MODELLING GROWTH OF SMALL CRACKS IN A POLYCRYSTAL

Abstract In this contribution we study the microstructurally short crack propagation phase in a grain structure of a duplex stainless steel (DSS). The grains in the DSS are either austenitic or ferritic. In order to take the material microstructure into account in the simulations we generate models of the grain structure using the Voronoi polygonization algorithm. The material behavior of the grains is modelled by crystal plasticity, hence, the crystallographic directions of the grains are modelled explicitly and evolution equations for the plastic slip along each slip-system are defined. Furthermore, the crystal plasticity model is enhanced, in the spirit of the disclocation model proposed by Navarro and De Los Rios [1], to include a crack propagation model taking into account non-local effects of the dislocation density (via the accumulated plastic slip) along a slip direction within a grain. Finally, numerical results are presented which show the influence of material hardening and slip directions on the propagation of a crack in an austenitic grain

On the multiscale modeling of duplex stainless steel

This thesis deals with the modeling and simulation ofthe influence of the material substructure on the macroscopicmechanical properties of duplex stainless steel (DSS). Twosubscale levels and their interaction are considered: Grainstructure (mesoscale) and crystallographic structure (microscale).Typical mesoscale parameters are the volume fraction, morphologyand material properties of the two phases (ferrite and austenite).A multiscale modeling approach is adopted, whereby it is assumedthat the macro- and mesoscales are separated such that it ispossible to model the subscale effects within a representativevolume element (RVE) and to obtain the macroscale response viavolume averaging (computational homogenization). A new microscalematerial model based on crystal (visco)plasticity and damage hasbeen developed as part of the thesis work. This model is used tostudy the evolution of damage within the grain structure of theRVE. The important issue of parameter identification of thematerial parameters in the crystal plasticity model is alsoconsidered. Specifically, the necessary macroscale experimentsneeded for obtaining a unique set of material parameter values isexploited. Finally, concurrent (FE$^2$) multiscale modelingsimulations are performed. Different types of plane stressconditions in 2D simulations and the computation of thecorresponding macroscale algorithmic tangent stiffness (ATS)tensor are discussed. The concurrent simulations are used forinvestigating the influence of cold--working on the macroscalemechanical properties for a thin metal sheet with stronglyinhomogeneous deformation and stress states