Isotropic Samples Research Articles

Evaluation of forming limit diagrams using Nakazima out-of-plane test and incremental forming process for two-phase magnesium–lithium alloy sheet

The focus of this paper is to investigate the formability of two-phase Mg–Li sheets through drawing forming limit diagrams (FLDs) obtained from Nakazima out-of-plane test and incremental forming process. For this goal, Mg LZ71 manufactured by casting and then as-cast ingot converted to plate by hot rolling, and finally, the sheets were annealed. Then, the microstructure of the LZ71 alloy was assessed by an optical microscope. Also, the mechanical properties of the LZ71 samples were investigated using uniaxial tensile tests. Furthermore, to achieve isotropic samples, anisotropy parameters of as-cast, as-rolled, and annealed samples were determined. Afterward, the formability of the produced LZ71 sheet was studied with the help of FLD. In the present investigation, the FLDs of the LZ71 sheet were obtained using two methods, including the Nakazima out-of-plane test and the incremental forming process. The results showed that Mg LZ71 has a dual-phase microstructure, which causes considerable formability. Also, the results showed that FLD of the Mg LZ71 sheet in incremental forming is higher than that of the conventional Nakazima test.

Three-dimensional patterned distribution of thermal conductivity in the volume for effective thermal concentration

For thermal devices and electronic systems, heat flow manipulation is critical to meet various needs. In this article, based on transformation thermodynamics, three-dimensional (3D) structures in the volume for patterned thermal conductivity were designed and fabricated. The 3D structures in the volume acting as heat flow routes can be achieved as pre-designed, providing a promising method to manipulate heat flow applied to the localized thermal concentration, thermal cloaking, etc. Simulation and experiments show that, by the 3D structures for thermal concentration, the heat flow could be concentrated to the pre-designed region, 7.0 K higher compared to the isotropic sample without any internal structures, and with a temperature gradient of over 1.3 K mm−1 to the surroundings (the heat source is 361 K), indicating an effective heat concentration. It also reveals that the heat flow could be accurately manipulated by carefully designing the geometric and material parameters of the internal structures, providing an alternative method to construct a 3D matrix for heat flow manipulation in the volume.

Resistance Matrix for an Anisotropic Hall Plate with Multiple Extended Asymmetric Contacts on the Boundary

The electrical properties of magnetic sensing devices fabricated from anisotropic materials are not easily extracted. Here we present a method for determining the resistance matrix for an anisotropic device with multiple electrical contacts placed in a perpendicular magnetic field. By using the methods developed by Van der Pauw and Wasscher, the analysis for the anisotropic system is reduced to the equivalent problem for an isotropic sample, which can then be solved using methods developed previously. As a result, the method works in the case of structures with an arbitrary number of asymmetric extended contacts at large magnetic field strength. In addition to the extraction of nonisotropic resistivities, the resistance matrix can be used to analyze the Hall effect for anisotropic plates.

Modified snapshot spectroscopic ellipsometry based on optical frequency-domain interferometry

We propose a modified snapshot spectroscopic ellipsometry based on optical frequency-domain interferometry. The proposed system employs only one high-order retarder and a Wollaston prism to analyze the changed state of polarization of the reflected light and can provide the maximum channel bandwidth of the channeled spectroscopic ellipsometry. The spectroscopic ellipsometric parameters of isotropic samples can be accurately measured in a measurement speed of 40 ms without mechanical or active modulation devices. The feasibility of the proposed spectroscopic ellipsometry is demonstrated by experiments.

Mathematical Model of Object Classifier based on Bayesian Approach

The paper claims that the primary importance in solving the classification problem is to find the conditions for dividing the General complexity into classes, determine the quality of such a bundle, and verify the classifier model. We consider a mathematical model of a non-randomized classifier of features obtained without a teacher, when the number of classes is not set a priori, but only its upper bound is set. The mathematical model is presented in the form of a statement of a minimax conditional extreme task, and it is a problem of searching for the matrix of belonging of objects to a class, and representative (reference) elements within each class. The development of the feature classifier is based on the synthesis of two-dimensional probability density in the coordinate space: classes-objects. Using generalized functions, the probabilistic problem of finding the minimum Bayesian risk is reduced to a deterministic problem on a set of non-randomized classifiers. At the same time, the use of specially introduced constraints fixes non-randomized decision rules and plunges the integer problem of nonlinear programming into a General continuous nonlinear problem. For correct synthesis of the classifier, the dispersion curve of the isotropic sample is necessary. It is necessary to use the total intra-class and inter-class variance to characterize the quality of classification. The classification problem can be interpreted as a particular problem of the theory of catastrophes. Under the conditions of limited initial data, a minimax functional was found that reflects the quality of classification for a quadratic loss function. The developed mathematical model is classified as an integer nonlinear programming problem. The model is given using polynomial constraints to the form of a General problem of nonlinear continuous programming. The necessary conditions for the bundle into classes are found. These conditions can be used as sufficient when testing the hypothesis about the existence of classes.

