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Öğe Comparison of the Method of Cells with the Mori-Tanaka in Predicting Damaged Elasto-Plastic behavior of Laminated Composite Materials(2001) Deliktas, Babur; Voyiadjis, George Z.Two commonly used micromechanical methods, namely the method of cell and the Mori-Tanaka averaging scheme are compared in this work. Their performance is evaluated in the reference to the mechanical behavior of the composite materials. Their respective homogenization schemes are compared and evaluated through the respective strain concentration tensors that are governed by these models. The damaged elasto plastic behavior of the laminated composite system (90) (8s) and (0/90) (4s) are analyzed and compared for the two methods for the case of uniaxial loading. The Lagoudas et. a (1991) modification of the Mori-Tanaka scheme as applied to elasto-plastic constitutive models is used in this work.Öğe Computer Technology for Enhancing Teaching and Learning Modules of Engineering Mechanics(Wiley, 2011) Deliktas, BaburImproving the quality of learning and teaching has always been in the interest of instructors in all fields of study. There have been tremendous efforts to this end. In this article, learning and teaching modules enhanced with computer technology are introduced. This approach is based on concept questioning and scenario building aided with interactive animation, simulation, and rich graphical content. Modules for the engineering mechanics course covering fundamental topics in Statics, Strength of Materials, and Dynamics are prepared by using the proposed approach. Some examples of the prepared modules are presented. Design of the course module and its evaluation from student's perspective are discussed. Based on evaluations using questionnaires by the students it can be inferred that this approach to teaching and learning helps students to increase their capacity to understand and instructors to convey their ideas more conveniently. (C) 2009 Wiley Periodicals, Inc. Comput Appl Eng Educ 19: 421-432, 2011; View this article online at wileyonlinelibrary.com/journal/cae; DOI 10.1002/cae.20321Öğe Consistent Non Local Coupled Damage Model and Its Application in Impact Response of Composite Materials(Springer-Verlag Wien, 2011) Voyiadjis, George Z.; Deliktas, Babur; Kattan, Peter I.In this work also the mechanics of small damage are also presented using a consistent mathematical and mechanical framework based on the equations of damage mechanics. In this regard, the new scalar damage variable is investigated in detail. The investigation in this work has been carried out for seeking a physical basis is sought for the damage tensor [M] that is used to link the damage state of the material with effective undamaged configuration. The approach presented here provides for a strong physical basis for this missing link. In particular, the authors have made an important link between the damage tensor and fabric tensors. Computational aspects of the presented theory are also discussed. Numerical integration algorithms, verification and validation process of the theory are discussed. The finite element simulations are also performed by implementing the presented model in the commercial finite element code ABAQUS [6.8.3] as a user defined subroutine (VUMAT).Öğe Consistent non local coupled damage model and its application in impact response of composite materials(Springer International Publishing, 2011) Voyiadjis, George Z.; Deliktas, Babur; Kattan, Peter I.In this work also the mechanics of small damage are also presented using a consistent mathematical and mechanical framework based on the equations of damage mechanics. In this regard, the new scalar damage variable is investigated in detail. The investigation in this work has been carried out for seeking a physical basis is sought for the damage tensor [M] that is used to link the damage state of the material with effective undamaged configuration. The approach presented here provides for a strong physical basis for this missing link. In particular, the authors have made an important link between the damage tensor and fabric tensors. Computational aspects of the presented theory are also discussed. Numerical integration algorithms, verification and validation process of the theory are discussed. The finite element simulations are also performed by implementing the presented model in the commercial finite element code ABAQUS [6.8.3] as a user defined subroutine (VUMAT). © CISM, Udine 2011.