Issue 35

T. Auger et alii, Frattura ed Integrità Strutturale, 35 (2016) 250-259; DOI: 10.3221/IGF-ESIS.35.29 258 Perspective : 3D agregate modeling with a crack A crack can be introduced in a polycrystalline model using the contact formulation (surface on surface contact). The local stress intensity factor can be calculated from the contour integration of the J integral. In Fig. 7, we show preliminary results on a 3 grain aggregate loaded with an initial crack. Crack propagation could be carried out after a fracture criterion based of the calculation of K ( )   G K EJ K decides which part of the microstructure is to be broken. Perspective : Fracture criterion for LME There is currently no theoretical framework able to describe the ductile to brittle transition seen in LME. As a starting point one can use the Griffith theory of brittle fracture [20] that connects the macroscopic fracture energy to a microscopic quantity the surface energy  s via the critical stress intensity factor (SIF) for cracking K G . It equates the energy release rate during the advance of a pre-existing crack (the drop in the stored elastic energy) to the energy cost required for the creation of two opposing surfaces. The minimum immediately gives the critical stress intensity factor for fracture (SIF): 4 1     s G K , with  the Poisson factor and  the shear modulus The adsorption of a liquid lowers the surface energy, in most cases, such that, as a general rule, there should be a lowering of the critical SIF for brittle cracking [21]. The search of a critical K c for cracking compatible with experiment would allow the calculation of an effective  s to be compared with a hypothetical measurement or calculation by atomistic methods. Figure 7: 10% deformed 3D-voronoï aggregate generated using Neper with a debound surface stopped at a triple line (preliminary result) [13] C ONCLUSION ME in the case of steels is conclusively shown here to consist in many cases of intergranular or interlath cracking. One often overlooked issue is the interplay between fracture and local plasticity making fracture surface often difficult to clearly assign to a microstructural element of the material. We have conclusively shown in this work that fractography needs to be completed with state-of-the-art nanoscale characterization tools (FIB sampling, EBSD, L