Optimizing the Post Sandvik Nanoflex material model using inverse optimization and the finite element method

This article describes an inverse optimization method for the Sandvik Nanoflex steel in cold forming\ud processes. The optimization revolves around measured samples and calculations using the Finite Element\ud Method. Sandvik Nanoflex is part of the group of meta-stable stainless steels. These materials are characterized\ud by a good corrosion resistance, high strength, good formability and crack resistance. In addition, Sandvik\ud Nanoflex has a strain-induced transformation and, depending on austenising conditions and chemical composition,\ud a stress-assisted transformation can occur. The martensite phase of this material shows a substantial aging\ud response. The inverse optimization is a sub-category of the optimization techniques. The inverse optimization\ud method uses a top down approach, as the name implies. The starting point is a prototype state where the current\ud state is to converge on. In our experiment the test specimen is used as prototype and a calculation result as\ud current state. The calculation is then adapted so that the result converges towards the test example. An iterative\ud numerical optimization algorithm controls the adaptation. For the inverse optimization method two parameters\ud are defined: shape of the product and martensite profile. These parameters are extracted from both calculation\ud and test specimen, using Fourier analysis and integrals. An optimization parameter is then formulated from\ud the extracted parameters. The method uses this optimization parameter to increase the accuracy of ”The Post”\ud material model for Sandvik Nanoflex. [1] The article will describe a method to optimize material models, using\ud a combination practical experiments, Finite Element Method and parameter extraction

Identification of plastic constitutive parameters at large deformations from three dimensional displacement fields

The aim of this paper is to provide a general procedure to extract the constitutive parameters of a plasticity model starting from displacement measurements and using the Virtual Fields Method. This is a classical inverse problem which has been already investigated in the literature, however several new features are developed here. First of all the procedure applies to a general three-dimensional displacement field which leads to large plastic deformations, no assumptions are made such as plane stress or plane strain although only pressure-independent plasticity is considered. Moreover the equilibrium equation is written in terms of the deviatoric stress tensor that can be directly computed from the strain field without iterations. Thanks to this, the identification routine is much faster compared to other inverse methods such as finite element updating. The proposed method can be a valid tool to study complex phenomena which involve severe plastic deformation and where the state of stress is completely triaxial, e.g. strain localization or necking occurrence. The procedure has been validated using a three dimensional displacement field obtained from a simulated experiment. The main potentialities as well as a first sensitivity study on the influence of measurement errors are illustrated

Numerical and experimental analysis of wrinkling during the cup drawing of an AA5042 aluminium alloy

The recent trend to reduce the thickness of metallic sheets used in forming processes strongly increases the likelihood of the occurrence of wrinkling. Thus, in order to obtain defect-free components, the prediction of this kind of defect becomes extremely important in the tool design and selection of process parameters. In this study, the sheet metal forming process proposed as a benchmark in the Numisheet 2014 conference is selected to analyse the influence of the tool geometry on wrinkling behaviour, as well as the reliability of the developed numerical model. The side-wall wrinkling during the deep drawing process of a cylindrical cup in AA5042 aluminium alloy is investigated through finite element simulation and experimental measurements. The material plastic anisotropy is modelled with an advanced yield criterion beyond the isotropic (von Mises) material behaviour. The results show that the shape of the wrinkles predicted by the numerical model is strongly affected by the finite element mesh used in the blank discretization. The accurate modelling of the plastic anisotropy of the aluminium alloy yields numerical results that are in good agreement with the experiments, particularly the shape and location of the wrinkles. The predicted punch force evolution is strongly influenced by the friction coefficient used in the model. Moreover, the two punch geometries provide drawn cups with different wrinkle waves, mainly differing in amplitude.The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) under project PTDC/EMS-TEC/1805/2012. The first author is also grateful to the FCT for the Postdoctoral grant SFRH/BPD/101334/2014 and P.D. Barros is grateful to the FCT for the PhD Grant SFRH/BD/98545/2013info:eu-repo/semantics/publishedVersio

Determination of the yield locus by means of temperature measurement

The paper presents a theoretical background of the thermo-graphical method of determining the\ud yield locus. The analytical expression of the temperature variation of the specimen deformed in the elastic\ud state is determined starting from the first law of thermodynamics. The experimental method for determining\ud the yield locus based on the Joule-Thompson effect is presented in detail. The analytical model is validated by\ud experiments. Based on these results, it is theoretically justified that the Joule-Thompson effect can be used in\ud the experimental determination of the yield locus. The thermo-graphical method has been used to determine\ud the yield locus of the AA 5182-0 aluminium alloy