Polyurea is a type of elastomer with excellent properties and a myriad of applications. Especially, the application in increasing the survivability of structures and buildings under impact loading is more attractive. Recently, modifying the properties of polyurea by mixing micro and nano particles into polyurea is becoming a new research hotspot. However, systematic study of the effect of particles size on the properties of polyurea-based composites has not been reported. Hence, in our present work, glass beads with diameters in the range from 3μm to 250μm were selected and composites with 20% volume fraction of glass beads were prepared. Microstructure of the composites was investigated by Scanning Electron Microscopy (SEM). Dynamic mechanical analysis was conducted using a TA Instruments DMA 2980 over the temperature range from -80°C to 50°C at various frequencies. The storage and loss moduli master curves for these composites were obtained through application of the time-temperature superposition (TTS). Ultrasonic properties were determined by a personal computer (PC) based ultrasonic system at room temperatures. Velocity and attenuation of longitudinal ultrasonic wave were measured. Consequently, complex longitudinal modulus was computed from these measurements. Evaluation of the effect of particle size on the properties of the composites mentioned above was presented.
Due to its excellent thermo-mechanical properties, polyurea is attracting more and more attention in blast-mitigating
applications. In order to enhance its capability of blast-induced stress-wave management, we seek to develop polyurea-based
composites in this work. Fly ash which consists of hollow particles with porous shell and low apparent density
was chosen as filler and a series of fly ash/polyurea composites with various fly ash volume fractions were fabricated.
The dynamic mechanical behavior of the composites was determined by a personal computer (PC) based ultrasonic
system in the 0.5-2MHz frequency range between -60°C to 30°C temperatures. Velocity and attenuation of both
longitudinal and shear ultrasonic waves were measured. The complex longitudinal and shear moduli were then computed
from these measurements. Combining these results provided an estimate of the complex bulk and Young’s moduli of the
fly ash/polyurea composites at high frequencies. These results will be presented and compared with those of pure
Acoustic impedance is a material property that depends on mass density and acoustic wave speed. An impedance
mismatch between two media leads to the partial reflection of an acoustic wave sent from one medium to another. Active
sonar is one example of a useful application of this phenomenon, where reflected and scattered acoustic waves enable
the detection of objects. If the impedance of an object is matched to that of the surrounding medium, however, the object
may be hidden from observation (at least directly) by sonar. In this study, polyurea composites are developed to facilitate
such impedance matching. Polyurea is used due to its excellent blast-mitigating properties, easy casting, corrosion
protection, abrasion resistance, and various uses in current military technology. Since pure polyurea has impedance
higher than that of water (the current medium of interest), low mass density phenolic microballoon particles are added to
create composite materials with reduced effective impedances. The volume fraction of particles is varied to study the
effect of filler quantity on the acoustic impedance of the resulting composite. The composites are experimentally
characterized via ultrasonic measurements. Computational models based on the method of dilute-randomly-distributed
inclusions are developed and compared with the experimental results. These experiments and models will facilitate the
design of new elastomeric composites with desirable acoustic impedances.
This work presents a new chiral composite composed of copper wires braided with Kevlar and nylon to form
conductive coils integrated among structural fiber. To create a fabric, these braids were woven with plain Kevlar
fiber. This yielded a composite with all coils possessing the same handedness, producing a chiral material. The
electromagnetic response of this fabric was first simulated using a finite element full-wave simulation. For the
electromagnetic measurement, the sample was placed between two lens-horn antennas connected to a Vector
Network Analyzer. The frequency response of the sample was scanned between 5.5 and 8GHz. The measured
scattering parameters were then compared to those of the simulated model. The measured parameters agreed well
with the simulation results, showing a considerable chirality within the measured frequency band. The new
composite combines the strength and durability of traditional composites with an electromagnetic design to create a
Acoustic stress waves can be guided to follow pre-determined paths in solids, using elastic anisotropy.
Recently, there has been intense interest to design materials and structures that can shield specific regions
within the material by redirecting the incident stress-waves along desired paths. Some of the proposed
techniques involve variable mass density and stiffness. We have designed a material with isotropic mass
density but highly anisotropic elasticity that can guide incident waves along desired trajectories. Harmonic excitations are imposed, and it is shown that the stress-wave energy would travel around a protected central region. The model is also evaluated using numerical simulations, which confirm that majority of the stress-wave energy is guided around the central cavity and is delivered exactly to the opposing face in a location corresponding to the incident excitation location.
Previous studies into the possibility of a plasmonic medium of a coiled conductor array in air have shown promise.
