Erik Saether

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• PhD
• Senior Research Engineer at National Aeronautics and Space Administration

This report investigates some issue in the prediction of buckling failure and plastic collapse in sandwich composites.

A multiscale modeling methodology that combines the predictive capability of discrete dislocation plasticity and the computational efficiency of continuum crystal plasticity is developed. Single crystal configurations of different grain sizes modeled with periodic boundary conditions are analyzed using discrete dislocation plasticity (DD) to obtain...

A Statistical Approach to Multiscale Modeling of Fracture in Metals at Microscale V. I. Yamakov1,2, E. Saether1, E. H. Glaessgen1 1NASA Langley Research Center, Hampton, VA 23681, U.S.A. 2National Institute of Aerospace, Hampton, VA 23666, USA. A statistical multiscale modeling approach to study atomistic mechanisms of fracture in aluminum is...

This paper describes a broad effort that is aimed at understanding the fundamental mechanisms of crack growth and using that understanding as a basis for designing materials and enabling predictions of fracture in materials and structures that have small characteristic dimensions. This area of research, herein referred to as Damage Science, emphasi...

This paper describes a broad effort that is aimed at understanding the fundamental mechanisms of crack growth and using that understanding as a basis for designing materials and enabling predictions of fracture in materials and structures that have small characteristic dimensions. This area of research, herein referred to as Damage Science, emphasi...

This paper describes a broad effort that is aimed at understanding the fundamental mechanisms of crack growth and using that understanding as a basis for designing materials and making more robust fracture predictions. This area of research, herein referred to as Damage Science, emphasizes the length scale regimes of the nano/micro-scale (10-9 to 1...

Methodologies for understanding the plastic deformation mechanisms related 10 crack propagation at the nano·, meso-and micro-length scales are being developed. These efforts include the development and application of several com putational methods including atomistic simulation, discrete dislocation plasticity, strain gradient plasticity and crysta...

Several efforts that are aimed at understanding the plastic deformation mechanisms related to crack propagation at the nano-, meso-and micro-length scales including atomistic simulation, discrete dislocation plasticity, strain gradient plasticity and crystal plasticity are discussed. The paper focuses on discussion of newly developed methodologies...

The coupling of molecular dynamics (MD) simulations with finite element methods (FEM) yields computationally efficient models that link fundamental material processes at the atomistic level with continuum field responses at higher length scales. The theoretical challenge involves developing a seamless connection along an interface between two inher...

Physics-based modeling of fracture begins at nanometer dimensional scales in which atomistic simulation is used to predict the formation, propagation, and interaction of fundamental damage mechanisms. These mechanisms include dislocation formation and interaction, interstitial void formation, and atomic diffusion. The development of these damage me...

Intergranular fracture is a dominant mode of failure in ultrafine grained materials. In the present study, the atomistic mechanisms of grain-boundary debonding during intergranular fracture in aluminum are modeled using a coupled molecular dynamics—finite element simulation. Using a statistical mechanics approach, a cohesive-zone law in the form of...

The coupling of molecular dynamics (MD) simulations with finite element methods (FEM) yields computationally efficient models that link fundamental material processes at the atomistic level with continuum field responses at higher length scales. The theoretical challenge involves developing a seamless connection along an interface between two inher...

The emerging field of nanomechanics is providing a new focus in the study of the mechanics of materials, particularly in simulating fundamental atomic mechanisms involved in the initiation and evolution of damage. Simulating fundamental material processes using first principles in physics strongly motivates the formulation of computational multisca...

A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain has been enhanced. The concurrent MD-FEM coupling methodology uses statistical averaging of the deformation of the atomistic MD domain to provide interface displacement boundary conditions to the surrounding continuum FEM r...

Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, current analysis is limited to small domains and increasing the size of the MD domain quickly presents intractable computational demands. A preferred approach to surmount this computational lim...

Intergranular fracture is a dominant mode of failure in ultrafine grained materials. In the present study, the atomistic mechanisms of grain-boundary debonding during intergranular fracture in aluminum are modeled using a coupled molecular dynamics – finite element simulation. Using a statistical mechanics approach, a cohesive-zone law in the form...

