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.

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.

Ballistic Impact Response of Composite Systems

Composite materials are widely used in many engineering applications and are an attractive for armor design because of their increased high toughness, impact resistance, stiffness, and strength-to-weight ratios and the ability to tailor their designs to applications. In this paper, numerical simulation of impact on composites is being performed to predict ballistic limit velocities and evaluate the delamination behavior of different composite systems. The normal impact and penetration of blunt rigid projectile on laminated composite targets was developed to estimate the velocity for which the projectile has complete penetration, the ballistic limits and energy absorbed while perforating a given piece of armor. A non-linear, explicit, three dimensional finite element commercial code (ABAQUS) is used to simulate the response of armor targets at V50 impact velocities. The armor test panel is modeled as a multi-layered laminated plate with different composite systems, thickness, and stacking sequence. The three failure modes that represent the three stages of the penetration process namely transverse shear, tensile fiber breakage, and delamination are identified. The ballistic limit curves for different materials, thickness, and orientations are determined. The target interlaminar stress distributions along the thickness are graphically represented. Strain energy, Plastic dissipation and Kinetic dissipation energy curves for the whole model were obtained including thickness effects.

On the Design of the Tubesheet and the Tubesheet-to-Shell Junction of a Fixed Tubesheet Heat Exchanger

On the hand of the (mechanical) design of an existing, very large, extreme fixed tubesheet heat exchanger, a C2-Hydrogenation reactor in a petrochemical plant, various code solutions are compared, with each other and with a Finite Element solution based on the Direct Route in Design by Analysis (EN 13445-3, Annex B). The codes and standards used in the investigation are ASME Section VIII, Division 1 (and 2), (TEMA), and EN 13445-3, Clause 13 and Annex J. ASME VIII/2, and TEMA are not appropriate for this design. ASME VII/1 and EN 13445-3 Clause 13 approaches are similar. Differences in maximum permissible pressures result partly from different nominal design stresses. The modern EN 13445-3 Annex J approach, being based on limit analysis theory, leads to very different, much more efficient results. The Direct Route in Design by Analysis confirms the EN 13445-3 Annex J results, but gives, at the same time, clear insight into the behaviour of the whole structure and the various maximum permissible pressure limiting details.

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.