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Traditional fracture theories infer damage at cracks (local field) by surveying loading conditions away from cracks (far field). This approach has been successful in predicting ductile fracture, but it normally assumes isotropic and homogeneous materials. However, myriads of manufacturing procedures induce heterogeneous microstructural gradients that can affect the accuracy of traditional fracture models. This work presents a microstructure‐sensitive finite element approach to explore the shielding effects of grain size and crystallographic orientation gradients on crack tip microplasticity and blunting. A dislocation density‐based crystal plasticity model conveys texture evolution, grain size effects, and directional hardening by computing the constraint from dislocation structures. The results demonstrate that the microstructure can act as a buffer between the local and far fields that affects the crack tip microplasticity variability. For nominal opening loading, grain size and texture affect the local ductility and induce a non‐negligible multiaxial plastic deformation. Furthermore, driving forces based on measuring displacements away from the crack tip are less affected by the microstructure, which suggests that traditional experimental methods smear out important crack tip variability. [ABSTRACT FROM AUTHOR]

Advances in higher resolution experimental techniques have shown that metallic materials can develop fatigue cracks under cyclic loading levels significantly below the yield stress. Indeed, the traditional notion of a fatigue limit can be recast in terms of limits associated with nucleation and arrest of fatigue cracks at the microstructural scale. Although fatigue damage characteristically emerges from irreversible dislocation processes at sub-grain scales, the specific microstructure attributes, environment, and loading conditions can strongly affect the apparent failure mode and surface to subsurface transitions. In this paper we discuss multiple mechanisms that occur during fatigue loading in the high cycle fatigue (HCF) to very high cycle fatigue (VHCF) regimes. We compare these regimes, focusing on strategies to bridge experimental and modeling approaches exercised at multiple length scales and discussing particular challenges to modeling and simulation regarding microstructure-sensitive fatigue driving forces and thresholds. We conclude by discussing some of the challenges in predicting the transition of failure mechanisms at different stress and strain amplitudes. [ABSTRACT FROM AUTHOR]

Fatigue crack initiation in the high cycle fatigue regime is strongly influenced by microstructural features. Research efforts have usually focused on predicting fatigue resistance against crack incubation without considering the early fatigue crack growth after encountering the first grain boundary. However, a significant fraction of the variability of the total fatigue life can be attributed to growth of small cracks as they encounter the first few grain boundaries, rather than crack formation within the first grain. This paper builds on the framework previously developed by the authors to assess microstructure-sensitive small fatigue crack formation and early growth under complex loading conditions. The scheme employs finite element simulations that explicitly render grains and crystallographic directions along with simulation of microstructurally small fatigue crack growth from grain to grain. The methodology employs a crystal plasticity algorithm in ABAQUS that was previously calibrated to study fatigue crack initiation in RR1000 Ni-base superalloy. This work present simulations with non-zero applied mean strains and geometric discontinuities that were not previously considered for calibration. Results exhibit trends similar to those found in experiments for multiple metallic materials, conveying a consistent physical description of fatigue damage phenomena. [ABSTRACT FROM AUTHOR]

The number of cycles required to form and grow microstructurally small fatigue cracks in metals exhibits substantial variability, particularly for low applied strain amplitudes. This variability is commonly attributed to the heterogeneity of cyclic plastic deformation within the microstructure, and presents a challenge to minimum life design of fatigue resistant components. This paper analyzes sources of variability that contribute to the driving force of transgranular fatigue cracks within nucleant grains. We employ crystal plasticity finite element simulations that explicitly render the polycrystalline microstructure and Fatigue Indicator Parameters (FIPs) averaged over different volume sizes and shapes relative to the anticipated fatigue damage process zone. Volume averaging is necessary to both achieve description of a finite fatigue damage process zone and to regularize mesh dependence in simulations. Results from constant amplitude remote applied straining are characterized in terms of the extreme value distributions of volume averaged FIPs. Grain averaged FIP values effectively mitigate mesh sensitivity, but they smear out variability within grains. Volume averaging over bands that encompass critical transgranular slip planes appear to present the most attractive approach to mitigate mesh sensitivity while preserving variability within grains. [ABSTRACT FROM AUTHOR]

Fatigue failure is a leading concern for many applications involving structures for transportation, manufacturing, medical devices, and electronic components. Recent advances in modeling and simulation, coupled with in situ experimental techniques, have enhanced the understanding required to distinguish and characterize mechanisms of fatigue crack formation and early growth at scales of underlying microstructure. In particular, microstructure substantially influences high cycle fatigue resistance and contributes to variability of the fatigue response. This paper reviews the confluence of recent experimental and modeling advances aimed at understanding and modeling of the formation and early growth of fatigue cracks with size on the order of dominant microstructure attributes. [ABSTRACT FROM AUTHOR]

