Variation Of Material Parameters Research Articles

Vibration analysis of rotating functionally graded graphene platelet reinforced composite shaft-disc system under various boundary conditions

A unified dynamic model of a rotating shaft-disc system with multilaminar functional graded graphene platelet reinforced composite (FG-GPLRC) disc is established in order to study the vibration characteristics of the system. Based on first-order shear deformation theory, Timoshenko Beam theory and differential quadrature finite element method, the discrete energy matrices of shaft and disc are obtained. Then the total differential equation of the system is procured by assembling the shaft and disc element matrices. The accuracy and reliability of the model are verified by comparing with the software simulation results. Besides, the influence of boundary conditions, GPL distribution types, GPL weight, disc layers and physical dimensions on the system vibration characteristics is analyzed. It reflects that the GPL distribution types and boundary conditions will make a certain difference to the frequency parameters of shaft-disc system. Furthermore, the vibration characteristics of different GPL distribution types are differently sensitive to the variation in material and geometric parameters.

On robust discretization methods for poroelastic problems: Numerical examples and counter-examples

Finite element approximations of poroelastic materials are nowadays used within multiple applications. Due to wide variation of possible material parameters, robustness of the considered discretization is important. Within this contribution robust of discretization schemes, initially developed for Biot’s theory, will be applied within the Theory of Porous Media. Selected numerical test-cases, special attention will be paid to incompressible and impermeable regimes, are conducted.

Analysis on influence factors for transmission measurements of absorbing and scattering media based on the internal optical-path of FTIR spectrometer

Fouriertransforminfrared(FTIR) spectrometer is commonly used to obtain spectral radiation characteristic parameters of various materials based on transmission or reflection mode. The analysis model for transmission measurements of absorbing and scattering media is established based on the internal optical-path of FTIR spectrometer. According to the experimental phenomenon observed in transmission measurements, the influence of several factors on measuring transmission spectra is studied, including geometric parameters of the measurement optical-path and spectral radiation properties of materials. Based on the verification of calculation model, the relationship between the detected transmittance by the internal optical-path of FTIR spectrometer and the optical thickness of materials is proposed in a similar form as Beer’s law. This study has certain guiding significance for the subsequent use of FTIR spectrometer in the measurements of radiation properties for absorbing and scattering materials.

Effective primary radiation damage model parameters for neutron irradiation experiments

Neutron irradiation damage is a key factor that needs to be studied for materials employed in nuclear systems. The athermal recombination-corrected displacement per atom (arc-dpa) model has been recently proposed for improving the prediction of the primary radiation damage of materials. Using neutron irradiation experimental data, the present work investigates the arc-dpa model parameters for various materials. The adjusted arc-dpa model gives calculation results consistent with measured data within ∼1σ experimental uncertainty. In addition to the optimization of parameters against experimental data, the 1σ reasonable regions of the two model parameters are determined for neutron irradiation damage. Based on the asymptotic value of one of the model parameters, an effective threshold displacement energy Ed∞ is defined. It is observed that Ed∞ is not sensitive to the widely used average threshold displacement energy Ed. The classical Norgett-Robinson-Torrens (NRT)-dpa model using Ed∞ is consistent with the arc-dpa model for neutron irradiation-induced displacement damage.

Positional Stability of Skyrmions in a Racetrack Memory with Notched Geometry

Magnetic skyrmions are chiral spin textures with attractive features, such as ultra-small size, solitonic nature, and easy mobility with small electrical currents that make them promising as information-carrying bits in low-power high-density memory, and logic applications. However, it is essential to guarantee the positional stability of skyrmions for reliable information extraction. Using micromagnetic simulations for the minimum energy path (MEP), we compute the energy barriers associated with stabilizing notches along a racetrack. We vary material parameters, specifically, the strength of the chiral Dzyaloshinskii-Moriya interactions (DMI), the notch geometry, and the thickness of the racetrack to get the optimal barrier height. We find that the reduction of skyrmion size as it squeezes past the notch gives rise to the energy barrier. We find a range of energy barriers up to ~ 45 kBT for a racetrack of 5 nm thickness that can provide years long positional lifetime of skyrmions for long-term memory applications while requiring a moderate amount of current (~ 10^10 A/m2) to move the skyrmions. Furthermore, we derive quasi-analytical equations to estimate the energy barrier. We also explore other pinning mechanisms, such as a local variation of material parameters in a region, and find that notched geometry provides the highest energy barrier. Our results open up possibilities to design practical skyrmion-based racetrack geometries for spintronics applications.

Realizing the thinnest hydrodynamic cloak in porous medium flow

Transformation mapping theory offers us great versatility to design invisible cloaks for the physical fields whose propagation equations remain invariant under coordinate transformations. Such cloaks are typically designed as a multi-layer shell with anisotropic material properties, which makes no disturbance to the external field. As a result, an observer outside the cloak cannot detect the existence of this object from the field disturbances, leading to the invisible effect in terms of field prorogation. In fact, for many prorogating fields, at a large enough distance, the field distortion caused by an object is negligible anyway; thus, a thin cloak is desirable to achieve near-field invisibility. However, a thin cloak typically requires more challenging material properties, which are difficult to realize due to the huge variation of anisotropic material parameters in a thin cloak region. For a flow field in a porous medium, by applying the bilayer cloak design method and integrating the inner layer with the obstacle, we successfully reduce the anisotropic multi-layer cloak into an isotropic single-layer cloak. By properly tailoring the permeability of the porous medium, we realize the challenging material parameters required by the ultrathin cloak and build the thinnest shell-shaped cloak of all physical fields up to now. The ratio between the cloak’s thickness and its shielding region is only 0.003. The design of such an ultrathin cloak may help to achieve the near-field invisibility and concealment of objects inside a fluid environment more effectively.

Evaluating the mechanical properties using high beam energy electron energy loss spectroscopy

The electron energy loss spectra of Ge, Si, Cu, MgO, Al, Ni, C (Diamond) were analyzed. The possibility of mechanical parameters of various materials evaluating using bulk plasmon energy in electron energy loss spectra is shown. The bulk modulus of Ge, Si, Cu, MgO, Al, Ni, C (Diamond) is calculated. It is shown that bulk modulus logarithm has a linear correlation with the bulk plasmon energy.

Fitting the Relaxation Modulus of Viscoelastic Materials Based on the IoT-Supported Mathematics Algorithm

In this paper, the theory of viscoelastic material relaxation modulus testing, testing method, and material relaxation modulus parameter solution method is studied. Based on the IoT sensor test devices and the Hertzian mathematics theory, the relationship between the compression amount, pressure, and relaxation modulus of a rigid spherical indenter pressed into a flat viscoelastic material is studied. In combination with the Boltzmann superposition principle, the relaxation modulus test method and theory for viscoelastic materials are proposed. A good fitting curve was obtained to solve the relaxation modulus parameters of various materials under the three-element Maxwell model. The experimental setup and the results of the parameter fitting were analyzed to verify the feasibility and accuracy of the results.

Direct exchange calculation for unstructured micromagnetic meshes

In micromagnetic simulations it is necessary to compute the effective field associated to the exchange interaction, which corresponds to applying the Laplace operator to the magnetization field. The most widely used approach for computing the exchange field is to apply finite-difference schemes to homogeneous Cartesian meshes. In many situations it would be computationally advantageous to perform micromagnetic simulation on inhomogeneous meshes, but doing so presents a challenge with regard to the calculation of the exchange field. Here we present the construction of an exchange field calculation method for inhomogeneous meshes. A novel second order differential operator method is introduced and applied to a series of analytical functions on irregular meshes and compared to existing methods, finding an accuracy improvement of up to 81% relative to the second order finite difference method. The method is then used in combination with the MagTense framework and tested on the μmag standard micromagnetics problems 3 and 4. Here we find that especially for tetrahedral meshes, the proposed method results in much faster convergence as function of mesh size. We demonstrate that the method converges stably even for meshes where some mesh elements have a volume as small as 1/8 compared to the base resolution. Finally, we show that the method works equally well for meshes with spatially varying material parameters.

Optimization and experimental validation of a bi-focal lens in the microwave domain

The design of photonic devices is usually done through analytical modeling or variation in geometry and material parameters to obtain the required functionalities. Here, we report the use of topology optimization to obtain a bi-focal lens that concentrates the electromagnetic field at different spatial positions depending on the wavelength. Numerical inverse design is carried out to obtain the permittivity layout, satisfying this objective. The resulting device is then 3D printed using two low-loss dielectrics, and experimental field mapping at microwaves demonstrates the ability to enhance the field locally at distinct locations for two separate frequencies.

Reliability evaluation of critical local buckling load on the thin walled cylindrical shell made of composite material

The goal of this research is to evaluate the variability of the buckling load in thin orthotropic cylindrical shells due to the deviation with respect to the nominal values, for both the geometrical and material properties. In this research also the fiber orientation i.e the alignment with respect to the cylinder axis was considered. The research is developed by comparing the results obtained through analytical formulas and numerical results (f.e.m.) for isotropic and orthotropic materials. The coefficient of variation for critical buckling load (for symmetrical and non-symmetrical deformation), assumed for comparing the standard deviation values, is linear with the variation of material and geometrical parameters. The standard deviation on critical buckling load considering both the material and geometrical variations and fiber orientation can be very well described by a surface. From these surfaces it is important to underline that to quantify the dispersion values on the critical buckling load, the material properties assume the fundamental role with respect to the fiber orientation. The main implications of these results are correlated to the fact that, for example, during the manufacturing process, the geometrical and material variability is more important than the fiber orientation in order to have a reliability component.

