Lutz Nasdala’s research while affiliated with Offenburg University of Applied Sciences and other places

The applicability of finite elements for molecular dynamic simulations depends on both the structure’s dimensions and the underlying force field type. Shell and continuum elements describe molecular structures only in an average sense, which is why they are not subject of this paper. In contrast, truss and beam elements are potentially attractive candidates when it comes to accurately reproducing the atomic interactions. However, special considerations are required for force fields that use not only two-body, but also multi-body potentials. For the example of bending and torsion energies it is shown how standard beam element models have to be extended to be equivalent to classical molecular dynamic simulations.

In order to understand the underlying mechanisms of inelastic material behavior and nonlinear surface interactions, which can be observed on macro-scale as damping, softening, fracture, delamination, frictional contact etc., it is necessary to examine the molecular scale. Force fields can be applied to simulate the rearrangement of chemical and physical bonds. However, a simulation of the atomic interactions is very costly so that classical molecular dynamics (MD) is restricted to structures containing a low number of atoms such as carbon nanotubes. The objective of this paper is to show how MD simulations can be integrated into the finite element method (FEM) which is used to simulate engineering structures such as an aircraft panel or a vehicle chassis. A new type of finite element is required for force fields that include multi-body potentials. These elements take into account not only bond stretch but also bending, torsion and inversion without using rotational degrees of freedom. Since natural lengths and angles are implemented as intrinsic material parameters, the developed molecular dynamic finite element method (MDFEM) starts with a conformational analysis. By means of carbon nan-otubes and elastomeric material it is demonstrated that this pre-step is needed to find an equilibrium configuration before the structure can be deformed in a succeeding loading step.

This paper presents a new approach for tire dynamic analysis. By using this method, transient response for rotating tires under various loading situations can be analyzed. The well-known model of ring on elastic foundations (REF) is utilized to model tires. The general forced solution of undamped inextensible vibration is derived by the use of a modal expansion technique as well as Meirovitch modal analysis method. Closed form transient response for the stationary constant point load case is obtained; for the case of damped vibration, the response of rotating tire is formulated by using the first-order matrix perturbation theory together with Meirovitch modal analysis method. The effects of damping on the tire response are investigated. The developed method has been validated by comparison with direct numerical integration results. Combined with a contact or interface model, the proposed methodology can be used to model the tire dynamic responses under any given road profile.

It is a challenging task in the design of automobile tires to predict lifetime and performance on the basis of numerical simulations. Several factors have to be taken into account to correctly estimate the aging behavior. This paper focuses on oxygen reaction processes which, apart from mechanical and thermal aspects, effect the tire durability. The material parameters needed to describe the temperature-dependent oxygen diffusion and reaction processes are derived by means of the time–temperature–superposition principle from modulus profiling tests. These experiments are designed to examine the diffusion-limited oxidation (DLO) effect which occurs when accelerated aging tests are performed. For the cord-reinforced rubber composites, homogenization techniques are adopted to obtain effective material parameters (diffusivities and reaction constants). The selection and arrangement of rubber components influence the temperature distribution and the oxygen penetration depth which impact tire durability. The goal of this paper is to establish a finite element analysis based criterion to predict lifetime with respect to oxidative aging. The finite element analysis is carried out in three stages. First the heat generation rate distribution is calculated using a viscoelastic material model. Then the temperature distribution can be determined. In the third step we evaluate the oxygen distribution or rather the oxygen consumption rate, which is a measure for the tire lifetime. Thus, the aging behavior of different kinds of tires can be compared. Numerical examples show how diffusivities, reaction coefficients, and temperature influence the durability of different tire parts. It is found that due to the DLO effect, some interior parts may age slower even if the temperature is increased.

Goal of this paper is the development of analytical design rules for bolted end-plate connections with an elastomeric intermediate layer as thermal separation. The actual stress distribution is converted to an equivalent mean stress and an associated effective height. The geometry dependence of the design compressive stresses can be described using a form factor.A finite element model is developed for validating the design rules taking into account geometric and material nonlinearities like large deformations, creep processes or the frictional contact. A major conclusion that can be drawn from the finite element simulation is that compressive stresses decrease towards the edges leading to a stress concentration between the bolts. Investigations of the long-term behavior demonstrate that the influence of creeping processes on the stress distribution can be neglected. On the other hand, it can be shown that the moment–rotation characteristic is strongly dependent on the pre-load of the bolts and the elastomer thickness.

Kraftfelder sind eine bewährte Methode für die Berechnung atomarer Strukturen wie Kohlenstoffnanoröhren. Die Bedeutung des Auftretens von Defekten für die mechanischen Eigenschaften der Kohlenstoffnanoröhren kann mit einem auf Kraftfeldern basierenden finiten Element nachgewiesen werden. Bei einer Belastung der Kohlenstoffnanoröhren kommt es in Abhängigkeit von der Last entweder zu Material- oder Stabilitätsversagen. Ein interessantes Ergebnis ist, dass bei Torsion Verzweigungsprobleme durch Defekte in Durchschlagsprobleme überführt werden. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)