in the presence and absence of a magnetic field. It has been found that the shape of surface inhomogeneities in the films changes upon annealing in the magnetic field applied along the film plane, which substantially affects, the surface anisotropy. In the films characterized by antiferromagnetic coupling, the easy axis of magnetization is induced only in the magnetic fields exceeding the saturation field. The induced-anisotropy constant estimated theoretically agrees well with those determined experimentally.     

高频磁场制备复合材料组织及性能研究

利用高频磁场制备Al-Si、Al-Fe复合材料,研究了试样长度对Al-24%Si复合材料显微组织及磁场强度分布的影响,以及在一定条件下改变磁场对Al-2%Fe复合材料的作用时间对其微观组织的影响。研究发现：两种复合材料的心部组织与表层组织明显不同,表层的显微硬度比基材要高出2~3倍;并且,试验材料形成了很好的过渡层,使初生相和基材得到很好的结合。     

Stress concentration degree affects spontaneous magnetic signals of ferromagnetic steel under dynamic tension load

Under the excitation of geomagnetic field, stress can induce spontaneous magnetic signals in ferromagnetic materials, which can be potentially applied to estimate the damage degree. In this research, the normal component of magnetic field, H_p(y), was measured during dynamic tension test on the surfaces of ferromagnetic specimens having different stress concentration factors, α constant amplitude sinusoidal tensile load cycles were applied on 18CrNi4A steel plate specimens whose α were respectively 1, 3 and 5. Then correlation of H_p(y) values’ changes with loading cycles reported. The results indicate that stress concentration factor, α extremely affects the magnetic signal, the absolute value of the gradient, |K|, increases with the increase of α. Meanwhile, |K| also goes up when loading cycle increases in the initial state. However, after a few cycles, those H_p(y) curves corresponding to different cycles become similar before cracking.     

Solidification and solidification structure control of weld metals

Summary

The influence of the shielding gas and laser irradiation conditions on porosity formation in CO₂ laser welding of aluminium alloy were investigated. Bead-on-plate welding tests were performed on 7 mm thick Al-Mg alloy (A5182) plates. The weld beads were inspected by X-ray and the number of pores was counted as a function of the pore diameter. We found that the number of pores was minimised when the dew point of the shielding gas was kept at as low as ?50°C at the welding nozzle tip. At a dew point of ?50°C the number of pores decreased when the flow rate of the shielding gas (Ar) was reduced and the nozzle diameter was increased, presumably because of less air mixing in the welding region. We also found that porosity formation was reduced when He/Ar mixtures were used as the shielding gas instead of pure Ar or He shielding gas. But porosity also depended on the defocused distance of the laser beam at a given laser power. With Ar shielding gas, the number of pores increased when the beam was focused above the specimen surface/while with He shielding gas it increased when the beam was focused in the weld pool. These dependencies may be attributed to the unstable keyhole formation due to laser absorption through the intense plasma formed on the specimen surface and due to the strong boiling of the molten metal respectively. The results indicate that both the hydrogen and the unstable keyhole behaviour cause porosity formation in laser welding of aluminium alloys. Under the optimised conditions, however, the number of pores was significantly reduced.     

Magnetic field shielding technique for pulsed remote field eddy current inspection of planar conductors

Pulsed remote field eddy current (PRFEC) can be used for detection of deeply buried flaws in non-magnetic flat plates. However, detection sensitivity for buried defect is low due to the great depth and the weak detecting signal. To improve the defect detection sensitivity, the PRFEC probe is optimized with the magnetic field shielding techniques in this work. On one hand, the exciting coil with shielding structure can focus the indirect magnetic field to penetrate through the plate. On the other hand, the detecting coil and the region between these two coils with shielding material can block and minimize the magnetic field energy diffusion along the direct coupling path. In order to investigate the shielding technique which blocks the direct coupling magnetic field, simulation studies using finite element method (FEM) has been first conducted, where three different PRFEC probes have been simulated and compared. Both detection abilities to defects in different directions and to thickness variation have been analyzed by using the second probe with shielding structure. The conclusions derived from the simulation study have been validated through experimental studies. Both simulation and experimental studies have indicated that the shielding technique can improve the detection sensitivity of subsurface defects.     

漏磁通检测中铁磁材料的磁化状态

讨论缺陷引起的附加磁场和外磁化场对铁磁材料的磁化作用,引入有效磁导率和平衡磁导率来描述铁磁材料不同区域的磁化状态,建立铁磁材料中的磁化状态及磁偶极子模型中的关荷密度与外磁化场强度、铁磁材料的内禀磁性质以及铁磁材料中磁路和缺陷几何尺寸的关系。     

金属材料直流磁场干摩擦的研究进展

磁场干摩擦主要是研究磁场强度参量对材料干摩擦磨损性能的影响机理和规律。系统综述了金属材料直流磁场干摩擦的研究过程、研究方法和相应研究成果。阐述了不同磁属性(抗磁性、顺磁性和铁磁性)材料的磁场干摩擦特性及其机理,得出材料磁导率与其磁场干摩擦学特性有一定对应关系,材料磁导率越大,磨损率和摩擦系数的减小趋势越明显,并指出了试验研究中遇到的问题,如应关注摩擦副材料选配、磁场施加装置、摩擦接触方式及装卡装置等。为了避开材料化学成分不同造成的干扰,设计了三种制备方案,发现通过热处理工艺改变微观组织相含量或添加铁、钴、镍等铁磁性元素进行化学成分设计等常规的材料制备方法,难以达到目的。最终利用奥氏体不锈钢1Cr18Ni9Ti较大的冷加工硬化特性,采用常温下锻造工艺制备了不同磁导率的几种相同成分材料,使磁导率成为关键变量。研究了磁场强度、材料磁导率与其磁场干摩擦特性的交互影响情况和规律,并展望了未来需要开展的研究方向。