Issue 35

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 256 S KETCH OF THE MODELING OF A CRACK PROPAGATION IN LME aving established the generic intergranular/interlath crack path character of steel’s LME, one wishes to model crack propagation into a framework able to take into account the polycrystalline aspect of real materials, the large plasticity observed before cracking in LME and potentially various specificities of the onset of cracking such as a cracking criterion that could be tuned to the specific environment under consideration. It requires one to incorporate grain boundaries (GB), 3D shapes of grains and a set of assembled grains forming an aggregate to represent the 3 dimensional aspect of a material as close as possible to a representative element volume. Realistic grains to neighbouring grains crystallographic relationships need to be incorporated as well in order to represent not only the texture of a material but to correctly assess the micromechanics near GBs. The plastic behavior of such a material is then modeled via a finite element implementation of crystalline plasticity. Debonding at GBs can be obtained through the contact formulation of a finite-element solver using a dedicated LME fracture criterion. 3D agregate modeling There are 2 possible ways to model 3D microstructures:  The first one is to generate virtual microstructures representative of an equiaxed microstructure, for example, by using the Voronoï tessellation technique to define the grain morphology and grain size distribution [13]. The advantage is the ease with which one can generate a virtual microstructure. The generation of grain orientation might not be representative of the local misorientation (especially if the orientation is selected randomly) but can be made representative of the crystallographic texture through constrained selection.  The other possibility is to use a 3D reconstruction of a real microstructure using either tomography by X-ray analysis [14] or destructive serial-sectioning by EBSD [15]. In this case, every material requires its own aggregate with a sufficient number of grains to be representative. Figure 5: a. Deformed 3D-voronoï aggregate generated using Neper [13] b. 3D-EBSD aggregate obtained by serial sectioning on 316LN [16] The boundaries are meshed using triangular elements. The volume mesh is created from the boundary surface with tetragonal (C3D4) elements. Crystalline plasticity The modeling of a material’s polycrystalline plasticity is carried out in the framework of finite transformations (small elastic distortions but large lattice rotations) following the kinematic decomposition proposed by Pierce et al [17] for single crystals. Crystal plasticity constitutive equations are used to model the plastic deformation in metals and alloys [18]. H