Effect of field independent surface barriers on hysteresis losses in type II superconductors

The behaviour of hysteresis losses, W(H_o, ΔH), per cycle and unit volume, as a function of the amplitude h_o and the normalized height of field independent surface barriers, ΔH/h_o, has been examined computationally. These model calculations reveal that, in many instances, the losses when a barrier is acting are higher than when no barrier is present. We present a catalogue of twelve sets of families of curves of W(h_o, ΔH)/W(h_o, O) versus ΔH_o for various fixed amplitudes $\text{h}{}_{\mathrm{o}}\text{? H}{}_1$, where H₁, is the field required for full penetration of the flux front. These twelve sets arise from considering, (i) infinite plane and cylinder geometry, (ii) half-wave and full-wave cycles where the applied magnetic field varies between 0 and 2h_o and between h_o and -h_o, (iii) symmetric barriers which oppose entry and exit of flux equally and asymmetric barriers which oppose entry only, and (iv) bulk pinning causing the critical current density j_c to be independent of the magnetic induction B and inversely proportional to B. This matrix generates sixteen combinations, but only twelve different cases.     

A COMPARISON OF THE STRAIN INTENSITY AND CYCLIC J APPROACHES TO CRACK GROWTH

Two parameters describing the growth of fatigue cracks are compared. They are the cyclic J integral ΔJ and the strain intensity expressed as an equivalent stress intensity ΔK_eq-. By referring to cyclic stress-strain data obtained from hysteresis loops in high strength ferritic steels at room temperature and austenitic and ferritic steels at elevated temperature it is shown that: (i) for short cracks the parameters are simply related and (ii) both parameters adequately link fatigue crack growth rates observed in the separate high strain fatigue (HSF) and linear elastic fracture mechanics (LEFM) regimes. Correction factors for thumbnail cracks and the conditions under which the relations need further modification are discussed.     

Low cycle fatigue resistance of Al–Li alloys

Abstract

Low cycle fatigue (LCF) resistance data from binary Al–Li, ternary Al–Li–Cu, and quaternary Al–Li–Cu–Mg alloys have been compiled and discussed. The LCF resistance is measured in terms of the variation of the number of reversals to failure 2N _fwith the plastic strain amplitude Δ? _p /2 as well as a modified average plastic strain energy per cycle (ΔW _p )_modified , obtained at different applied total strain amplitudes (Δ? _t /2). The data show the effects of microstructural features, namely dominant strengthening precipitates and the degree of recrystallisation as well as crystallographic texture. Lithium content, when in excess of 2·5 wt-%in aluminium decreases the low cycle fatigue resistance the most. The degree of aging, the degree of recrystallisation, and the degree of texture also influence the LCF resistance; among which the effect of the degree of aging is the most pronounced. The effects of lithium content in aluminium solid solution and strengthening precipitates obtainable by the change in the Li/Cu ratio are found to be marginal. Based on a modified total cyclic plastic strain energy till fracture, it is shown that most of the Al–Li alloys exhibit degradation in their LCF resistance in both hypotransition (higher fatigue lives) and hypertransition (lower fatigue lives) regions. Such degradation is attributed to the combined effects of mechanical fatigue, strain localisation through dislocation–precipitate interaction, environmental effects, and finally strain localisation through the high angle grain boundaries. In comparison with the currently used 2XXX and 7XXX series aluminium alloys, Al–Li alloys require substantial improvement before they can be considered for fatigue critical applications.     

From Manipulation of Giant Magnetoimpedance in Thin Wires to Industrial Applications

We present results on our studies of soft magnetic properties and Giant magnetoimpedance, GMI, effect in thin microwires at elevated frequencies paying special attention to tailoring the GMI effect and achievement of high GMI effect with low hysteretic behavior. We measured magnetic field, H, dependence of real part, of the longitudinal wire impedance up to frequency, f, 4 GHz in Co-rich microwires. Amorphous microwires of appropriate composition exhibit extremely soft magnetic properties with low coercivity (generally below 10 A/m) with well defined magnetic anisotropy field, H _k . We report a number of interesting for sensor applications phenomena such as stress-impedance effect and stress sensibility of overall hysteresis loop shape. Field dependence of the off-diagonal voltage response of pulsed GMI effect in nearly-zero magnetostriction (λ _s≈?3×10^?7) microwires exhibits anti-symmetrical shape, suitable for industrial applications. We observed that the magnetic field dependence of GMI ratio can be tailored either controlling magnetoelastic anisotropy of as-prepared microwires or by heat treatment. Composite character of such microwires results in the appearance of additional magnetoelastic anisotropy. We found that if the surface anisotropy is not circumferential, then the magnetization, and consequently, the MI curve Z(H) present hysteresis. This hysteresis can be suppressed by application of sufficiently high DC bias current I _B that creates a circumferential bias field H _B.     

