bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelling of Damage Evolution in Laminated Viscoelastic Composites
Authors: Allen D. H. , Lo D. C. , Zocher M. A.
Published: July 2008

A computational finite element model is developed herein for the purpose of predicting the evolution of delaminations in a laminated continuous fiber polymer matrix composite subjected to monotonically increasing loading. Because the temperature of interest is near the glass transition temperature of the matrix, the lamina are modelled as orthotropic linear viscoelastic materials. Crack growth is accommodated in the prediction via the incorporation of a cohesive zone model. Although the resulting algorithm is nonlinear and computationally intensive, it is demonstrated herein to be tenable for predicting delamination evolution. To illustrate this point, results are obtained for a quasi-isotropic [00,450,90°, -45° 12 laminate subjected to in-plane loading, as well as a crossply [9006/003], laminate subjected to out-of-plane loading. While results obtained herein are encouraging, they also indicate that further research is warranted to study potential anomalies.
A Damage Mechanics Treatment of Creep Failure in Rock Salt
Authors: Chan K. S. , Bodner S. R. , Fossum A. F. , Munson D. E.
Published: July 2008

Recent progress in the formulation of a constitutive model for describing coupled creep and damage development in rock salt is summarized. The constitutive model is based on the assumption that both dislocation slip and creep damage in the form of microcracks with possible wing-tips contribute to the macroscopic inelastic strain rate. The relevant kinetic equations, flow law, and damage evolution equation are presented. Evaluations are made of the coupled creep and fracture model against the measured creep response of clean salt from the Waste Isolation Pilot Plant (WIPP) site. In addition, the development of creep damage and the rupture of WIPP salt subjected to either triaxial compression or indirect tension have been analyzed to evaluate several creep rupture criteria.
An Elastoplastic Damage Model for Concrete Subjected to Sustained High Temperatures
Authors: Miao Shuke , Wang Ming L.
Published: July 2008

Quasi-brittle materials, such as concrete, exhibit plastic deformation and stiffness degradation under applied loading conditions. To predict the mechanical behavior of quasi-brittle materials, it may be necessary to combine elastoplasticity theories and continuum damage mechanics. An elastoplastic damage model is proposed within a framework that automatically meets the restrictions of thermodynamics. Both rate independent and rate dependent formulations are given. Specific evolution equations are given for a scalar isotropic assumption and results are compared with experimental data on concrete subjected to sustained high temperatures. The correlations between experiments and analytical solution in terms of plastic deformation and stiffness degradation are in good agreement.
On the Interface Debonding Models
Authors: Chaboche J. L. , Girard R. , Levasseur P.
Published: July 2008

Interface debonding models offer the possibility to simulate numerically the interface crack growth, and the eventual contact friction behaviour on the cracked area. The paper considers models in the framework of an Interface Damage Mechanics, relating displacement discontinuities across the interface to the corresponding tractions, through various constitutive and damage equations. The approach is based on the initial works by Needleman (1987) and Tvergaard (1990a, 1990b).

After a brief review of the existing models, two new versions are proposed and discussed in detail, with the purpose of simulating mixed mode progressive decohesion, followed by a contact/friction behaviour after complete separation, and to guarantee the continuity between the two phases. The first modified model considers coupling terms in the elastic behaviour with damage. The second one introduces the possibility for sliding effects, obeying the Coulomb criterion, during the decohesion phase.

The initial Tvergaard model and the second modified model are shown to be derivable from a state potential. Moreover, the thermodynamic framework is used to choose some characteristics of the Model II under a compressive behaviour. Under tension, the models reduce to the classical one. A three-dimensional generalization is given in the Appendix. In addition, the numerical simulation capabilities of the interface debonding models are illustrated by two examples: the fibre-matrix-interface decohesion in a Metal Matrix composite with a bridged matrix crack and the delamination of a DCB (Double Cantilever Beam) specimen.
Modelling of Damage Behaviour of a Short-Fiber Reinforced Composite Structure by the Finite Element Analysis Using a Micro-Macro Law
Authors: Guo Guang , Fitoussi Joseph , Baptiste Didier , Sicot Nathalie , Wolff Christian
Published: July 2008

The objective of this paper is composed of two parts: (1) presentation of the damageable elastic law for randomly reinforced composites based on the micro-macro relationship of Mori and Wakashima type (1990); and (2) calculation of composite structures by a finite element code, ABAQUS, using this damage model.