Multi-functional sandwich composites are proposed to integrate the previously separate objectives of radiation shielding, structural integrity, damage tolerance, thermal insulation, and debris/micrometeoroid shielding into a viable structural design. Effective radiation shielding is provided through the selection of hydrogen-rich polymeric material...

Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, current analysis is limited to small domains and increasing the size of the MD domain quickly presents intractable computational demands. A preferred approach to surmount this computational lim...

A multiscale continuum-atomistic model for crack propagation through grain-boundary in aluminum is presented. The crack grows as an edge crack in the continuum environment subject to mode I loading conditions and penetrates into the atomistic environment. The continuum environment is simulated using finite-element (FE) method with incorporated cohe...

The dynamics and energetics of intergranular crack growth along a flat grain boundary in aluminum is studied by a molecular-dynamics simulation model for crack propagation under steady-state conditions. Using the ability of the molecular-dynamics simulation to identify atoms involved in different atomistic mechanisms, it was possible to identify th...

Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, increasing the size of the MD domain quickly presents intractable computational demands. A robust approach to surmount this computational limitation has been to unite continuum modeling procedu...

Multiscale modeling methods for the analysis of fracture in metallic microstructures are discussed. Molecular dynamics models are used to analyze grain-boundary sliding and fracture in an aluminum bicrystal model. A bilinear traction-displacement relationship that may be embedded into cohesive zone finite elements for microscale problems is extract...

A traction–displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics (MD) simulations. An MD model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat S99 [1 1 0] symmetric tilt...

A multiscale modeling strategy is developed to study grain boundary fracture in polycrystalline aluminum. Atomistic simulation is used to model fundamental nanoscale deformation and fracture mechanisms and to develop a constitutive relationship for separation along a grain boundary interface. The nanoscale constitutive relationship is then paramete...

Multiscale modeling methods for the analysis of fracture in metallic microstructures are discussed. Molecular dynamics models are used to analyze grain-boundary sliding and fracture in an aluminum bicrystal model. A bilinear traction-displacement relationship that may be embedded into cohesive zone finite elements for microscale problems is extract...

22.1 Introduction Uneven stress distribution and stress localization during deformation are the key factors for fracture and failure in polycrystalline and nanocrystalline metals. The inhomogeneous polycrystalline microstructure that consists of grains of different size and shape joined together at different angles and forming various types of grai...

Uneven stress distribution and stress localization during deformation are the key factors for fracture and failure in polycrystalline and nanocrystalline metals. The inhomogeneous polycrystalline microstructure that consists of grains of different size and shape joined together at different angles and forming various types of grain-boundaries (GB)...

A molecular-dynamics model for crack propagation under steady-state conditions is used to study dynamic instabilities along a grain boundary in aluminum that occur when the crack speed approaches 1/3 of the material's Rayleigh wave speed. Instead of crack branching, as is characteristic for a crack propagating in a homogeneous environment, the inst...

Multiscale modeling methods for the analysis of metallic microstnictures are discussed. Both molecular dynamics and the finite element method are used to analyze crack propagation and stress distribution in a nanoscale aluminum bicrystal model subjected to hydrostatic loading. Quantitative similarity is observed between the results from the two ver...

Recent research conducted under NASA LaRC's Creativity and Innovation Program has led to the development of an initial approach for a hierarchical fracture mechanics. This methodology unites failure mechanisms occurring at different length scales and provides a framework for a physics-based theory of fracture. At the nanoscale, parametric molecular...

In this paper, a multiscale modelling strategy is used to study the effect of grain-boundary sliding on stress localization in a polycrystalline microstructure with an uneven distribution of grain size. The development of the molecular dynamics (MD) analysis used to interrogate idealized grain microstructures with various types of grain boundaries...

Aliphatic polymers were identified as optimum radiation polymeric shielding materials for building multifunctional structural elements. Conceptual damage-tolerant configurations of polyolefins have been proposed but many issues on the manufacture remain. In the present paper, we will investigate fabrication technologies with e-beam curing for inclu...

The use of organic polymeric materials has been proposed as a means of increasing the shielding performance of space structures as an improvement over the use of aluminum alloys. The TransHab wall structure using polyurethane foams and ceramic fabrics with a polymeric bladder is one example. Analysis has shown the highest performing organic polymer...

