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entary exercises for nuclear safety research are to appraise the structural integrity of reactor pressure vessels and their components.[1,2] Reliable assessment of structural integrity prevails as the basic key problem for safety and fitness for service scrutinization of a wide class of engineering structures.[3] The structural integrity of these engineering components is traditionally gaged by employing different defect assessment methodologies based on established fracture mechanics approaches.[2,4] The fracture surface geometry/topography (in both two and three dimensions) imparts the fundamental guidance about the toughness characteristics of any material and the state through which it has failed.[5] Fractography is one of the major techniques to understand different fracture features of a material. Fractographic information of a material is commonly acquired using a scanning electron microscope (SEM). Therefore, fracture surface geometries/morphologies retain and memorize the imprint of entire deformation process/path undergone in a material.[6–10] Current study quantitatively analyzes the tensile fractographic features in two dimensions of a 20MnMoNi55 steel deformed at

ARPAN DAS is with the Mechanical Metallurgy Division, Materials Group, Bhabha Atomic Research Centre, Department of Atomic Energy, Mumbai 400 085, Maharashtra, India. Contact emails: [email protected], [email protected] Manuscript submitted August 24, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

ambient environment with different initial specimen thicknesses in order to comprehend the close connection between deformation, fracture, and the related fractographic features. Figures 1(a) through (c) schematically conceptualize the basic ductile fracture micromechanisms of a material. The involvement of stress-state and stress concentrations due to necking is also noted. Ductile fracture in polycrystals occurs most often as a consequence of microvoid nucleation, growth, coalescence, and finally link-up.[11–16] Hence, it is natural to link material fracture to parameters that describe the evolution of microvoids through a micromechanical model (such as Gurson[11]). Gurson proposed a model based on a micromechanical approach, which describes the growth of spherical cavities and its influence on material behavior. Microvoids nucleate from inclusions, second-phase particles, dispersoids, dislocation pile-ups, twin boundaries, shear bands, grain boundaries, grain boundary triple junctions, etc. in metal matrix and grow under the influence of hydrostatic tensile stress and favorable plastic strain.[11–13] In other words, there is a requirement of critical free energy change (i.e., DGc) to nucleate a microvoid inside a material. The microvoids, once nucleated, exhibit rapid growth (under tension) along the preferential crystallographic direction in material.[17,18] Void nucleation, in general, depends on particle strength, size, shape, and the hardening exponent of matrix material.[11–13] The nucleation mechanism can be strain controlled or stress cont