Several mechanical tests on the short-fiber reinforced composite, such as bending, torsion, and the combination of bending and torsion, have been performed experimentally and simulated. A good correlation is obtained between experiment and simulation.
Thermodynamical Modeling of Elastic-Plastic Damage and Experimental Validation of Damage Potential
Authors: Hayakawa K. , Murakami S.
Published: July 2008

Formulation of constitutive and damage evolution equations of elastic-plastic damage materials are first discussed. In order to facilitate the description of the experimental damage surface in axialtorsional stress space, Gibbs thermodynamic potential is employed. The damage surface is expressed as a function of the damage conjugate force which is easily related to stress tensor. Then, the resulting equations are applied to the experimental results of the spheroidized graphite cast iron. The change in elastic moduli due to damage development and the initial and the subsequent damage surface expressed in stress space are described well by the proposed theory. Finally, experimental verification of the existence of a damage potential and the corresponding normality law is performed. By noting that the normality law must be discussed in the space of damage conjugate, the direction of the damage rate vector on each loading path is identified from the change in the elastic modulus due to damage. The resulting direction of the damage rate at each loading path coincides sufficiently well with the normal of the damage surface, and hence, the proposed damage surface is certified to be identical to the damage potential.
Localized Fracture in Inelastic Polycrystalline Solids under Dynamic Loading Processes
Authors: Lodygowski Tomasz , Perzyna Piotr
Published: July 2008

The main objective of the paper is the investigation of adiabatic shear band localized fracture phenomenon in inelastic solids during dynamic loading processes. This kind of fracture can occur as a result of an adiabatic shear band localization generally attributed to a plastic instability implied by micro-damage and thermal softening during dynamic plastic flow processes.

The theory of thermoviscoplasticity is developed within a framework of the rate type covariance material structure with a finite set of internal state variables. The theory takes into consideration the effects of micro-damage mechanism and thermomechanical coupling. The micro-damage mechanism has been treated as a sequence of nucleation, growth, and coalesence of microcracks. The micro-damage kinetics interacts with thermal and load changes to make failure of solids a highly rate, temperature, and history dependent, nonlinear process. The dynamic failure criterion within localized shear band region is proposed. The relaxation time is used as a regularization parameter. By assuming that the relaxation time tends to zero, the rate independent micro-damage mechanism is considered.

Rate dependency (viscosity) allows the spatial differential operator in the governing equations to retain its ellipticity, and the initial-value problem is well posed. The viscoplastic regularization procedure assures the stable integration algorithm by using the finite element method. Particular attention is focused on the well-posedness of the evolution problem (the initial-boundary value problem), as well as on its numerical solutions. Convergence, consistency, and stability of the discretised problem are discussed. The Lax equivalence theorem is formulated and conditions under which this theorem is valid are examined.

Utilizing the finite element method and ABAQUS system for regularized elastoviscoplastic model, the numerical investigation of the three-dimensional dynamic adiabatic deformation in a particular body at nominal strain rates ranging from 10-1-104 s-1 is presented. Three particular examples have been considered; namely, a dynamic adiabatic process for a thin-walled steel tube and dynamic adiabatic and quasi-static processes for a thin steel plate. In each case, a thin shear band region of finite width which undergoes significant deformations and temperature rise has been determined. Its evolution until occurrence of final fracture has been simulated. Numerical results are compared with available experimental observation data.
Numerical Analysis of Interface Fracture in Concrete Using a Lattice-Type Fracture Model
Authors: van Mier J. G. M. , Vervuurt A.
Published: July 2008

A key parameter in (numerical) models for simulating fracture of concrete at the meso-level is the behaviour of the interfacial transition zone between aggregate and matrix. In normal concrete the interface is a very porous thin layer with relatively low strength and stiffness. However, more dense interfaces can be obtained when different types of aggregates are used, or by means of selected additives to the cement matrix. Using a lattice-type fracture model of concrete, the effect of the interface properties on the global stress-strain behaviour is studied. To this end, splitting type tests on single particle specimens are conducted. Different types of aggregates are included. From the analyses, the sensitivity of the crack patterns to variations in interface strength and stiffness was obtained. Comparison with experimental observations is quite favourable. Next to the single particle specimens, concrete plates were tested, containing broken aggregates of the same material as used in the single particle plates. The effect of the type and size of the aggregates on the fracture geometry (fractal dimension) is analyzed. The results indicate that improved mechanical performance of composites may be obtained by carefully engineering the interface, taking care that an optimal ratio of weak to strong microstructural elements is present.
Coupled Inelasticity and Damage Model for Metal Matrix Composites
Authors: Kawai M.
Published: July 2008

