On the Properties of Anisotropic Piezoelectric and Fiber Reinforced Composite Materials

A new procedure based on constructing orthonormal tensor basis using the form-invariant expressions which can easily be extended to any tensor of rank n. A new decomposition, which is not in literature, of the stress tensor is presented. An innovational general form and more explicit physical property of the symmetric fourth rank elastic tensors is presented. The new method allows to measure the stiffness and piezoelectricity in the elastic fiber reinforced composite and piezoelectric ceramic materials, respecively, using a proposed norm concept on the crystal scale. This method will allow to investigate the effects of fiber orientaion, number of plies, material properties of matrix and fibers, and degree of anisotropy on the stiffness of the structure. The results are compared with those available in the literature for semiconductor compounds, piezoelectric ceramics and fiber reinforced composite materials.

Shear Piezoresistive Response of a Graphite/Silicone Suspension

A shear piezoresistive effect has been observed for micrographite particles suspended in uncured silicone elastomer. A phenomenological formulation of piezoresistivity is presented and an experimental approach is discussed within this paper. The experimental objective is to extract two material parameters, fully describing the piezoresistance effect in deformed isotropic materials. A rheometer in the cone-and-plate configuration provides well-defined oscillatory shear flow of the suspension; it also measures rheological characteristics of the suspension. The piezoresistive response is probed using interdigitated electrodes, which are attached to the rheometer plate. The electrodes are arranged in parallel-to-flow and perpendicular-to-flow orientations. The signal acquired from two such orthogonal electrode pairs can be combined in a way to exclude any contribution of volumetric deformations to the piezoresistance signal. The experimental results indicate a second harmonic relationship between the mechanical oscillation and the resistive response. These two-probe measurement results represent the first observations of a non-volumetric deformation contribution to the piezoresistivity of viscoelastic liquid suspensions.

Effects of Texture on Thermoelectric Power in Ti-6Al-4V

Ti-6A1-4V alloy exhibits a very strong anisotropic texture caused by the existence of a preferred crystallographic orientation in the polycrystalline microstructure. This crystallographic alignment can result in anisotropic behavior of the material so that the material properties are different depending on whether they are measured in perpendicular or parallel direction. In addition to this morphological anisotropy, due to the dominantly hexagonal grain structure, the Ti-6A1-4V alloy also exhibited a substantial thermoelectric anisotropy. This study was conducted to investigate the effect of thermoelectric anisotropy on the thermoelectric power measurements in a highly textured Ti-6A1-4V specimen using a completely nondestructive technique based on the Seebeck effect. The result shows the thermoelectric power dependence associated with texturing and the macroscopic grain structure in a rolled Ti-6A1-4V specimen, which was annealed at 710°C for 2 hours and slowly cooled. The measurements clearly demonstrated that the intrinsic sensitivity of the thermoelectric contact technique is a very useful tool that could be exploited for quantitative nondestructive (QND) material characterization.

Effect of Carbon Nanofiber on Thermal and Tensile Properties of Polypropylene

In the present study, effect of vapor grown carbon nanofiber on the mechanical and thermal properties of polypropylene was investigated. Firstly, nanofibers were dry-mixed with polypropylene powder and extruded into filaments by using a single screw extruder. Then the tensile tests were performed on the single filament at the strain rate range from 0.02/min to 2/min. Experiments results show that both neat and nano-phased polypropylene were strain rate strengthening material. The tensile modulus and yield strength both increased with increasing strain rate. Experimental results also show that infusing nanofiber into polypropylene can increase tensile modulus and yield strength, but decrease the failure strain. At the same time, thermal properties of neat and nano-phased polypropylene were characterized by TGA. TGA results have showed that the nanophased system is more thermally stable. At last, a nonlinear constitutive equation has been developed to describe strain rate sensitive behavior of neat and nano-phased polypropylene.

