Sun-Myung Kim

The mechanical properties of porous MAX phase, namely porous Ti2AlC with aligned ellipsoid-like pores is studied in this study. Micromechanics based representative element volume (RVE) and finite element (FE) method are adopted in modeling porous Ti2AlC. A damage-plasticity coupled constitutive model considering different tensile and compressive behaviors is used in modeling the inelastic behavior of porous Ti2AlC under uniaxial compression. The effects of porosity volume fraction and pore size… 더보기

The mechanical properties of porous MAX phase, namely porous Ti2AlC with aligned ellipsoid-like pores is studied in this study. Micromechanics based representative element volume (RVE) and finite element (FE) method are adopted in modeling porous Ti2AlC. A damage-plasticity coupled constitutive model considering different tensile and compressive behaviors is used in modeling the inelastic behavior of porous Ti2AlC under uniaxial compression. The effects of porosity volume fraction and pore size on elastic moduli and compressive behavior of the porous Ti2AlC systems are investigated. Numerical results show that unlike typical porous ceramic, porous Ti2AlC behaves like a quasi-brittle material under uniaxial compression, and the damage of porous Ti2AlC is mainly driven by localized tensile failure. 더보기 취소

Carbon nanotubes (CNTs) have recently been integrated in the most widely used material in the world ‘concrete’ for improving its mechanical properties and fracture resistance. This paper computationally investigates the pull-out behaviour of a single CNT from cement. The effects of: 1) CNT-cement interfacial shear strength, stiffness, and fracture energy; 2) the cement mechanical properties; 3) CNT’s mechanical properties, aspect ratio, and surface area to volume ratio on the pull-out… 더보기

Carbon nanotubes (CNTs) have recently been integrated in the most widely used material in the world ‘concrete’ for improving its mechanical properties and fracture resistance. This paper computationally investigates the pull-out behaviour of a single CNT from cement. The effects of: 1) CNT-cement interfacial shear strength, stiffness, and fracture energy; 2) the cement mechanical properties; 3) CNT’s mechanical properties, aspect ratio, and surface area to volume ratio on the pull-out strength from a cement matrix are investigated through simulating the single straight CNT pull-out. A coupled elastic-plastic-damage constitutive model is adopted to simulate the behaviour of the cement matrix, whereas the continuum shell model is used to simulate the elastic behaviour of CNT. The surface-based cohesive behaviour is employed for modelling the interface between CNT and cement matrix. It is shown that CNT pull-out is mainly governed by the interfacial fracture energy, and not the interfacial shear strength. 더보기 취소

In this paper an elastoplastic finite element model for stainless-steel/bronze interpenetrating phase composites (IPCs) is proposed. 3D representative volume elements (RVEs) are created based on the microstructures of the stainless-steel/bronze IPCs and the corresponding finite element models are used to study the mechanical and thermal expansion properties. The predicted effective elastic moduli and coefficient of thermal expansion are compared with those obtained from micromechanics based… 더보기

In this paper an elastoplastic finite element model for stainless-steel/bronze interpenetrating phase composites (IPCs) is proposed. 3D representative volume elements (RVEs) are created based on the microstructures of the stainless-steel/bronze IPCs and the corresponding finite element models are used to study the mechanical and thermal expansion properties. The predicted effective elastic moduli and coefficient of thermal expansion are compared with those obtained from micromechanics based homogenization methods. The Gurson–Tvergaard–Needleman (GTN) constitutive model is adopted to investigate the influence of voids located in the bronze phase on elastoplastic and evolutionary damage behavior of the IPCs under uniaxial tension. The FE results have very good correlation with the experimental data, and the effects of thermal residual stress, void growth and nucleation on the flow properties of the IPCs are discussed. 더보기 취소

Oxidative aging is one of the most important factors in reducing the fatigue damage resistance of aging-susceptible materials such as bituminous materials and asphalt concrete. This study proposes a mechanistic-based phenomenological oxidative aging (or oxidative hardening) model by introducing a physically-based oxidative aging internal state variable which captures the effect of aging on the viscoelastic, viscoplastic, and viscodamage responses of aging-susceptible materials, especially… 더보기

