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George W. Woodruff School of Mechanical Engineering

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College of Engineering

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Advanced Mechanical Bipedal Experimental Robotics Lab

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Computational Reactor and Medical Physics Laboratory

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Cryo Lab

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Environmentally Conscious Design and Manufacturing

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Fusion Research Center

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Publication Search Results

Now showing 1 - 10 of 39

Multiscale Modeling of Hydrogen Embrittlement

(Georgia Institute of Technology, 2022-04-04) Zirkle, Theodore

Hydrogen embrittlement is a long-standing issue in materials science and engineering with a multitude of competing hypotheses and theories. Despite advances in experimental and computational capabilities, common understanding of contributing phenomena has not yet been achieved. Hence, a more complete understanding of hydrogen embrittlement processes operating at multiple length and time scales is still an open challenge that justifies the current research. In this thesis, a unique approach is taken to incorporate a wide range of experimental, computational, and analytical approaches across multiple length scales to produce a mechanistically motivated hydrogen embrittlement model for fracture and fatigue. This research describes and simulates the complex interplay between hydrogen, hydrogen-related defects, dislocations, and dislocation substructures. The model is developed in a crystal plasticity context and implemented in a finite element framework to simulate the hydrogen embrittlement of austenitic stainless steels, structural materials important in energy applications. The proposed research extends current understanding through the development of: i. a physically-based crystal plasticity model developed to capture the evolution of dislocation substructure and material behavior during cyclic loading, ii. a hydrogen transport and trapping model that considers dislocation-mediated transport mechanisms and a more complete set of hydrogen traps, and iii. a fully coupled chemo-mechanical model to capture the effects of hydrogen in reducing crack tip ductility, leading to embrittlement effects.
HIGH THROUGHPUT MECHANICAL PROPERTY CHARACTERIZATION OF STRUCTURAL ALLOYS USING SPHERICAL MICROINDENTATION

(Georgia Institute of Technology, 2021-07-27) Bhat, Anirudh Srinivas

Spherical indentation has been shown to be a reliable high throughput alternative to capture the complete elastic-plastic response of polycrystalline metal alloys when using the Pathak-Kalidindi (P-K) protocol. However, yielding initiates subsurface and due to the hydrostatic pressure associated with the constraint of the surrounding material, this occurs at a higher stress than it would for the same material under uniaxial load. Hence, the indentation stress is higher than uniaxial stress and the two are related by a scaling factor, referred to as the constraint factor. In the current work, some of the open questions in the use of spherical indentation to extract uniaxial stress-strain curves are addressed. The dependence of the mechanical properties of the material and the indentation strain on the constraint factor is investigated using FEA and experiments. Based on the P-K indentation strain definition, revisions are proposed to the classical equations for: 1) the representative uniaxial strain, which relates the indentation strain to an equivalent uniaxial strain and 2) the non-dimensional strain, which relates the indentation strain to the constraint factor. From these revised definitions, an inverse method has been developed using FEA simulations to estimate the uniaxial stress-strain from spherical indentation stress-strain curves. The inverse method is verified with FEA generated indentation stress-strain curves and experiments conducted on Al7050 and Al6061 samples of varying strengths and hardening behaviors. Finally, spherical indentation of a material exhibiting anisotropic mechanical response is studied. The uniaxial and spherical indentation mechanical response of samples of Inconel 718 that were additively manufactured using electron beam melting are critically evaluated. The variation of the experimentally obtained elastic spherical indentation response as a function of the orientation of the material is verified using FEA simulations.
Hydrogen effects on dislocation structures and interactions

(Georgia Institute of Technology, 2020-12-02) Costello, Luke L.

Hydrogen embrittlement (HE) is a complex process, in which the interactions of H atoms, vacancies, and dislocations lead to a macroscopic loss of ductility. Although this phenomenon is commonly observed, its microscopic origins remain unclear. In this thesis we study the process of HE, starting from the microscale, using atomistic simulations and modeling to connect with higher length scales. Passing information from physically realistic atomic scale simulations allows for improved understanding of the underlying mechanisms of H embrittlement and specifically how effects attributed to H contribute at the meso and macroscales. We study these contributions in three parts. First, a method for computing the distribution of H around an edge dislocation is presented and compared to an alternative approach. The presented method is then exercised in an example, studying the effect of H on the stacking fault width (SFW) of an extended edge dislocation. It is shown that H acts to decrease the SFW. Further, only the H very locally around and inside the dislocation cores and stacking fault contribute significantly to the observed decrease in the SFW. We then turn our focus to the role of H on the stabilization and clustering of vacancies. A new model is developed for the production of excess vacancies by plastic deformation. This model is integrated into an existing macroscale computational framework. Lastly, the role of H on the formation energy of vacancy clusters is studied using a hybrid molecular statics / Monte Carlo method. These calculations show that H tends to decrease the formation energy of vacancy clusters which leads to a decrease in the critical cluster size for void nucleation.
The concurrent atomistic-continuum method: Advancements and applications in plasticity of face-centered cubic metals

