MECHAMODE · Mechanism based modelling of plastic deformation

Periodic Report Summary - MECHAMODE (Mechanism based modelling of plastic deformation)

Project context and objectives The project MECHAMODE deals with the formulation, implementation and application of constitutive equations for newly developed metallic sheet metals. Specific emphasis is put on the optimisation of manufacturing processes for advanced high strength steel and magnesium products. The objectives are as follows: - providing the scientific foundation for developing plasticity models; - validation and verification of the models by respective mechanical tests; - application of the models to metal forming processes. Novel structural materials like magnesium alloys and Twinning Induced Plasticity (TWIP) steels are considered. The project focuses on the numerical modelling of anisotropic plastic deformation and deformation twinning. An outcome of the project is - besides a better understanding of the deformation mechanisms on the micro-scale - a robust and fast simulation procedure for industrial applications. Project results Within the project, the following scientific tasks have been performed: - Equipment for conducting hydraulic bulge tests has been installed and instrumented. It enables the determination of a sheet stress-strain response under balanced biaxial loading conditions. A specific die has been designed and fabricated; - A biaxial tester allowing for variable biaxial tensile loading has been set up and instrumented; - Hole expansion experiments have been conducted using different steel and magnesium sheets. Finite element (FE) simulations revealed the effects of the chosen plastic potential; - The visco-plastic self-consistent (VPSC) code was made available through the Graduate Institute of Ferrous Technology (GIFT, South Korea). Predictions of the mechanical response of polycrystalline aggregates can now be performed efficiently; - A novel plastic potential based on two linear transformations has been implemented in two different FE codes and verified; - Numerical localisation analyses on sheet strip samples have been performed in order to quantify the effects of temperature and strain rate; - A series of mechanical tests has been carried out including tensile, biaxial, balanced biaxial, simple shear, disk compression, forming limit and hole expansion experiments on magnesium alloy; - A new computational optimisation strategy based on coupon specimen tests and FE-simulations has been developed and applied to magnesium sheet material; - An industrial forming operation has been successfully simulated based on the above mentioned methods; - Thermal effects in forming operations have been studied using fully-coupled thermo-mechanical simulations. Parallelisation of the simulations was realised to reduce computation time. Besides the technical part of the project, complementary social accomplishments have been achieved by the fellow, including: - Three semesters of Korean language course participation as well as additional lessons offered by a private teacher during the outgoing phase; - Short course 'fracture mechanics' and a single semester course 'finite elements in the mechanics of materials' were lectured; - Lectures on German culture were given in the context of UNESCO's Cross Cultural Awareness Program at elementary schools in Pohang; - A two month research stay at the Centre des Matériaux, Mines ParisTech (France); - Two invited lectures have been given by the fellow at Electricité de France (EdF) on 'local approach to fracture'. Potential impact It is expected that the technical results achieved so far will be used in a fully coupled model for magnesium sheet forming. This class of technical alloys has poor formability at room temperature and thus have to be treated at elevated temperatures. Numerical predictions help to define the minimal necessary temperature to conduct a successful forming operation, which in turn helps to save energy. The strategic importance of this field was underlined by signing a ‘memorandum of understanding’ between POSCO (South Korea) and Helmholtz-Zentrum Geesthacht (HZG, Germany) in October 2011. According to his agreement, both parties will cooperate in various fields of magnesium technology.

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