Epitaxial Orientation and Planar Hall Effect for MnAs Films Grown on GaAs(001)

We study the planar Hall effect in high quality thin ferromagnetic films of MnAs grown on GaAs(001) exhibiting hysteresis due to the hindered rotation of the magnetic moment in the plane. The saturation magnetic field H _s, which is necessary to align the magnetization along the hard axis, depends sensitively on the epitaxial orientation of the film. By using out-of-plane magnetic fields directions, we show that H _s is strongest along the direction of the c-axis of the MnAs crystal, thus demonstrating the importance of the crystal field anisotropy for the planar Hall effect.     

Magnetic Properties of Zn0.8(Fe0.1,Co0.1)O Diluted Magnetic Semiconductors: Experimental and Theoretical Investigation

Structural and magnetic properties of Zn_0.8(Fe_0.1, Co_0.1)O bulk diluted magnetic semiconductor have been investigated using X-ray diffraction (XRD) and magnetic measurements. TEM (Transmission Electron Microscopy) images confirmed the high crystallinity and grain size of Zn_0.8(Fe_0.1,Co_0.1)O powder, the samples were characterized by energy dispersive spectroscopy (EDS) to confirm the expected stoichiometry. This sample has been synthesized by co-precipitation route. The study of magnetization hysteresis loop measurements infers that the bulk sample of Zn_0.8(Fe_0.1,Co_0.1)O shows a well-defined hysteresis loop at T _c (200?K) temperature, which reflects its ferromagnetic behavior. Hydrogenation treatment was used for the control of phase separation. Based on first-principles spin-density functional calculations, using the Korringa?CKohn?CRostoker method (KKR) combined with the coherent potential approximation (CPA), the ferromagnetic state energy was calculated and compared with the local-moment-disordered (LMD) state energy. The mechanism of hybridization and interaction between magnetic ions in Zn_0.8(Fe_0.1,Co_0.1)O is also investigated.     

Low cycle fatigue behaviour of dual-phase steel with different volume fractions of martensite

Completely reversed low cycle fatigue tests were carried out at a constant strain rate of 10^?2s^?1 on 3 mm thick sheet specimens of a dual-phase steel treated to give 1.5 to 28% martensite without changing the carbon content. Hysteresis loop shape, stress/strain response and plastic strain energy as a function of applied cycles are analysed for different microstructures. Strain/life and plastic strain energy per cycle ($\text{Δ}\text{W}\text{?}{}_{\mathrm{<mtext>p</mtext>}}$)/life (2N_f) plots are discussed in terms of microstructure. It is shown that during cycling the shape of the hysteresis loop continuously changes at lower volume fractions of martensite whereas it remains more or less constant for microstructures with a higher percentage of martensite. A Coffin-Manson type of plot between log ($\text{Δ}\text{W}\text{?}{}_{\mathrm{<mtext>p</mtext>}}$) and log (2N_f) is found to be applicable to test results of dual-phase steel with a wide range of martensite contents and is thus more versatile than the plot between log (Δ?_p/2) and log (2N_f) for predicting the fatigue life.     

REPRESENTATION OF CYCLIC PROPERTIES OF AUSTENITIC STEELS WITH PLASTICITY-INDUCED MARTENSITIC TRANSFORMATION

This study is part of a wider research project on the cyclic properties, energy cumulation and fatigue life of metastable austenitic steels undergoing a martensitic transformation induced by plastic straining.
This paper considers the representation of the σ–ε hysteresis loop over a wide range of strain. A novel, power-function model of cyclic elastic–plastic material behaviour was used. The model allows the occurrence of a cyclic yield point and the characteristic inflection point of the CSS curve, which separates the single-phase (austenite) region from the two-phase (austenite + martensite) one. The plastic strain corresponding to the inflection point is assumed to be a material constant and is termed the martensitic transformation cyclic limit ε1 . The generalization of the model made possible the representation of cyclic softening of the two-phase material.
In addition, the study chose a measurement technique that assisted the estimation of the cyclic plastic strain (ε1 ) inducing the martensitic transformation. The crossed magnetomechanical (Villari) effect was shown to be applicable in detecting the nucleation and estimating the increase of the α'-martensite content.
The identification was performed making use of experimental results obtained from an AISI 304 high nickel content steel. The tests were performed under both increasing and constant plastic strain amplitude. The measured quantities were: total strain εt elastic strain εe , plastic strain εp , stress σ and hysteresis loop area ΔW. The results justify the assumed model.     

