On the experimental and numerical investigation of clay/epoxy nanocomposites

Due to increasing use of clay/epoxy nanocomposites in industry, investigation of mechanical properties of clay nanocomposites has become of great interest. While the stiffening mechanism of clay nanocomposites is well documented, there is still not a clear understanding about how addition of clays affect the fracture behavior of clay/epoxy nanocomposites. The main aim of this paper is to measure and explain the effect of clays on ductility reduction of these nanocomposites. First, epoxy and clay/epoxy nanocomposites with different clay weight ratio were built. Then, the damage parameters of epoxy and clay/epoxy nanocomposites were measured by variation of the elasticity modulus. Based on loading–unloading experiments, the Lemaitre damage parameters for epoxy and clay/epoxy nanocomposites were extracted. Crack initiation and propagation in dog-bone sample were simulated for epoxy and clay/epoxy nanocomposites using the eXtended Finite Element Method (XFEM). The comparison between experimental and numerical results shows that the proposed method can predict the crack initiation location and propagation path in clay/epoxy nanocomposites.     

Uncertainty treatment in forced response calculation of mistuned bladed disk

Mistuning, imperfections in cyclical symmetry of bladed disks is an inevitable and perilous occurrence due to many factors including manufacturing tolerances and in-service wear and tear. It can cause some unpredictable phenomena such as mode splitting, mode localization and dramatic difference in forced vibration response. In this paper first, a method is presented which calculates the forced vibration response of a mistuned system based on an exact relationship between tuned and mistuned systems. Then, the genetic algorithm is used for solving an optimization problem to find the worst-case response of bladed-disk assembly. The second part tries to find methods to reduce the system worst-case response. Intentional mistuning which breaks the nominal symmetry of a tuned bladed disk and rearranging the bladed-disk assembly are introduced and used to reduce the system worst-case response. Finally, a two degree of freedom per blade simplified model with 56 blades is used to demonstrate the capabilities of the techniques in reducing the worst response of the bladed-disk system.     

An Improved Model for Fiber Kinking Analysis of Unidirectional Laminated Composites

This paper focuses on the fiber-kinking failure mode of unidirectional laminated composites under the compressive loading. An available stress based fiber-kinking model is explained and improved on the bases of strain concept. In the improved model, a new fracture surface is considered and the stresses are updated according to this new fracture surface. By taking the advantage of damage variables, the models are implemented into a finite element code and the results of numerical analysis such as prediction of kink band angles are discussed in details and compared with the available results in the literature. It is shown that the predicted kink band angles using the improved model are in good agreement with the experimental results.     

Numerical thermal analysis of nanofluid flow through the cooling channels of a polymer electrolyte membrane fuel cell filled with metal foam

Improvement in the cooling system performance by making the temperature distribution uniform is an essential part in design of polymer electrolyte membrane fuel cells. In this paper, we proposed to use water-CuO nanofluid as the coolant fluid and to fill the flow field in the cooling plates of the fuel cell stack by metal foam. We numerically investigated the effect of using nanofluid at different porosities, pore sizes, and thicknesses of metal foam, on the thermal performance of polymer electrolyte membrane fuel cell. The accuracy of present computations is increased by applying a three-dimensional modeling based on finite-volume method, a variable thermal heat flux as the thermal boundary condition, and a two-phase approach to obtain the distribution of nanoparticles volume fraction. The obtained results indicated that at low Reynolds numbers, the role of nanoparticles in improvement of temperature uniformity is more dominant. Moreover, metal foam can reduce the maximum temperature for about 16.5 K and make the temperature distribution uniform in the cooling channel, whereas increase in the pressure drop is not considerable.     

Effect of size on the dynamic behaviors of atomic force microscopes

Microsystem Technologies - Accurate mathematical modeling and simulation of cantilever dynamics are crucial to design and fabrication of the atomic force microscope (AFM). Thickness of AFM...     