Abstract: The formation and early growth of fatigue cracks in the high cycle fatigue regime is influenced by microstructure features such as grain size and morphological and crystallographic texture. However, most fatigue models do not predict the influence of the microstructure on early stages of crack formation, or they employ parameters that should be calibrated with experimental data from specimens with microstructures of interest. These post facto strategies are adequate to characterize materials, but they are not fully appropriate to aid in the design of fatigue-resistant engineering alloys. This paper presents a modeling framework that facilitates relative assessment of fatigue resistance among different microstructures, including the microstructurally small crack growth regime. The scheme employs finite element simulations based on crystal plasticity that explicitly render the microstructure and a methodology that estimates transgranular growth of microstructurally small cracks on a grain-by-grain basis, including consideration of growth within grains and stress redistribution as the cracks extend. The methodology is implemented in ABAQUS and is calibrated to study fatigue crack initiation of a bimodal grain size distribution found in RR1000 powder processed Ni-base superalloys for aircraft gas turbine disk applications. The transition to intergranular failure is also considered. [Copyright &y& Elsevier]

A probabilistic finite element model is implemented to estimate microstructurally small fatigue crack growth in bridge steel welds. Simulations are based on a microstructure-sensitive crystal plasticity model to quantify fatigue indicator parameters (FIPs) at the slip system level and a fatigue model that relates FIPs to fatigue lives of individual grains. Microstructures from three weld zones, namely, fusion zone (FZ), heat affected zone (HAZ), and base metal (BM), are constructed based on their microstructural attributes such as grain morphology, size, and orientation. Statistical volume elements (SVEs) are generated and meshed independently for the three welding zones. Each grain within the SVEs is divided into several slip bands parallel to crystallographic planes. During the loading process, cracks nucleate at the slip bands (SBs) with the largest FIP next to the free surface. The crack extension path is assumed to be transgranular along SBs and the number of cycles required to crack the neighbor grain is calculated by the corresponding FIP-based crack growth rate equation. The simulation process is carried out using ABAQUS with a user defined subroutine UMAT for crystal plasticity. After the calibration of the constitutive model and irreversibility parameters, numerical simulations for small crack growth in three zones are presented. The crack length vs. the predicted fatigue resistance shows significant differences in the mean values and variability among the three weld zones. [ABSTRACT FROM AUTHOR]

• A novel approach to predict fatigue live by decoupling microstructure and notch effects, • An analytical formulation to quantify crack growth from notches without running crystal plasticity. • A novel vectorial FIP that accounts multiple slip system activation, Damage from small manufacturing defects often go unnoticed until fatigue cracks have grown beyond repairability. These cracks initiate at defects with dimensions on par with the microstructure length scale (e.g., 5–200 µm deep), which affects fatigue variability and renders most engineering prognosis methods inapplicable. This work develops a novel microstructure-sensitive formulation that reduces computational efforts by decoupling geometric and microstructural contributions to fatigue cracking. Crystal plasticity finite element models with and without geometry induced strain gradients were considered to assess the role of defects independently from the microstructure. The analysis results in a fatigue life analytical formulation whose parameters depend on the microstructure and defect morphology. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides some background and historical review of how microstructure-sensitive finite element simulations can play a role in understanding the effects of stress amplitude, R-ratio, and microstructure on fatigue crack formation and early growth at notches, including pores and non-metallic inclusions for Ti alloys and Ni-base superalloys. The simulations employ fatigue indicator parameters (FIPs) computed over finite volumes that relate to processes of fatigue crack formation and early growth at the scale of individual grains. It is argued that both coarse scale (uncracked, mesoscale) and fine scale FIPs (computed in the vicinity of cracks in single grains or crystals) serve as a driving force for crystallographic fatigue crack growth, and correlate directly with the cyclic crack tip displacement (CTD). Furthermore, variability in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) responses is computationally assessed using multiple statistical volume elements and the distribution of FIPs of extreme value character. The concepts of marked correlation functions and weighted probability density functions are reviewed as a means to quantify the role of multiple microstructure attributes that couple to enhance the extreme value FIPs in the HCF regime. An algorithm for estimation of the cumulative probability distribution of cycles for crack formation and growth from notches in HCF and VHCF is also described. [Copyright &y& Elsevier]

• Integrating physics-based models for predicting fatigue crack nucleation. • Modelling and experiments in small volumes. • Validation of crack nucleation orientation and nucleation life statistical distribution. • Use of non-local FIPs in microstructure-sensitive models with gradients. The failure of micron-scale metallic components presents significant variability as a result of their size being comparable to microstructural length scales. Indeed, these components do not represent the bulk of the material but correspond to statistical volume elements (SVEs). This work investigates the role of SVEs on fatigue crack nucleation with a novel comparison between microbeam experiments and microstructure-sensitive simulations. We recreate multiple microstructural computational realizations to estimate fatigue crack nucleation lives and orientations by means of physics-based crystal plasticity models. We demonstrate a unique approach to validate microstructure sensitive models and quantify the fatigue crack stochasticity associated with small volumes. [ABSTRACT FROM AUTHOR]