Multiscale modeling provides an efficient means of interrogating deformation and fracture of metallic materials at the micro-and nano-scales. In the current implementation, molecular dynamics simulations are used to determine the mechanisms of fracture of aluminum at atomic length scales. Information obtained from molecular dynamics simulations may...

A self-consistent set of relationships is developed for the physical properties of single walled carbon nanotubes (SWCN) and their hexagonal arrays as a function of the chiral vector integer pair, (n,m). Properties include effective radius, density, principal Young's modulus, and specific Young's modulus. Relationships between weight fraction and v...

Multiscale modeling provides an efficient means of interrogating deformation and fracture of metallic materials at the micro-and nano-scales. In the current implementation, molecular dynamics simulations are used to determine the mechanisms of fracture of aluminum at atomic length scales. Information obtained from molecular dynamics simulations may...

Carbon nanotubes naturally tend to form crystals in the form of hexagonally packed bundles. An accurate determination of the effective mechanical properties of nanotube bundles is important in order to assess potential structural applications such as reinforcement in future composite material systems. Although the intratube axial stiffness is on th...

Perfect crystals of carbon nanotubes tend to form aligned bundles that assume a hexagonal packing configuration in a minimum energy state. The theoretical constitutive relation for these defect-free crystals is highly anisotropic with a large axial stiffness due to a network of strong delocalized carbon–carbon bonds and transverse properties that a...

A set Of relationships is developed for selected physical properties of single-walled carbon nanotubes (SWCN) and their hexagonal arrays as a function of nanotube size in terms of the chiral vector integer pair, (n,m). Properties include density, principal Young's modulus, and specific Young's modulus. Relationships between weight fraction and volu...

Carbon nanotubes naturally tend to form crystals in the form of hexagonally packed bundles or ropes that should exhibit a transversely isotropic constitutive behavior. Although the intratube axial stiffness is on the order of 1 TPa due to a strong network of delocalized bonds, the intertube cohesive strength is orders of magnitude less controlled b...

A set of relationships is developed for selected physical properties of single-walled carbon nanotubes (SWCN) and their hexagonal arrays as a function of nanotube size in terms of the chiral vector integer pair, (n,m). Properties include density, principal Young’s modulus, and specific Young’s modulus. Relationships between weight fraction and volu...

Carbon nanotubes naturally tend to form crystals in the form of hexagonally packed bundles or ropes that should exhibit a transversely isotropic constitutive behavior. Although the intratube axial stiffness is on the order of 1 TPa due to a strong network of delocalized bonds, the intertube which is controlled by weak, nonbonding van der Waals inte...

A general predictive methodology for determining residual strength in impact damaged composite laminates has been developed and incorporated into a computer code designated RESTRAN (REsidual STRength ANalysis). RESTRAN is a finite element based design tool that can analyze composite structures with arbitrary three-dimensional (3-D) geometry, loadin...

Structural optimization methods in MSC /NASTRAN are used to size substructures and to reduce the weight of a composite sandwich cryogenic tank for future launch vehicles. Because the feasible design space of this problem is non-convex, many local minima are found. This non-convex problem is investigated in detail by conducting a series of analyses...

A general methodology for deriving explicit element stiffness matrices in hybrid stress formulations is extended to incorporate nonconstant material properties over the element domain for nonlinear elastic analysis. The technique utilizes special stress field transformations to simplify the stiffness definition together with an assumed variation of...

The hybrid stress method has demonstrated many improvements over conventional displacement-based formulations. A main detraction from the method, however, has been the higher computatational cost in forming element stiffness coefficients due to matrix inversions and manipulations as required by the technique. By utilizing permissible field transfor...

A general methodology for deriving explicit element stiffness matrices in hybrid stress formulations has recently been developed by the author. The technique utilizes special stress field transformations to eliminate the complementary energy matrix inherent to hybrid formulations, thus simplifying the stiffness definition such that an explicit eval...

Two versions of a { 1, 2} higher order laminate plate theory are derived for elastodynam-ics and used to compute natural frequencies and mode shapes for homogeneous and laminated plates. The theories include deformations due to transverse shear and transverse normal stretching and account for rotary and thickness-motion inertia. Analytic natural fr...

Two versions of a { 1, 2} higher order laminate plate theory are derived for elastodynam-ics and used to compute natural frequencies and mode shapes for homogeneous and laminated plates. The theories include deformations due to transverse shear and transverse normal stretching and account for rotary and thickness-motion inertia. Analytic natural fr...

