Output-only analysis of structures with closely spaced poles

In the framework of the operational modal analysis, several approaches have been developed for estimating the modal parameters, i.e., natural frequencies, damping ratios, and mode shapes. Specifically, a technique capable to evaluate the biased (i.e., unscaled by a constant or an almost constant function) frequency response functions, FRFs, has been proposed. Assuming that only the responses of the structure are disposable, the technique allows one to estimate biased FRFs starting from the power spectral densities, PSDs, and applying the Hilbert transform. This paper deals with the estimates of the modal analysis parameters mentioned above. It is possible to obtain each single mode shape, from the singular vectors achieved by applying the singular value decomposition to the FRF matrix evaluated at the spectral line corresponding to the selected natural frequency. A special attention will be devoted to structures with coupled modes, i.e., closely spaced modes. Once the FRFs have been obtained, the natural frequencies and damping ratios could be achieved either in the frequency domain or in the time domain. Experimental tests, carried out on beams, plates and on the AB-204 helicopter blade, will be presented.     

A magnetometer of weak quasi-stationary and high-frequency fields on resonator microstrip transducers with thin magnetic fields

A high-sensitivity magnetometer for simultaneous measurements of three components of a weak quasi-stationary or high-frequency magnetic-field vector was developed and investigated. Microstrip structures that are based on irregular resonators serve as the magnetometer transducers. An anisotropic thin-film magnetic structure is used as the sensing element. This structure consists of two thin magnetic films that are prepared by magnetron sputtering of a Ni₇₅Fe₂₅ permalloy target and separated by a silicon monoxide layer. It is demonstrated that the transducer exhibits the maximum sensitivity, when the easy magnetization axis of the film structure is orthogonal to the polarization direction of the pumping microwave magnetic field in the microstrip resonator and at an optimal value of a constant magnetic bias field and its optimal deflection from the pumping-field polarization direction which is parallel to it. The magnetometer is characterized by a wide dynamic range of measured magnetic fields, 10^–10–10^–4 Т, and a wide frequency range, 10^–1–10⁵ Hz.     

Temperature dependence dynamical permeability characterization of magnetic thin film using near-field microwave microscopy

A temperature dependence characterization system of microwave permeability of magnetic thin film up to 5 GHz in the temperature range from room temperature up to 423 K is designed and fabricated as a prototype measurement fixture. It is based on the near field microwave microscopy technique (NFMM). The scaling coefficient of the fixture can be determined by (i) calibrating the NFMM with a standard sample whose permeability is known; (ii) by calibrating the NFMM with an established dynamic permeability measurement technique such as shorted microstrip transmission line perturbation method; (iii) adjusting the real part of the complex permeability at low frequency to fit the value of initial permeability. The algorithms for calculating the complex permeability of magnetic thin films are analyzed. A 100 nm thick FeTaN thin film deposited on Si substrate by sputtering method is characterized using the fixture. The room temperature permeability results of the FeTaN film agree well with results obtained from the established short-circuited microstrip perturbation method. Temperature dependence permeability results fit well with the Landau-Lifshitz-Gilbert equation. The temperature dependence of the static magnetic anisotropy H(K)(sta), the dynamic magnetic anisotropy H(K)(dyn), the rotational anisotropy H(rot), together with the effective damping coefficient α(eff), ferromagnetic resonance f(FMR), and frequency linewidth Δf of the thin film are investigated. These temperature dependent magnetic properties of the magnetic thin film are important to the high frequency applications of magnetic devices at high temperatures.     

Noise reduction of ultrasonic Doppler velocimetry in liquid metal experiments with high magnetic fields

The last decades have seen a number of liquid metal experiments on the interaction of magnetic fields with the flow of electrically conducting fluids. The opaqueness of liquid metals requires non-optical methods for inferring the velocity structure of the flow. Quite often, such experiments are carried out in the presence of high electrical currents to generate the necessary magnetic fields. Depending on the specific purpose, these currents can reach several kiloamperes. The utilized switching mode power supply can then influence seriously the measurement system by electromagnetic interference. A recent experiment on the azimuthal magnetorotational instability (AMRI) has shown that a hydrodynamically stable Taylor–Couette flow becomes unstable under the influence of a high azimuthal magnetic field. An electrical current along the axis of the experiment with up to 20 kA generates the necessary field to destabilize the flow. We present experimental results of this AMRI experiment carried out at the PROMISE facility with an enhanced power supply. For this setup, we discuss the elaborate measures that were needed to obtain a reasonable signal-to-noise ratio of the ultrasonic Doppler velocimetry (UDV) system. In dependence on various parameter variations, some typical features of the observed instability, such as the energy content, the wavelength, and the frequency are analyzed and compared with theoretical predictions.     