Cross-Sectional Nanoindentation of Alumina Thin Films Deposited by Pulsed Laser Deposition Process

Alumina is a widely used ceramic material due to its high hardness, wear resistance and dielectric properties. The study of phase transformation and its correlation to the mechanical properties of alumina is essential. In this study, interfacial adhesion properties of alumina thin films are studied using cross-sectional nanoindentation (CSN) technique. Alumina thin films are deposited at 200 and 700 °C, on Si (100) substrates with a weak Silica interface, using pulsed laser deposition (PLD) process. Effect of annealing on the surface morphology of the thin films is studied using atomic force microscopy. Xray diffraction studies revealed that alumina thin films are amorphous in nature at 200 °C and polycrystalline with predominant gamma alumina phase at 700 °C.

Characterization of Mechanical Properties of Carbon Nanotubes in Copper-Matrix Nanocomposites

Carbon nanotubes have received considerable attention because of their excellent mechanical properties. In this study, carbon nanotube - copper composites have been sintered by a mechanical mixing process. The interfacial bonding between nanotubes and the copper matrix was improved by coating nanotubes with nickel. Sintered pure copper samples were used as control materials. The displacement rate of nanotube-copper composites was found to increase at 200°C whereas that of nickel-coated nanotue-copper composites significantly decreased. The incorporation of carbon nanotubes and nickel-coated carbon nanotubes in the copper matrix decreased friction coefficients and increased the time up to the onset of scuffing compared with those of pure copper specimens.

Bending Behavior of Plain-Woven Fabric Air Beams: Fluid-Structure Interaction Approach

A swatch of plain-woven fabric was subjected to biaxial tests and its material characterization was performed. The stress-strain relations of the fabric were determined and directly used in finite element models of an air beam, assumed constructed with the same fabric, subjected to inflation and bending events. The structural responses to these events were obtained using the ABAQUS-Explicit[1] finite element solver for a range of pressures including those considered typical in safe operations of air inflated structures. The models accounted for the fluid-structure interactions between the air and the fabric. The air was treated as a compressible fluid in accordance with the Ideal Gas Law and was subjected to adiabatic constraints during bending. The fabric was represented with membrane elements and several constitutive cases including linear elasticity and hyperelasticity were studied. The bending behavior for each constitutive case is presented and discussions for their use and limitations follow.

A Stochastic Model of Oxidation Mechanism on High Temperature Corrosion of Stainless Steel

To interpret the role of diffusion and reaction process, a cellular automaton model, which combines the surface growth and internal oxidation, was developed to explain the oxidation mechanism of stainless steels in high temperature corrosive liquid metal environment. In this model, three main processes, which include the corrosion of the substrate, the diffusion of iron species across the oxide layer and precipitation of iron on the oxide layer, are simulated. The diffusion process is simulated by random walk model. Mapping between present model and Wagner theory has been created. The gross features concerning the evolution of the involved process were founded.

Self-Sensing of Stresses and Strains Through the Dielectrostriction Effect in Composites

Any dielectric material would vary its dielectric properties with deformation. By measuring these variations one could monitor stresses or strains with no mechanical interface with a load-bearing member. This effect, called dielectrostriction, can be formulated as a linear relation between the stress/strain and the dielectric response of a material. A planar capacitor on a rigid substrate is utilized to monitor the dielectrostriction effect. A rosette of such sensors can be located on the surface or embedded in the monitored part. A four-sensor rosette measures principal directions and difference of principal strains. Overall, this sensing technology shows a good potential for Non-Destructive Evaluation (NDE) and structural health monitoring of composite materials. This work provides theoretical background and experimental study of dielectrostriction response in polycarbonate, polyethylene, acrylic, and carbon nanotube composite materials.

Characterization Studies on the SOFC Anode Material Using RSM Technique

The characteristics of the Ni/YSZ anode material for the solid oxide fuel cells (SOFCs) were investigated in order to study the relation between the porosity and the conductivity of the cell. Response Surface Methodology (RSM) was used for the study. The anode material was prepared with NiO and YSZ along with the graphite as pore former. The experiments were performed based on a central composite design matrix, which yielded nine sets of experiments. Porosity and conductivity measurements were performed on the sintered and reduced anode material. Using the measured values as output variables, statistical analysis was performed. The results indicated that the porosity increases by reducing the sintering temperature values, while the conductivity values were on the reverse scale. The conductivity values increase with increasing temperature. Using RSM technique, a ideal point was found in order to obtain a desired porosity volume and conductivity value.