Öğe Formulation of strain gradient plasticity with interface energy in a consistent thermodynamic framework(Pergamon-Elsevier Science Ltd, 2009) Voyiadjis, George Z.; Deliktas, BaburIn this work, the strain gradient formulation is used within the context of the thermodynamic principle, internal state variables, and thermodynamic and dissipation potentials. These in turn provide balance of momentum, boundary conditions, yield condition and flow rule, and free energy and dissipative energies. This new formulation contributes to the following important related issues: (i) the effects of interface energy that are incorporated into the formulation to address various boundary conditions, strengthening and formation of the boundary layers, (ii) nonlocal temperature effects that are crucial, for instance, for simulating the behavior of high speed machining for metallic materials using the strain gradient plasticity models, (iii) a new form of the nonlocal flow rule, (iv) physical bases of the length scale parameter and its identification using nano-indentation experiments and (v) a wide range of applications of the theory. Applications to thin films on thick substrates for various loading conditions and torsion of thin wires are investigated here along with the appropriate length scale effect on the behavior of these structures. Numerical issues of the theory are discussed and results are obtained using Matlab and Mathematica for the nonlinear ordinary differential equations (NODE) which constitute the boundary value problem. This study reveals that the micro-stress term has an important effect on the development of the boundary layers and hardening of the material at both hard and soft interface boundary conditions in thin films. The interface boundary conditions are described by the interfacial length scale and interfacial strength parameters. These parameters are important to control the size effect and hardening of the material. For more complex geometries the generalized form of the boundary value problem using the nonlocal finite element formulation is required to address the problems involved. (C) 2008 Elsevier Ltd. All rights reserved.Öğe Friction coefficient evaluation using physically based viscoplasticity model at the contact region during high velocity sliding(Springer Wien, 2010) Voyiadjis, George Z.; Deliktas, Babur; Faghihi, Danial; Lodygowski, AdamMany physical systems require the description of mechanical interactions across interfaces if they are to be successfully analyzed. One of the well-known examples of such a system in the engineering world is the metal to metal friction. This is a complex process that needs to be adequately identified by a constitutive relation in order to better facilitate the design components in severe contact stress applications. In this paper, the formulation of Molinari et al.'s work (J Tribology Trans ASME 35-41, 1999) is revisited in order to investigate the coefficient of dry friction for steel on steel in the high velocity range using physically based viscoplastic constitutive relations. First some of the errors in the work are corrected, and their results are regenerated. The phenomenological constitutive relation used in Molinari et al. (J Tribology Trans ASME 35-41, 1999) is then replaced by the physically based viscoplastic model used in this paper. This constitutive model is implemented into ABAQUS (Analysis User's Manual, 2008) as user-defined subroutine as VUMAT in order to obtain the stress-strain curves at different strain rates and various temperatures. It is shown that the material responses obtained from the simulation using the physically based constitutive viscoplastic model agree well with the real behavior of the metals. Comparing this proposed work with that of Molinari et al. (J Tribology Trans ASME 35-41, 1999) one observes that the proposed theory and constitutive model are superior to the one presented by Molinari et al. This is specifically the case for the artificial shape of softening in the curve.Öğe Mechanics of strain gradient plasticity with particular reference to decomposition of the state variables into energetic and dissipative components(Pergamon-Elsevier Science Ltd, 2009) Voyiadjis, George Z.; Deliktas, BaburThe results of recent studies on strengthening and hardening mechanisms for micro/nano structured materials indicate that the use of only one type of energetic or dissipative description may be insufficient to accurately describe the size effects exhibited in metallic components. Therefore, it is important to incorporate more than one description of the thermodynamic processes into the modeling in order to have a better understanding of the hardening and strengthening mechanisms for micro/nano structured materials. The research presented here is based on this deficiency and the goal is to develop a strain gradient theory based on the decomposition of the state variables into energetic and dissipative components. This, in turn, endowed the constitutive equations to have both energetic and dissipative gradient length scales l(en) and l(dis) respectively. The effect of the material microstructural interface between two materials is also incorporated into the proposed formulation. Hence four material length scales are introduced: two for the bulk and the other two for the interface. The resulting formulation exhibits the following important physical phenomena: (i) standard energetic hardening associated with plastic strain and nonlocal energetic hardening associated with plastic strain gradients; (ii) size dependent increase in yield