Georgios I. Giannopoulos

On the coupling of axial and shear deformations of single-walled carbon nanotubes and graphene: a numerical study

This paper presents a numerical investigation for the evaluation of elastic coupling of extension and twist in single-walled carbon nanotubes. The carbon nanotubes are modelled according to their atomistic structure. The harmonic approximation is utilized for describing the interaction potential energies. The force field is simulated via suitable straight and torsional springs that express the interatomic interactions and interconnect nodes placed on the atomic positions. By using appropriate boundary conditions, the nanotubes are loaded axially and the resulted twist is numerically predicted using finite element procedures. The numerical outcomes reveal that the stretching deformation of single-walled carbon nanotubes leads to their torsion only for the chiral case. A complete parametric study with respect to geometric characteristics of nanotubes shows that the coupling is strongly dependent on the diameter, chiral angle, and nanotube length. To explain these observations, the inherent graphene sheet geometries used for the generation of single-walled carbon nanotubes are also studied under extension. The results prove similar coupling phenomena occur between extension and in-plane shear. Comparisons with results provided in the open literature are also performed, where possible.

A Numerical Investigation on the Influence of Steel Fiber Shape and Interface Strength in Reinforced Concrete

Not all steel fiber reinforced concrete composites are equally effective in enhancing structural performance. Their mechanical behaviour strongly depends upon the reinforcement morphology as well as the properties of the interface lying between steel reinforcement and concrete matrix. Using bone-shaped short (BSS) steel fibers, instead of conventional straight short (CSS) steel fibers, to reinforce concrete has demonstrated their potential in improving toughness, ductility and energy absorbing capacity under impact significantly and simultaneously. Accomplishing a strong steel–concrete interface leads to a slight increase in composite strength but simultaneously to a significant decrease in its toughness. Due to the sensitivity of steel reinforced concrete performance on these complex geometric and material parameters, the development of a numerical tool capable of simulating accurately the composite mechanical behaviour and thus leading to optimized design solutions is desirable. The physical problem of the present work involves a typical concrete composite uniformly reinforced with steel fibers subjected to tensional loading. A micromechanical non-linear finite element formulation is utilized in order to predict the load transfer characteristics and the failure process. A linear material behaviour is assumed for the steel component; a non-linear multi-crack material response is used to describe concrete while a mix-mode bilinear behaviour is utilized for the interface providing separation of primary material phases. Numerical results are presented in terms of the global design parameters. The influence of the fiber end shape, the interface strength and the fiber volume fraction on the composite strength and toughness is addressed and consequently optimized design preferences arise.

Thermomechanical Interfacial Crack Closure: A BEM Approach

In this paper a sub-regional boundary element formulation is proposed for the treatment of general two-dimensional steady-state and time-dependent thermoelastic crack closure problems considering friction and thermal resistance along the crack faces. These problems are solved by an incremental-iterative scheme since the extent and the status of the contact zone are not known in advance. The assumption of pressure-dependent thermal contact increases the degree of non-linearity and couples the thermal and mechanical fields. The present work is focused on fracture problems situated on the interface of dissimilar isotropic solids under combined mechanical and thermal loads.

Finite Element Modeling of Nanotubes

Abstract This chapter presents an integrated computational method for the prediction of the nanoscale mechanical response of nanotubes. According to the proposed numerical approach, the nanotubes are modeled according to their equilibrium atomistic structure while standard structural mechanics principles are adopted. Utilizing molecular theory, the interatomic interactions within the nanotube are simulated via suitable spring elements that connect the nodes placed at the atomic positions. The calculation of elastic properties as well as natural frequencies requires linear approximation, whereas nonlinear formulation is utilized for the investigation of static or dynamic elastoplastic deformations. For the prediction of vibrational problems, the stiffness and mass matrices are constructed according to the nodal positions to make the dynamic equilibrium equation solvable. The natural frequencies and the corresponding modes of vibration are derived by solving the eigenvalue problem for different support conditions. The modeling is regenerative and can provide simulations for different geometric characteristics of the nanotubes. The effectiveness of the proposed computational method is demonstrated by comparisons with relevant results from the literature.