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Justin Wilkerson - One of the best experts on this subject based on the ideXlab platform.
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the role of elastic and plastic anisotropy in intergranular spall Failure
Acta Materialia, 2019Co-Authors: Thao D Nguyen, Darby J Luscher, Justin WilkersonAbstract:Abstract Recent mesoscale experimental observations of dynamic Ductile Failure [1,2] have demonstrated a strong relationship between grain boundary (GB) misorientation and the likelihood of Failure initiation along said GB. This correlation has been attributed to inherent GB weakness of particular misorientation. Here we discuss the role played by mechanics, i.e. elastic and plastic anisotropy, on the experimental observation [1,2]. We make use of a recently developed framework for modeling dislocation-based crystal plasticity and Ductile Failure of single crystals under dynamic loading (CPD-FE) [3]. Polycrystals are studied at the mesoscale level through the explicit resolution of individual grains, i.e. resolving each individual grain's size, shape, and orientation. In our simulations, Failure naturally localizes along the GBs with no necessity for ad hoc rules governing damage nucleation. We carry out a few thousand mesoscale calculations, systematically varying the misorientation angles of the GB in the computational microstructure. Despite the fact that we neglect the possibility of variations in inherent GB weakness, our simulations agree favorably with the experimental observations, implying that stress concentration generated by elastic and plastic anisotropies across GBs is a dominant governing factor in this phenomenon. Lastly, we find that misorientation angle is an insufficient GB descriptor to predict the likelihood of intergranular spall Failure, which is better understood through the consideration of additional GB degrees of freedom.
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a dislocation based crystal plasticity framework for dynamic Ductile Failure of single crystals
Journal of The Mechanics and Physics of Solids, 2017Co-Authors: Thao D Nguyen, Darby J Luscher, Justin WilkersonAbstract:Abstract A framework for dislocation-based viscoplasticity and dynamic Ductile Failure has been developed to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. An averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Additionally, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in (Wilkerson and Ramesh, 2014), which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.
Thao D Nguyen - One of the best experts on this subject based on the ideXlab platform.
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the role of elastic and plastic anisotropy in intergranular spall Failure
Acta Materialia, 2019Co-Authors: Thao D Nguyen, Darby J Luscher, Justin WilkersonAbstract:Abstract Recent mesoscale experimental observations of dynamic Ductile Failure [1,2] have demonstrated a strong relationship between grain boundary (GB) misorientation and the likelihood of Failure initiation along said GB. This correlation has been attributed to inherent GB weakness of particular misorientation. Here we discuss the role played by mechanics, i.e. elastic and plastic anisotropy, on the experimental observation [1,2]. We make use of a recently developed framework for modeling dislocation-based crystal plasticity and Ductile Failure of single crystals under dynamic loading (CPD-FE) [3]. Polycrystals are studied at the mesoscale level through the explicit resolution of individual grains, i.e. resolving each individual grain's size, shape, and orientation. In our simulations, Failure naturally localizes along the GBs with no necessity for ad hoc rules governing damage nucleation. We carry out a few thousand mesoscale calculations, systematically varying the misorientation angles of the GB in the computational microstructure. Despite the fact that we neglect the possibility of variations in inherent GB weakness, our simulations agree favorably with the experimental observations, implying that stress concentration generated by elastic and plastic anisotropies across GBs is a dominant governing factor in this phenomenon. Lastly, we find that misorientation angle is an insufficient GB descriptor to predict the likelihood of intergranular spall Failure, which is better understood through the consideration of additional GB degrees of freedom.
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a dislocation based crystal plasticity framework for dynamic Ductile Failure of single crystals
Journal of The Mechanics and Physics of Solids, 2017Co-Authors: Thao D Nguyen, Darby J Luscher, Justin WilkersonAbstract:Abstract A framework for dislocation-based viscoplasticity and dynamic Ductile Failure has been developed to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. An averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. Additionally, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in (Wilkerson and Ramesh, 2014), which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.
Fadi Aldakheel - One of the best experts on this subject based on the ideXlab platform.
