Alireza V. Amirkhizi

An experimentally-based viscoelastic constitutive model for polyurea, including pressure and temperature effects

Presented here are the results of a systematic study of the viscoelastic properties of polyurea over broad ranges of strain rates and temperatures, including the high-pressure effects on the material response. Based on a set of experiments and a master curve developed by Knauss (W.G. Knauss, Viscoelastic Material Characterization relative to Constitutive and Failure Response of an Elastomer, Interim Report to the Office of Naval Research (GALCIT, Pasadena, CA, 2003.) for time–temperature equivalence, we have produced a model for the large deformation viscoelastic response of this elastomer. Higher strain-rate data are obtained using Hopkinson bar experiments. The data suggest that the response of this class of polymers is strongly pressure dependent. We show that the inclusion of linear pressure sensitivity successfully reproduces the results of the Hopkinson bar experiments. In addition, we also present an equivalent but approximate model that involves only a finite number of internal state variables and is specifically tailored for implementation into explicit finite-element codes. The model incorporates the classical Williams–Landel–Ferry (WLF) time–temperature transformation and pressure sensitivity (M.L. Williams, R.F. Landel, and J.D. Ferry, J. Am. Chem. Soc., 77 3701 (1955)), in addition to a thermodynamically sound dissipation mechanism. Finally, we show that using this model for the shear behaviour of polyurea along with the elastic bulk response, one can successfully reproduce the very high strain rate pressure–shear experimental results recently reported by Jiao et al. (T. Jiao, R.J. Clifton and S.E. Grunschel, Shock Compression of Condensed Matter 2005 (American Institute of Physics, New York, 2005.).

Dynamic Mechanical Analysis of Fly Ash Filled Polyurea Elastomer

In this work, the material properties of a series of fly ash/polyurea composites were studied. Dynamic mechanical analysis was conducted to study the effect of the fly ash volume fraction on the composites mechanical properties, i.e., on the materials frequency- and temperature-dependent storage and loss moduli. It was found that the storage and loss moduli of the composite both increase as the fly ash volume fraction is increased. The storage and loss moduli of the composites relative to those of pure polyurea initially increase significantly with temperature and then slightly decrease or stay flat, attaining peak values around the glass transition region. The glass transition temperature (measured as the temperature at the maximum value of the loss modulus) shifted toward higher temperatures as the fly ash volume fraction increased. Additionally, we present the storage and loss moduli master curves for these materials obtained through application of the time-temperature superposition on measurements taken at a series of temperatures.

Numerical calculation of electromagnetic properties including chirality parameters for uniaxial bianisotropic media

Through the use of conductive straight wires or coils the electromagnetic properties of a composite material can be modified. The asymmetric geometry of the coils creates an overall chiral response. The polarization vectors rotate as an electromagnetic wave travels through such a medium. To calculate the chirality of a medium prior to its manufacturing, we developed a method to extract all four electromagnetic material parameter tensors for a general uniaxial bianisotropic composite based on the numerical simulation of the electromagnetic fields. Our method uses appropriate line and surface field averages in a single unit cell of the periodic structure of the composite material. These overall field quantities have physical meaning only when the microscopic variation of the electromagnetic fields in the scale of the unit cell is not important, that is when the wavelength of interest is significantly larger than the maximum linear dimension of the unit cell. The overall constitutive relations of the periodic structure can then be obtained from the relations among the average quantities.

Experimental Characterization of Chiral Uniaxial Bianisotropic Composites at Microwave Frequencies

This paper presents an experimental procedure for retrieving the effective constitutive parameters of chiral materials. Unlike past research that primarily deals with isotropic materials, this study considers a lossy uniaxial bianisotropic slab with a nonzero chirality along its axial direction. First, plane-wave scattering off the uniaxial slab in a free-space environment is studied analytically. This forward analysis gives insight into the problem and the choice of proper independent measurements required for the inverse process, i.e., retrieving the slab constitutive parameters from its S-parameters. Based on this analysis, three sets of co-polarized and cross-polarized S-parameters are required, including both the transmission and reflection coefficients of the slab. Given the measured scattering data, the complex permittivity, permeability, and chirality tensors are determined numerically using the results of the analytic study. To test the performance of the new retrieval method, an array of 2 × 56 long, metallic helices is designed and fabricated for operation at C-band. Having the same handedness, the helices are closely spaced and held in parallel to each other in a wooden frame in order to create an effective uni-axial chiral medium. A conventional transmission/reflection setup measures the array scattering parameters, which are fed into the retrieval process to obtain the effective parameters. The measured parameters well model the array scattering response, exhibiting a significant averaged chirality of ~0.4 over 5.5-8.7 GHz and a plasmonic behavior at ~7.1GHz.