Multiaxial constitutive models to describe fully coupled inelastic deformation and damage of unidirectional fiber-reinforced metal matrix composites are developed from a phenomenological point of view. A damage-coupled kinematic hardening model for transversely isotropic materials is first formulated in the invariant form on the basis of the internal variable approach. An isotropic hardening model coupled with damage is then constructed by assuming a particular representation of the kinematic hardening variable. The extent of composite damage is described using a scalar variable, and a different damage growth rate depending on the orientation of composite is considered through a fourth-rank anisotropic tensor. The rates of damage and hardening are affected by the magnitudes of the relevant internal variables, respectively, so that a full coupling between inelasticity and damage can be predicted. An analytical expression for the creep rupture time under a constant stress condition can be derived from the isotropic hardening model. This expression of the creep life depends on both the time when the minimum creep rate is attained and the degree of damage at that instant. Numerical simulations have been performed to elucidate the characteristics of the present formulation.
Intergranular Fracture under Creep-Fatigue Interaction
Authors: Nielsen Helle S. , Tvergaard Viggo
Published: July 2008

For metals under cyclic loading at elevated temperatures the interaction of fatigue failure with intergranular creep failure is studied by a micromechanical model. Focus here is on a range of relatively slow cycling, where creep damage dominates, or rapid cycling after that some creep damage has developed. A numerical unit cell model analysis for a small region around a grain boundary cavity is used to study the acceleration of the damage process due to the development of fatigue damage in the ligament between neighbouring grain boundary cavities. The prediction of fatigue damage is incorporated in a cyclic plasticity model, and dislocation creep is represented as power-law creep. Also the effect of grain boundary diffusion on the cavity growth rate is accounted for in some cases.
A Thermomechanical Model for Airfield Concrete Pavement under Transient High Temperature Loadings
Authors: Ju J. W. , Zhang Y.
Published: July 2008

A simple one-dimensional thermo-micromechanical constitutive and damage model is proposed for airfield concrete pavement under rapid transient high temperature loadings due to vectored thrust. The subsurface temperature fields and time histories, the pore pressure histories, the thermoplastic stress-strain relations with void effects are investigated for both constant and varying thermal properties.

In particular, if the thermal properties are constant, the resulting temperature fields and histories can be solved numerically by utilizing the Duhamel's theorem. On the other hand, if the thermal properties are varying with rising temperatures, then the resulting temperature histories can be solved numerically by the explicit finite difference method. Furthermore, based on the ASME Steam Tables and the temperature histories, we are capable of predicting the pore pressure fields and histories. In accord with the theory of linear thermoelasticity, the strain histories and the stress-strain relations at different depths and time accounting for void effects are subsequently derived. Finally, a simple dynamic crack growth model due to high pore pressure is considered. A preliminary damage criterion to characterize the delamination of concrete pavement at various depths is also proposed.
Models for Compressible Elasto-Plasticity Based on Internal State Variables
Authors: Marin E. B. , McDowell D. L.
Published: July 2008

A general constitutive framework is presented for rate-independent porous plasticity. This framework is formulated using combined nonlinear isotropic-kinematic hardening rules along with void growth models which can be either implicitly embedded within a pressure dependent yield function or explicitly established based on a micromechanical model. A void nucleation criterion is also introduced in the framework. An associative flow rule is first set forth which implicitly introduces general void growth models. Correlations are presented with experimentally determined porosity evolution in circumferentially notched specimens subjected to various superimposed hydrostatic pressures. The framework is then generalized to include explicit void growth models with plastic flow which are either associative or non-associative with the yield function. Single element computations are carried out using these model frameworks to compare the behavior predicted by specific implicit and explicit void growth models.
Dynamics of Stochastic Damage Evolution
Authors: Silberschmidt Vadim V.
Published: July 2008

Characteristic features of damage accumulation under arbitrary stochastic conditions are studied in terms of continuum damage mechanics (CDM). A uniaxial tension case is chosen for a simplicity of discussion and clearness of results' interpretations. Modification for a kinetic equation of damage evolution for stochastic conditions is proposed. Numerical algorithms for three types of stochasticity—(a) additional noise (fluctuations in external load), (b) inner noise (as result of the non-uniform evolution of ensembles of micro-defects) and (c) combination of previous two factors—are obtained. Introduction of a local failure criterion via a threshold damage concentration allows the time-to-fracture distributions and their change with the noise intensity to be analyzed.
Damage Evolution and Damage Surface of Elastic-Plastic-Damage Materials under Multiaxial Loading
Authors: Murakami S. , Hayakawa K. , Liu Y.
Published: July 2008

