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A reduced micromorphic model is formulated in the scope of crystal plasticity and crystalline cleavage damage. The finite strain formulation utilizes a single additional microvariable that is used to regularize localized inelastic deformation mechanisms. Damage is formulated as a strain-like variable to fit the generalized micromorphic microslip and/or microdamage based formulation. Strategies of treating slip and damage simultaneously and separately as non-local variables are investigated. The model accounts for size-effects that simultaneously affect the hardening behavior and allow to predict finite width damage localization bands. The results show that the micromorphic extension introduces extra-hardening in the vicinity of grain boundaries and slip localization zones in polycrystals. At the single crystal level slip band width is regularized. Two ways of dealing with damage localization were identified: An indirect method based on controlling width of slip bands that act as initiation sites for damage and a direct method in which damage flow is regularized together with or separately from plastic slip. Application to a real martensitic steel microstructure is investigated.

Ferritic–austenitic duplex stainless steels are known to offer favorable combinations of good mechanical properties and corrosion resistance to be used for structural purposes. Lean duplex grades have already been introduced for consideration to replace standard 18–8 austenitic stainless steels in various industrial applications. Ferrite and austenite represent different deformation behaviors and contribute to the resulting fracture toughness characteristics. Micromechanical crystal plasticity based assessment of this cleavage fracture behavior is the subject area of current work. The objective is to bridge cleavage fracture models to full field crystal plasticity imaging based modeling of microstructures of ferritic–austenitic duplex stainless steels. The goal is to introduce means to computationally assess the effects of different multi-phase microstructural morphologies to cleavage fracture toughness and develop both the respective constitutive and cleavage fracture modeling capabilities. Such means can be used as an aid to develop better cleavage resistant, while in the current context lean, steel grades. Three different steels are investigated with differing austenite phase morphologies, and their behavior with respect to fracture mechanical response evaluated by micromechanical modeling. The effect of austenite fraction and morphology in terms of improving fracture toughness, a critical parameter concerning these steel grades and improvement of their cleavage fracture properties, is investigated. Deleterious features such as large ferrite grain size and microstructural property mismatching are identified and their implications to fracture toughness and development of applicable modeling capabilities for ferritic–austenitic duplex steels discussed. Simple design task of varying the austenite phase fraction is performed using synthetic microstructural modeling and the results evaluated with respect to their influence on the fracture toughness ductile-to

High cycle fatigue (HCF) is a frequently limiting failure mechanism of machine elements and modern high strength steels. Present day design rules rely on semi-empirical methods, guidelines and utilization of macroscopic analysis means in origin, such as fracture mechanics. The resulting challenge is that short crack regime, critical for HCF in terms of lifetime of components and products, is somewhat poorly handled. This is an outcome of the fact that the present means and methodologies do not explicitly account for effects arising from material microstructure, an oversight micromechanics aims to rectify. Micromechanical modeling operating on fatigue at the scale of material microstructure necessitates the introduction of suitable means to describe the mechanisms of cyclic plastic deformation and microstructural morphologies, considered critical for HCF especially at the early stages of micro-crack nucleation and damage evolution towards and within the short crack regime. In current work, a crystal plasticity based approach with combined hardening is utilized to capture the respective deformation response utilizing full field modeling. The modeling is carried out for both simplified prior austenite grain like microstructures as well as complex imaging based martensitic quenched and tempered steel microstructural models. A fully coupled damage modeling scheme is introduced to track damage nucleation and evolution at the scale of the studied microstructures. Crack closure is included within the approach to track behavior of microstructure scale defects under, e.g., fully reversed loading, more realistically. Model calibration is addressed and application cases involving damage and crack growth both under monotonic and cyclic loading are presented. The results demonstrate how the coupling of damage to crystal plasticity modeling can be utilized to identify and track the evolution of microstructure scale damage mechanisms in complex martensitic microstructures.