N. Bontcheva

Numerical investigation of the damage process in metal forming

Abstract The damage process during metal forming is investigated. The coupling of the deformation, thermal and damage processes is taken into account. A finite element program is developed and simple upsetting is simulated. The influence of geometry and friction on rupture is investigated.

Influence of the induced anisotropy in powder-based materials

Development of anisotropy, induced during processes of compaction of metal powders and plastic deformation of sintered metals is considered. A mechanical model, taking into account the induced anisotropy is proposed. The application of the finite element (FE) method for the proposed model is given and a FE code is created. Numerical simulation of compaction and of forging processes are given, showing the influence of the induced anisotropy on the relative density distribution and on the form of the end product.

Initiation of plastic localization in powder-based materials

Experiments show that plastic localization bands may occur in materials during metal-forming processes. As plastic strains during metal-forming processes are much larger than the elastic ones, the rigid-plastic model is assumed. Coupled temperature and damage effects are taken into account. The necessary conditions for initiation of shear and delatation localization bands under plain nonhomogeneous plastic flow are obtained from the conditions for bifurcation of the rates of the process parameters. Numerical examples show the initiation of localization bands during metal-forming processes.

ELASTIC PLASTIC PROPERTIES OF MULTIPHASE COMPOSITE BASED ON NON HOMOGENEOUS METAL MATRIX – MODELLING AND SIMULATION

Abstract A new approach for estimating the mechanical properties of multiphase composite is proposed. The Representative Volume Element (RVE) of the material consists of two different metal matrices and arbitrary number of elastic hardening phases. Multistep homogenization procedures are applied, accounting for influence of the constituent properties, sizes, volume fractions and microstructural distributions. Modified Mori-Tanaka homogenization technique for micropolar media and Budiansky self-consistent method are used for estimation the overall properties of the composite. The theoretical model is applied to description of the properties of precipitation hardening rapidly solidified alloy of the system AlFeVSi with high content of Fe and Si.

Multiscale modeling of multiphase "in situ" composite

The elastic-plastic behaviour of rapidly solidified Al based (FeSi)-enriched alloys containing intermetallic compounds is considered. A new multilevel mechanical model for the “in situ” composite is proposed considering the aluminium matrix as a micropolar elastic plastic Cosserat material and the hardening phases as pure elastic ones. A two steps homogenization procedure is applied to obtain the overall properties of the multiphase “in situ“ composite, taking into account the existence of different sizes of intermetallic inclusions. A variational approach is applied to evaluate the equivalent stress on macro level at the transition from micro to macro scale. The model is developed using information provided by microstructural investigations and EDX analysis. The multistage bulk material manufacturing process from rapid solidified powders or ribbons is simulated using the Finite Element Method. The model is implemented as user subroutines into the FE code MARC. Numerical simulations are provided, corresponding to different values of metal forming parameters. The influence of the different inclusions sizes on the hardening behavior is discussed. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

THERMOELASTIC STRESS ANALYSIS FOR LINEAR ELASTIC BODIES

The stress analysis for elastic cylinders made of polymethylmethacrylate (PMMA) under cyclic loading is provided by using FEM. Under the assumption that the elastic moduli depend linearly on the temperature, new fomulae, generalizing the well known Kelvins formula, are given such that we can estimate the temperature variation on the external surface of a homogeneous ideal cylinder. Using suitable numerical methods we obtain theoretical results which fit the experimental values for the temperature variation