Damage evolution and fundamental aspects of damage surface of a spheroidized graphite cast iron are observed. This paper is intended to discuss the applicability of the irreversible thermodynamics theory to the modeling of elastic-plastic-damage materials. By performing damage tests on the tubular specimens of the material under proportional and nonproportional loading, onset and development of material damage are first detected by measuring Acoustic Emission (AE) counts. The change in Young's modulus, Poisson's ratio and shear modulus is also observed in the tests to correlate them with the damage development in the material. The existence and the development of the damage surface, together with the condition of loading, unloading and neutral loading, are elucidated to facilitate the identification of the fundamental aspects of thermodynamics theory of the elastic-plastic-damage materials. The effects of the hydrostatic stress on the deformation and damage equations are also discussed.
Damage of Plain Concrete due to Thermal Incompatibility of its Phases
Authors: Sumarac D. , Krasulja M.
Published: July 2008

The present paper focuses on analytical and experimental investigation of plain concrete deterioration due to internal thermal stresses. Using fracture mechanics and micromechanics approach the crack size is defined as a function of temperature increase. In this type of material cracks appear at the interface of aggregate and cement paste because of the mismatch of the coefficients of thermal expansion. Assuming uniform distribution of cracks, analytical expression for Young's modulus as a function of temperature increase is obtained. Experimental investigation of two series of specimens confirms analytical predictions. This research challenges usually accepted assumptions that moderate temperature change doesn't affect mechanical properties of plain concrete.
Recovery and Healing of Damage in WIPP Salt
Authors: Chan K. S. , Bodner S. R. , Munson D. E.
Published: July 2008

Creep damage in rock salt, which generally manifests in the form of microcracks, can be recovered or healed when subjected to sufficiently high pressures and temperatures. In this paper, the phenomena of damage recovery and healing in rock salt are treated using the continuum damage mechanics approach. A healing term is formulated and incorporated into an existing constitutive model for describing coupled creep, fracture, and healing in rock salt. The constitutive model is then evaluated against experimental data of rock salt from the Waste Isolation Pilot Plant (WIPP) site. Satisfactory results are obtained between model calculations and experimental measurements of axial, lateral, and volumetric strains, as well as acoustic wave velocity and attenuation recovered during damage healing. Furthermore, analyses of experimental data revealed that healing anisotropies exist in WIPP salt. Most, though not all, of the healing anisotropies can be modeled with an appropriate power-conjugate equivalent stress, kinetic equation, and evolution equation for damage healing based on a scalar damage variable. This work represents the first attempt to simulate healing of damaged intact WIPP salt; however, further evaluations and improvement of the model can be anticipated.
Surface Damage Modeling of Oxidized Metal Matrix Composite Laminates under Axial and Transverse Tension
Authors: Lagoudas Dimitris C. , Xu Shouze , Ma Xinzheng
Published: July 2008

The oxidation effects on surface crack development of oxidized Ti-15-3 and fl-21S Metal Matrix Composite (MMC) unidirectional laminates under applied axial or transverse tension loadings are investigated in the present study. An approximate mechanical model is established and a stress analysis of an oxidized laminate with surface cracks is performed. To simplify the analysis, motivated by experimental observations, a periodic distribution of surface cracks in the oxide layer is assumed. The critical value of the applied mechanical load, which is necessary for the information of new cracks as a function of crack density, is calculated for given oxide layer thickness. The effective stiffness reduction, caused by the formation of the surface cracks, is also evaluated. Finally, the influence of gradient effective properties of the metal matrix underneath the oxide layer, caused by the diffusion of oxygen, on the stress field is considered.
An Inadequacy in a Common Micromechanics Approach to Analysis of Damage Evolution in Composites
Authors: Talreja Ramesh , Akshantala Nagendra V.
Published: July 2008

This article examines a commonly used micromechanics approach to evolution of damage in composites. In this approach an initial crack pattern is assumed and on the basis of the local stress fields and a (brittle) fracture criterion, further evolution of the same crack pattern is determined. We question this assumed self-similarity in the damage mode by considering two alternative crack modes, one of which would result in deviation from the assumed crack pattern. Using a simple maximum stress criterion for formation of brittle cracks, it is found that the self-similarity breaks down before saturation of cracking in the assumed crack pattern. This seemingly inherent inadequacy in the damage evolution analysis is discussed.
Continuum Damage Mechanics Theory and Application-Part I: Theory
Authors: Wohua Zhang , Valliappan S.
Published: July 2008