Evaluation of fatigue parameters estimation methods with regard to specific ranges of fatigue lives and relevant monotonic properties

Evaluations of methods for estimation of fatigue parameters from materials’ monotonic properties and comparisons of estimated and experimental fatigue lives are regularly performed for complete range of fatigue lives and materials with wide range of monotonic properties. In some cases this causes in significant averaging of results of evaluations and possibly erroneous conclusions on estimation methods accuracy. A comprehensive evaluation of three selected, estimation methods was performed using established methodology on an extensive number of detailed fatigue datasets on unalloyed, low-alloy and high alloy steels. Results provide detailed insight on the accuracy of the selected estimation methods and their applicability for the estimation of fatigue parameters of various materials divided to low and high strength subgroups and for low- and high-cycle fatigue regimes.

Stress peaks, stiffening and back-flow in bilayer poro-elastic metamaterials

In the present work, we present a linear poroelastic model for a quasi-static oscillating bilayer beam made by two thick strata in perfect bond at their interface, characterized by different elastic moduli and permeability, both directly influenced by porosity. The solution is built up by considering the structure as a three-dimensional object and is obtained analytically for the full coupling of mass balance and mechanical equations, by imposing weak boundary conditions on the sole forces emerging at the lateral outermost surfaces, neglecting the inertial effects. After verifying the accuracy of the solution by comparing theoretical outcomes and numerical results from Finite Element simulations, ad hoc sensitivity analyses were then carried out in order to investigate the effects on the stress distribution and fluid flow due to prescribed inhomogeneities related to variations of material and geometrical parameters across the beam thickness, by keeping constant the total volume of the porous solid skeleton and applying self-equilibrated and oscillating in time axial forces and bending moments at two opposite sides of the structure. Some dynamic analyses were also performed to highlight proper periods and frequencies ranges where pulse loadings ensured the quasi-static response of the system, however discussing interferences due to transient regimes for selected cases on the basis of numerical analyses. Finally, we show that, by playing with ratios between elastic moduli, thickness and porosity of the two layers, a number of counterintuitive or even non-conventional behaviors can be observed, which give rise to unexpected back-flow phenomena and anomalous fluid content distributions, as well as to stress peaks kindled by moving the layer interface and overall stiffening that could be all exploited for the design of new multi-objective poroelastic metamaterials.

Influence of Material Parameters on the Contact Pressure Characteristics of a Multi-Disc Clutch.

As an essential part of the transmission, the life of the clutch directly affects the stability of the transmission. In this paper, a finite element model and a thermodynamic numerical model of a multi-disc clutch are established to investigate the influence of material parameters on the contact pressure distribution. The pressure distribution index (PDI) is firstly proposed to evaluate the pressure difference among friction pairs. Moreover, the correctness of the numerical model is verified by the clutch static pressure experiment. The results show that increasing the elastic modulus and Poisson’s ratio of the backplate can effectively improve the uniformity of the contact pressure. However, the variations in material parameters of other clutch components can not easily smooth the pressure difference. Therefore, optimized material parameters for the clutch are proposed, where the maximum pressure and temperature differences are reduced by about 27.2% and 10.3%, respectively.

Improving failure rates in pulsed SOT-MRAM switching by reinforcement learning

Finding and optimizing robust schemes for field-free switching remains a challenging problem in spin-orbit torque magnetoresistive random access memories. In this work reinforcement learning is employed for the optimization of switching schemes for such memory cells. A cell is switched purely electrically by applying pulses to two orthogonal metal wires. It is shown that a neural network model trained on a fixed material parameter set is suitable to determine optimal pulse sequences for reliable switching in the presence of thermal fluctuations, material parameter variations and reduction of the current to a sub-critical value. Multiple realizations of switching by means of simulation prove the reliability of magnetization reversal based on the pulse sequences found via reinforcement learning and show that the failure rate due to material parameter variations in these memory devices can be significantly reduced.

New Inverse Method for Determining Uniaxial Flow Properties by Spherical Indentation Test

The spherical indentation test has been successfully applied to inversely derive the tensile properties of small regions in a non-destructive way. Current inverse methods mainly rely on extensive iterative calculations, which yield a considerable computational costs. In this paper, a database method is proposed to determine tensile flow properties from a single indentation force-depth curves to avoid iterative simulations. Firstly, a database that contain numerous indentation force-depth curves is established by inputting varied Ludwic material parameters into the indentation finite elements model. Secondly, for a given experimental indentation curve, a mean square error (MSE) is designated to evaluate the deviation between the experimental curve and each curve in the database. Finally, the true stresses at a series of plastic strain can be acquired by analyzing these deviations. To validate this new method, three different steels, i.e. A508, 2.25Cr1Mo and 316L are selected. Both simulated indentation curves and experimental indentation curves are used as inputs of the database to inversely acquire the flow properties. The result indicates that the proposed approach provides impressive accuracy when simulated indentation curves are used, but is less accurate when experimental curves are used. This new method can derive tensile properties in a much higher efficiency compared with traditional inverse method and are therefore more adaptive to engineering application.