(Georgia Institute of Technology, 2016-11-09) Xu, Shuozhi

Metal plasticity is a multiscale phenomenon that is manifested by irreversible microstructure rearrangement associated with nucleation, multiplication, interaction, and migration of dislocations. Long range elastic interactions between dislocations and other crystal defects are important to describe, along with the nonlocal, nonlinear dislocation core field. These requirements necessitate multiscale modeling techniques which (i) describe certain lattice defects and their interactions using fully resolved atomistics, (ii) preserve the net Burgers vector and associated long range stress fields of curved mixed character dislocations in a sufficiently large continuum domain in a fully three-dimensional model, and (iii) employ the same governing equations and interatomic potentials in both atomistic and continuum domains to avoid the usage of phenomenological parameters/criteria and ad hoc procedures for passing dislocation segments between the two domains. One such approach is the concurrent atomistic-continuum (CAC) method. Unlike many other concurrent multiscale approaches, the continuum domain in CAC admits motion of dislocations and intrinsic stacking faults through a lattice without necessity of adaptive mesh refinement while employing an underlying interatomic potential as the only constitutive relation and is thus a suitable tool for dislocation-mediated metal plasticity phenomena. In this dissertation, the CAC method is advanced in multiple aspects and applied in a series of problems in plasticity of face-centered cubic (FCC) metals. First, four significant advancements in the CAC method have been made: (i) new types of finite elements are developed which yields a more accurate stacking fault energies and core structure in coarse-grained atomistic descriptions of dislocations, (ii) zero temperature, quasistatic CAC approaches are formulated to enable the constrained multiscale optimization for a sequence of non-equilibrium dislocation configurations in metals, (iii) mesh refinement schemes for both dynamic fracture and curved dislocation migration are implemented, and (iv) the code efficiency is improved using parallelized object-oriented programming. Subsequently, this enhanced CAC method is employed to study multiple plasticity problems in a variety of FCC metals, including screw dislocation cross-slip in Ni, edge dislocation bowing out from a row of collinear obstacles in Al, dislocation multiplication from Frank-Read sources in Cu, Ni, and Al, as well as sequential slip transfer of curved dislocations across a Σ3{111} coherent twin boundary and a Σ11{113} symmetric tilt grain boundary in Cu, Al, and Ni. This work makes significant contributions to the fields of mechanics of materials and multiscale modeling. It is anticipated that the finding in this dissertation will improve physical understanding of dislocation-mediated plastic deformation processes in FCC metals and may assist in formulating constitutive laws and rules used in computational techniques at higher length scales.
Exploration of forward and inverse protocols for property optimization of Ti-6Al-4V

