Browsing by Author "McDowell, David L."

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    Mesoscale cyclic crystal plasticity with dislocation substructures
    (Elsevier, 2017-06-21) Castelluccio, Gustavo M.; McDowell, David L.
    Constitutive formulations have increasingly focused on physically-based approaches that are less phenomenological and incorporate information from multiple scales. Most dislocation-based plasticity approaches reflect many-body dislocation physics without considering the length scales introduced by the self-organization of dislocations into mesoscale structures. These structures promote internal stresses or back stresses that are heterogeneous and long-range in nature and play a critical intermediary role in distinguishing the stress at micro- and nano-scales under cyclic loading. We present a framework that explicitly incorporates length-scales and evolution laws associated with mesoscale dislocation substructures such as cells and persistent slip bands (PSBs) in metallic materials under cyclic loading. A physically-based formulation for the back stress based on the Eshelby inclusion formalism is introduced that explicitly depends on morphology of mesoscale dislocation structures. The approach employs material parameters that can be measured or computed at lower length scales to contrast the response of models and experiments for multiple single crystals orientation and polycrystals for a wide range of strains.
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    A rationale for modeling hydrogen effects on plastic deformation across scales in FCC metals
    (Elsevier, 2018-07-11) Castelluccio, Gustavo M.; Geller, Clint B.; McDowell, David L.
    Although there have been many investigations on the effects of hydrogen on the plastic deformation of metals, an intense debate continues about the physical mechanisms responsible. Most puzzling is the fact that hydrogen appears to be able to both harden and soften FCC metals, depending on the loading conditions. In addition, experiments have shown that hydrogen affects slip system activity differentially, resulting in shear localization of plastic deformation. The work reported in this paper employs a physics-based crystal plasticity model to reproduce the macroscopic response of hydrogen-charged FCC metals through the hydrogen effects on dislocation interactions proposed herein. Different micro-scale mechanisms by which hydrogen may affect plastic deformation are considered, and their resulting macroscopic stress-strain responses under monotonic and cyclic loading are compared. The results support the conclusion that hydrogen screening of dislocations alone cannot explain all the observed macroscopic responses. Instead, it is argued that hydrogen can promote hardening or softening through an increase in glide activation energy and a reduction in dislocation line tension.
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    Sensitivity of polycrystal plasticity to slip system kinematic hardening laws for Al 7075-T6
    (Elsevier, 2017-01-22) Hennessey, Conor; Castelluccio, Gustavo M.; McDowell, David L.
    The prediction of formation and early growth of microstructurally small fatigue cracks requires use of constitutive models that accurately estimate local states of stress, strain, and cyclic plastic strain. However, few research efforts have attempted to systematically consider the sensitivity of overall cyclic stress-strain hysteresis and higher order mean stress relaxation and plastic strain ratcheting responses introduced by the slip system back-stress formulation in crystal plasticity, even for face centered cubic (FCC) crystal systems. This paper explores the performance of two slip system level kinematic hardening models using a finite element crystal plasticity implementation as a User Material Subroutine (UMAT) within ABAQUS (Abaqus unified FEA, 2016) [1], with fully implicit numerical integration. The two kinematic hardening formulations aim to reproduce the cyclic deformation of polycrystalline Al 7075-T6 in terms of both macroscopic cyclic stress-strain hysteresis loop shape, as well as ratcheting and mean stress relaxation under strain- or stress-controlled loading with mean strain or stress, respectively. The first formulation is an Armstrong-Frederick type hardening-dynamic recovery law for evolution of the back stress [2]. This approach is capable of reproducing observed deformation under completely reversed uniaxial loading conditions, but overpredicts the rate of cyclic ratcheting and associated mean stress relaxation. The second formulation corresponds to a multiple back stress Ohno-Wang type hardening law [3] with nonlinear dynamic recovery. The adoption of this back stress evolution law greatly improves the capability to model experimental results for polycrystalline specimens subjected to cycling with mean stress or strain. The relation of such nonlinear dynamic recovery effects are related to slip system interactions with dislocation substructures.
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    Two-way coupled modeling of dislocation substructure sensitive crystal plasticity and hydrogen diffusion at the crack tip of FCC single crystals
    (Elsevier, 2025-01-01) Gu, Tang; Wang, Baolin; Zhu, Ting; Castelluccio, Gustavo M.; McDowell, David L.
    Dislocation substructure-sensitive crystal plasticity (DSS-CP) modeling accounts for the evolution of mesoscale structures using dislocation-based parameters informed by experiments and computation at various lower length scales. To a first-order approximation, DSS-CP model parameters are affected by hydrogen (H) concentration, accounting for both H-dependent yield strength and strain hardening rate. This H-affected DSS-CP model is two-way coupled with H-diffusion to explore both effects of plastic deformation on H-diffusion and effects of H on yield strength and strain hardening in the DSS-CP model. Crack tip simulations are performed for face-centered cubic (FCC) metals under monotonic loading conditions with and without H. Enhanced maximum plastic deformation in the vicinity of the crack tip (i.e., localization or intensification of plastic strain) and crack tip opening displacement (CTOD) are predicted in the presence of H, consistent with experimental observations. In spite of increased initial strength due to H, subsequent reduction of the rate of strain hardening in the presence of H is shown to enhance localization of crack tip plasticity. Furthermore, this modeling framework predicts that higher H-diffusivity (leading to a larger H-affected zone) will enhance the crack tip plasticity, making use of the two-way coupling algorithm implemented in this work. On the other hand, we find that the H-sensitivity of crack tip strain localization response, based only on modification of model parameters, is too weak to explain typical experimental observations. This points to the need to develop more advanced DSS-CP constitutive relations that consider highly complex dislocation interactions with point defects.