Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: ''as received graphite'' (ARG), ''expanded graphite,'' (EG) and ''graphite nanoplatelets'' (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of $10 nm. Solution-based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix.
To facilitate the design and development of porous metals, simulation of their mechanical behavior is essential. As an alternative to complex tomography procedures, a methodology has been developed to construct a simulated microstructure that retains the essential features of the experimental material. The target material is a moderate porosity titanium foam that is being developed as a bone implant material. The methodology applies stereology theory to a foaming process based on growth of pressurized pores. Three-dimensional (3D) pore size and pore distribution information is derived from 2D sections for a sample with low porosity, early in the foaming process. A 3D microstructure is developed based on the 3D location and size distribution of the pores by use of a computational procedure. Pores are allowed to grow and coalesce in a simple simulated foaming process to achieve microstructures of higher porosity. These data have been used as inputs to write scripts of I-DEAS to create 3D finite element models which are then examined for basic global and local mechanical properties.
The porous microstructures of metallic foams cause microscopic stress and strain localization under deformation which reduces the damage tolerance and therefore limits application of the materials. In this paper, the deformation of a relatively low porosity porous titanium is examined using two-dimensional (2D) plane strain and three-dimensional (3D) finite element models to identify the accuracy and limitations of such simulations. To generate the finite element models, a simulated microstructure was created based on micrographs of an experimental material. Compared to the 2D models, the 3D models require smaller model size to obtain convergent results. The macroscopic responses predicted by the 3D models are in reasonable agreement with experimental results while the 2D models underestimated the response. In addition, 3D models predicted more uniform microscopic field variable distributions. 2D models predicted higher probability of Von Mises stress and equivalent plastic strain exceeding a certain value and therefore overestimate the failure probability of the material.
By introducing a flux-controlled memristor model with absolute value function, a 5D multistable four-wing memristive hyperchaotic system (FWMHS) with linear equilibrium points is proposed in this paper. The dynamic characteristics of the system are studied in terms of equilibrium point, perpetual point, bifurcation diagram, Lyapunov exponential spectrum, phase portraits, and spectral entropy. This system is of the group of systems that have coexisting attractors. In addition, the circuit implementation scheme is also proposed. Then, a secure communication scheme based on the proposed 5D multistable FWMHS with disturbance inputs is designed. Based on parametric modulation theory and Lyapunov stability theory, synchronization and secure communication between the transmitter and receiver are realized and two message signals are recovered by a convenient robust high-order sliding mode adaptive controller. Through the proposed adaptive controller, the unknown parameters can be identified accurately, the gain of the receiver system can be adjusted continuously, and the disturbance inputs of the transmitter and receiver can be suppressed effectively. Thereafter, the convergence of the proposed scheme is proven by means of an appropriate Lyapunov functional and the effectiveness of the theoretical results is testified via numerical simulations.
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