Size Scale Effect on Generated Electric Potential of Nano Composite Electrical Generators

A multiscale approach is pursued for modeling the size-scale effect on generated electric potential by nanocomposite electrical generators of ZnO nanowires. A core-surface model is used for capturing the effect of size-scale on elastic modulus of ZnO NWs. In this model, a surface with different elastic modulus as of the core of NW was considered. Using linear elasticity and axisymmetric configuration of this problem, closed form governing equations are derived in cylindrical coordinate system. Parametric studied are performed for sample cases to demonstrate application of the developed model. It is shown that ZnO nanowires with larger aspect ratio and smaller diameters have higher performance and can produce higher electric potential.

Buckling of Stiffened Curved Panels and Cylindrical Shells: A Study of Comparison Among Various Theories

Formulation is derived for buckling of the circular cylindrical shell with multiple orthotropic layers and eccentric stiffeners acting under axial compression, lateral pressure, and/or combinations thereof, based on Sanders-Koiter theory. Buckling loads of circular cylindrical laminated composite shells are obtained using Sanders-Koiter, Love, and Donnell shell theories. These theories are compared for the variations in the stiffened cylindrical shells. To further demonstrate the shell theories for buckling load, the following particular case has been discussed: Cross-Ply with N odd (symmetric) laminated orthotropic layers. For certain cases the analytical buckling loads formula is derived for the stiffened isotropic cylindrical shell, when the ratio of the principal lamina stiffness is F = E2/E1 = 1. Due to the variations in geometrical and physical parameters in theory, meaningful general results are complicated to present. Accordingly, specific numerical examples are given to illustrate application of the proposed theory and derived analytical formulas for the buckling loads. The results derived herein are then compared to similar published work.

Polyurea With Hybrid Polymer Grafted Nanoparticles: A Parametric Study

The basis of this research is to mitigate shock through material design. In this work, we seek to develop an understanding of parametric variations in polyurea-based nano-composite materials through experimental characterization and computational modeling. Blast-mitigating applications often utilize polyurea due to its excellent thermo-mechanical properties. Polyurea is a microphase-separated segmented block copolymer formed by the rapid reaction of an isocyanate component and an amine component. Block copolymers exhibit unique properties as a result of their phase-separated morphology, which restricts dissimilar block components to microscopic length scales. The soft segments form a continuous matrix reinforced by the hard segments that are randomly dispersed as microdomains. The physical properties of the separate phases influence the overall properties of the polyurea. While polyurea offers a useful starting point, control over crystallite size and morphology is limited. For compositing, the blending approach allows superb control of particle size, shape, and density; however, the hard/soft interface is typically weak for simple blends. Here, we overcome this issue by developing hybrid polymer grafted nanoparticles, which have adjustable exposed functionality to control both their spatial distribution and interface. These nano-particles have tethered polymer chains that can interact with their surrounding environment and provide a method to control well defined and enhanced nano-composites. This approach allows us to adjust a number of variables related to the hybrid polymer grafted nanoparticles including: core size and shape, core material, polymer chain length, polymer chain density, and monomer type. In this work, we embark on a parametric study focusing on the effect of silica nanoparticle size, polymer chain length, and polymer chain density. Preliminary results from experimental characterization and computational modeling indicate that the dynamic mechanical properties of the material can be significantly altered through such parametric modifications. These efforts are part of an ongoing initiative to develop elastomeric composites with optimally designed compositions and characteristics to manage blast-induced stress-wave energy.

Molecular Dynamics Study of Anatase TiO2 Nanoparticles in Water and Vacuum Environments

Nanoparticles are nanometer sized metallic oxides which possess enhanced properties that are desirable to a wide range of industries. In this study, we investigate structural and surface properties of anatase TiO2 nanoparticles in vacuum and water environments using molecular dynamics simulations. The particle sizes ranged from 2 to 6 nm and simulations were performed at 300 K. Surface energy of the particles in vacuum was seen to be higher than that of the particles in water by about 100% for the smaller particles (i.e. 2 and 3nm) and about 60% for the larger particles (i.e. 4 to 6 nm). Surface energy of the particles in both environments, is seen to increase to a maximum (optimum value) as the particle size increases after which no further significant increase is observed. In vacuum, studies carried out at temperatures ranging from 300–2500 K showed a high dependence of surface energy on temperature. The estimated surface tension of water is seen to agree quite well with that of experiments.

Three Dimensional Dynamic Analysis of a Piezoelectric Valveless Micropump: Effects of Working Fluid

Coupled structural and fluid flow analysis of a piezoelectric valveless micropump is carried out for liquid transport applications. The valveless micropump consists of trapezoidal prism inlet/outlet elements; the pump chamber, a thin structural layer (Pyrex glass) and a piezoelectric element (PZT-5A), as the actuator. Two-way coupling of forces and displacements between the solid and the liquid domains in the systems are considered where actuator deflection and motion causes fluid flow and vice-versa. Flow contraction and expansion (through the trapezoidal prism inlet and outlet respectively) generates net fluid flow. The pressure, velocity, flow rate and pump membrane deflections of the micropump are investigated for six different working fluids (acetone, methanol, ethanol, water, and two hypothetical fluids). For the compressible flow formulation, an isothermal equation of state for the working fluid is employed. Three-dimensional governing equations for the flow fields and the structural-piezoelectric bi-layer membrane motions are considered. Comparison of the pumping characteristics of the micropumps operating with different working fluids can be utilized to optimize the design of MEMS based micropumps in drug delivery and biomedical applications.

