In advanced high strength steels, crack propagation and fracture is preceeded by damage initiation and propagation, yet, the nature of the plasticity mechanisms leading to damage are debated. To fully unravel the plasticity-to-damage mechanisms, we present a novel integrated experimental-numerical nanomechanical framework for testing ultra-thin specimens, yielding (i) full 3D reconstruction of grain/phase shapes and orientations, (ii) front&rear-sided, high-resolution, microstructure-correlated SEM-DIC strain fields, and (iii) one-to-one comparison to numerical strain fields computed with (advanced) crytal plasticity. Results on martensite ‘bridges’ show that limited plasticity results in martensite damage whereas significant plasticity prevents damage; analysis reveals the key role of ‘substructure boundary sliding’ in martensite on damage initiation.
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Themes: Ductile Fracture Under Complex Loading
INFLUENCE OF LARGE STRAIN REVERSE LOADING ON DYNAMIC STRAIN LOCALIZATION AND FAILURE OF DUCTILE METALLIC RODS
A bespoke real time strain control setup is constructed to apply the reverse loading directly to the gauge section of 304L stainless steel specimen up to a maximum strain level of ±0.16. The subsequent tensile tests of the reverse loaded specimens are performed from quasi-static to high strain rates of 1000 /s. A higher strain reverse loading significantly influences the development of necking instabilities, with smaller strain to necking inception, higher local stress in the necking zone, and higher local strain rate up to failure. An analysis of the local stress-strain relationship indicates that the reverse loaded 304L rod shows good impact energy absorption up to failure, which agrees with the ductile fracture surfaces of the 304L materials with reverse loading.
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A GURSON-TYPE LAYER MODEL FOR DUCTILE POROUS SOLIDS CONTAINING ARBITRARY ELLIPSOIDAL VOIDS WITH ISOTROPIC AND KINEMATIC HARDENING
Extensions of Gurson’s model for porous ductile materials have been done by Madou and Leblond (2012) for general ellipsoidal cavities made of rigid-plastic materials, and Morin et al. (2017), for spherical voids with rigid-hardenable matrices. The aim of this work is to provide a homogenized criterion for porous ductile materials incorporating both void shape effects and isotropic and kinematic hardening. A sequential limit-analysis is performed on an ellipsoidal representative volume made of some rigid-hardenable material, containing a confocal ellipsoidal cavity. The overall plastic dissipation is obtained by using the velocity field proposed by Leblond and Gologanu (2008) and that satisfies conditions of homogeneous strain rate on an arbitrary family of confocal ellipsoids. The heterogeneity of hardening is accounted for by discretizing the cell into a finite number of ellipsoids between each of which the quantities characterizing hardening are considered as homogeneous. The model is finally assessed through comparison of its predictions with the results of micromechanical finite element simulations. The numerical and theoretical overall yield loci are compared for various distributions of isotropic and kinematic pre-hardening with a very good agreement.
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A NON-LOCAL GURSON MODEL WITH TWO FRACTURE-MECHANISM ASSOCIATED LENGTH SCALES: SUPPORTED BY NUMERICAL ANALYSES AND EXPERIMENTS
An extension of Gurson’s porous plasticity model capable of preventing pathological strain localization, and describing crack initiation and propagation under both shearing and tension is investigated. This paper separates the progression of shear failure and flat dimple rupture based on the assumption that these two failure mechanisms are governed by different characteristic length scales, a deviatoric and a dilatational length scale, respectively. A set of numerical analyses is presented which brings out the effects of these length scales on the development of e.g. cup-cone and slant fracture. Guided by the outcome of the numerical study, a set of tests has been designed and carried out for calibration of these length scales.
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