SOFT-FOCUSING IN ANISOTROPIC INDEFINITE MEDIA THROUGH HYPERBOLIC DISPERSION

Materials that exhibit negative refraction may have many novel applications. We seek to evaluate the possibility of soft-focusing of microwave signals using a medium with an indeflnite (hyperbolic) anisotropic permittivity tensor. We fabricated a 147mm thick and 220mm wide Styrofoam sample with an embedded array of 12-gauge brass wires of 6.35mm lattice spacing. Two single-loop antennas were used to approximately generate a transverse magnetic (TM) point source and the associated detector. Using an Agilent 8510C Vector Network Analyzer (VNA), the frequency spectrum was scanned between 7 and 9GHz. Relative gain or loss measurements were taken at equal spatial steps behind the sample. A scanning robot was used for automatic scanning in the x, y, and z directions, in order to establish the focusing patterns. The signal amplitudes measured in the presence and absence of the sample were compared. The robot was controlled using LabVIEW y , which also collected the data from the VNA and passed it to MATLAB z for processing. A soft focusing spot was observed when the antennas were placed in two difierent symmetric conflgurations with respect to the sample. These results suggest a method for focusing electromagnetic waves using negative refraction in indeflnite (hyperbolic) anisotropic materials.

Composites with mechanically tunable plasmon frequency

This paper summarizes our efforts to create a composite material with a mechanically tunable plasmon frequency at the microwave band. The permittivity of the composite changes sign at the plasmon frequency. Such composites, therefore, can be used as electromagnetic filters. Theoretically, an array of non-magnetic, metallic wire coils has been shown to have a plasmon behavior that is dependent on the wire thickness, coil inner diameter, pitch and coil spacing. Here, a material is made out of an array of coils placed within a non-metallic frame, and the material plasmon frequency is tuned through altering the pitch. The coils are arranged with alternating handedness to create an effective, non-chiral medium. A transmit/receive setup is used to characterize the electromagnetic behavior of the composite. The setup consists of a vector network analyzer and two horn antennas, which are used to measure the scattering parameters of the material. These parameters are then used to calculate the permittivity. The results show an increase in the plasmon frequency with increase in the pitch. Increasing the pitch 30%, from 3 to 3.9 mm, results in a corresponding increase from 6.3 to 7.5 GHz in the frequency. (Some figures in this article are in colour only in the electronic version)

Controlling acoustic-wave propagation through material anisotropy

Acoustic-wave velocity is strongly direction dependent in an anisotropic medium. This can be used to design composites with preferred acoustic-energy transport characteristics. In a unidirectional fiber-glass composite, for example, the preferred direction corresponds to the fiber orientation which is associated with the highest stiffness and which can be used to guide the momentum and energy of the acoustic waves either away from or toward a region within the material, depending on whether one wishes to avoid or harvest the corresponding stress waves. The main focus of this work is to illustrate this phenomenon using numerical simulations and then check the results experimentally.

Polyurea-Based Composites: Ultrasonic Testing and Dynamic Mechanical Properties Modeling

Many scientists and researchers study polyurea due to its excellent blast-mitigating properties. In this work, we have studied two polyurea composite systems with filler materials intended to improve dynamic mechanical properties. The two filler materials are milled glass and fly ash. The shape and quantity of filler significantly affect the dynamic mechanical properties of the composite. Ultrasonic tests were conducted on samples with both fillers. The volume fraction of the inclusions was varied to study the effect of filler quantity on mechanical properties. Moreover, computational models based on the methods of dilute-randomly-distributed inclusions and periodically-distributed inclusions were created to improve our understanding of polyurea-based composites and serve as tools for estimating the dynamic mechanical properties of similar composite material systems. The experimental and computational results were compared and show good agreement. The experiments and modeling have been conducted to facilitate the design of new elastomeric composites with desirable impact- and blast-mitigating properties.

Properties of Elastomer-based Particulate Composites

In this work, an attempt has been made to develop fly ash filled polyurea matrix composites with low density and good dynamic mechanical behavior. Fly ash (105μm –149μm in diameter) was introduced into polyurea, and its volume fraction was varied to study its effects on the overall properties of the composites. Scanning electron microscopy was used to observe the morphology of the composites. The storage and loss moduli of the composites were determined using dynamic mechanical analysis (DMA) from -80 to 70°C at low frequencies and using ultrasonic measurements at high frequencies under ambient conditions. Results showed that fly ash particles were distributed homogeneously in the polyurea matrix, and the density of the composites decreased as the volume fraction of fly ash increased. Compared to neat polyurea, increases in storage and loss moduli at high temperature were achieved by increasing fly ash content. The peak in the ratio of the moduli of the composites system over that of neat polyurea occurred near glass transition temperature Tg. The speed of sound in the composites increased with increasing fly ash content. Longitudinal modulus and acoustic impedance had similar trends.

The Effect of Stoichiometric Ratio on Viscoelastic Properties of Polyurea

Polyurea is a commonly utilized elastomer due to its excellent thermo-mechanical properties. In this study, the polyurea is synthesized using Versalink P-1000 (Air Products) and Isonate 143 L (Dow Chemicals). The diisocynate blocks generally assemble into hard domains embedded in the soft matrix, creating a lightly cross-linked heterogenous nano-structure. We seek to evaluate the effect of the stoichiometric ratio of the two components on the viscoelastic properties of the resultant polyurea. By altering the ratio, polyurea samples with different stoichiometric variations are made. In order to approximate the mechanical properties of polyurea for a wide frequency range, master curves of storage and loss moduli are developed. This is achieved by time-temperature superposition of the dynamic mechanical analysis (DMA) data, which is conducted at low frequencies and at temperatures as low as the glass transition. Furthermore, in order to access the effect of the stoichiometric ratio on the relaxation mechanisms in the polyurea copolymer system, continuous relaxation spectra of all the stoichiometric variations are calculated and compared.