A modular designed ultra-high-vacuum spin-polarized scanning tunneling microscope with controllable magnetic fields for investigating epitaxial thin films

A room-temperature ultra-high-vacuum scanning tunneling microscope for in situ scanning freshly grown epitaxial films has been developed. The core unit of the microscope, which consists of critical components including scanner and approach motors, is modular designed. This enables easy adaptation of the same microscope units to new growth systems with different sample-transfer geometries. Furthermore the core unit is designed to be fully compatible with cryogenic temperatures and high magnetic field operations. A double-stage spring suspension system with eddy current damping has been implemented to achieve ≤5 pm z stability in a noisy environment and in the presence of an interconnected growth chamber. Both tips and samples can be quickly exchanged in situ; also a tunable external magnetic field can be introduced using a transferable permanent magnet shuttle. This allows spin-polarized tunneling with magnetically coated tips. The performance of this microscope is demonstrated by atomic-resolution imaging of surface reconstructions on wide band-gap GaN surfaces and spin-resolved experiments on antiferromagnetic Mn(3)N(2)(010) surfaces.     

Technique for high axial shielding factor performance of large-scale, thin, open-ended, cylindrical Metglas magnetic shields

Metglas 2705M is a low-cost commercially available, high-permeability cobalt-based magnetic alloy, provided as a 5.08-cm wide and 20.3-μm thick ribbon foil. We present an optimized construction technique for single-shell, large-scale (human-size), thin, open-ended cylindrical Metglas magnetic shields. The measured dc axial and transverse magnetic shielding factors of our 0.61-m diameter and 1.83-m long shields in the Earth's magnetic field were 267 and 1500, for material thicknesses of only 122 μm (i.e., 6 foil layers). The axial shielding performance of our single-shell Metglas magnetic shields, obtained without the use of magnetic shaking techniques, is comparable to the performance of significantly thicker, multiple-shell, open-ended Metglas magnetic shields in comparable-magnitude, low-frequency applied external fields reported previously in the literature.     

Simultaneous measurement of magnetization and magnetostriction in 50 T pulsed high magnetic fields

To simultaneously perform magnetization and magnetostriction measurements in high magnetic fields, a miniaturized device was developed that combines an inductive magnetometer with a capacitive dilatometer and, therefore, it is called "dilamagmeter." This combination of magnetic and magnetoelastic investigations is a new step to a complex understanding of solid state properties. The whole system can be mounted in a 12 mm clear bore of any cryostat usually used in nondestructive pulsed high field magnets. The sensitivity of both methods is about 10(-5) A m(2) for magnetization and 10(-5) relative changes in length for striction measurements. Measurements on a GdSi single crystal, which are corrected by the background signal of the experimental setup, agree well with the results of steady field experiments. All test measurements, which are up until now performed in the temperature range of 4-100 K, confirm the perfect usability and high stability in pulsed fields up to 50 T with a pulse duration of 10 ms.     

Near-field observation of carrier diffusion in GaAs quantum structures under high magnetic fields

Photoluminescence from a two‐dimensional electron‐gas system in GaAs single hetero‐structures was investigated using a scanning near‐field optical microscope (SNOM) operated at cryogenic temperatures under high magnetic fields. The local intensity of the luminescence increased 600‐fold that at 0 T as the magnetic field was increased up to 6 T. The enhancement depended on the spatial resolution of the SNOM. These characteristics are explained by the suppression of the diffusion of photocarriers caused by the Lorentz force in magnetic fields.     

轴类镀层结构材料力学性能声学表征方法研究

针对轴类镀层结构材料力学性能无损表征方法开展理论和实验研究,对于表面工程的质量检测与评价具有重要意义。利用自主开发的超声显微测量系统与PVDF线聚焦探头,对不同厚度的轴类镀层材料进行散焦测量。针对轴类试件表面波传播路径与散焦距离的非线性关系,分析了线聚焦探头几何尺寸与散焦距离和试件轴径间的匹配关系,优化检测参数,并在V(f,z)分析法的基础上,采用相位相关分析法获得轴类镀层材料的实验频散曲线。结合镀层材料声波传播特性,采用基于模拟退火的粒子群优化算法(PS-B-SA)将理论频散曲线与实验频散曲线相拟合,反演轴类镀层结构材料的声学参数,进而表征镀层材料的弹性常数。多个不同厚度轴类镀层结构材料弹性常数及镀层厚度的反演结果与实际值相吻合,表明该测量方法可靠、结果准确。该研究成果为轴类镀层结构力学性能无损检测提供了技术手段,也为工程材料表面工程加工性能的评价提供了新方法。     

Surface magnetization measurement with a spin-polarized metastable He beam under high magnetic fields of 0-5 T

An apparatus for measuring the surface magnetization with a spin-polarized metastable helium atom (He(*)) beam under external magnetic fields of 0-5 T was developed. The He(*) beam, spin polarized by a sextupole magnet, was directed to the sample placed in the bore of 5 T superconducting magnet. A zero-field spin flipper was used for switching the polarity of beam polarization. A Stern-Gerlach analysis indicated the beam polarization of nearly 100% and the spin flipping efficiency of >90%. A surface magnetization curve was successfully measured from 0 to 5 T for an FeCu(100) surface.     

