Multiscale modelling of ductile fracture in heterogeneous metallic alloys

(eng) The development of new heterogeneous metallic materials with high resistance, such as multiphase steels, a/b Ti-alloys, or Al-alloys involving precipitate free zones, is often impeded by the lack of understanding and control of the damage and fracture mechanisms. The objective of the thesis has been to develop a multiscale modelling strategy in order to unravel the relationships between the micromechanisms of ductile fracture and the microstructure of heterogeneous alloys. The present work also contributes to enhancing the predictive capabilities of ductile fracture models regarding structural integrity assessment. More precisely, we have developed new extensions of a Gurson-type constitutive model describing the growth and coalescence of spheroidal voids, possibly not oriented parallel to the main loading direction. These extensions are essentially (i) a description of the void rotation and deformation under shear, (ii) a generalization of the Thomason criterion for the onset of coalescence accounting for the localization plane orientation, (iii) a new geometrical model for the internal necking process. The full model has been implemented in a home-made finite element code and validated towards finite element unit-cell calculations. Based on this model, we have investigated the influence of the microstructural features of homogeneous metallic materials on the different fracture mechanics parameters characterizing the fracture resistance such as JIc, CTOD, CTOA, plastic zone size, and JR. The multiscale modelling tool developed has been also applied to heterogeneous alloys with a specific focus on 7xxx aluminium alloys involving precipitate free zones along the grain boundaries and displaying a competition between transgranular and intergranular fracture modes. The study has contributed to a better understanding of the effect of the microstructural features, flow properties of the grain, and loading conditions on the preferential fracture mode, fracture strain tearing resistance, and crack path.