Papers by Author: Andreas Öchsner

Abstract: The vibrational behavior of defected graphene sheets was investigated via finite element analysis. The simulations were carried out for perfect and imperfect nanosheets. This study was conducted to examine the influence of vacant sites on these nanostructures. In the current study, a graphene sheet is considered as a space frame. The natural frequency and corresponding mode shapes of the perfect and defective nanosheets were obtained and compared. Results are presented as diagrams stating the natural frequency of graphene sheets with respect to the amount of vacancy defects. The results indicate that the natural frequency of nanosheets reduced by introducing atomic defects to the configuration of these nanomaterials. Such impurities lower the vibrational stability of graphene sheets.

Abstract: This study deals with the investigation of the tensile and shear behavior of connected carbon nanotubes (CNTs) with parallel longitudinal axes by performing several computational tests. In particular, the effect of imperfections on the mechanical properties, i.e. Young’s modulus and shear modulus, of these nanoconfigurations was analyzed. For this purpose, straight hetero-junctions were simulated in their perfect form and different boundary conditions were considered. In the second phase the three most likely atomic defects, i.e. impurities (doping with Si atoms), vacant sites (carbon vacancy) and introduced perturbations of the ideal geometry in different amounts to the perfect models, were simulated. Finally, the mechanical properties of imperfect hetero-junctions were numerically evaluated and compared with the behavior of perfect ones. It was concluded that the existence of any type of imperfections in the structure of connected CNTs leads to a reduction in the Young’s modulus as well as the shear modulus, and as a result, lower stiffness of these straight nanostructures.

Abstract: Perfect and spiral configurations of carbon nanotubes (CNTs) were modeled by a commercial finite element package and their tensile behavior was studied. Computational tests with cantilevered boundary conditions were performed to evaluate their Young’s modulus. It was concluded that the existence of any imperfection, spiral shape in particular, in the structure of perfect CNTs results in a remarkable reduction in the stiffness. It was also revealed that the Young’s modulus of perfect CNTs decreases by introducing spiral distortion.

Abstract: New applications for carbon nanotubes (CNTs) are emerging every day. CNTs are mostly used as the reinforcing phase in polymer composites. Recently, their application in improving the conductivity of these composites has attracted a lot of researchers. Considering helping to have a more realistic view of their reinforcing ability, this paper investigates the effect of nanotubes arbitrary orientations on the reinforced composite thermal conductivity. Two cases, i.e. the case when all the fibers are aligned and the case when the fibers are distributed with arbitrary orientations have been studied. Also, the effect of volume fraction value on the reinforcing capability of the inclusions is investigated. It is shown that the fibers orientation has an unfavorable effect on the composites conductivity and decreases it in comparison with the case when all the fibers are aligned and parallel to the heat flux. Furthermore, increasing the volume fraction also increases the thermal conductivity.

Abstract: In this paper, two different beam elements (i.e. according to the Bernoulli beam and Timoshenko beam theory) for the modeling of the behavior of carbon nanotubes are applied. Finite element models are developed for this study with variation of chirality for both zig-zag and armchair configurations of CNTs. The deformations from the finite element simulations are subsequently used to predict the elastic stiffness and the critical buckling load in terms of material and geometric parameters. Furthermore, the dependence of mechanical properties on the kind of beam element and the mesh density is also compared. Based on the obtained results, Youngs modulus and critical buckling load of structures using Timoshenko beams are clearly lower than the Bernoulli beam approach for all chiralities.

Abstract: In this study, numerous types of straight hetero-junction carbon nanotubes (CNTs) and their fundamental CNTs were investigated by the finite element method (FEM). By applying the FEM, the shear behavior of these hetero-junctions was obtained thorough numerical simulation. The behavior of hetero-junctions and their constituent CNTs were investigated. The investigations revealed that the twisting angle of straight hetero-junction CNTs lies within the range of twisting angle of their fundamental CNTs. In addition, change of boundary conditions did not significantly change the value of obtained twisting angle of hetero-junctions. It was also concluded that the shear behavior of straight hetero-junctions and their constituent CNTs increases by increasing the chiral number of both armchair and zigzag CNTs. The current study provides a better insight towards the prediction of straight hetero-junction CNTs behavior.

Abstract: The finite difference method is applied to derive approximate solutions for the elasto-plastic bending of Euler-Bernoulli beam problems. The investigations are restricted to a simple exemplary configuration, i.e. a straight cantilevered beam with constant rectangular cross-section and linear-elastic/ideal-plastic material properties, loaded by a constant distributed load. Only finite difference approximations of second-order accuracy are considered and special emphasis is given to the influence of the load step, the number of layers, and the number of nodes. Based on comparisons with the analytical solution, clear recommendations can be given on the required parameters to obtain a certain accuracy in the numerical approach.

Abstract: Heat conductivity is a well-known energy transfer method and it is here applied to the study of metal foams and laser processing. Metallic hollow sphere structures (MHSS), a relatively new group of advanced composite materials, combine the advantages of cellular metals without major scattering of their material parameters. They are characterised by high geometry reproduction leading to relatively constant mechanical and physical properties.The laser processing technology provides not only a laser cutting but also a laser soldering procedure. Within this work a laser cutting process is applied to MHSS. Laser beam cutting is a highly efficient technique to cut materials, because the relatively small amount of heat affects only a small heating zone.Numerical simulation is used in order to define proper process parameters for a large variety of MHSS. The finite element method based simulation covers material parameters as well as process parameters like the cutting velocity. Heat conduction and convection are taken into account and the phase change from solid to liquid state as well. Within the simulations the concept of representative volume element (RVE) is applied. The temperature distribution is the fundamental result.

Abstract: This contribution investigates the numerical solution of the steady-state heat conduction equation. The finite difference method is applied to simple formulations of heat sources where still analytical solutions can be derived. Thus, the results of the numerical approach can be related to the exact solutions and conclusions on the accuracy obtained. In addition, the numerical implementation of different forms of boundary conditions, i.e. temperature and flux condition, is compared to the exact solution. It is found that the numerical implementation of coordinate dependent sources gives the exact result while temperature dependent sources are only approximately represented. Furthermore, the implementation of the mentioned boundary conditions gives the same results as the analytical reference solution.

Abstract: Two configurations of perfect single walled carbon nanotubes (armchair and zigzag) were simulated based on the finite element method. Then, three most likely defects (Si-doping, carbon vacancy and perturbation) were introduced to the models to represent defective forms of single walled carbon nanotubes (SWCNTs). Finally, the vibrational properties of perfect and defective carbon nanotubes were evaluated and compared. The results showed that SWCNTs have a natural frequency with a rather high value between 18.69 and 24.01 GHz. In the consideration of the natural frequency of the defective SWCNTs, it was also observed that the existence of any type of defects or irregularities leads to a lower value of natural frequency and vibrational stability. Simple mathematical relations which express the change in natural frequency versus the percentage of the defect were also presented. This can be very useful to realistically estimate the influence of defects of different amounts on the vibrational behavior of carbon nanotubes.