A systematic formulation for isotropic and anisotropic elasto-plastic analysis of static and dynamic damage mechanics problems based on the principle of thermodynamics is presented in this article. These formulations are developed by the combination of concepts between the internal energy dissipation and the damage elasto-plasticity energy potential in terms of a set of internal state variables for explanation of microstructural changes in damaged materials. The main studies in this article are related to isotropic and anisotropic elasto-plastic damage mechanics problems, as well as dynamics of damaged materials and structures. The theory of damage mechanics has also been extended to study the behaviour of isotropic and anisotropic damaged materials subjected to dynamic loading.
Continuum Damage Mechanics Theory and Application-Part II: Application
Authors: Wohua Zhang , Valliappan S.
Published: July 2008

The theoretical formulations of continuum damage mechanics theory developed in Part I have been applied into numerical algorithms of the finite element technique for isotropic and anisotropic elasto-plastic analysis of static and dynamic damage mechanics problems. A statistics method to estimate isotropic and anisotropic damage state in materials has been taken into account of finite element analysis. Numerical examples pertaining to a footing on isotropic and anisotropic damaged ground and a thick walled cylinder subjected to internal pressure have been included to illustrate the applicability of the formulations postulated in this study.
A Coupled Viscoelasticity/Damage Model with Application to Swirl-Mat Composites
Authors: Abdel-Tawab Khaled , Weitsman Y. Jack
Published: July 2008

A comprehensive thermodynamics framework is proposed for modeling the continuum damage behavior of viscoelastic materials. The thermodynamic framework is formulated in a general form that accounts for the possibility of a permanent deformation component in the behavior. Therefore, the total strain is decomposed into viscoelastic and permanent components. The effect of damage on the constitutive behavior is introduced through direct coupling between damage and the internal state variables representing deformation of the undamaged (virgin) material. This approach accounts for time-dependent damage as well as damage-induced changes in material symmetry. The proposed viscoelastic constitutive model is formulated for the case of damage coupled with linear viscoelasticity and is applied to experimental data for swirl-mat polymeric composites.
Effect of the Opening/Closure of Microcracks on the Nonlinear Behavior of a 2D C-SiC Composite under Cyclic Loading
Authors: Morvan J. M. , Baste S.
Published: July 2008

The nonlinear mechanical behavior of a 2D ceramic matrix composite is investigated under cyclic loading by combined use of an ultrasonic method and strain measurements. This method allows the accurate determination of the loss of stiffness of the material under tensile stress and which part of the global strain is either elastic or inelastic. It appears that the loss of stiffness during monotonic loading is mainly due to transverse microcracks density and that the variation of the elastic response of the composite during cyclic loading is the result of the opening/closure of these cracks. In order to take this effect into account, an internal variable that represents the damage accumulated along the loading is introduced. By using the analytical expressions of the effective properties of a cracked solid, the nonlinear behavior of a 2D C-SiC is fully predicted under both monotonic and cyclic loading. The constitutive laws of the crack density and the opening/closure variable describe the three dimensional changes in elasticity under cyclic loading as well as the inelastic strains along the whole test.
A Statistical Model of Microvoids' Evolution in Rate Sensitive Ductile Materials-Solution for High Stress Triaxiality Cases
Authors: Li H. L. , Huang Z. P.
Published: July 2008

Statistical behavior ofthe microvoids' evolution process in ductile materials is investigated in this article with the emphasis on the discussion about the effect of strain rate-sensitivity of the matrix material. The viscoplastic constitutive relation of the matrix material is assumed to have the overstress form proposed by Perzyna. In the case of high stress triaxiality, the nucleation rate and the growth rate of microvoids are discussed. Then, based on the balance law of microvoids' number density, differential equations governing the evolution of microvoids are obtained and solved with the help of the two kinetic equations describing the nucleation rate and the void growth rate. Thus, the size distribution functions of microvoids are found from the theoretical analysis, and the effect of strain rate-sensitivity of the matrix on the evolution of microvoids is examined. The present theoretical prediction provides a reasonable explanation of the experimental phenomena observed by previous researchers.
Inelastic Effects of Microcracking: A New Continuum Damage Model
Authors: Cheng D. F. , Cox S. J. D. , Barr T. D.
Published: July 2008