Operation of calorimeters based on vacuum and gas photodetectors in high intensity magnetic fields

Recent attempts to operate counters for calorimetric use inside high intensity magnetic fields have resulted in interesting developments both in the construction of the counters and in the design of the vacuum photosensitive devices. In particular, a new one-stage photomultiplier has been developed. The present status of the development of gas photodiodes will finally be illustrated.     

The static axial compression of tall hollow cylinders with high interfacial friction

Hollow cylinders of initial height to diameter ratios between 1:1 and 2:1, with central holes of various diameters, were compressed axially under dry, quasi-static conditions. In the initial phase of compression three modes of flow could be identified, distinguished by single (I), double (II) or triple (III) barrels on the free surface.During continued compression, mode III degenerated into an unstable process similar to the collapse of thin-walled cans, whilst specimens initially displaying mode II changed to mode I or retained their initial form. Those cylinders that initially displayed mode I were found to retain this form throughout their subsequent compression.The deformation characteristics outlined above are shown to be dependent on the initial height to diameter ratio and the size of the bore.A preliminary axi-symmetric upper bound analysis is presented, but the simple models used are shown to be insufficient to represent accurately the complex phenomena observed. It is suggested that finite-element elastic-plastic analysis may be more appropriate.     

球形磁性磨料磁力研磨ZrO2材料机理研究

在磁力研磨加工ZrO 2材料过程中,分析了单颗磁性磨料在加工区域内的受力情况,并对研磨压力的形成进行探讨,利用公式推导计算研磨压力,通过研磨压力的大小分析了磁力光整加工中材料的去除机理,包括脆性断裂去除、塑性变形去除和粉末化去除。通过白光干涉仪、扫描电子显微镜等分析检测仪器对磁力研磨加工后的工件表面进行检测,可知在其他条件相同时,磁力研磨加工后的工件材料精度高于传统的草轮抛光精度,可达到0.59μm。     

利用物质磁化性质定性分析邻间对苯二酚同分异构体

研究了一项新的分析技术"电磁感应式磁化定性分析装置".采用该技术定性分析了邻、间、对苯二酚同分异构体样品,监测了3种苯二酚溶液在一段时间内的氧化过程.该分析技术是利用不同单组分样品的磁化性质不同的原理,采用检测电磁感应电动势的方法、测定样品磁化后所产生的附加磁场变量强度进行定性分析的,通过实验表明,该分析技术可以定性分析邻、间、对苯二酚同分异构和大分子量样品,还可以监测样品的动态氧化还原过程.     

The magnetic fields of eddy currents with the relative movement of point magnetic poles and a magnetized conductor

A numerical solution by the integral equation method of the model of point magnetic poles, which move with a constant velocity over a magnetized conducting half space, is given. The proposed model can be useful for the qualitative understanding and preliminary estimation of the magnetic fields of eddy currents that arise, e.g., during the magnetic testing of moving rails.     

Indication of magnetic fields with the use of galvanic currents in magnetic-powder nondestructive testing

A difference in the potentials in a nonuniform magnetic field is shown to occur across electrodes that are placed into a ferromagnetic suspension, which is used in magnetic-powder nondestructive testing (NDT). This phenomenon may be used for the detection of flaws in ferromagnetic products.     

Friction behavior of magnetorheological fluids with different material types and magnetic field strength

Magnetorheological (MR) fluid is a type of a smart material that can control its mechanical properties under a magnetic field. Iron particles in MR fluid form chain structures in the direction of an applied magnetic field, which is known as MR effect, resulting in variation of stiffness, shear modulus, damping and tribological characteristics of MR fluid. As MR effect depends on the density of particles in the fluid or the strength of a magnetic field, the experiments are conducted to evaluate the friction property under reciprocating motion by changing the types of MR fluid and the strength of a magnetic field. The material of aluminum, brass, and steel are chosen for specimen as they are the most common material in mechanical applications. The surfaces of specimen are also observed by optical microscope before and after experiments to compare the surfaces with test conditions. The comparing results show that the friction coefficient increases as the strength of a magnetic field increases in regardless of types of MR fluid or the material. Also the density of particle in MR fluid affects the friction characteristic. The results from this research can be used to improve the performance of mechanical applications using MR fluid.     

高速切削温度场的三维有限元模拟

基于高速切削实际工艺分析,通过合理设置边界条件,建立高速切削下工件-刀具相互作用的三维数值模型,采用有限元法模拟了使用涂层刀具高速切削45钢时的温度场分布状况,并比较不同的切削速度、不同的刀-屑界面摩擦因数下刀具的温度场分布,模拟结果与试验结果具有较好的一致性.     

Evaluation of strength variation of domes and cylinders with different winding angle of type III high pressure storage vessel

Journal of Mechanical Science and Technology - Type III vessels are used to store gases at high pressure and, for efficient use, should be as light as possible without compromising stability....