Aphrodite Ktena

A Preisach model identification procedure and simulation of hysteresis in ferromagnets and shape-memory alloys

Abstract A Preisach model able to adjust to different systems with hysteresis is presented. The related identification scheme involved uses data from a major hysteresis curve and a least-squares error minimization procedure for the parameters of the characteristic density. The output sequence, f ( t ), is obtained by integrating the characteristic probability density function, ρ ( α , β ), of the elementary hysteresis operators, γ ab , operating on the input sequence u ( t ) over the Preisach plane. Once the appropriate operator is chosen and the Preisach plane adjusted accordingly, the parameters of the characteristic density are determined via a least-squares procedure minimizing the error between the experimental major curve and the calculated one. Results using two different scalar operators are discussed. Further, the reliability of the procedure is assessed by considering experimental data regarding two different magnetic samples and a shape-memory alloy sample.

Vector Preisach modeling (invited)

While the scalar Preisach model has long served as a description of hysteretic processes, the response of ferromagnetic materials is essentially vector in character and scalar models are likely to fail in important respects for many advanced applications. A 2D vector Preisach model has been developed for the simulation of magnetic recording and playback for oriented media. The objectives of the model are (i) faithful reproduction of both scalar and rotational measurements made on bulk material and (ii) calculational efficiency for use in the simulations. Past work on vector hysteresis models is reviewed. The present model is a superposition of Preisach models with different orientation directions. On the basis of the properties of the single‐domain uniaxial particle, the effect of the transverse field is included as the boundary between ‘‘up’’ and ‘‘down’’ magnetization on the Preisach plane is calculated; then the reversible rotation is calculated as well. The performance of the model in describing measured bulk properties is presented.

A Smart ZIGBEE Based Wireless Sensor Meter System

A temperature, humidity and light meter has been designed and implemented using open standards technology and commercial components in order to monitor environmental indoor and outdoor conditions. At the heart of the smart meter lies a microprocessor processor using the ZigBee protocol to communicate the data transmitted by commercial wireless sensors. A prototype has been built and preliminary measurements have been taken. The potential of this configuration is being discussed.

Vector Preisach modeling and recording applications

The authors report that recording and playback on a variety of high energy tape media have been simulated using a new 2-D vector Preisach model in a self-consistent calculation of the magnetization. Simulation results are compared against vibrating sample magnetometer (VSM) bulk measurements and experimental erasure data. >

Identification of 1D and 2D Preisach models for ferromagnets and shape memory alloys

Abstract A least-squares parameter fitting procedure is used in conjunction with one-dimensional (1D) and two-dimensional (2D) formulations of the Preisach formalism. The model can use any operator from a selection of 1D and 2D hysteresis operators appropriate for the modeling of hysteresis in ferromagnets, and shape memory alloys. The performance of the hysteresis operators is evaluated, and their applicability is discussed. The model is applied to experimental data from ferromagnetic samples, and shape memory alloys. The results suggest that the proposed approach can be a useful and adjustable tool in the modeling of hysteresis in various systems regardless of the underlying physical mechanism.

On the Universality of the Dependence of Magnetic Parameters on Residual Stresses in Steels

A method for the monitoring of residual stress distribution in steels has been developed based on non-destructive magnetic permeability measurements. The dependence of differential permeability on residual stresses induced through a controlled process of applied tensile and compressive stress in the elastic region, of all three zones of the welded metal, yields the magnetic stress calibration curve (MASC). MASC is obtained on flawless welded steel plates and can be measured for any grade of ferromagnetic steels. A surface MASC correlates the magnetic permeability with the spatial stress distribution, as determined by the X-Ray Diffraction Bragg-Brentano diffraction. A bulk MASC correlates the bulk magnetic permeability with residual stresses, as determined by the neutron diffraction. The resulting calibration curves, obtained for several grades of ferromagnetic steels, have a sigmoid shape but are unique for each grade of steels. Normalizing the magnetic permeability and the stress values against the differential permeability measured at the yield point and yield stress, respectively, the dependence of the local magnetic permeability on residual stresses for all different tested grades of steels results in a universal curve relating magnetic and elastic properties of steels at the macroscopic level.

Stress Dependent Magnetization and Vector Preisach Modeling in Low Carbon Steels

Measurements of magnetic properties have been performed on commercially available low carbon steels (Armco and non-oriented electrical steels) in order to construct and test a Preisach model for materials under stress. The measurements include hysteresis loops of samples under tensile stress taken during loading and after unloading at given strain levels. A vector Preisach model using the SW model as the elementary operator, superposition to account for easy axes dispersion and a weighed mixture of normal distributions to account for the long-range interactions established through the magnetoelastic coupling is proposed. The model reproduces the effect of applied stress, verifying the experimental finding that the permeability of a material under tensile stress decreases. The Preisach density is constructed in such a way as to account for the strong magnetostatic interactions observed in the high induction regions in the case of built-in stresses, so the model is also able to capture the effect of the residual compressive stresses in unloaded materials. The use of the differential permeability and its first derivative are proposed as parameters to be monitored in NDT applications.

Preisach modeling of AFC magnetic recording media

The ability of the Preisach formalism to model hysteresis processes in antiferromagnetically coupled (AFC) recording media is investigated in this paper. Two modeling approaches are proposed, each one using a weighed mixture of two normal bivariate probability function densities as the characteristic density and either classical or appropriately modified one-dimensional or two-dimensional hysteresis operators. The models are identified for three samples of different lower layer thickness, t/sub L/. In all cases, the resulting weighing factor w/sub L/ correlates with t/sub L/ as do some of the density parameters with material parameters. The calculated major loops compare well with the experimental data while the discrepancy of the calculated minor loops at fields lower than the coercive field indicates the limitations of the proposed models.

Residual Stress Analysis in Nonoriented Electrical Steel Sheets by Barkhausen Noise Measurements

The nondestructive magnetic Barkhausen noise (MBN) technique has been used to study the effect of residual stress in nonoriented electrical steels. The MBN response is presented and evaluated with respect to thermomechanical treatments. The ensuing microstructure was studied using scanning electron microscopy, transmission electron microscopy, and hardness measurements. The stress-dependent MBN measurements exhibit good correlation with XRD stress measurements made by the d - sin2 ψ method, using Cr-Ka radiation, and (211) reflections. It has been found that there is a good correlation between MBN response and variations in microstructure, which affects mechanical properties.

The effect of viscoelastic brain on the dynamic characteristics of the human skull-brain system

SummaryIn this paper, we present a solution to the problem of free vibrations of the human head system taking into account the dissipative behaviour of the brain. The mathematical model is based on the three-dimensional theory of viscoelasticity and the representation of the displacement field in terms of the Navier eigenvectors. The frequency equation is solved numerically and the results for eigenfrequencies and damping coefficients are presented for various geometrical and physical parameters of the system. The results obtained are in excellent agreement with the measured eigenfrequencies, and the predicted damping coefficients are within the order of magnitude of the measured ones. From the proposed analysis we have obtained the information that the role of the viscoelastic neck as well as the viscoelastic properties of the skull-brain system have to be simultaneously taken into account in the study of the frequency spectrum of the human head. The analysis of the realistic model is under preparation.