Methoden zur Darstellung inelastischen Materialverhaltens bei kleinen Deformationen

Available from TIB Hannover: RR 2040(1994,5) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Phenomenological model for the macroscopical material behavior of ferroelectric ceramics

A thermodynamically consistent previous termphenomenologicalnext term model for the simulation of the macroscopic behavior of ferroelectric polycrystalline ceramics is presented. It is based on the choice of microscopically motivated internal state variables, which describe the texture and the polarization state of the polycrystal. Saturation states are defined for the internal state variables. The linear material behavior is modelled by a transversely isotropic piezoelectric constitutive law, where the anisotropy is history dependent. For non-linear irreversible processes, a switching function and associated evolution rules are applied, satisfying the principle of maximum ferroelectric dissipation. Saturation is modelled by the use of energy-barrier functions in the electric enthalpy density function. Numerical examples demonstrate the capability of the proposed model, to predict the typical experimental results

Aspekte der Kontinuumsmechanik und Materialtheorie Festschrift zum 60. Geburtstag von Professor Peter Haupt

SIGLEAvailable from TIB Hannover: RR 2040(1998,1) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Diskussion plastischer Instabilitaeten eines Druckbehaelters mit Hilfe der Methode der Finiten Elemente

Im vorliegenden Bericht wurde mittels des Finite-Elemente Programms ABAQUS der Einfluss geometrischer und thermischer Imperfektionen auf den Versagensdruck von Druckbehaeltern bestimmt. Im Rahmen der Theorie grosser Deformationen wurde ausgehend von einem Materialmodell der Thermoplastizitaet Versagen durch plastische Instabilitaet berechnet. Die dazu notwendigen Materialparameter wurden anhand einachsiger Zugversuche identifiziert. Die Parameter wurden so angepasst, dass das Einschnuerverhalten der Zugprobe gut wiedergegeben wurde. Ausgehend vom Zugstab wurde der Einfluss vom Imperfektionen auf das Einschnuerverhalten diskutiert. In 2D- und 3D-Rechnungen wurde gezeigt, dass bei Druckbehaeltern durch ausreichend grosse Imperfektionen eine Verformungslokalisierung erzielt werden kann. Erwartungsgemaess zeigte sich, dass der maximal ertragbare Innendruck sowie der Beginn der Verformungslokalisierung von der Groesse der Imperfektion abhaengt. (orig.)In this report the influence of geometrical and thermal imperfections on the load-carrying capacity of pressure vessels has been studied by means of the ABAQUS finite element code. Using a thermo-plasticity model for finite strains failure due to plastic instability has been investigated. The parameters needed for this model were identified with uniaxial tensile tests. The parameters were fitted in such a way that necking of the tensile rod was described in a satisfactory manner. Starting from the tensile rod the influence of imperfections on necking of a structure is discussed. Axisymmetric and three dimensional calculations showed that for a sufficient size of the imperfection necking can be obtained. As expected, the maximum internal pressure at failure and the beginning of localization of plastic deformations depend on the size of the imperfection. (orig.)SIGLEAvailable from TIB Hannover: ZA 5141(5362) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Experimentelle Ermittlung der Gleichgewichtsspannung bei viskoplastischen Materialien

Using strain controlled uniaxial loading histories, two viscoplasticity laws, formulated by means of two different so called overstresses, are discussed. According to the first viscoplasticity law the equilibrium stress is defined to be a rate independent funcional of the strain history. In the second viscoplasticity law the equilibrium stress is related to the so called static stress. This equilibrium stress can be determined by the property that during a relaxation experiment the actual stress, the static stress and the equilibrium stress converge to the same limit value. It is shown that the two viscoplasticity laws predict different responses for strain controlled loadings. In order to examine the capabilities of the two viscoplasticity laws, uniaxial tension-compression tests including hold time periods have been carried out. It turns out that the material behavior of bronze, zircaloy and three stainless steels can be described more accuratly by the first viscoplasticity law. On the other hand the material response of unalloyed aluminium and an aluminium alloy can be represented better by the second viscoplasticity law. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Untersuchungen zum dynamischen Verhalten mikrostrukturierter Membranen mit Hilfe der Methode der finiten Elemente

A sensor consisting of microstructured membranes which is a tool for measuring quantities such as temperature, strain, or density of a surrounding fluid medium by ultrasound, i.e. without any contant (e.g. by a wire connection), is in a stage of development at the Institute for Microstructure Technology at the Karlsruhe Research Center. In this report we study the dynamical properties of the sensor by means of the finite element method. First, the existence of the eigenmode, which is needed for the measurements, and the possibility to excite it is proved with suitable chosen finite element models resulting from extensive preliminary examinations. Then the dependence of the corresponding eigenfrequency on material- and geometry parameters is examined by variation of these parameters. Hence it follows, that the analytical formula, which is used to evaluate the experimental measurements, is valid for certain material combinations only. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Theoretische und experimentelle Untersuchungen zur zyklischen Thermoviskoplastizitaet

The aim of the present paper is to investigate viscoplasticity laws of Chaboche type exhibiting pure kinematic hardening. The evolution-equations describing kinematic hardening include a term which is dependent on the temperature rate. The investigations are performed with reference to the stainless steel AISI 316L(N). The material parameters appearing in the constitutive equations are determined on the basis of isothermal experiments while responses predicted by the model are calculated for non-isothermal deformation processes in order to check the capabilities of the model. Experimental results indicate that both Young's modulus and yield stresses decrease with increasing temperature. Also, for monotonic loading as well as for cyclic loading conditions essential hardening effects in the stress-strain plots were observed. Relaxation effects and thus the value of the corresponding overstress decrease with increasing temperature. After temperature controlled cyclic loading conditions the material response shows fading memory properties with respect to past deformation histories. Calculated responses compared with corresponding experimental findings demonstrate the good performance of the constitutive model in describing the material behaviour during the first cycle of the loading history. However, with increasing number of cycles there are differences between experimental results and calculated model predictions. The reason for this behaviour follows from the fact that no cyclic hardening effects are incorporated in the constitutive model. Some kind of deviations are observed as well for cases where the absolute value of the stress and the value of the temperature are increasing at the same time. (orig.)20 refs.Available from TIB Hannover: ZA 5141(5958) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Simulation eines Abformvorgangs

In this report the demolding of microstructures, where the mold insert is produced by the LIGA process, is studied by means of the ABAQUS finite element code. Mechanisms which can lead to failure during the demolding process were studied with respect to geometry and material properties of the compounds. It could be shown that the failure can be expected at the transition from the microstructure to the residual layer. The intensity of loading in this region depends on the thickness of the residual layer and the combination of the polymer (mold material) with the substrate. As an essential result, the use of a substrate has a negative influence on the demolding process for thick residual layers. For thin residual layers, it turns out, that the choice of the substrate material has a strong influence on the strength of the stress in the transition region. (orig.)SIGLEAvailable from TIB Hannover: ZA 5141(5418) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman