Decrease In Loss Factor Research Articles

Energy Dissipation during Vibrations of Heterogeneous Composite Structures: 3. Numerical Experiments

The influence of the relative thickness of a “stiff” viscoelastic polymer layer and the orientation of reinforcing layers of the bearing layer on the values of their natural frequencies and loss factors of damped vibrations of an unsupported two-layered composite plate has been investigated. It has been established that each natural vibration mode of a two-layered plate corresponds to a certain effective relative thickness of an isotropic viscoelastic polymer layer. A further increase in the relative thickness of plates with an orthotropic bearing layer depending on ambient temperature may result in both an increase and decrease in natural frequencies without any considerable change in their energy dissipation. For plates with a monoclinic bearing layer, the increase in relative thickness of the isotropic viscoelastic polymer layer is accompanied by both a decrease and increase of loss factor for different vibration modes. It is shown that a change in the stacking sequence of reinforcing layers in the bearing layer brings about mutual transformation domains of coupled vibration modes. It has been established that the mutual transformation of the natural shapes of coupled modes of an unsupported quasi-uniform rectangular plate vibrations appears if one of the natural shapes has an even number of quarter-wavelengths in at least one of the plate directions, whereas another natural shape has an odd number of those. It has been demonstrated that the temperature-frequency relationship of elastic-dissipative characteristics of the “stiff” viscoelastic polymer has a considerable effect upon natural frequencies and loss factors of all the investigated vibration modes for an unsupported rectangular two-layered plate.

Electro-optical properties of poly(N-vinyl carbazole) nanoclay composites

In this work, different weight percentages of nanoclay (NC) (2% and 5%) and N-vinyl carbazole were prepared by chemical polymerization. For investigation of structure, optical and morphologies of samples, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy and scanning electron microscopy were used, respectively. Moreover, dielectric properties of composites were examined using dielectric analysis system and vector network analyzer system at high frequency. It was seen that the values of the real and imaginary dielectric constants and loss factor decrease with doping nanoclay into poly(N-vinyl carbazole) (PNVC). The conductivity properties of all samples were studied, and “s” parameter was calculated. Up to 1.7 GHz, “s” parameter value correlated with Jonscher power law, and between 1.7 GHz and 20 GHz, “s” parameter value correlated with superlinear power law. Also, the atomic polarization behaviors in the high-frequency regions have been obtained for PNVC and PNVC doped with 2%, 5% NCs at 25 °C.

Prediction of Statistical Energy Analysis Parameters in Thermal Environment

An algorithm is presented to predict the statistical energy analysis (SEA) parameters of structures in a thermal environment. An energy flow model considering thermal effects is established by the finite element method, and the SEA parameters of the structure under a thermal environment are obtained by a power injection method. The proposed algorithm is applied to a simply supported L-shaped folded plate under rain-on-the-roof excitation. The accuracy of proposed method is verified by the travelling wave approach, and the influence of thermal effects on the SEA parameters is further investigated. The results show that, when only the effect of temperature-dependent material properties is considered, both the coupling loss factor (CLF) and the modal density have an opposite trend with the changing of the elastic modulus and shear modulus; when only the effect of additional stiffness due to thermal stresses is considered, the internal loss factor (ILF) and CLF decrease with the increase of temperature; when both the effects of the temperature-dependent material properties and the additional stiffness due to thermal stresses are taken into account, the ILF and CLF decrease with the increase of temperature. Furthermore, when the structure is exposed to a nonuniform thermal environment, the modal density, ILF, and CLF will decrease with the increase of the temperature gradient.