A numerical study on heat transfer enhancement and design of a heat exchanger with porous media in continuous hydrothermal flow synthesis system

The aim of this study is to use a new configuration of porous media in a heat exchanger in continuous hydrothermal flow synthesis (CHFS) system to enhance the heat transfer and minimize the required length of the heat exchanger.For this purpose,numerous numerical simulations are performed to investigate performance of the system with porons media.First,the numerical simulation for the heat exchanger in CHFS system is validated by experimental data.Then,porous media is added to the system and six different thicknesses for the porous media are examined to obtain the optimum thickness,based on the minimum required length of the heat exchanger.Finally,by changing the flow rate and inlet temperature of the product as well as the cooling water flow rate,the minimum required length of the heat exchanger with porous media for various inlet conditions is assessed.The investigations indicate that using porous media with the proper thickness in the heat exchanger increases the cooling rate of the product by almost 40％and reduces the required length of the heat exchanger by approximately 35％.The results also illustrate that the most proper thickness of the porous media is approximately equal to 90％ of the product tube's thickness.Results of this study lead to design a porous heat exchanger in CHFS system for various inlet conditions.     

Parametric study and numerical analysis of empty and foam-filled thin-walled tubes under static and dynamic loadings

In this paper the crushing behavior of thin-walled tubes under static and dynamic loading is investigated. First, a finite element (FE) model for empty thin-walled tube was constructed and validated by available experimental and numerical data. The comparison between the FE results and the existing numerical solutions as well as the available experimental results showed good agreements. Next, a model for the foam was adopted and implemented in an in-house FE code. The implemented isotropic foam model was then used to simulate the behavior of foam-filled tubes under both static and dynamic loadings. Good agreement was observed between the results from the model with those obtained by analytical relations and experimental test data. The validated FE model was then used to conduct a series of parametric studies on foam-filled tapered tubes under static and dynamic loadings. The parametric studies were carried out to determine the effect of different parameters such as the number of oblique sides, foam density and boundary conditions on crushing behavior of rectangular tubes. The characteristic included deformed shapes, load–displacement, fold length and specific energy absorptions.     

An approach for simulating microstructures of polycrystalline materials

In this paper, an approach is identified using concepts in molecular dynamics (MD) and discrete element method (DEM) to generate the microstructure of polycrystalline materials. Using the proposed methods, different types of particles with different grain size and volume fraction in the real material, can be easily generated. It is assumed that the particles can be randomly packed together into a simulation region, by defining artificial interaction forces among them. Such forces may be either adopted from Van der Waals potential energy, or Hooke pair and gravity forces. The proposed method has proved to be fast due to the fact that the algorithm has been implemented on graphical processing units (GPU). Utilizing the Voronoi tessellation method, the set of the generated discrete grains have been altered to space-filling, adjoining polyhedrons with respect to the real geometry. Moreover, as an advantage, the boundary and the interface region of the microstructures were modeled.     

Finite-element modeling of rate dependent mechanical properties in nanocrystalline materials

A generalized self-consistent polycrystal model is used to study the mechanical properties of polycrystalline metals as the grain size decreases from the ultra-fine size to the nanometer scale. The model takes each oriented grain and its immediate grain boundary to form a pair. Then by making use of a composite model, the nonlinear behavior of the nanocrystalline polycrystal is determined. The finite-element method is employed in conjunction with the unit cell of the composite to investigate the rate-dependent tensile behavior of the system. A dislocation density based constitutive equation is used to describe the plastic flow behavior of the grain interior. The boundary phase is assumed to have the mechanical properties of quasi-amorphous material. The constitutive equations for both grain interior and boundary phase are implemented into a finite-element program and the results of the calculations are compared with previously published experimental data. For some cases, an optimization procedure was used to tune some parameters of the model in order to decrease the distance between the calculated and experimental stress–strain curves. The agreement between results indicates the suitability of the updated model for nanocrystalline materials.