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

Inelastisches Materialverhalten ist die Folge einer st?ndigen Umordnung von chemischen und physikalischen Bindungen. Am Beispiel eines f?r Elastomerwerkstoffe entwickelten mikromechanischen Ersatzmodells wird gezeigt, dass sich auf Grundlage der rein elastischen Potentialfunktionen des Kraftfeldansatzes, also ohne Verwendung von D?mpfer- oder Reibelementen,Ph?nomene wie der Mullins-Effekt oder Hysteresen simulieren lassen. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The molecular structure of a material determines its mechanical, thermal and chemical properties. Thus, to better understand characteristic mechanical properties like damping behavior or softening, in principle, one just has to model the interactions of a sufficiently large number of atoms. Various force field approaches have been proposed for that purpose, which are based on molecular-dynamic simulations, or rather quantum-mechanical ab initio calculations. They provide the potential energy of a structure in dependence of sort and number of chemical and physical bonds. In general, the different energy forms can be represented by nonlinear normal, bending and torsional springs which suggests the use of a finite element code. However, standard finite elements like truss, beam or shell elements are not very applicable because of the interaction of many atoms and, considered from a mechanical perspective, the absence of rotational degrees of freedom. For example, a bending of beam elements would lead to unrealistic constraints of neighboring molecular groups. In order to overcome this disadvantage, a new 4-node finite element is introduced, which uses only translational degrees of freedom and therefore is capable of representing the different energy forms exactly.

Like any other geometric structure or building, carbon nanotubes may break down due to either material failure or structural failure. In this paper, it is shown that the failure mechanism-of carbon nanotubes not only depends on the type and direction of loading but also on the location and number of defects. For the finite element simulations we use a new 4-node finite element without rotational degrees of freedom based on the force field method. For the examples shown here, mainly a single-walled (10,10) armchair nanotube with different Stone-Wales defects, the material parameters are directly taken from the DREIDING force field. For carbon nanotubes subject to tension a kind of material failure, i. e. a breaking of bonds, can be observed. For carbon nanotubes subject to bending, an interesting question is whether they fail due to a breaking of bonds in the tension zone, which would be similar to the tension experiment, or due to a snap-through of bonds in the compression zone. From our FE simulations, it can be concluded that neither of these two failure mechanisms, but local buckling in the compression zone can be observed. From a mechanical point of view, however, it is not a pure bifurcation problem because the buckles are formed relatively slowly which corresponds more to a snap-through problem. For carbon nanotubes subject to torsion, we have to distinguish between bifurcation problems which are the case for defect-free nanotubes and snap-through problems which can be observed for those with defects. In all cases the Stone-Wales defects are responsible for a reduction of the maximum load, about 10 % for tension and bending, and up to 30 % for torsion.

Tire shoulder endurance is one of the main concerns of tire designers as well as the end users. The separation of the tire shoulder will lead to tire failure and usually causes safety problems of vehicles. This work presents an initial analysis on the cause and effect for a typical heavy-duty radial tire structure. Structure parameters and material selection are considered as the main factors affecting the tire shoulder endurance. Stress and strain distributions in the area of the tire shoulder are obtained by using finite element analysis. Rebar elements are used to model the belt as well as the carcass, while the rubbers are modeled by solid elements. Temperature, frequency and strain scan tests are conducted to obtain the dynamical mechanical properties of the rubbers in the shoulder area. In addition, stress-strain properties are obtained through tensile tests. Parameter study is performed to find the optimum compound combination, based on the minimum local shear stress/strains and heat generation. It is argued that the tire material arrangement and compound design should be optimized according to the stress and strain fields of the tire.