Effect of Bi-Co co-doping on the microstructure and magnetic properties of NiMgCuZn ferrites

Ni0.15−xMg0.1Cu0.3Zn0.45BixCoxFe2−xO4 (x = 0, 0.01, 0.02, 0.03) ferrites were prepared by sol-gel auto-combustion method. The X-ray diffraction confirmed the formation of the single-phase spinel structure when x ≤ 0.02. The microstructures were examined by SEM. The liquid phase produced by high temperature sintering promoted the grain growth and decreased the porosity. Magnetic measurements showed that saturation magnetization decreased continuously with the substitution of Bi and Co contents, whereas, the initial permeability increased with increase in Bi-Co content up to 0.02 followed by a decrease. In addition, Incorporation of Bi-Co resulted in the increase in quality factor and the decrease in loss factor, which implies that NiMgCuZn ferrite doped with Bi-Co is suitable for high frequency applications. Moreover, the Ni0.13Mg0.1Cu0.3Zn0.45Bi0.02Co0.02Fe1.98O4 sample exhibited the largest density, the highest initial permeability, and the lowest loss.

The influence of combined active fillers on the properties of elastomeric materials for eco-friendly tyres

Reinforcing of elastomers by the addition of various nanofillers has led to significant improvements in elastomer properties, and therefore, to the increase of potentials for use as suitable materials for special applications. The aim of this study was to investigate the effects of combinations of two active fillers (carbon black and synthesized silica (50/0; 35/15; 25/25; 15/35; 0/50; phr)) on the morphology, curing, as well as thermal, dynamic-mechanical and mechanical properties of nanocomposites based on the styrene-butadiene rubber as the network precursor. Based on SEM results, the largest agglomerates were observed in the structure of the nanocomposite containing 25 phr of carbon black and 25 phr of SiO2. Determination of curing characteristics indicated that the minimum torque at 100, 150 and 160?C had higher magnitudes for the compounds with silica filler, which is in accordance with the tendency of SiO2 to form filler-filler interactions. Values of the scorch time and the optimal time of crosslinking increased by increasing the silica phr content in the fillers mixture. It could be thus supposed that the fraction of occluded and trapped chains in the structure of elastomer reinforced by SiO2 is higher due to the almost 2-fold higher value of the silica NDBP number as compared to the NDBP number of carbon black particles. On the basis of MDSC results, it was assessed that the addition of combined active fillers had negligible effects on the glass transition temperature, registered at about -47?C for all tested samples. The obtained DMA data, concerning storage moduli in the temperature range from -20 to 0?C, (characteristic for "Payne" effect) are very significant for predictions of the properties of the tyre tread such as the slip resistance on the road on ice and in the wet conditions. The increase of silica content in the combined filler induced a decrease of the dynamic mechanical loss factor of prepared nanocomposites in the temperature range from 40 to 80?C. The obtained results are very important for the structuring of elastomeric materials aimed for production of tyres with reduced rolling resistance and fuel consumption. The elastomer containing the highest content of carbon black showed the highest value of the tensile strength (21.8 MPa). The abrasion resistance of the synthesized nanocomposites increased as the silica content was reduced in the combined fillers.

Buckling and free vibration analysis of skew sandwich plates with viscoelastic core and functionally graded material constraining layer

The present study is concerned with the free vibration and buckling analysis of a skew sandwich plate with a viscoelastic material core fixed between a functionally graded material constraining layer and a base layer of elastic material. The sandwich plate theory is followed to obtain the governing equations of motion in which the displacement fields of the viscoelastic core are assumed to have a linear variation between those of the two face layers. Finite element method based on first-order shear deformation theory is used to develop the governing equations of motion of the plate. The effects of different parameters such as skew angle, aspect ratio, thickness ratio, and volume fraction index on static and dynamic characteristics of the plate are examined. The increase in the skew angle has increasing effect on both natural the frequencies and critical buckling loads, whereas the fundamental loss factor decreases. The volume fraction index and various boundary conditions also have significant effects on the static and dynamic behavior of the plate.