Microcracking is an important mechanism of damage accumulation in stressed solids. Two contributions to deformation can be distinguished: modification of the bulk effective elastic behaviour as cracks are introduced; and accumulation of an inelastic offset for each crack on formation. Equivalent medium approximations consider the former. In this study we have developed a new formulation to model the latter. The change in strain energy at the instant of formation of each individual microcrack is used to calculate the incremental inelastic strain increase during monotonic loading. This leads to integral equations describing the damage state, in which stress and strain are functions of damage. The macroscopic behaviour is path dependent. We obtain an explicit form of the stress-strain relation as a function of both intrinsic and extrinsic variables (damage and loading history) in order to test the model using experimental data for uniaxial tests on rock samples. Quantitative damage estimation is made through monitoring acoustic emissions emitted as microcracks form, which are related to crack dimensions through an independently determined scaling relation. The new model successfully reproduces the experimental stress-strain behaviour through failure, and also allows secondary effects, such as residual strain or hysteresis to be modelled, which is not possible with the equivalent elastic medium approach. The results suggest that (a) the incremental damage approach is needed to fully model behaviour in compression; (b) inelastic effects arising from cracking must be considered as well as modifications to the sample stiffness; and (c) acoustic emission parameters can be used in quantitative modelling of inelastic behaviour of cracked solids.
On a Damaged Hyperelastic Medium: Mullins Effect with Irreversible Strain
Authors: Andrieux F. , Saanouni K.
Published: July 2008

A theoretical model is proposed to describe the damaged hyperelastic behavior of filled rubber-like solids. Special emphasis is laid on two specific phenomena: the Mullins effect (i.e., strain induced stress softening) and irreversible (i.e., permanent) strain. Often observed in the behavior of the hyperelastic rubber-like materials, these two phenomena are supposed to be exclusively induced (and then governed) by isotropic damage (cavities and holes) initiation and growth. Using the classical Continuum Damage Mechanics (CDM) together with the generalized normality rule, a constitutive model is proposed to account for these two effects of damage. After a choice of constitutive functions, the obtained model is applied to four different loading paths classically studied in the case of this type of filled elastomeric material.
Aspects of Testpiece Design Responsible for Errors in Cyclic Plasticity Experiments
Authors: Lin J. , Dunne F. P. E. , Hayhurst D. R.
Published: July 2008

A uniaxial cyclic plasticity copper testpiece has been simulated, using the Continuum Damage Mechanics based finite element programme Damage XX, under conditions of cyclic plasticity at temperatures of 20°C and 500°C. The results obtained have been compared with the results of experiments carried out under the same conditions.

It has been shown that inhomogeneous fields of strain and stress are generated in the gauge length region of the testpiece, which result from standard cyclic plasticity testpiece features, namely, the blend radius linking testpiece shank and gauge length, and the extensometer ridges. The influence of these features on the inhomogeneity is exacerbated by the short gauge length which is necessary in cyclic plasticity testpieces to prevent buckling.

At the temperatures of 20°C and 500°C, cracks were predicted to occur, and found experimentally, at the extensometer ridges. Testpiece failure at 20°C occurred outside the gauge length; and at 500°C failure took place from cracks growing within the gauge length region. The formation and growth of the latter gauge length crack was accurately predicted.

The results obtained clearly demonstrate the dependence of cyclic plasticity data on testpiece design and suggest that the range of scatter observed experimentally in cyclic plasticity testing for nominally identical testing conditions may result from variations in testpiece design. This is of some concern, and the establishment of a methodology which overcomes these problems will be the subject of a future paper.
Effective Elastic Compliances and Engineering Constants for Damaged Isotropic Solids
Authors: Lee Usik
Published: July 2008

In continuum damage mechanics, damaged solids have been represented by the effective elastic stiffness into which local damages are smoothly smeared. In parallel to the effective elastic stiffness representation, damaged solids may be represented in terms of effective elastic compliance. It then becomes easier to derive the effective engineering constants (i.e., effective elastic moduli and Poisson's ratios) for damaged solids, all in closed forms, from the effective elastic compliance rather than from the effective elastic stiffness. Thus, in this paper, by using a continuum modeling approach previously developed by the author based on both the principle of strain energy equivalence and the equivalent elliptical micro-crack representation of a local damage, the effective elastic compliance and effective engineering constants are derived in closed forms for both damaged two- and three-dimensional isotropic solids. They are derived in terms of the undamaged (virgin) elastic properties and a scalar damage variable.
Constitutive Relations for Elastic Microcracked Bodies: From a Lattice Model to a Multifield Continuum Description
Authors: Mariano Paolo Maria , Trovalusci Patrizia
Published: July 2008