A continuum magneto-mechanical model for magnetorheological elastomers

Magnetorheological elastomers (MREs) consist of micron-sized magnetizable particles embedded in a rubber matrix. Properties of these magneto-sensitive materials are changed reversibly upon application of external magnetic fields. They exhibit highly non-linear magneto-mechanical response which allows developing new devices and applications. However, the coupled magneto-mechanical behavior makes mathematical modeling of MREs quite complicated. So development of a reliable constitutive framework is essential for further understanding of this coupled behavior as well as simulation of the systems that utilize MREs. In this paper, a finite strain continuum model is developed for MREs where the effect of magnetization on material stiffness is directly introduced in the material shear modulus. It is shown that this approach simplifies the constitutive models and also perceives the magnetic saturation of MREs. Moreover, the coupled effects of magnetization, deformation and particle-chains orientation on the mechanical response are also taken into account in the introduced parameter. This reduces the number of material parameters, the required experimental tests for parameters identification and also simplifies the mathematical formulation of the developed constitutive equations which is beneficial for numerical formulations. A systematic two-step method is then introduced for material parameters identification which assures uniqueness of the parameters set. The predictive capabilities of the proposed model are examined via available mechanical and magneto-mechanical experimental data on both isotropic and anisotropic MRE samples at different configurations of magnetic field and loading with respect to the preferred direction of the samples. It is shown that the model can well predict the magneto-mechanical response of MREs at different deformation modes and magnetic fields.

Geochronology and Geochemistry of Gabbros from Moa‐Baracoa Ophiolitic Massif, Eastern Cuba: Implication for Early Cretaceous SSZ Magmatism

AbstractA series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto‐Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates (Iturralde‐Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back‐arc basin, forearc basin or plume setting (e.g. Giunta et al., 2009; Marchesi et al., 2016; Kerr et al., 1999). The Moa‐Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa (20°35′ 17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence (CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U‐Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2 (47.22 wt%–49.25 wt%), TiO2 (0.13wt%–0.26 wt%) and K2O (0.04 wt%–0.08 wt%) and high MgO (8.22 wt%–16.72 wt%), Cr (1337–2065 μg/g) and Ni (192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt% to 21.98 wt%, and 0.71 wt% to 1.87 wt%, respectively. In the SiO2 versus Zr/TiO2∗0.0001 diagram, the samples plot in the sub‐alkaline basalt field but show calc‐alkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents (ΣREE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE (LREE, (La/Yb)N = 0.34–0.50) and flat heavy REE (HREE, (Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies (Eu/Eu∗ = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle‐normalized trace element diagram. Geochemical features, such as high values of Ba/La (30–38) and low Th/Yb (0.02–0.03), medium values of (Ta/La)PM (0.30–0.35) and (Hf/Sm)PM (0.58–0.80), indicate enrichment by slab‐derived fluids and subduction‐related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran (Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the Moa‐Baracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.

A setup to measure the Seebeck coefficient and electrical conductivity of anisotropic thin-films on a single sample.

This work documents an all-in-one custom setup that allows us to measure the in-plane Seebeck coefficients and electrical conductivities of anisotropic thin film samples close to room temperature. Both pairs, S∥ and σ∥ and S⊥ and σ⊥, can be measured using four contacts on the same sample, reducing measurement time and minimizing potential sources of error due to aggregating data from several distinct samples. The setup allows us to measure the electrical conductivity of isotropic samples using the well-known van der Pauw method. For samples with in-plane anisotropy, the two components σ∥ and σ⊥ can be extracted from the same type of measurements by performing additional calculations. Using the same contacts, the Seebeck coefficient along one direction is measured using a differential steady-state method. After rotating the sample by 90°, the orthogonal Seebeck component can be measured. In order to show the generality of the method, we measure different types of samples, from metal references to oriented doped conjugated polymers.

Transversely isotropic magnetoactive elastomers: theory and experiments

The current contribution is concerned with the development of novel constitutive equations for anisotropic magnetoactive elastomers (MAEs). A hyperelastic material model for an incompressible magnetoelastic medium representing transversely isotropic MAEs has been developed to investigate their response behavior in the presence of the applied magnetic field while undergoing finite deformation. Transversely isotropic MAE samples in circular cylindrical geometry with 15% iron particle volume fraction are then fabricated and experimentally tested to measure their permeability and torque–twist response. The experimental results have then been effectively utilized to identify the constant material parameters in the proposed material model. Finally, the accuracy and validity of the proposed constitutive equations are demonstrated through the comparison of the simulation and experimental results.