Model updating of the vehicle-track coupled system based on in-situ dynamic measurements

With the rapid development of subway transportation, it has brought great convenience to us, but the induced vibration has seriously influenced our daily life. The simulation models are the major approach to investigate this problem. However, due to the discreteness and randomness of the physical parameters of various materials and inappropriate simplifications of the finite element (FE) model, the simulated results cannot reflect the actual dynamic responses accurately. To solve this issue, we carried out systematic investigations as follows: (1) conducting in-situ dynamic measurements of the track structure in an operating subway line; (2) establishing a simulation model of the vehicle-track coupled system; (3) proposing the multi-objective functions according to the simulated and measured results; (4) designing and implementing the sensitivity analysis based on the Latin Hypercube Sampling (LHS); (5) devising and performing the model updating process based on the multi-island genetic algorithm (MIGA). The results reveal that the updated model can simulate the dynamic responses accurately, and the simulation results have an excellent agreement with the in-situ measurements. Overall, the developed method in this work can obviously improve the accuracy of the dynamic simulation of the track structure and provide a way for the model updating and vibration analysis of the vehicle-track coupled system.

A parallel space-time multigrid method for the eddy-current equation

We expand the applicabilities and capabilities of an already existing space-time parallel method based on a block Jacobi smoother. First we formulate a more detailed criterion for spatial coarsening, which enables the method to deal with unstructured meshes and varying material parameters. Further we investigate the application to the eddy-current equation, where the non-trivial kernel of the curl operator causes severe problems. This is remedied with a new nodal auxiliary space correction. We proceed to identify convergence rates by local Fourier analysis and numerical experiments. Finally, we present a numerical experiment which demonstrates its excellent scaling properties.

Multi-Objective Optimization of Tree Trunk Axes in Glulam Beam Design Considering Fuzzy Probability-Based Random Fields

Abstract Deterministic design and a priori parameters are used in traditional optimization approaches. The material characteristics of solid wood are not deterministic in reality. Hence, realistic optimization and simulation methods need to take the uncertainties of parameters into account. The uncertainty characteristics of wood are mainly originated in natural variation. In addition to this, incertitudes from lack of knowledge are inherent. Accordingly, the aleatoric approach of randomness can be expanded to a polymorphic uncertainty model. Fuzzy probability-based randomness is used in this work. Therefore, the epistemic approach of fuzziness is taken into account. The distribution functions of random variables are parametrized by fuzzy variables. So coupling of both, aleatoric and epistemic uncertainties, is involved. Interactions of fuzzy variables and crosscorrelations of random variables are considered among and within the parameters. Crosscorrelated random fields are used to represent spatial variation of material parameters. The autocovariance structures are modeled structurally dependent on the tree trunk axes. Finite element method is applied as deterministic basic solution of a loaded timber structure. A local orthotropic material formulation with respect to specifically located tree trunk axes is used. The optimal positions of the tree trunk axes for each wooden log are examined as design parameters. Polymorphic uncertainty is used to describe a priori parameters. The developed methods for uncertainty analysis are embedded in an automated and parallelized optimization processing. An analysis of a two-tier glulam beam, according to a purlin of a timber roof construction, is shown as numerical example for the optimization framework.

Optimization of a Spin-Orbit Torque Switching Scheme Based on Micromagnetic Simulations and Reinforcement Learning.

Spin-orbit torque memory is a suitable candidate for next generation nonvolatile magnetoresistive random access memory. It combines high-speed operation with excellent endurance, being particularly promising for application in caches. In this work, a two-current pulse magnetic field-free spin-orbit torque switching scheme is combined with reinforcement learning in order to determine current pulse parameters leading to the fastest magnetization switching for the scheme. Based on micromagnetic simulations, it is shown that the switching probability strongly depends on the configuration of the current pulses for cell operation with sub-nanosecond timing. We demonstrate that the implemented reinforcement learning setup is able to determine an optimal pulse configuration to achieve a switching time in the order of 150 ps, which is 50% shorter than the time obtained with non-optimized pulse parameters. Reinforcement learning is a promising tool to automate and further optimize the switching characteristics of the two-pulse scheme. An analysis of the impact of material parameter variations has shown that deterministic switching can be ensured for all cells within the variation space, provided that the current densities of the applied pulses are properly adjusted.