strength which is characterized by the dissipative strengthening associated with plastic strain gradient rate; (iii) the effect of interfacial yield strength and hardening; (iv) description of the boundary layer; and (v) the effect of the different boundary conditions. The problem is solved analytically by using for example, a thin film on elastic substrate under uniaxial uniform tension or a single phase bicrystal under uniform tension where the interface represents the grain boundary. The interface model here describes the internal boundary of the plastic region and characterizes the physical understanding of the dislocation mechanics at the interface between two phases. The results of the analytical results indicate that the proposed theory qualitatively captures the overall physical behavior. However there is strong debate in the literature on the choice of accurate physical boundary conditions at the elastic plastic boundaries. Therefore, more elaborate studies are needed for better assessment of the boundary conditions of the higher-order strain gradient plasticity theories. (C) 2009 Elsevier Ltd. All rights reserved.Öğe Modeling of strengthening and softening in inelastic nanocrystalline materials with reference to the triple junction and grain boundaries using strain gradient plasticity(Springer Wien, 2010) Voyiadjis, George Z.; Deliktas, BaburThe work presented here provides a generalized structure for modeling polycrystals from micro- to nano-size range. The polycrystal structure is defined in terms of the grain core, the grain boundary and the triple junction regions with their corresponding volume fractions. Depending on the size of the crystal from micro to nano, different types of analyses are used for the respective different regions of the polycrystal. The analyses encompass local and nonlocal continuum or crystal plasticity. Depending on the physics of the region dislocation-based inelastic deformation and/or slip/separation is used to characterize the behavior of the material. The analyses incorporate interfacial energy with grain boundary sliding and grain boundary separation. Certain state variables are appropriately decomposed into energetic and dissipative components to accurately describe the size effects. This new formulation does not only provide the internal interface energies but also introduces two additional internal state variables for the internal surfaces (contact surfaces). One of these new state variables measures tangential sliding between the grain boundaries and the other measures the respective separation. Additional entropy production is introduced due to the internal subsurface and contacting surface. A multilevel Mori-Tanaka averaging scheme is introduced in order to obtain the effective properties of the heterogeneous crystalline structure and to predict the inelastic response of a nanocrystalline material. The inverse Hall-Petch effect is also demonstrated. The formulation presented here is more general, and it is not limited to either polycrystalline- or nanocrystalline-structured materials. However, for more elaborate solution of problems, a finite element approach needs to be developed.Öğe Nonlocal gradient-dependent modeling of plasticity with anisotropic hardening(Pergamon-Elsevier Science Ltd, 2010) Voyiadjis, George Z.; Pekmezi, Gerald; Deliktas, BaburThis work addresses the formulation of the thermodynamics of nonlocal plasticity using the gradient theory. The formulation is based on the nonlocality energy residual introduced by Eringen and Edelen (1972). Gradients are introduced for those variables associated with isotropic and kinematic hardening. The formulation applies to small strain gradient plasticity and makes use of the evanescent memory model for kinematic hardening. This is accomplished using the kinematic flux evolution as developed by Zbib and Aifantis (1988). Therefore, the present theory is a four nonlocal parameter-based theory that accounts for the influence of large variations in the plastic strain, accumulated plastic strain, accumulated plastic strain gradients, and the micromechanical evolution of the kinematic flux. Using the principle of virtual power and the laws of thermodynamics, thermodynamically-consistent equations are derived for the nonlocal plasticity yield criterion and associated flow rule. The presence of higher-order gradients in the plastic strain is shown to enhance a corresponding history variable which arises from the accumulation of the plastic strain gradients. Furthermore, anisotropy is introduced by plastic strain gradients in the form of kinematic hardening. Plastic strain gradients can be attributed to the net Burgers vector, while gradients in the accumulation of plastic strain are responsible for the introduction of isotropic hardening. The equilibrium between internal Cauchy stress and the microstresses conjugate to the higher-order gradients frames the yield criterion, which is obtained from the principle of virtual power. Microscopic boundary conditions, associated with plastic flow, are introduced to supplement the macroscopic boundary conditions of classical plasticity. The nonlocal formulation developed here preserves the classical assumption of local plasticity, wherein plastic flow direction is governed by the deviatoric Cauchy stress. The theory is applied to the problems of thin films on both soft and hard substrates. Numerical solutions are presented for bi-axial tension and simple shear loading of thin films on substrates. (C) 2010 Elsevier Ltd. All rights reserved.