Oxidative aging is one of the most important factors in reducing the fatigue damage resistance of aging-susceptible materials such as bituminous materials and asphalt concrete. This study proposes a mechanistic-based phenomenological oxidative aging (or oxidative hardening) model by introducing a physically-based oxidative aging internal state variable which captures the effect of aging on the viscoelastic, viscoplastic, and viscodamage responses of aging-susceptible materials, especially bituminous materials. The proposed aging model is formulated as a function of the diffused oxygen content and temperature evolution which ties the mechanical response of aged material to the underlying physics happening during the oxidative aging of asphalt concrete. Phenomenologically, the evolution of the aging internal state variable in asphalt concrete is related to the rate of carbonyl formation during the aging process of the asphalt binder. It is argued that oxidative aging mostly affects the viscous behavior of the aged material, such that the viscosity model parameters in the coupled nonlinear-viscoelastic, viscoplastic, and viscodamage constitutive models are defined as a function of the aging state variable. The qualitative capabilities of the model in capturing the effect of aging on mechanical response of asphalt concrete are verified against a wide range of simulations that include single creep, creep-recovery, repeated creep-recovery, monotonic tension and compression, uniaxial tensile and compressive loading–unloading, and relaxation tests as well as against the rutting performance simulations of an asphalt layer. It is shown that the proposed aging model predicts proper trends for the effect of oxidative hardening on the various mechanical properties of the asphalt concrete, such as the increase in the stiffness and strength, the decrease in ductility, and early initiation and rapid evolution of damage with aging. 더보기 취소

Concrete is considered as a 3-phase composite material; mortar matrix, aggregates, and interfacial transmission zone (ITZ). In order to investigate the contribution of each phase to the strength and damage response of concrete, 2-D and 3-D meso-scale simulations based on a coupled plasticity-damage model are carried out. The aggregates are modeled as a linear-elastic material, whereas the mortar matrix and ITZ are modeled using a coupled plasticity-damage model with different tensile and… 더보기

Concrete is considered as a 3-phase composite material; mortar matrix, aggregates, and interfacial transmission zone (ITZ). In order to investigate the contribution of each phase to the strength and damage response of concrete, 2-D and 3-D meso-scale simulations based on a coupled plasticity-damage model are carried out. The aggregates are modeled as a linear-elastic material, whereas the mortar matrix and ITZ are modeled using a coupled plasticity-damage model with different tensile and compressive mechanical behavior. Aggregate shape, distribution, and volume fraction are considered as simulated variables. The effect of the ITZ thickness and the strength of the ITZ and mortar matrix are also evaluated. It is shown that the behavior of concrete is merely dependent on the aggregate distribution and the strength of the mortar matrix, but dependent on aggregate shape, size, and volume fraction, and the thickness and strength of the ITZ. 더보기 취소

A coupled plasticity-damage model for plain concrete is presented in this paper. Based on continuum damage mechanics (CDM), an isotropic and anisotropic damage model coupled with a plasticity model is proposed in order to effectively predict and simulate plain concrete fracture. Two different damage evolution laws for both tension and compression are formulated for a more accurate prediction of the plain concrete behavior. In order to derive the constitutive equations and for the easiness in… 더보기

A coupled plasticity-damage model for plain concrete is presented in this paper. Based on continuum damage mechanics (CDM), an isotropic and anisotropic damage model coupled with a plasticity model is proposed in order to effectively predict and simulate plain concrete fracture. Two different damage evolution laws for both tension and compression are formulated for a more accurate prediction of the plain concrete behavior. In order to derive the constitutive equations and for the easiness in the numerical implementation, in the CDM framework the strain equivalence hypothesis is adopted such that the strain in the effective (undamaged) configuration is equivalent to the strain in the nominal (damaged) configuration. The proposed constitutive model has been shown to satisfy the thermodynamics requirements. Detailed numerical algorithms are developed for the finite element implementation of the proposed coupled plasticity-damage model. The numerical algorithm is coded using the user subroutine UMAT and then implemented in the commercial finite element analysis program Abaqus. Special emphasis is placed on identifying the plasticity and damage model material parameters from loading–unloading uniaxial test results. The overall performance of the proposed model is verified by comparing the model predictions to various experimental data, such as monotonic uniaxial tension and compression tests, monotonic biaxial compression test, loading–unloading uniaxial tensile and compressive tests, and mixed-mode fracture tests. 더보기 취소