A continuum model suitable for the description of microcracked bodies is shown. The influence of microcracks on the mechanical behavior of the body is estimated through a microstructural field added to the displacement one. This field represents the perturbation to the regular displacement field due to the presence of microcracks. It is an observable quantity; its rate must satisfy appropriate balance equations. The problem of deriving constitutive relations for such a model at least in the linear elastic case is dealt with. Constitutive equations are deduced from a lattice model using an integral identification procedure based on the equivalence in terms of virtual work, without resorting to limit processes. The discrete model considered is made of two superposed lattices; the first one is constituted of material points connected by elastic links; the second one is made of empty closed shells interacting between themselves and with the first lattice. As sample test, a one-dimensional problem is shown.
Crack Opening Displacement and the Associated Response of Laminates with Varying Constraints
Authors: Varna Janis , Akshantala Nagendra V. , Talreja Ramesh
Published: July 2008

This paper reports an experimental study of the opening displacement behavior of 90-ply cracks in a class of composite laminates where the constraint to cracking is varied systematically. The study is motivated by a synergistic approach to damage mechanics proposed by Talreja (1996) where the crack opening displacement forms a key micromechanically determined parameter in a continuum damage formulation of the composite response. It is found that although the opening displacement shows scatter from point to point along a crack length, its average over the crack length forms a robust parameter. This parameter, when introduced as the damage influence coefficient in a continuum description of damage, allows satisfactory predictions of the elastic moduli changes due to transverse cracks in the considered class of laminates with varying constraints.
On Homogeneous and Localized Deformation in Water-Infiltrated Soils
Authors: Pietruszczak S.
Published: July 2008

This paper addresses the issue of the mechanical behaviour of fluid-infiltrated soils in the presence of localized deformation. The formulation of the problem is based on an averaging procedure and the conditions of full as well as partial saturation are considered. The case when both constituents (i.e., the intact and shear band material) respond in undrained manner is examined first. Later, the problem is reformulated by allowing for an exchange of fluids in constituents due to dissipation of the excess pore pressure gradient. The formulation is applied to study the influence of anisotropy on the onset of localized deformation and to investigate the sensitivity of mechanical characteristics to the initial degree of saturation.
Damage Mechanics and Fatigue Life Assessment of Composite Materials
Authors: Talreja Ramesh
Published: July 2008

The present state of fatigue life assessment of composite materials is largely empirical. This paper discusses how the discipline of damage mechanics can elevate this status to a mechanisms-based methodology. Specific topics addressed are mechanisms of fatigue damage, characterization of damage and its evolution, criticality of damage and prediction of fatigue life. Data and analysis results for cross ply laminates are used to illustrate and discuss the topics.
Constitutive Modeling of Material Damage for Fatigue Failure Prediction
Authors: Chow C. L. , Wei Y.
Published: July 2008

This paper presents a constitutive modeling of material damage capable of characterizing fatigue damage and plastic damage for rate-independent materials under multiaxial loading with the theory of damage mechanics. First, an internal state variable known as the damage variable is introduced with two scalars to characterize material degradation due to the change of material microstructures under fatigue. Then, depending upon the level of applied stress in a material element, the damage accumulation is postulated to undergo two processes, the accumulation of fatigue damage and plastic damage. Two surfaces, fatigue damage surface and plastic damage surface, are proposed to quantify fatigue damage initiation and plastic damage initiation. The damage-coupled constitutive equations of elasticity and plasticity plus two corresponding damage evolution equations are formulated based on the irreversible thermodynamics. Finally, a failure criterion based on the overall damage accumulation is postulated to govern crack initiation and propagation in material elements.

A test program to determine the necessary material parameters is also provided. These parameters are considered intrinsic material properties required as the finite element input for predicting damage accumulation under fatigue loading. As an example, experiments are carried out to measure a set of damage parameters for aluminum alloy 2024-T3. The measured parameters are then used to demonstrate systematically how the damage parameters can be effectively evaluated.
Damage Mechanics and Metal Fatigue: A Discriminating Perspective
Authors: McDowell D. L.
Published: July 2008

The effects of microstructure on the development and growth of small fatigue cracks in structural components play a strong role in dictating the class of mechanics approaches which are most viable in their treatment. The development of multiple small crack embryos associated with distributed cyclic plastic strain processes motivates a Continuum Damage Mechanics (CDM) description rather than a fracture mechanics treatment. The influence of material heterogeneity and the proximity to the free surface on the fatigue process in wrought alloys is significant and must be addressed in CDM. Sequential processes of early crack growth past heterogeneously distributed microstructural barriers, small crack coalescence and propagation of a dominant flow must be addressed through a consistent scheme which identifies damage driving forces as well as accessible propagation paths through the microstructure. Size effects in fatigue are related to the accessibility of low resistance microstructure paths to formation and progression of fatigue damage and the likelihood of coalescence of distributed crack systems. In this paper, we outline some of the key physical aspects of the problem and then discuss considerations for the application of damage mechanics concepts to the problem of fatigue crack nucleation and small crack growth.
Assessing Machine Volition: An Ordinal Scale for Rating Artificial and Natural Systems
Authors: Chadderdon George L.
Published: July 2008