Kraftfeldansätze für die Berechnung atomarer Strukturen können in die Finite Elemente Methode eingebettet werden. Standard-Elemente sind jedoch nicht in der Lage, die Atomphysik adäquat wiederzugeben. Anhand von Berechnungen von Kohlenstoffnanoröhren wird gezeigt, dass ein neues 4-Knoten-Element geeignet ist, atomare Strukturen zuverlässig zu berechnen. Auch Defekte in der Struktur der Kohlenstoffnanoröhren können damit berücksichtigt werden. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Zur Berechnung von Nanostrukturen wie z.B. Kohlenstoff-Nanoröhren stehen eine Reihe von Kraftfeld-Ansätzen (force fields) zur Verfügung, die sich aus molekulardynamischen Untersuchungen bzw. quantenmechanischen “ab initio” Berechnungen herleiten lassen. Sie liefern die potentielle Energie einer Struktur in Abhängigkeit von der Art und Anzahl der chemischen und physikalischen Bindungen. Die verschiedenen Energieformen lassen sich grundsätzlich mit nichtlinearen Normalkraft-, Biegemomenten- und Torsionsmomentenfedern vergleichen, so dass der Einsatz von Finite Elemente Programmen nahe liegt. Aufgrund der Interaktion mehrerer Atome und der Tatsache, dass Atome keine Rotationsfreiheitsgrade im mechanischen Sinne besitzen, ist die Anwendung von Standard-Elementen wie Stab-, Balken- oder Schalenelementen problematisch. Beispielsweise führt die Verdrehung von Balkenelementen zu unerwünschten Zwängungen in angrenzenden Molekülgruppen. Aus diesem Grund wird eine neues 4-Knoten-Element entwickelt, das ausschließlich Verschiebungen als Freiheitsgrade benutzt und somit in der Lage ist, die verschiedenen Energieformen exakt abzubilden. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis for tire dynamic response rolling over an obstacle is important to study automobile NVH (Noise-Vibration-Harshness), determine vehicle fatigue load, investigate combined longitudinal and sideslip properties, and develop ABS system on uneven roads. Based on the model of Ring on the Elastic and Viscoelastic Foundation (REF) and its analytical solution previously developed, the rolling contact problem between tire/flat and tire/cleat is dealt with in this paper. The static contact problem is treated as the first step to show the effectiveness and accuracy of the model. Then, the time domain simulation of tire rolling contact on uneven roads is conducted. Meirovitch modal analysis method and first-order matrix perturbation theory are applied to obtain the general forced response of damping REF vibration. An effective numerical quadrature method is developed to obtain the time-varying modal coordinates of the system under various loading conditions. Numerical examples of a tire rolling over a cleat are given to verify the developed method. It is found that both damping and velocity have strong effects on tire response over a cleat and the frequency of dynamic load is mainly controlled by the first tire mode.

In this work, the mechanical properties of aged rubbers were investigated. Dynamic properties of aged rubbers with various aging times, temperatures and pre-strains were tested. Several kinds of filled rubber specimens relevant to a heavy-duty radial tyre were prepared to be prestrained in an in-house rig. The prestrained specimens were then put into an aging oven to accelerate aging. The aging times were chosen to be 24–240 h. After aging, static and dynamic mechanical tests are performed on these specimens. Some experimental results are presented and discussed to demonstrate the importance of rubber age for its time-dependent properties, especially the Payne Effect.

Die diskrete Modellierung von Faserverbundstrukturen führt zu einem sehr großen numerischen Aufwand, so dass die Materialeigenschaften von Fasern und Matrix in der Regel gemittelt werden müssen. Bei der Berechnung von Sauerstoffdiffusions- und -reaktionsvorgängen ergibt sich die Schwierigkeit, dass aufgrund der Oxidation die Massenbilanz nicht erfüllt werden kann. Die Güte des gemittelten Materialparametersatzes hängt neben den Ausgangsmaterialien auch wesentlich davon ab, ob die Diffusion in oder quer zur Faserrichtung betrachtet wird.

Durch die steigenden Anforderungen hinsichtlich Wirtschaftlichkeit und Komplexität an moderne Bauten rückt die Berücksichtigung nichtlinearer Werkstoffeigenschaften im Rahmen der Berechnung und Lebensdaueranalyse immer weiter in den Blickpunkt der Anwendung. Ein Bereich, in dem das nichtlineare Tragverhalten wesentliche Auswirkungen auf Bemessung und Konstruktion der Bauteile hat, ist der Stahl-/Stahlbeton-Verbundbau. In diesem Artikel wird ein elasto-plastisches Schädigungsmodell für den Beton vorgestellt. Die Anwendung dieses Modells erfolgt im Rahmen der Simulation des Tragverhaltens eines Kopfbolzendübels eines Verbundträgers. Dieser Kopfbolzendübel ist auf den Stahlträger aufgeschweißt und dient der Schubübertragung zwischen Stahlträger und Stahlbetonplatte.