Dynamic Optimization of Constrained Layer Damping Structure for the Headstock of Machine Tools with Modal Strain Energy Method

Dynamic stiffness and damping of the headstock, which is a critical component of precision horizontal machining center, are two main factors that influence machining accuracy and surface finish quality. Constrained Layer Damping (CLD) structure is proved to be effective in raising damping capacity for the thin plate and shell structures. In this paper, one kind of high damping material is utilized on the headstock to improve damping capacity. The dynamic characteristic of the hybrid headstock is investigated analytically and experimentally. The results demonstrate that the resonant response amplitudes of the headstock with damping material can decrease significantly compared to original cast structure. To obtain the optimal configuration of damping material, a topology optimization method based on the Evolutionary Structural Optimization (ESO) is implemented. Modal Strain Energy (MSE) method is employed to analyze the damping and to derive the sensitivity of the modal loss factor. The optimization results indicate that the added weight of damping material decreases by 50%; meanwhile the first two orders of modal loss factor decrease by less than 23.5% compared to the original structure.

Structural, electrical, dielectric and magnetic properties of Mn-Nd substituted CoFeO3 nano sized multiferroics

A series of MnxCo1−xFe1−yNdyO3 (where x=0.0–1.0 & y=0.0–0.1) multiferroic nanocrystals was synthesized via sol-gel auto-combustion technique. The structure was confirmed by X-ray diffraction (XRD) while morphology was investigated by scanning electron microscopy (SEM). The electrical resistivity was observed to increase from 2.14×107 to 8.77×109Ω-cm and activation energy was found to increase from 0.64 to 0.75eV, while the drift mobility decreased from 4.75×10−13 to 1.27×10−15cm2V−1S−1 by the substitution of Mn and Nd contents. The dielectric constant, dielectric loss and dielectric loss factor decrease with frequency and Mn-Nd contents. The saturation magnetization was increased from 34 to 70emug−1 while the coercivity decreased from 705 to 262Oe with the increase of substituents. The increase in electrical resistivity and saturation magnetization while decrease in dielectric parameters and coercivity make these nanomaterials suitable for applications in microwave devices and longitudinal magnetic recording media.

Effect of Rye Bran Particles on Structure Formation Properties of Rye Dough and Bread

Rye possesses higher nutritional value compared to wheat; and has recently gained increasing attention. However, rye is still a challenging matrix regarding structure characterization and processing. The effects of different concentrations (0.0–20.0%) and sizes (125–250 and 355–500 µm) of added rye bran particles, as well as processing methods on dough formation and bread structure were investigated. Rheological studies on dough showed decrease of loss factor by 11.0% whereas the complex shear modulus increased by 114.0%. Baking tests showed volume reduction as well as increased crumb hardness by bran addition. Xylanase addition and bran particles up to 15.0% showed similar rheological and baking performance like rye without bran and xylanase. These findings highlight the importance of arabinoxylans as main structure-forming component of rye dough and their influence on mechanical properties. Practical Applications With increasing demand worldwide to nutritious and alternative bread tastes, rye bran represents a natural ingredient with high content in fiber. In contrast to traditional rye bread, the enhancement of rye flour with bran increases water absorption which influences shelf life positively in addition to nutritional enrichments. However, due to poor structure formation based on the molecular structure of the rye proteins and the increased amount of the polysaccharide arabinoxylan, rye dough is still a challenging matrix regarding structure characterization and processing. The addition of further ingredients such as rye bran makes the way of processing even more difficult. In this paper, the way of rye bread production enriched with bran is investigated. The paper demonstrates impressively the role of mechanical treatment of bran as well the role of enzymes to minimize the influence of bran on final dough and bread structure.

