Bending dual-phase steel: A finite element analysis

Bending of dual-phase (DP) steels has introduced new issues concerning the analysis by finite element (FE) modelling. During the investigation of the microstructure of a DP1000 steel, it is found that there is crack formation at lower bending angles and elongations than would be expected based on the forming limit curves (FLCs). The results of an FE analysis with a continuum material model without taking into account damage formation and subsequent softening do not show behaviour similar to what is seen in actual microstructures. The prediction of the FE analysis will therefore not be a good representation of the bendability of the DP steel. An addition to the continuum analysis is made by implementing a damage and softening model. Localization effects due to the softening of the material arise in places similar to the real material. However, mesh sensitivity is still present for different mesh sizes, despite the addition of a delocalization parameter, and alter the results of the simulations. To improve the analysis of DP steels, a new representation of the microstructure is implemented in the finite element model. A zone with a Voronoi diagram is used as a representative volume element (RVE) for the DP steel. The diagram is embedded in the continuum sheet analysed in the simulation and represents both the ferrite and the martensite phases. The interaction of the ferrite and martensite phase in the embedded Voronoi diagram determines the strain patterns that develop during the bending simulation. An investigation of different Voronoi diagrams on the former force versus former displacement is made. It is not necessary to introduce damage and softening to create the strain patterns which are seen in the real material. However, material degradation is still required to obtain the softening behaviour at high former displacement.

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2015-08-25

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master thesis

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