During highly dynamic and ballistic loading processes, large inelastic deformation associated with high strain rates leads, for a broad class of heterogeneous materials, to degradation and failure by localized damage and fracture. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element predictions of ballistic response are considerably affected by the mesh size. This gives rise to non-physical description… 더보기

During highly dynamic and ballistic loading processes, large inelastic deformation associated with high strain rates leads, for a broad class of heterogeneous materials, to degradation and failure by localized damage and fracture. However, as soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element predictions of ballistic response are considerably affected by the mesh size. This gives rise to non-physical description of the ballistic behavior and mesh-dependent ballistic limit velocities that may mislead the design of ballistic resistant materials. This paper is concerned with the development and numerical implementation of a coupled thermo-hypoelasto-viscoplastic and thermo-viscodamage constitutive model within the laws of thermodynamics in which an intrinsic material length scale parameter is incorporated through the nonlocal gradient-dependent damage approach. This model is intended for impact and ballistic penetration and perforation problems of heterogeneous metallic targets such as metal matrix composites with dispersed particles at decreasing microstructural length scales. An evolution equation for the material length scale as a function of the material microstructural features (e.g. mean grain size in polycrystalline materials or particle size and inter-particle spacing in metal matrix composites), course of plastic deformation, strain hardening, strain-rate hardening, and temperature is presented. It is shown through simulating plugging failure in ballistic penetration of high-strength steel targets of different thicknesses by a hard blunt projectile that the length scale parameter plays the role of a localization limiter allowing one to obtain meaningful values for the ballistic limit velocity independent of the finite element mesh density. 더보기 취소

There are various ways to connect the GFRP deck and girder which have been adopted as systems worldwide: including the use of a shear connector, and mechanical connection systems that utilize bolts or clamps. Recently, there have been studies on the systems to connect the GFRP deck and steel girder using a bonding agent. This paper proposes a new connection system, which is different from the GFRP deck-to-girder connection system that has been applied so far. In addition to evaluate the… 더보기

There are various ways to connect the GFRP deck and girder which have been adopted as systems worldwide: including the use of a shear connector, and mechanical connection systems that utilize bolts or clamps. Recently, there have been studies on the systems to connect the GFRP deck and steel girder using a bonding agent. This paper proposes a new connection system, which is different from the GFRP deck-to-girder connection system that has been applied so far. In addition to evaluate the structural performance of the proposed connection system, it is compared with the bolted mechanical connection system via testing and FEM analysis under the same conditions. 더보기 취소

Cold-formed steel members such as beams and columns have the great flexibility of cross-sectional profiles and sizes available to structural steel designers. However, this flexibility makes the selection of the most economical section difficult for a particular situation. In this study, micro genetic algorithm (MGA) is used to find an optimum cross section of cold-formed steel channel and lipped channel columns under axial compression. Flexural, torsional and torsional–flexural buckling of… 더보기

Cold-formed steel members such as beams and columns have the great flexibility of cross-sectional profiles and sizes available to structural steel designers. However, this flexibility makes the selection of the most economical section difficult for a particular situation. In this study, micro genetic algorithm (MGA) is used to find an optimum cross section of cold-formed steel channel and lipped channel columns under axial compression. Flexural, torsional and torsional–flexural buckling of columns and flat-width-to-thickness ratio of web, flange and lip are considered as constraints. The design curves are generated for optimum values of the thickness, the web flatdepth-to-thickness ratio, the flange flat-width-to-thickness ratio for columns. As numerical results, the optimum design curves are presented for various load level and column lengths. 더보기 취소