New insights into the material chemistry of polycaprolactone-grafted cellulose nanofibrils/polyurethane nanocomposites

Polycaprolactone (PCL) was grafted to TEMPO-oxidized nanocellulose (TONCs) through a classical ring-opening polymerization reaction mediated by the surface TONC hydroxyl and carboxyl groups. The PCL increased the thermal stability and hydrophobicity without compromising the crystallinity. When TONCs and PCL-grafted TONCs (PTONC) were compared with respect to their perfusion within a segmented polyurethane matrix (CLPU), PTONC dispersed far better as evidenced by increased storage modulus and Young’s modulus. The mechanical strength of the PTONC nanocomposites was nearly that of unmodified TONCs while at a low content (<3 wt%). Furthermore, PTONC in CLPU promoted micro-phase separation of the matrix leading to a smaller decrease in loss factor and elongation at break for the nanocomposites, highly superior to unmodified TONCs. Thus, PTONC as a reinforcement agent enhanced the mechanical properties and ductility of CLPU.

Role of Ni–Co co-substitution in improving electrical and dielectric properties of La-based multiferroics nanomaterials

The synthesis of a material having ferroelectric and ferromagnetic properties in the same phase is a challenging task. In the present work, lanthanum based multiferroic materials, with composition La1−xCoxFe1−yNiyO3 (where, x=0.0–0.5 and y=0.0–1.0), have been synthesized via sol–gel auto-combustion method. The phase of the synthesized materials was confirmed by the X-ray diffraction (XRD) analysis, while the surface morphology and particle size were determined by the scanning electron microscopy (SEM) analysis. The DC electrical resistivity and activation energy were observed to increase from 2.14×107 to 3.04×1010Ωcm and 0.64 to 0.77eV, respectively with the increase in concentration of substituents. The drift mobility decreases from 3.27×10−13 to 2.80×10−16cm2V−1S−1. The dielectric constant, dielectric loss and dielectric loss factor decrease with the increase in frequency (1MHz to 3GHz) and Ni–Co content as well. The electrical and dielectric results are in good agreement with each other. The increase in electrical resistivity and decrease in dielectric parameters make these materials suitable for applications in microwave devices.

Microwave dielectric characterization of Saudi Arabian date palm biomass during pyrolysis and at industrial frequencies

The knowledge of the dielectric property is essential to design and develop microwave technology for processing biomass materials. This paper focuses on the measurement of dielectric properties of three different Saudi Arabian date palm biomass (seed, leaf and stem) from room temperature to ∼800°C and at six different frequencies (397–2986MHz) using a cavity perturbation method. The result showed that all date palm biomass samples were poor microwave absorbers from room temperature till 500°C. The dielectric constant and loss factor dropped slightly in the drying region due to removal of moisture and was almost constant in pyrolysis region. However, a significant increase in dielectric constant and loss factor occurred in temperature between 500 and 750°C, except for leaf biomass. Penetration depth and tangent loss had an opposite behavior with the rise in temperature. In conclusion, the dielectric properties might not only depend on the temperature and frequency but also on the physical and chemical characteristics of the biomass material. For instance, it was interesting to know that date leaf showed a sharp decrease in loss factor from temperature 475°C, which was opposite to that observed in case of date seed and stem. The data can be very useful in designing, modeling and simulation of microwave processing system for date palm biomass.

Prediction of process cheese instrumental texture and melting characteristics using dielectric spectroscopy and chemometrics

This study evaluated the potentiality of dielectric spectroscopy as a tool to predict the functional properties of process cheese. Dielectric properties of process cheese were collected over the frequency range 0.2 to 3.2GHz at 25°C. Dielectric spectra of process cheese were collected using a high-temperature, open-ended dielectric probe connected to a vector network analyzer. The present study was conducted using 2 sets of commercial process cheese formulations and a set of specially formulated process cheese. For the all the process cheese samples analyzed, a decrease in dielectric constant and dielectric loss factor was observed as the incident frequency increased. Partial least square regression (PLSR) and multilayer perceptron neural network models were developed using the dielectric spectra of process cheese to predict the hardness (gf), melting point (°C), and modified Schreiber melt diameter (mm) of process cheese. The prediction models were validated using the full cross-validation method. The ratio of prediction error to deviation was greater than 2 for melt diameter and hardness, indicating a good practical utility of the PLSR prediction models. The predictability of multilayer perceptron neural network was less than the PLSR models and could be due to the small number of training samples in the data sets. Dielectric spectroscopy coupled with PLSR could be a useful tool for the nondestructive measurement of functional properties of process cheese.