(Georgia Institute of Technology, 2016-07-26) Priddy, Matthew William

The modeling and simulation of advanced engineering materials undergoing mechanical loading requires accurate treatment of relevant microstructure features, such as grain size and crystallographic texture, to determine the heterogeneous response to deformation. However, many models constructed for this purpose are not being fully realized in their predictive capability. Additionally, physics-based models can be combined with bottom-up deductive mappings and top-down inductive decision paths to increase their utility in materials selection and optimization. However, connecting these types of models or algorithms with experiments, rapid inverse property/response estimates, and design decision-making via integrated workflows has yet to become well-established for materials design and/or development. One material system primed for this type of concurrent advancement is alpha+beta titanium alloys, because its resultant microstructure and mechanical properties are highly dependent on material processing and composition. This dissertation seeks to advance a materials design process for fatigue resistance, strength, and elastic stiffness of Ti-6Al-4V through the advancement of various computational tools, as well as the integration of simulation-based tools and high-throughput experimental datasets. The microstructure-sensitive crystal plasticity finite element method (CPFEM) is utilized to explicitly account for the grain structure and crystallographic texture of Ti-6Al-4V. To improve the predictive capability of the CPFEM model, high throughput spherical indentation experimental datasets are used for model calibration because of their ability to extract elastic and plastic individual phase and grain properties from multiphase materials such as titanium alloys. The CPFEM can be used to capture the microstructure heterogeneity on fatigue crack driving forces, but these types of simulations are computationally expensive. Instead, an explicit integration of the relevant constitutive relations in the CPFEM model are combined with the materials knowledge system (MKS) approach for generating spatially local results of polycrystalline materials. These bottom-up simulation methods provide macroscopic properties from microstructure-level model inputs. For materials design, it is important to determine the inverse -- microstructure-level information from the macroscopic response -- which is referred to as top-down modeling. The Inductive Design Exploration Method (IDEM) offers a systematic approach to combining bottom-up simulations with top-down inductive design search. In this dissertation, a generalized framework of the IDEM is implemented to assess multi-objective design scenarios specific to the microstructure-sensitive datasets generated in this work.Th e general approach presented in this dissertation integrates CPFEM simulations with experimental spherical indentation for model refinement and also combines CPFEM with the MKS for computational-efficient generation of local quantities. These advancements are the basis for accelerated decision-support for materials design exploration when merged with the IDEM. Although performed with alpha+beta titanium, individual elements of the framework can be applied to a variety of engineering alloys for tasks such as extraction of model parameters from spherical indentation experiments, coupling MKS with crystal plasticity constitutive relations, and performing a top-down inductive design search with polycrystalline datasets.
Modeling the effects of shot-peened residual stresses and inclusions on microstructure-sensitive fatigue of Ni-base superalloy components

(Georgia Institute of Technology, 2014-04-28) Musinski, William D.

The simulation and design of advanced materials for fatigue resistance requires an understanding of the response of their hierarchical microstructure attributes to imposed load, temperature, and environment over time. For Ni-base superalloy components used in aircraft jet turbine engines, different competing mechanisms (ex. surface vs. subsurface, crystallographic vs. inclusion crack formation, transgranular vs. intergranular propagation) are present depending on applied load, temperature, and environment. Typically, the life-limiting features causing failure in Ni-base superalloy components are near surface inclusions. Compressive surface residual stresses are often introduced in Ni-base superalloy components to help retard fatigue crack initiation and early growth at near surface inclusions and shift the fatigue crack initiation sites from surface to sub-surface locations, thereby increasing fatigue life. To model the effects of residual stresses, inclusions, and microstructure heterogeneity on fatigue crack driving force and fatigue scatter, a computational crystal plasticity framework is presented that imposes quasi-thermal eigenstrain to induce near surface residual stresses in polycrystalline Ni-base superalloy IN100 smooth specimens with and without nonmetallic inclusions. In addition, the effect of near surface inclusions in notched Ni-base superalloy components on MSC growth and fatigue life scatter was investigated in this work. A fatigue indicator parameter (FIP)-based microstructurally small crack (MSC) growth model incorporating crack tip/grain boundary effects was introduced and fit to experiments (in both laboratory air and vacuum) for the case of 1D crack growth and then computationally applied to 3D crack growth starting (1) from a focused ion beam (FIB) notch in a smooth specimen, (2) from a debonded inclusion located at different depths within notched components containing different notch root radii, and (3) from inclusions located at different depths relative to the surface in smooth specimens containing simulated shot peened induced residual stresses. Computational predictions in MSC growth rate scatter and distribution of fatigue life were in general accordance with experiments. The general approach presented in this Dissertation can be used to advance integrated computational materials engineering (ICME) by predicting variation of fatigue resistance and minimum life as a function of heat treatment/microstructure and surface treatments for a given alloy system and providing support for design of materials for enhanced fatigue resistance. In addition, this framework can reduce the number of experiments required to support modification of material to enhance fatigue resistance, which can lead to accelerated insertion (from design conception to production parts) of new or improved materials for specific design applications. Elements of the framework being advanced in this research can be applied to any engineering alloy.
Microstructure-sensitive simulation of shock loading in metals

(Georgia Institute of Technology, 2014-04-02) Lloyd, Jeffrey T.

A constitutive model has been developed to model the shock response of single crystal aluminum from peak pressures ranging from 2-110 GPa. This model couples a description of higher-order thermoelasticity with a dislocation-based viscoplastic formulation, both of which are formulated for single crystals. The constitutive model has been implemented using two numerical methods: a plane wave method that tracks the propagating wave front; and an extended one-dimensional, finite-difference method that can be used to model spatio-temporal evolution of wave propagation in anisotropic materials. The constitutive model, as well as these numerical methods, are used to simulate shock wave propagation in single crystals, polycrystals, and pre-textured polycrystals. Model predictions are compared with extensive existing experimental data and are then used to quantify the influence of the initial material state on the subsequent shock response. A coarse-grained model is then proposed to capture orientation-dependent deformation heterogeneity, and is shown to replicate salient features predicted by direct finite-difference simulation of polycrystals in the weak shock regime. The work in this thesis establishes a general framework that can be used to quantify the influence of initial material state on subsequent shock behavior not only for aluminum single crystals, but for other face-centered cubic and lower symmetry crystalline metals as well.
Multiscale modeling and design of ultra-high-performance concrete

(Georgia Institute of Technology, 2013-07-11) Ellis, Brett D.