This work serves to evaluate the possibility of creating a mechanically-tunable composite filter at low frequencies.
Copper springs were created with varying starting pitches using a coil winder. These springs were then embedded
into a flexible host polymer. The mechanical and electromagnetic properties of each spring design were predicted
and tested. Two horn antennas were used to characterize the overall electromagnetic (EM) properties of the
composite. The pitch of each spring was increased mechanically through application of force to the entire polymermetal
composite at equal intervals, with an EM test completed at each step. Using an Agilent 8510C Vector
Network Analyzer (VNA), the frequency spectrum within the microwave range was scanned. Relative amplitude
and phase measurements were taken at equal frequency and pitch steps. With no polymer surrounding the springs,
plasmon turn-on frequencies were observed to span the microwave bands as the pitch of the springs were increased.
Similar results are expected with the springs embedded in a polymeric matrix. These results suggest a method of
creating a mechanically-tunable composite filter for use at low frequencies.
Fly ash, which consists of hollow particles with porous shells, was introduced into polyurea elastomer. A one-step
method was chosen to fabricate pure polyurea and the polyurea matrix for the composites based on Isonate® 2143L
(diisocyanate) and Versalink® P-1000 (diamine). Scanning electron microscopy was used to observe the fracture
surfaces of the composites. Particle size and volume fraction were varied to study their effects on the tensile properties
of the composites. The tensile properties of the pure polyurea and fly ash/polyurea (FA/PU) composites were tested
using an Instron load frame with a 1 kN Interface model 1500ASK-200 load cell. Results showed that fly ash particles
were distributed homogeneously in the polyurea matrix, and all of the composites displayed rubber-like tensile behavior
similar to that of pure polyurea. The tensile strength of the composites was influenced by both the fly ash size and the
volume fraction. Compared to the largest particle size or the highest volume fraction, an increase in tensile strength was
achieved by reducing particle size and/or volume fraction. The strain at break of the composites also increased by using
fine particles. In addition, the composites filled with 20% fly ash became softer. These samples showed lower plateau
strength and larger strain at break than the other composites.
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.
Materials that exhibit negative refraction demonstrate physical phenomena that may be used for novel applications. This
work serves to evaluate the possibility of hyperbolic focusing due to an indefinite anisotropic permittivity tensor. Two
single-loop antennas were used to approximately achieve a transverse magnetic (TM) point source and detector. Using
an Agilent 8510C Vector Network Analyzer (VNA), the frequency spectrum was scanned between 7 and 9 GHz.
Relative gain or loss measurements were taken at equal spatial steps around the center of the sample. A scanning robot
allowed for the automatic scanning of the space behind the sample in the x, y, and z directions, to establish the focusing
patterns, and to compare the signal amplitudes in the presence and absence of the sample. The robot was controlled using
LabVIEW, which also collected the data from the VNA and passed it to Matlab for processing. A soft focusing spot was
observed when the antennas were placed in a symmetric configuration with respect to the sample. These results suggest a
method of focusing electromagnetic waves using negative refraction in indefinite materials.
We have incorporated arrays of conductive electromagnetic scattering elements such as straight copper wires and copper coils into fiber-reinforced polymer composites, resulting in materials with required structural and further electromagnetic functionality. The scattering elements provide controlled electromagnetic response for tasks such as filtering and may be used to tune the overall index of refraction of the composite. Integration of these metallic elements into traditional fiber-reinforced polymer composites has introduced other opportunities for multifunctionality in terms of self-healing, thermal transport and perhaps sensing applications. Such functionalities are the result of fiber/wire integration through textile braiding and weaving, combined with a new polymer matrix that has the ability to heal internal cracking through thermo-reversible covalent bonds. Multifunctional composites of this kind enhance the role of structural materials from mere load-bearing systems to lightweight structures of good thermo-mechanical attributes that also have electromagnetic and other functionalities.
We are studying the incorporation of electromagnetic effective media in the form of arrays of metal scattering elements, such as wires, into polymer-based or ceramic-based composites. In addition to desired structural properties, these electromagnetic effective media can provide controlled response to electromagnetic radiation such as RF communication signals, radar, and/or infrared radiation. With the addition of dynamic components, these materials may be leveraged for active tasks such as filtering. The advantages of such hybrid composites include simplicity and weight savings by the combination of electromagnetic functionality with necessary structural functionality. This integration of both electromagnetic and structural functionality throughout the volume of the composite is the distinguishing feature of our approach. As an example, we present a class of composites based on the integration of artificial plasmon media into polymer matrixes. Such composites can exhibit a broadband index of refraction substantially equal to unity at microwave frequencies and below.