Characterization and modeling of temperature effect on the shear mode properties of magnetorheological elastomers

The temperature dependency of the viscoelastic properties of magnetorheological elastomers (MREs) operating in the shear mode was experimentally investigated under broad ranges of strain amplitude, excitation frequency, and applied magnetic field. Experiments were performed with isotropic MRE samples with volume fraction of carbonyl iron particles dispersed in the silicone rubber matrix under controlled temperature, ranging from to together with different levels of magnetic field density ( to ). The results revealed significant effect of temperature on the linear and nonlinear viscoelastic properties of MREs, in addition to the effects of strain amplitude, rate, and applied magnetic flux density. The temperature dependency of mechanical properties of an unfilled rubber matrix was also measured, which served as a reference. The shear properties of the MRE revealed reductions in both the storage and loss moduli with increasing temperature. The temperature dependency of the MRE, however, diminished under shear strain in excess of the critical strain. The results also revealed more pronounced temperature effect at higher temperatures and excitation frequencies, while it was relatively lesser with increasing magnetic field. Increase in temperature resulted in enhanced magnetorheological effect, while a clear trend with regard to the excitation frequency could not be established. Finally, a phenomenological model has been developed to predict the storage and loss moduli of the MRE as a function of excitation frequency, applied magnetic flux density, and temperature. The superb performance of the proposed model in predicting viscoelastic moduli under various operating and environmental conditions has been demonstrated through comparison of the experimental and simulation results.

Influence of sample thickness, curvature and notches on the Charpy impact strength - An approach to standardise the impact strength of curved test specimens and biological structures

The specimen geometry has a significant influence on the Charpy impact strength. This is often a problem when it comes to the analysis of materials that can only be prepared with a curved shape, or which, like some biological structures, are naturally given in curved form like nutshells. The question repeatedly arises to what extent the curvature influences the impact strength. Hence, the present study deals with the dependence of the specimen geometry like specimen thickness, curvature and notches on the impact properties of concrete, gypsum, polyurethane (PUR) and epoxy-based (EP) samples. Increasing sample thickness from 2 to 10 mm resulted in increased toughness for brittle materials, whereas more ductile materials did not show any significant change in toughness. Increasing span length showed an increase of the unnotched impact strength for brittle gypsum, while toughness increased for a more ductile PUR sample. For the curved specimens both, the unnotched and the notched Charpy impact strength could be shown to increase with a reduction of the specimen radius (higher curvature) while the notch-sensitivity was not significantly affected. Material-specific linear relationships between the impact strength and the curvature of the samples were found. These relationships were used to create a model to calculate the influence of the curvature on the impact strength. For all materials, the characteristic values of a normalised flat standard sample could be well predicted from measured values of curved specimens. This calculation approach was used to predict the impact strength of an isotropic biological sample (sweet potato) concerning a flat sample and different curved specimens. The results show a good trend with the measured values.

Application of Rietveld refinement in orientated material structures

The phase analysis based on the X-ray diffraction method is a well-known procedure for determining the phases crystalline in the sample. The phase identification is simple in case of the isotropic powder samples, however the standard investigation protocol cannot always be used, e.g. in case if the presence of anisotropy like preferred orientation (PO) is observed. In case of steels, the diffracted intensity of a given phases is influenced by the directional phase development having a needle character caused by the martensitic transformation; some reflections can even disappear from the diffractograms as a consequence of the strong PO. One impacting consequence of PO is over quantitative evaluation, percentages close to actual values are obtained only if diffracted intensity is integrated for the whole measured 2θ range for each phase and anomalous intensities caused by PO are corrected.Incorrect quantitative values were obtained by using the full-profile analysis method in the course of our earlier investigations, because no correction method had been available for the recognition and evaluation of the orientated structures in phase-identification of metals. By using the Rietveld refinement method for whole measured 2θ range, both the PO and the size-strain broadening effects are accounted for. Strain results from stress developing in the lattice and can be taken into consideration in the calculated model and results are corrected to accurately describe the real structure. In the course of our research work, orientated, complex structures developing in the Cu-base alloys were investigated by using the Rietveld refinement method. The quantity of phases was determined in the different samples in which clear signs of the used technological processes could be observed in the structures. The character of orientation appearing in the structure was observed, evaluated and the necessary corrections were performed.