Effects of extrusion temperature on the rheological, dynamic mechanical and tensile properties of kenaf fiber/HDPE composites

The effects of extrusion processing temperature on the rheological, dynamic mechanical analysis and tensile properties of kenaf fiber/high-density polyethylene (HDPE) composites were investigated for low and high processing temperatures. The rheological data showed that the complex viscosity, storage and loss modulus were higher with high processing temperature. Complex viscosities of pure HDPE and 3.4wt% composite with zero shear viscosity of ⩽2340Pas were shown to exhibit Newtonian behavior while composites of 8.5 and 17.5wt% with zero shear viscosity ⩾30,970Pas displayed non-Newtonian behavior. The Han plots revealed the sensitivity of rheological properties with changes in processing temperature. An increase in storage and loss modulus and a decrease in mechanical loss factor were observed for 17.5wt% composites at high processing temperature and not observed at low processing temperature. Processing at high temperature was found to improve the tensile modulus of composites but displayed diminished properties when processed at low processing temperature especially at high fiber content. At both low and high processing temperatures, the tensile strength and strain of the composite decreased with increased content of the fiber.

Synthesis, electrical and dielectric characterization of cerium doped nano copper ferrites

The nanosized CuFe2−xCexO4 (x=0.0, 0.2, 0.4, 0.6, 0.8) ferrites doped with cerium are synthesized by chemical co-precipitation method. The synthesized materials are characterized by XRD, FTIR, TGA and SEM. XRD analysis of cerium substituted copper ferrites confirms the cubic spinel structure. The average crystallite size calculated by using Scherrer's formula ranges from 37 to 53nm. The values of cell constant and cell volume vary with the dopant concentration. These variations can be explained in terms of their ionic radii. The DC electrical resistivity, measured by two point probe method, increases with increase in dopant concentration while it decreases with rise in temperature exhibiting semiconductor behaviour. Energy of activation of these ferrites is calculated by using Arrhenius type resistivity plots. Dielectric measurements of the synthesized compounds show exponential decrease in dielectric constant and dielectric loss factor with increase in frequency. This indicates the normal dielectric behaviour of ferrites.

A nonlinear finite element model using a unified formulation for dynamic analysis of multilayer composite plate embedded with SMA wires

In this study, a new nonlinear finite element model is presented in the frame work of Carrera’s Unified Formulation (CUF) for the dynamic analysis of SMA hybrid composite considering the instantaneous phase transformation and material nonlinearity effects, for every point on the plate. The CUF unify many theories in a unified form which can be differed by the order of expansion and definition of the variables in the thickness direction. The Brinson’s SMA constitutive equation is used to model the behavior of SMA wires. The governing equations are derived using the Reissner Mixed Variational Theorem (RMVT) in order to enforce the interlaminar continuity of transverse shear and normal stresses between two adjacent layers. A transient finite-element-based method beside an iterative incremental procedure is presented to study the dynamic response of multilayered composite plate embedded with SMA wires. A suppressed vibration of the plate is observed, which is due to the energy dissipation of SMA wires. The parametric effects like length-to-thickness ratio, plate aspect ratio and also the effect of different boundary conditions, upon the loss factors are investigated. Results show that as the length-to-thickness ratio and also the plate aspect ratio increases, the loss factor decreases.

Dielectric properties of CaTiO3 coatings prepared by plasma spraying

This paper presents a study of dielectric properties, namely the relative permittivity and loss factor dependence on the frequency of a weak electric field. Perovskite CaTiO3 was studied in the form of coatings and self-supporting plates made by plasma spraying. A conventional gas stabilised plasma gun (GSP) as well as a water stabilised plasma gun (WSP) were employed. It was observed that plasma sprayed titanates exhibit a strong relaxation of permittivity and loss factor decrease with a frequency rise. These properties are influenced by spray technique and spraying parameters, but the relaxation character in general is preserved in all cases. The volume resistivity of the samples was studied as well. Several aspects of the structural features of plasma deposits, especially the phase composition, porosity character, and their influence on dielectric properties are discussed.