Thermophysical and Magnetic Properties of Carbon Beads Containing Nickel Nanocrystallites

Ferromagnetic and superparamagnetic nickel nanocrystallites, stabilized in a carbon matrix, were prepared by a three-step procedure including formation of a Ni acrylamide complex, followed by frontal polymerization and pyrolysis of the polymer at various temperatures. It was found that the procedure applied enables fabrication of magnetic beads containing metallic nanocrystallites embedded in a carbon matrix. The size of the crystallites, their morphology, volume fraction, and magnetic properties can be tailored by the pyrolysis temperature. The size of the crystallites affects their behavior in an external magnetic field, i.e., a heating process is the most effective for a sample pyrolyzed at 873 K. The revealed H ⁿ-type dependence of the temperature increase rate, (dT/dt) _t=0, on the amplitude of the magnetic field indicates the presence of both superparamagnetic and ferromagnetic particles in all the samples studied since n > 2. For the superparamagnetic particles, the heating mechanism is associated with Néel relaxation. For the lower values of the magnetic field amplitude, H <  H ₀, the relaxation losses dominate whereas for the opposite case, H >  H ₀, the magnetic hysteresis is the main source of thermal energy losses. The composites containing magnetic Ni nanocrystallites entrapped in a carbon matrix can be potentially applied for hyperthermia treatment.     

Anisotropic effects in the magnetoelastic damping of electrolytic nickel in a direct magnetic field

In the present work a quantitative study of the dependence of the e.m.f. U_W induced in a search coil which surrounds a ferromagnetic sample performing torsional oscillations at a constant amplitude on the amplitude and the frequency of the oscillations is made. The dependence of U_W on the intensity of the longitudinal magnetic field H is also studied. On the basis of these studies experimental arguments are presented for the existence of magnetoelastic hysteresis and macro-currents. A supplementary mechanism through which the energy of the vibrating system can be dissipated is also presented.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).     

FATIGUE BEHAVIOR OF A RAIL STEEL

The fatigue behavior of a hot-rolled, control-cooled, plain carbon eutectoid rail steel has been characterized. The data include monotonic and cyclic stress-strain curves, low cycle fatigue data and near-threshold fatigue crack growth rate behavior in air and in vacuo. The effects of environment and mean stress on the near-threshold fatigue crack growth rates of rail steel are significant. At a low stress ratio (R), ΔK_o is lower in vacuum (7 MPa √m) than in moist air (10 MPa √m). At high R, ΔK_o is higher in vacuum (6 MPa √m) than in air (4 MPa √m). The beneficial effect of moist air on FCGR at low ΔK and low R is attributed to an increase in closure due to fracture surface roughness and oxide film.     

The hysteresis loops of magnetic ferrites and the processes involved in their formation

It is shown that a specimen's microstructure affects the form of its magnetization curves but notM _s, the saturation magnetic moment per cm³ norH _s, the applied field strength required for saturation. A procedure is then given for the precise determination ofM _s andH _s using data derived from the hysteresis loops of polycrystalline isotropic specimens. The procedure is simple to use and avoids the laborious extrapolation requirements of the “law of approach”. Finally, the magnetization processes involved in forming the hysteresis loops betweenH _s and —H _s are described.     

AN ANALYSIS OF LOW CYCLE FATIGUE BASED ON HYSTERESIS ENERGY

Abstract— Cyclic deformation behaviour of medium carbon steel, armco iron and copper has been investigated. A model based on the energy absorbed for the fracture process at the crack tip, given in the form (Δw*_p)^y where Δw*_p is the hysteresis energy at the crack front and the exponent y decides the energy spent for the fracture process, is proposed for the crack growth in low cycle fatigue. The value of the exponent y has been found to be around 0·26 to 0·28 for medium carbon steel and armco iron and around 0·52 for copper. These values of y give good prediction of the relations between the total fracture energy and the stress amplitude and the total fracture energy versus the fatigue life. The energy based approach has been extended to predict the cumulative damage in a single step stress variation. Good correlation has been obtained between the experimental data and the theoretical prediction.     