Prediction of the sound absorption behavior of nonwoven fabrics: Computational study and experimental validation

Prediction of acoustic characteristics of porous media from the microstructure is the first step in designing novel materials with enhanced acoustic properties. This paper aims to provide insights into the microstructure of nonwoven fabrics and demonstrate how it collectively dictates their acoustical macro-behavior. To this end, fibers of different fineness were spun on an industrial-scale compact melt spinning line. Fibers were then fed into a laboratory needle punching line to produce nonwoven fabrics. Microstructural properties of fabrics were obtained using morphological analysis of their 3D realistic images obtained using X-ray micro-computed tomography (µCT). The sound absorption coefficient (SAC) of samples was measured using a two-microphone impedance tube. Moreover, to investigate the effect of fabric microstructure on sound absorption behavior, virtual fibrous structures with different thicknesses, porosities, fiber diameters, and through-plane fiber orientations were simulated. Fluid resistivity was calculated by simulation of airflow through the microstructure of generated virtual structures and solving the Stokes equations. Finally, the sound absorption spectrum of nonwoven fabrics was predicted using the empirical model of Delany-Bazley-Miki, and the results were compared with experimental data. Very good agreement was observed between the results of the proposed model and the experimental data. The results indicated that for samples with the same porosity of 91% and fiber diameter of 16 µm, an increase in fabric thickness up to 2 cm leads to an increase in SAC, while further increase in thickness does not improve SAC at high-frequency bands. Moreover, with increasing fabric thickness, the SAC peak shifts from the high-frequency range to the low-frequency range. It was found that for samples with a thickness of 3 cm, decreasing fiber diameter from 32 µm to 16 µm enhances SAC at low- and medium-frequency ranges, while a further decrease in fiber diameter adversely affects the SAC. It was also found that there is an optimal porosity for a nonwoven fabric in view of maximizing its sound absorption performance. The results indicated that SAC decreases with decreasing the angle between the dominant fiber orientation and sound incidence direction. It was concluded that 3D uniform isotropic samples exhibit the highest SAC.

Long-Lasting Molecular Orientation Induced by a Single Terahertz Pulse.

We present a novel, previously unreported phenomenon appearing in a thermal gas of nonlinear polar molecules excited by a single THz pulse. We find that the induced orientation lasts long after the excitation pulse is over. In the case of symmetric-top molecules, the time-averaged orientation remains indefinitely constant, whereas in the case of asymmetric-top molecules the orientation persists for a long time after the end of the pulse. We discuss the underlying mechanism, study its nonmonotonous temperature and amplitude dependencies, and show that there exist optimal parameters for maximal residual orientation. The persistent orientation implies a long-lasting macroscopic dipole moment, which may be probed by even harmonic generation and may enable deflection by inhomogeneous electrostatic fields.

An Adaptive BRDF Fitting Metric

AbstractWe propose a novel image‐driven fitting strategy for isotropic BRDFs. Whereas existing BRDF fitting methods minimize a cost function directly on the error between the fitted analytical BRDF and the measured isotropic BRDF samples, we also take into account the resulting material appearance in visualizations of the BRDF. This change of fitting paradigm improves the appearance reproduction fidelity, especially for analytical BRDF models that lack the expressiveness to reproduce the measured surface reflectance. We formulate BRDF fitting as a two‐stage process that first generates a series of candidate BRDF fits based only on the BRDF error with measured BRDF samples. Next, from these candidates, we select the BRDF fit that minimizes the visual error. We demonstrate qualitatively and quantitatively improved fits for the Cook‐Torrance and GGX microfacet BRDF models. Furthermore, we present an analysis of the BRDF fitting results, and show that the image‐driven isotropic BRDF fits generalize well to other light conditions, and that depending on the measured material, a different weighting of errors with respect to the measured BRDF is necessary.

Development and Control of Magnetorheological Elastomer-Based Semi-active Seat Suspension Isolator Using Adaptive Neural Network

The present study aims at development of a magnetorheological elastomer (MRE) based semi-active seat suspension isolator and its adaptive control using neural network (NN) control scheme. Isotropic MRE samples with 25% volume fraction of iron particles have been fabricated and then characterized under shear mode using a rotary magneto-rheometer to obtain MRE’s viscoelastic properties (shear storage and loss moduli) under different levels of applied magnetic flux density. Results reveal a significant change in the storage and loss moduli with respect to the varied magnetic field. The viscoelastic properties of the MRE are then utilized to design an MRE-based seat suspension isolator in order to attenuate the transmitted vibration to the driver. For this purpose, the modeling of the seat incorporated with the MRE-based isolator is derived and subsequently, a novel NN control scheme is proposed for the semi-active control of the MRE-based isolator. The convergence and stability of the proposed control strategy have been mathematically verified using the Lyapunov method. Finally, the performance of the proposed control strategy is compared with those obtained using passive and widely used sky-hook controllers under different types of excitation including harmonic motion, road bump, and random profile. It is shown that the proposed NN controller considerably mitigates the vibration of the driver seat and outperforms the passive and skyhook controllers over the frequency range of interest.