This paper presents a new method for deck-to-girder connections of glass fibre reinforced polymer (GFRP)
bridge decks. To design the connection system, a GFRP deck in a rectangular cross-sectional shape is considered. The size of the shear bolts, bolt location, bolt spacing, and the size and type of stiffening plate are the variables considered in the design. The behavior of the proposed deck-to-girder connection system was analysed using commercial finite element analysis software. The… 더보기

This paper presents a new method for deck-to-girder connections of glass fibre reinforced polymer (GFRP)
bridge decks. To design the connection system, a GFRP deck in a rectangular cross-sectional shape is considered. The size of the shear bolts, bolt location, bolt spacing, and the size and type of stiffening plate are the variables considered in the design. The behavior of the proposed deck-to-girder connection system was analysed using commercial finite element analysis software. The failure of the connection system was checked by the Tsai–Hill criterion. Effective deck-to-girder connection details for GFRP bridge decks are identified and presented based on the results of the analysis.
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The main objective of this paper is to design a viable GFRP (glass fiber reinforced plastics) deck profile for highway girder bridges. E-glass fibers with vinylester resin were selected for the material design. Five GFRP stacking patterns having different material compositions were designed. A total of 100 coupon specimens were tested to verify the material properties of the proposed GFRP patterns. A multi-cellular cross-sectional shape was proposed for the deck profile. The proposed profile… 더보기

The main objective of this paper is to design a viable GFRP (glass fiber reinforced plastics) deck profile for highway girder bridges. E-glass fibers with vinylester resin were selected for the material design. Five GFRP stacking patterns having different material compositions were designed. A total of 100 coupon specimens were tested to verify the material properties of the proposed GFRP patterns. A multi-cellular cross-sectional shape was proposed for the deck profile. The proposed profile was optimized to obtain a viable cross-sectional shape that is especially suitable for the pultrusion process. This paper also presents design of a GFRP deck for prototype of a steel I-girder bridge using the proposed deck profile. 더보기 취소

An important advantage of cold-formed steel is the great flexibility of cross-sectional profiles and sizes available to structural steel designers. However, this flexibility makes the selection of the most economical section difficult for a particular situation. In this study, a micro Genetic Algorithm (μ-GA) is used to find an optimum cross-section of cold-formed steel beams. The μ-GA is one of the improved form of GAs, to reduce iteration and computing resources by using small populations… 더보기

An important advantage of cold-formed steel is the great flexibility of cross-sectional profiles and sizes available to structural steel designers. However, this flexibility makes the selection of the most economical section difficult for a particular situation. In this study, a micro Genetic Algorithm (μ-GA) is used to find an optimum cross-section of cold-formed steel beams. The μ-GA is one of the improved form of GAs, to reduce iteration and computing resources by using small populations. The design curves are generated for optimum values of the thickness and the web flat-depth-to-thickness ratio for unbraced beams under uniformly distributed load. As numerical results, the optimum design curves are presented for various load level. 더보기 취소

This paper presents the design and analysis of GFRP (glass fiber reinforced polymer) decks for the girder bridges. In the material design, Eglass fibers with vinyl ester resin are assumed for the materials. Five GFRP plates having different stacking patterns and material compositions are designed and pultruded. Total 100 specimens are tested for tension, compression, shear, and flexure. Based on the material properties determined in the material test, structural shape of deck profile having… 더보기

This paper presents the design and analysis of GFRP (glass fiber reinforced polymer) decks for the girder bridges. In the material design, Eglass fibers with vinyl ester resin are assumed for the materials. Five GFRP plates having different stacking patterns and material compositions are designed and pultruded. Total 100 specimens are tested for tension, compression, shear, and flexure. Based on the material properties determined in the material test, structural shape of deck profile having multi-cellular tube section is proposed. The proposed profile is finally optimized by using a genetic algorithm-based optimization procedure. Based on the results of this study, viable GFRP patterns and cross-sectional dimensions of deck profile with deck-to-girder connections suitable for the deck renewal projects are proposed. Using the proposed cross-sectional shape and GFRP patterns, a design of GFRP deck for the typical steel I-girder bridge is also presented in this paper. 더보기 취소