外加磁场环境下45钢疲劳过程磁记忆特征及其损伤模型

为明确外加磁场环境下铁磁构件疲劳损伤过程磁记忆信号变化规律,以45钢为对象,开展了地磁场(40 A/m)及外加磁场(200、400、600 A/m)环境下的低周疲劳实验。获取了各疲劳损伤阶段的磁场强度法向分量H_p(y)及梯度值K,提取了H_p(y)极差值H_p(y)_Rv和K最大值K_max,结合经典疲劳损伤模型进行了磁特征损伤累积参数拟合,构建了疲劳损伤模型。结果表明：随着疲劳循环周次增加,各数据采集点的H_p(y)值、应力集中部位K值和H_p(y)_Rv值逐渐增大。相同循环周次下,随着外加磁场强度的增大,各数据采集点H_p(y)值和H_p(y)_Rv值逐渐增大,且对应于疲劳断裂阶段的K_max值逐渐增加。所构建的基于H_p(y)_Rv的疲劳损伤模型拟合度较高。本文可为铁磁构件疲劳损伤评估提供技术方法支持。     

Effect of Short-Time Magnetic Relaxation on Hysteresis Curves in Single-Crystal Bi2Sr2CaCu2Ox

We have studied the role of short relaxation time on magnetic hysteresis in a Bi₂Sr₂CaCu₂O _x single crystal. Effects were examined by comparing isothermal magnetization curves obtained over a range of temperatures as a function of the applied magnetic field, H, for magnetic field ramp rates of 3 and 30 Oe/s. At low temperature, a second magnetization peak, H _p, was produced by differences in magnetic relaxation occurring over a short period during the initial stage of relaxation. The data suggested an absence of pinning crossover for H H _p.     

Giant Magnetostriction and Flux Jumps in Superconducting Nb<Subscript>3</Subscript>Al Polycrystalline Slab

No Heading Transverse and longitudinal magnetostriction of a large polycrystalline slab of Nb₃Al superconductor was investigated by the strain gauge technique. At 4.2 K and in an external magnetic field of several Tesla the vertical width of transverse magnetostriction hysteresis loop was found to be of an order of 10^–4. In a wide range of temperatures and external magnetic fields we observed the phenomenon of giant jumps of both transverse and longitudinal magnetostriction caused by thermomagnetic avalanches. The shapes of the magnetostriction hysteresis loops are also influenced by the phenomenon of the second maximum of the critical current density, called as a peak or fishtail effect. Experimental results can be understood in the framework of the model of the magnetostriction induced by pinning forces.PACS numbers: 74.70.Ad, 75.80.+q     

High temperature magnetic properties of nanocrystalline Sn0∙95Co0∙05O2

Structural and magnetic properties of Sn_0?95Co_0?05O₂ nanocrystalline and diluted magnetic semiconductors have been investigated. This sample has been synthesized by co-precipitation route. Study of magnetization hysteresis loop measurements infer that the sample of Sn_0?95Co_0?05O₂ nanoparticle shows a well-defined hysteresis loop at 300 K temperature, which reflects its ferromagnetic behaviour. We confirmed the roomtemperature intrinsic ferromagnetic (FM) semiconductors by ab initio calculation, using the theory of the functional of density (DFT) by employing the method of Korringa–Kohn–Rostoker (KKR) as well as coherent potential approximation (CPA, explain the disorder effect) to systems. The ferromagnetic state energy was calculated and compared with the local-moment-disordered (LMD) state energy for local density approximation (LDA) and LDA–SIC approximation. Mechanism of hybridization and interaction between magnetic ions in Sn_0?95Co_0?05O₂ is also investigated. To explain the origin of ferromagnetic behaviour, we give information about total and atoms projected density of state functions.     

Effects of annealing on magnetic properties of Fe78B13Si9

We have annealed amorphous Fe₇₈B₁₃Si₉ samples at temperaturesT _a = 355 to 460 ° C for timet _a = 10 to 60 min, in an atmosphere with purging of nitrogen gas. After thermal treatment, we examined the hysteresis loop, DSC curve, and the X-ray diffractogram of each sample. The coercivityH _c, the peak inductionB _max, and the lossW _h were analysed to study the affects of annealing. Stress-relief and partial crystallization are two important parameters in determining optimal annealing conditions.