Finite deformation analysis of isotropic magnetoactive elastomers

In this study, the large deformation analysis of the magnetoactive elastomers based on continuum mechanics approach has been conducted. First, the governing differential equations for the spatial configuration are presented. Stored energy density function defined with respect to the invariants of the right or left Cauchy–Green deformation tensor and the magnetic field induction vector is adopted to develop a material model for finite deformation of isotropic magnetoactive elastomers (MAEs). An isotropic magnetoactive sample with 15% iron particle volume fraction is then fabricated, and a test setup has been designed to measure its magnetic permeability. Using an advanced magnetorheometer, quasi-static tests are then carried out on MAE circular cylindrical samples to find their torque-twist response under various magnetic fields. The experimental results are then effectively utilized to identify the unknown parameters in the proposed material model. The accuracy of the proposed constitutive model in predicting the response behavior of the MAE is then demonstrated through comparison of theoretical results with those obtained experimentally.

Re-examining the evidence of the Hercules–Corona Borealis Great Wall

ABSTRACT In the lambda cold dark matter paradigm of cosmology, anisotropies larger than 260 Mpc should not exist. However, the existence of the Hercules–Corona Borealis Great Wall (HCB) is purported to challenge this principle by some with an estimated size exceeding 2000 Mpc. Recently, some have challenged the assertion of the existence of the HCB, attributing the anisotropy to sky exposure effects. It has never been explained why the original methods purporting the existence of the HCB produce anisotropies, even if sky-exposure effects are taken into account. In this paper, I apply the methods of the original papers purporting the existence of the HCB in various Monte Carlo simulations that assume isotropy to analyse the empirical meaning of the significance levels of the original tests used. I find that, although the statistical tests at first glance show significant anisotropies present in the suspect sample, Monte Carlo simulations can easily reproduce the sample in most cases, and if not, the differences can be accounted for by other statistical considerations. An updated sample raises the probability of drawing the observed clustering from an isotropic sample ten-fold in some cases. Thus the statistical tests used in prior studies overestimate the significance of the observed anisotropy, and an updated sample returns even less significant probabilities. Given the ability to reproduce the observed anisotropy in Monte Carlo simulations, the new, higher probabilities of being drawn from isotropy for an updated sample, and the work of previous papers attributing anisotropies to sky-selection effects, the existence of the HCB must be treated as doubtful at best.

Ultrasonic characterization of anisotropic rocks: Impact of transducers’ size and geometry on data interpretation

Typical ultrasonic laboratory measurements of rock physical properties are conducted with ultrasonic transducers that are relatively large compared to the rock sample. We have determined the impact of the transducer size on the resulting velocity estimations. To improve data interpretation, we explore two complementary avenues: (1) explicitly account for the finite size of the transducers as part of the data interpretation/correction workflow, rather than assuming point sources, and (2) reducing the effective size of the transducers at the hardware design level. Both approaches as well as their combination have been tested successfully on multiple data sets including artificial homogeneous and isotropic samples as well as natural anisotropic rocks such as shales.

Analytic solutions for calculating the surface inclination of isotropic media and bare substrates by using reflection-based generalized ellipsometry

Using ellipsometry for curved-surface characterization requires the knowledge of the surface normal vector in order to determine material-related surface parameters like refractive index, layer thickness, or birefringence of the surface material at the incidence point, because the recorded signal depends on both the (unknown) surface normal vector n→ and material-related surface characteristics. It is convenient in ellipsometry to parametrize the surface normal vector by the angle of incidence θ and the azimuthal rotation angle ϕ. Depending on the design of the ellipsometer, there may be two angles, ϕ1 and ϕ2, necessary, which describe the azimuthal rotation before and after the light is reflected off the sample, respectively. The authors present analytic formulas to determine ϕ1 and ϕ2 for optically isotropic samples using generalized ellipsometry. The resulting measurement uncertainty is lower than that of previously known methods. Furthermore, the authors provide an analytic formula to calculate θ from the ellipsometric angles Ψ and Δ for bare substrates with known refractive index N1=n1−ik1. The formulas have been evaluated with experimental data acquired with a conventional and an imaging retroreflection-based return-path ellipsometer.