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phase field modeling of fracture in porous plasticity a variational gradient extended eulerian framework for the macroscopic analysis of Ductile Failure
Computer Methods in Applied Mechanics and Engineering, 2016Co-Authors: Christian Miehe, Fadi Aldakheel, Daniel Kienle, Stephan TeichtmeisterAbstract:Abstract This work outlines a rigorous variational-based framework for the phase field modeling of fracture in isotropic porous solids undergoing large elastic–plastic strains. It extends the recent works of Miehe et al., [33,53] to a particular formulation of isotropic porous plasticity. The phase field approach regularizes sharp crack surfaces within a pure continuum setting by a specific gradient damage modeling with geometric features rooted in fracture mechanics. A gradient plasticity model for porous plasticity with a simple growth law for the evolution of the void fraction is developed, and linked to a Failure criterion in terms of the local elastic–plastic work density that drives the fracture phase field. It is shown that this approach is able to model basic phenomena of Ductile Failure such as cup–cone Failure surfaces in terms of only two material parameters on the side of damage mechanics: a critical work density that triggers the onset of damage and a shape parameter that governs the postcritical damage up to fracture. The formulation includes two independent length scales which regularize both the plastic response as well as the crack discontinuities. This allows to design damage zones of Ductile fracture to be inside of plastic zones or vice versa, and guarantees on the computational side a mesh objectivity in post-critical ranges. The key aspect that allows to construct a variational theory for porous plasticity at fracture is the use of an Eulerian constitutive setting, where the yield function is formulated in terms of the Kirchhoff stress. Here, we exploit the fact that this stress approximates an effective stress that drives the plasticity in the matrix of the porous solid. The coupling of gradient plasticity to gradient damage is realized by a constitutive work density function that includes the stored elastic energy and the dissipated work due to plasticity and fracture. The latter represents a coupled resistance to plasticity and damage, depending on the gradient-extended internal variables which enter the plastic yield function and the fracture threshold function. The canonical theory proposed is shown to be governed by a rate-type minimization principle, which fully determines the coupled multi-field evolution problem, and provides inherent symmetries with regard to a finite element implementation. The robust computational setting proposed includes (i) a general return scheme of plasticity in the spectral space of logarithmic principal strains and dual Kirchhoff stresses, (ii) the micromorphic regularization of the gradient plastic evolution and (iii) a history-field-driven update of the linear phase field equation.
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phase field modeling of fracture in multi physics problems part ii coupled brittle to Ductile Failure criteria and crack propagation in thermo elastic plastic solids
Computer Methods in Applied Mechanics and Engineering, 2015Co-Authors: Christian Miehe, Martina Hofacker, Lisamarie Schanzel, Fadi AldakheelAbstract:Abstract This work presents a generalization of recently developed continuum phase field models from brittle to Ductile fracture coupled with thermo-plasticity at finite strains. It uses a geometric approach to the diffusive crack modeling based on the introduction of a balance equation for a regularized crack surface and its modular linkage to a multi-physics bulk response developed in the first part of this work. This evolution equation is governed by a constitutive crack driving force. In this work, we supplement the energetic and stress-based forces for brittle fracture by additional forces for Ductile fracture. These are related to state variables associated with the inelastic response, such as the amount of plastic strain and the void volume fraction in metals, or the amount of craze strains in glassy polymers. To this end, we define driving forces based on elastic and plastic work densities , and barrier functions related to critical values of these inelastic state variables. The proposed thermodynamically consistent framework of Ductile phase field fracture is embedded into a formulation of gradient thermo-plasticity, that is able to account for material length scales such as the width of shear bands. It is applied to two constitutive model problems. The first is designed for the analysis of brittle-to-Ductile Failure mode transition in the dynamic Failure analysis of metals . The second is constructed for a quasi-static analysis of crazing-induced fracture in glassy polymers . A spectrum of simulations demonstrates that the use of barrier-type crack driving forces in the phase field modeling of fracture, governed by accumulated plastic strains in metals or crazing strains in polymers, provide results in very good agreement with experiments.
Christian Miehe - One of the best experts on this subject based on the ideXlab platform.