Ultra-High-Performance Concretes (UHPCs) are a promising class of cementitious materials possessing mechanical properties superior to those of Normal Strength Concretes (NSCs). However, UHPCs have been slow to transition from laboratory testing to insertion in new applications, partly due to an intuitive trial-and-error materials development process. This research seeks to addresses this problem by implementing a materials design process for the design of UHPC materials and structures subject to blast loads with specific impulses between 1.25- and 1.5-MPa-ms and impact loads resulting from the impact of a 0.50-caliber bullet travelling between 900 and 1,000 m/s. The implemented materials design process consists of simultaneous bottom-up deductive mappings and top-down inductive decision paths through a set of process-structure-property-performance (PSPP) relations identified for this purpose. The bottom-up deductive mappings are constructed from a combination of analytical models adopted from the literature and two hierarchical multiscale models developed to simulate the blast performance of a 1,626-mm tall by 864-mm wide UHPC panel and the impact performance of a 305-mm tall by 305-mm wide UHPC panel. Both multiscale models employ models at three length scales – single fiber, multiple fiber, and structural – to quantify deductive relations in terms of fiber pitch (6-36 mm/revolution), fiber volume fraction (0-2%), uniaxial tensile strength of matrix (5-12 MPa), quasi-static tensile strength of fiber-reinforced matrix (10-20 MPa), and dissipated energy density (20-100 kJ/m²). The inductive decision path is formulated within the Inductive Design Exploration Method (IDEM), which determines robust combinations of properties, structures, and processing steps that satisfy the performance requirements. Subsequently, the preferred material and structural designs are determined by rank order of results of objective functions, defined in terms of mass and costs of the UHPC panel.
A study on the influence of microstructure on small fatigue cracks

(Georgia Institute of Technology, 2012-05-09) Castelluccio, Gustavo Marcelo

In spite of its signiﬁcance in industrial applications, the prediction of the inﬂuence of microstructure on the early stages of crack formation and growth in engineering alloys remains underdeveloped. The formation and early growth of fatigue cracks in the high cycle fatigue regime lasts for much of the fatigue life, and it is strongly inﬂuenced by microstructural features such as grain size, twins 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 thesis considers ﬁnite element computational models that explicitly render the microstructure of selected FCC metallic systems and introduces a fatigue methodology that estimates transgranular and intergranular fatigue growth for microstructurally small cracks. The driving forces for both failure modes are assessed by means of fatigue indicators, which are used along with life correlations to estimate the fatigue life. Furthermore, cracks with meandering paths are modeled by considering crack growth on a grain-by-grain basis with a damage model embedded analytically to account for stress and strain redistribution as the cracks extend. The methodology is implemented using a crystal plasticity constitutive model calibrated for studying the eﬀect of microstructure on early fatigue life of a powder processed Ni-base RR1000 superalloy at elevated temperature under high cycle fatigue conditions. This alloy is employed for aircraft turbine engine disks, which undergo a thermomechanical production process to produce a controlled bimodal grain size distribution. The prediction of the fatigue life for this complex microstructure presents particular challenges that are discussed and addressed. The conclusions of this work describe the mechanistic of microstructural small crack. In particular, the fatigue crack growth driving force has been characterized as it evolves within grains and crosses to other grains. Furthermore, the computational models serve as a tool to assess the eﬀects of microstructural features on early stages of fatigue crack formation and growth, such as distributions of grain size and twins.
Microstructure-sensitive weighted probability approach for modeling surface to bulk transition of high cycle fatigue failures dominated by primary inclusions

(Georgia Institute of Technology, 2011-05-19) Salajegheh, Nima

In this thesis, we pursue a simulation-based approach whereby microstructure-sensitive finite element simulations are performed within a statistical perspective to examine the VHCF life variability and assess the surface initiation probability. The methodology introduced in this thesis lends itself as a cost-effective platform for development of microstructure-property relations to support design of new or modified alloys, or to more accurately predict the properties of existing alloys.