TEMPERATURE DEPENDENT DIELECTRIC BEHAVIOR OF NANOCRYSTALLINE Ca FERRITE

Dielectric behaviour of Nanocrystalline CaFe2O4 ferrite synthesized by advanced sol- gel method has been investigated as a function of frequency at different temperatures. Rietveld profile refinement of the XRD pattern confirms formation of cubic spinel structure of the specimen.The dispersion in dielectric behavior of CaFe2O4 ferrite sample has been observed in the temperature range of 100-250˚C as a function of frequency in the range 75 kHz to 10 MHz Both the real value of dielectric constant (ɛ′) and the dielectric loss factor (tanδ) decrease with frequency. This decrease in the values of ɛ′ and tanδ could be explained on the basis of available ferrous, i.e. Fe2+ , ions on octahedral sites such that beyond a certain frequency of applied electric field the electronic exchange between the ferrous and ferric ions i.e. Fe2+↔Fe3+ cannot follow the applied alternating electric field.

Effects of Mesh Density on Static Load Performance of Metal Mesh Gas Foil Bearings

Metal mesh materials have been used successfully in vibration isolators and bearing dampers due to their superior friction or hysteresis damping mechanism. These materials are formed to metal mesh (or wire mesh) structures in ring-shape by compressing a weave of metal wires, in general. Recently, oil-free rotating machinery implement metal mesh structures into hydrodynamic gas foil bearings by replacing bump strip layers with them, to increase its bearing structural damping. A metal mesh foil bearing (MMFB) consists of a top foil and support elastic metal mesh pads installed between a rotating shaft and a housing. The present research presents load capacity tests of a MMFB at rotor rest (0 rpm) and 30 krpm for three metal mesh densities of 13.1%, 23.2%, and 31.6%. The metal mesh pad of test MMFB is made using a stainless steel wire with a diameter of 0.15 mm. Test rig comprises a rigid rotor with a diameter of 60 mm supported on two ball bearings at both ends and test MMFB with an axial length of 50 mm floats on the rotor. Static loads is provided with a mechanical loading device on test MMFB and a strain gauge type load cell measures the applied static loads. A series of static load versus deflection tests were conducted for selected metal mesh densities at rest (0 rpm). Test data are compared to further test results of static load versus journal eccentricity recorded at the rotor speed of 30 krpm. Test data show a strong nonlinearity of bearing deflection (journal eccentricity) with static load, independent of rotor spinning. Observed hysteresis loops imply significant structural damping of test MMFB. Measured journal deflections at 0 rpm are in similar trend to recorded journal eccentricities at the finite rotor speed; thus implying that the MMFB performance depends mainly on the metal mesh structures. The paper also estimates linearlized stiffness coefficient and damping loss factor of test MMFB using the measured static load versus deflection test data at 0 rpm and 30 krpm. The results show that the highest mesh density of 31.6% produces highest linearlized stiffness coefficient and damping loss factor. With rotor spinning at 30 krpm, the linearlized stiffness coefficient and damping loss factor decrease slightly, independent of metal mesh densities. The present test data will serve as a database for benchmarking MMFB predictive models.

Static and Dynamic Mechanical Properties of Chlorobutyl Rubber Composites with Variable Sulfur/Accelerator Ratio

The Composite based on CIIR and variable sulfur/accelerator ratio were prepared by compounding and vulcanizing process. Tensile properties and dynamic mechanical properties of CIIR were studied. The results indicate that tensile strength and loss factor are influenced by sulfur/ accelerator ratio. Tensile strength and modulus are improved with increasing sulfur/accelerator ratio while loss factor decrease with increasing sulfur/ accelerator ratio.