Abstraet-A higher-order shell theory is developed for elastodynamic analysis of orthotropic shells. The theory accounts for all basic deformations including transverse shear and transverse normal strains and stresses. The theory is developed in orthogonal curvilinear coordinates in which the reference surface components of the displacement vector v...

The prediction of residual strength in damaged composite structures is complicated by the need to accurately characterize internal damage states and to mathematically simulate the material behavior which displays a variety of damage mechanisms and failure modes. Numerous analyses have been developed which, in general, have been restricted to specif...

The widespread application of adhesively bonded joints has necessitated the development of methodology to predict ultimate static joint strength and service life under cyclic loading. Due to the complexity of mathematically modelling adhesive joint response, analytical treatments are limited to highly idealized joint configurations, simplified assu...

A user-deflined subroutine has been developed to implement special 2- D and 3-D layered continuum elements in the commercial finite element proprarn ABAQUS. These elements are specially configured to accurately predict interface stresses in adhesively bonded joints and are formulated using the hybrid stress technique to explicitly enforce stress eq...

The hybrid stress method has demonstrated many improvements over conventional displacement-based elements. A main detraction from the method, however, has been the higher computational cost in forming element stiffness coefficients due to matrix inversions and manipulations as required by the technique. By utilizing special transformations of initi...

A user-defined subroutine has been developed to implement a higher-order three-node facet shell finite element in the commercial finite element program ABAQUS. The element, designated HOT3, is based on a ( 1,2) -order plate theory specifically formulated to model thin and thick composite laminates and exhibits improved accuracy compared to conventi...

Two versions of a (1,2) higher-order laminate plate theory are derived for elastodynamics and used to compute natural frequencies and mode shapes for homogeneous and laminated plates. The theories include deformations due to transverse shear and transverse normal stretching and account for rotary and thickness-motion inertia. Analytic natural frequ...

A variational higher-order theory involving all transverse strain and stress components is proposed for the analysis of laminated composite plates. Derived from three-dimensional elasticity with emphasis on developing a viable computational methodology, the theory is well suited for finite element approximations as it incorporates both Co and C-' c...

A variational higher-order theory involving all transverse strain and stress components is proposed for the analysis of laminated composite plates. Derived from three dimensional elasticity with emphasis on developing a viable computational methodology, the theory is well suited for finite element approximations as it incorporates both C0 and C-1....

This report details the analytical solution of orthotropic governing equations of elasticity for a four-layer laminate containing an embedded crack. Surface loads are incorporated into the analysis and their influence on the strain energy release rate at the crack tip is computed.

A methodoloy for certification testing of composite structures is developed and a detailed description of the methodology is presented. Test data interpretation methodology is also developed. The methodology is demonstrated on existing composite structures. Based on the results of this investigation, composite structure certification testing proced...

A methodology for certification testing of composite structures is developed and a detailed description of the methodology is presented. Test data interpretation methodology is also developed. The methodology is demonstrated on existing composite structures. Based on the results of this investigation, composite structure certification testing proce...

this report presents a strength analysis for laminated and/or metallic plates bolted together by many fasteners. This analysis has been programmed and designated as the SAMCJ (Strength Analysis of Multi-fastened Composite Joints) computer code. SAkMCJ incorporates a finite element method using special elements developed in this program to predict f...

A design guide was developed for bolted composite structural joints. The guide Includes general design guidelines for the various joint parameters, an analytical design methodology, a description of the analytical design tools, an illustration of the use of corresponding computer codes (SASCJ and SAMCJ), and a listing of the computer codes. The pro...

This report presents a strength analysis for laminated and/or metallic plates bolted together by a single fastener. The analysis has been programmed to be the SASCJ (Strength Analysis of Single Fastener Composite Joints) computer code. SASCJ includes:a two dimensional analysis of doubly-connected anisotropic plates accounting for finite plate dimen...

This report presents a strength analysis for laminated and/or metallic plates bolted together by a single fastener. The analysis includes a two dimensional analysis of doubly-connected anisotropic plates accounting for finite plate dimensions, a fastener analysis that approximately accounts for the local three dimensional effects, and a progressive...