Öğe Role of strain concentration factors in predicting the inelastic behavior of laminated composite material(Elsevier Sci Ltd, 2009) Deliktas, BaburTwo commonly used micromechanical methods, namely the method of cells and Mori-Tanaka averaging scheme are compared through the respective strain concentration factors that are governed by these models. The anisotropic damage model is incorporated into one of these averaging schemes in order to predict the inelastic behavior of the composite material. A coupled computational algorithm for plasticity and damage is presented. In order to show the reliability of the damage model and the algorithms the laminated composite system of (90)((8s)) and (0/90)((4s)) is analyzed through the use of the Mori-Tanaka averaging scheme. Published by Elsevier Ltd.Öğe Simulation of perforation and penetration in metal matrix composite materials using coupled viscoplastic damage model(Elsevier Sci Ltd, 2009) Deliktas, Babur; Voyiadjis, George Z.; Palazotto, Anthony N.In the first part of the two companion papers, theoretical formulation of the multiscale micromechanical constitutive model that couples the anisotropic damage mechanism with the viscoplastic deformation is presented. In the second part of these companion papers the numerical simulation of the computational aspects of the theory are elaborated. The perforation and penetration problem of metal matrix composites (MMCs) due to high impact loading is simulated. in this sense, the computational aspects of the developed theory are elaborated here. First. the verification of the developed model is performed through its numerical implementation in order to test the model predictions of the material characteristic tests. This encompasses uniaxial monotonic loading and unloading under different strain rates, uniaxial cyclic loading, and uniaxial loading and relaxation. The verified material routine of the developed model is then implemented in the explicit finite element code ABAQUS via the user defined subroutine VUMAT at each integration point in order to analyze the projectile impact and penetration into laminated composite plates. (C) 2009 Elsevier Ltd. All rights reserved.Öğe Theoretical and Experimental Characterization for the Inelastic Behavior of the Micro-/Nanostructured Thin Films Using Strain Gradient Plasticity With Interface Energy(Asme, 2009) Voyiadjis, George Z.; Deliktas, BaburThin film technology is pervasive in many applications, including microelectronics, optics, magnetic, hard and corrosion resistant coatings, micromechanics, etc. Therefore, basic research activities will be necessary in the future to increase knowledge and understanding and to develop predictive capabilities for relating fundamental physical and chemical properties to the microstructure and performance of thin films in various applications. In basic research, special model systems are needed for quantitative investigation of the relevant and fundamental processes in thin film material science. Because of the diversity of the subject and the sheer volume of the publications, a complete a review of the area of the current study, is focused particularly on the experimental and theoretical investigations for the inelastic behavior of the micro-/nanostructured thin films. [DOI: 10.1115/1.3183774]Öğe Thermodynamically consistent coupled viscoplastic damage model for perforation and penetration in metal matrix composite materials(Elsevier Sci Ltd, 2009) Voyiadjis, George Z.; Deliktas, Babur; Palazotto, Anthony N.Accurate modeling and efficient analysis of the metal matrix composite materials failure mechanism during high velocity impact conditions is still the ultimate goal for many researchers. The objective is to develop a micromechanical constitutive model that can effectively simulate the high impact damage problem of the metal matrix composite materials. Therefore in this paper, a multiscale micromechanical constitutive model that couples the anisotropic damage mechanism with the viscoplastic deformation is presented here as a solution to this situation. This coupled viscoplastic damage model is formulated based on thermodynamic laws. Nonlinear continuum mechanics is used for this heterogeneous media that assesses a strong coupling between viscoplasticity and anisotropic damage. It includes the strong directional effect of the fiber on the evolution of the back stress and the development of the viscoplastic strain in the material behavior for high velocity impact damage related problems. (C) 2009 Elsevier Ltd. All rights reserved.