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phase field modeling of fracture in porous plasticity a variational gradient extended eulerian framework for the macroscopic analysis of Ductile Failure
Computer Methods in Applied Mechanics and Engineering, 2016Co-Authors: Christian Miehe, Fadi Aldakheel, Daniel Kienle, Stephan TeichtmeisterAbstract:Abstract This work outlines a rigorous variational-based framework for the phase field modeling of fracture in isotropic porous solids undergoing large elastic–plastic strains. It extends the recent works of Miehe et al., [33,53] to a particular formulation of isotropic porous plasticity. The phase field approach regularizes sharp crack surfaces within a pure continuum setting by a specific gradient damage modeling with geometric features rooted in fracture mechanics. A gradient plasticity model for porous plasticity with a simple growth law for the evolution of the void fraction is developed, and linked to a Failure criterion in terms of the local elastic–plastic work density that drives the fracture phase field. It is shown that this approach is able to model basic phenomena of Ductile Failure such as cup–cone Failure surfaces in terms of only two material parameters on the side of damage mechanics: a critical work density that triggers the onset of damage and a shape parameter that governs the postcritical damage up to fracture. The formulation includes two independent length scales which regularize both the plastic response as well as the crack discontinuities. This allows to design damage zones of Ductile fracture to be inside of plastic zones or vice versa, and guarantees on the computational side a mesh objectivity in post-critical ranges. The key aspect that allows to construct a variational theory for porous plasticity at fracture is the use of an Eulerian constitutive setting, where the yield function is formulated in terms of the Kirchhoff stress. Here, we exploit the fact that this stress approximates an effective stress that drives the plasticity in the matrix of the porous solid. The coupling of gradient plasticity to gradient damage is realized by a constitutive work density function that includes the stored elastic energy and the dissipated work due to plasticity and fracture. The latter represents a coupled resistance to plasticity and damage, depending on the gradient-extended internal variables which enter the plastic yield function and the fracture threshold function. The canonical theory proposed is shown to be governed by a rate-type minimization principle, which fully determines the coupled multi-field evolution problem, and provides inherent symmetries with regard to a finite element implementation. The robust computational setting proposed includes (i) a general return scheme of plasticity in the spectral space of logarithmic principal strains and dual Kirchhoff stresses, (ii) the micromorphic regularization of the gradient plastic evolution and (iii) a history-field-driven update of the linear phase field equation.
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phase field modeling of fracture in multi physics problems part ii coupled brittle to Ductile Failure criteria and crack propagation in thermo elastic plastic solids
Computer Methods in Applied Mechanics and Engineering, 2015Co-Authors: Christian Miehe, Martina Hofacker, Lisamarie Schanzel, Fadi AldakheelAbstract:Abstract This work presents a generalization of recently developed continuum phase field models from brittle to Ductile fracture coupled with thermo-plasticity at finite strains. It uses a geometric approach to the diffusive crack modeling based on the introduction of a balance equation for a regularized crack surface and its modular linkage to a multi-physics bulk response developed in the first part of this work. This evolution equation is governed by a constitutive crack driving force. In this work, we supplement the energetic and stress-based forces for brittle fracture by additional forces for Ductile fracture. These are related to state variables associated with the inelastic response, such as the amount of plastic strain and the void volume fraction in metals, or the amount of craze strains in glassy polymers. To this end, we define driving forces based on elastic and plastic work densities , and barrier functions related to critical values of these inelastic state variables. The proposed thermodynamically consistent framework of Ductile phase field fracture is embedded into a formulation of gradient thermo-plasticity, that is able to account for material length scales such as the width of shear bands. It is applied to two constitutive model problems. The first is designed for the analysis of brittle-to-Ductile Failure mode transition in the dynamic Failure analysis of metals . The second is constructed for a quasi-static analysis of crazing-induced fracture in glassy polymers . A spectrum of simulations demonstrates that the use of barrier-type crack driving forces in the phase field modeling of fracture, governed by accumulated plastic strains in metals or crazing strains in polymers, provide results in very good agreement with experiments.
Imad Barsoum - One of the best experts on this subject based on the ideXlab platform.
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tension torsion fracture experiments part i experiments and a procedure to evaluate the equivalent plastic strain
International Journal of Solids and Structures, 2013Co-Authors: Jonas Faleskog, Imad BarsoumAbstract:Abstract Ductile Failure experiments on a double notched tube (DNT) specimen subjected to a combination of tensile load and torque that was applied at a fixed ratio is presented. The experimental results extend those in Barsoum and Faleskog (2007a) down to zero stress triaxiality. A new and robust evaluation procedure for such tests is proposed, and a simple relation for the equivalent plastic strain at Failure for combined normal and shear deformation, respectively, is developed. Tests were carried out on the medium strength medium hardening steel Weldox 420, and the high strength low hardening steel Weldox 960. The experimental results unanimously show that Ductile Failure not only depends on stress triaxiality, but is also strongly affected by the type of deviatoric stress state that prevails, which can be quantified by a stress invariant that discriminates between axisymmetric stressing and shear dominated stressing, e.g., the Lode parameter. Additional experiments on round notch bar (RNB) specimens are recapitulated in order to give a comprehensive account on how Ductile Failure depends on stress triaxiality, ranging from zero to more than 1.6, and the type of stress state for the two materials tested. This provides an extensive experimental data base that will be used to explore an extension of the Gurson model that incorporates damage development in shear presented in Xue et al. (2013) (Part II).
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tension torsion fracture experiments part i experiments and a procedure to evaluate the equivalent plastic strain
International Journal of Solids and Structures, 2013Co-Authors: Jonas Faleskog, Imad BarsoumAbstract:Ductile Failure experiments on a double notched tube (DNT) specimen subjected to a combination of ten-sue load and torque that was applied at a fixed ratio is presented. The experimental results ex ...