Fatigue Life Prediction of Microstructures

The formation and early growth of fatigue cracks in the high cycle fatigue regime is influenced by microstructuctural features such as grain size and morphological and crystallographic texture. However, most fatigue models do not predict the influence of the microstructure on early stages of crack formation, or they employ parameters that should be calibrated with experimental data from specimens with microstructures of interest. These post facto strategies are adequate to characterize materials, but they are not fully appropriate to aid in the design of fatigue-resistant engineering alloys. This paper presents a modeling framework that facilitates relative assessment of fatigue resistance among different microstructures. The scheme employs finite element simulations that explicitly render the microstructure and a methodology that estimates transgranular fatigue growth for microstructurally small cracks on a grain-by-grain basis, including consideration of growth within grains (embedded analytically) and stress redistribution as the cracks extend. The methodology is implemented using a crystal plasticity algorithm in ABAQUS and calibrated to study fatigue crack initiation of a bimodal grain size distribution found in RR1000 powder processed Ni-base superalloys for turbine disk applications.

Inhomogeneous Deformation Examples and Applications of Light Activated Shape Memory Polymers

Light Activated Shape Memory Polymers (LASMP) are recently developed innovative materials defined by their capacity to store a deformed (temporary) shape and recover an original (parent) shape. This change in shape and the return to original shape is achieved by exposing the polymer to light at different wavelengths. These unique properties have led to the use of LASMP’s in a wide variety of applications. These SMP’s have a great potential in the biomedical industry as well as the aerospace industry. In the past, the authors have introduced a constitutive model to model the mechanics of these LASMP [1] and used it to solve a few cases of boundary value problems of interest. In this paper, the developed model is used to solve some other inhomogeneous deformation boundary value problems.

Estimating the ‘In-Service’ Modulus of Elasticity and Length of Polyester Mooring Lines via a Non Linear Viscoelastic Model Governed by Fractional Derivatives

In this paper a non linear viscoelastic model governed by fractional derivatives is presented for modeling the in-service behavior of polyester mooring lines. In the formulation an iterative approach utilizing the Gauss-Newton minimization algorithm in conjunction with the catenary equations used to determine the static modulus of elasticity and the effective length of polyester mooring lines corresponding to calm sea conditions. Upon establishing the accuracy of the static modulus via comparison with field data, the catenary equations and the offshore platform’s position versus time are used to identify the polyester strain under developed-sea conditions. In this manner, time histories of stress and strain for polyester ropes in service conditions are obtained. Then, a non linear viscoelastic model involving fractional derivative terms is used to capture the in service polyester line behavior. For this, the tension of the proposed model corresponding to the actual polyester strain is compared at each time step to the tension obtained from the field data. Finally, the parameters of the proposed model are derived by minimizing the error in the least-squares sense over a large number of data points using the Levenberg-Marquardt algorithm. The numerically derived force-strain relationship is found to be in reasonable agreement with supplementary field and laboratory experimental data, the field data pertain to an offshore structure moored in position using polyester mooring lines operated in the Gulf of Mexico during Hurricane Katrina (August of 2005).

The Effect of Microstructural Morphologies on the Effective Electromechanical Properties of Piezoelectric Particle Composites

This study introduces a micromechanical model that incorporates detailed microstructures for analyzing the effective electro-mechanical properties, such as piezoelectric and permittivity constants as well as elastic moduli, of piezoelectric particle reinforced composites. The studied composites consist of polarized spherical piezoelectric particles dispersed into a continuous and elastic polymeric matrix. A micromechanical model generated using three-dimensional (3D) continuum elements within a finite element (FE) framework. For each volume fraction (VF) of particles, realization with different particle sizes and arrangements were generated in order to represent microstructures of a particle composite. We examined the effects of microstructural morphologies, such as particle sizes and distributions, and particle volume fractions on the overall effective electro-mechanical properties of the active composites. The overall electro-mechanical properties determined from the present micromechanical model were compared to those generated using the Mori-Tanaka, self-consistent, and simplified unit-cell micromechanical models.

Development and Validation of a Non-Linear Viscoelastic Viscoplastic Stress Model for a PFCB/PVDF Fuel Cell Membrane

Proton exchange membranes (PEMs) in operating fuel cells are subjected to varying thermal and hygral loads while under mechanical constraint imposed within the compressed stack. Swelling during hygrothermal cycles can result in residual in-plane tensile stresses in the membrane and lead to mechanical degradation or failure through thinning or pinhole development. Numerical models can predict the stresses resulting from applied loads based on material characteristics, thus helping to guide the development of more durable membrane materials. In this work, a non-linear viscoelastic stress model based on the Schapery constitutive formulation is used with a Zapas-Crissman viscoplastic term to describe the response of a novel membrane material comprised of a blend of perfluorocyclobutane (PFCB) ionomer and polyvinylidene fluoride (PVDF). Uniaxial creep and recovery tests are used to establish the time dependent linear viscoelastic modulus as well as the fitting parameters for the non-linear viscoelastic viscoplastic model. The stress model is implemented in a commercial finite element code, Abaqus®, to predict the response of a membrane subjected to mechanical loads. The stress model is validated by comparing predicted and experimental responses for membranes subjected to stress relaxation and multiple step creep loads in uniaxial tension.