Volition, although often poorly defined, is a concept of interest and utility to both philosophers and researchers in artificial intelligence. In this article, a definition of volition is proposed and a functionally defined, physically grounded ordinal scale and a procedure by which volition might be measured are put forward: a type of Turing test for volition, but motivated by an explicit analysis of the concept being tested and providing results that are graded, rather than Boolean, so that candidate systems may be ranked according to their degree of volitional endowment. It is proposed that volition is a functional, aggregate property of certain physical systems and it is defined as the capacity for adaptive decision-making. The scale, similar in scope to Daniel Dennett's Kinds of Minds scale, is then outlined, as well as a set of progressive “litmus tests” for determining where a candidate system falls on the scale. Such a formulation may be useful for understanding volition and assessing the progress made in engineering intelligent, autonomous artificial organisms.
Extremism Propagation in Social Networks with Hubs
Authors: Franks Daniel W. , Noble Jason , Kaufmann Peter , Stagl Sigrid
Published: July 2008

One aspect of opinion change that has been of academic interest is the impact of people with extreme opinions (extremists) on opinion dynamics. An agent-based model has been used to study the role of small-world social network topologies on general opinion change in the presence of extremists. It has been found that opinion convergence to a single extreme occurs only when the average number of network connections for each individual is extremely high. Here, we extend the model to examine the effect of positively skewed degree distributions, in addition to small-world structures, on the types of opinion convergence that occur in the presence of extremists. We also examine what happens when extremist opinions are located on the well-connected nodes (hubs) created by the positively skewed distribution. We find that a positively skewed network topology encourages opinion convergence on a single extreme under a wider range of conditions than topologies whose degree distributions were not skewed. The importance of social position for social influence is highlighted by the result that, when positive extremists are placed on hubs, all population convergence is to the positive extreme even when there are twice as many negative extremists. Thus, our results have shown the importance of considering a positively skewed degree distribution, and in particular network hubs and social position, when examining extremist transmission.
Violence Against Women With HIV Risk and Recent Criminal Justice System Involvement
Authors: Weir Brian W. , Bard Ronda S. , O'Brien Kerth , Casciato Carol J. , Stark Michael J.
Published: July 2008

This research note examines the prevalence and correlates of intimate partner violence (IPV) and other violence (OV) among women ( N = 529) at risk for HIV and with histories of criminal justice system involvement. The 3-month prevalences of IPV and OV were 31.2% and 18.7%, respectively. IPV was associated with having a current main partner, substance use, sexual risk behavior, trading sex, anxiety, depression, and lower self-esteem. OV was associated with no current employment or schooling, unstable housing, drug use, trading sex, anxiety, depression, and lower self-esteem. The high prevalence of violence demonstrates the need for intervention in this population; the correlates show that effective interventions must address the complex issues in these women's lives.
Physical and Sexual Covictimization From Dating Partners
Authors: Katz Jennifer , Moore Jessica , May Pamela
Published: July 2008

Covictimized women experience both physical and sexual forms of abuse. The purpose of the present research was to compare covictimized women to those who experienced physical violence only or unwanted sexual activity only from dating partners. Data were collected from two samples of female undergraduates in heterosexual relationships. Covictimization was associated with less general satisfaction (Study 1) and sexual satisfaction (Study 2), more arguing (Study 1) and verbal conflict (Study 2), and more partner psychological aggression (Studies 1 and 2). Overall, findings suggest that dating partner covictimization may be a distinct type of interpersonal abuse that warrants further research.
Coping as a Mediator and Moderator Between Intimate Partner Violence and Symptoms of Anxiety and Depression
Authors: Calvete Esther , Corral Susana , Estévez Ana
Published: July 2008

This study examines the role of coping as both a moderator and a mediator of the association between intimate partner violence and women's mental health. A sample of 298 women who had suffered physical aggression completed measures of physical and psychological abuse, coping responses, and symptoms of anxiety and depression. Tests of moderation consistently indicated that coping responses did not moderate the impact of intimate partner violence on symptoms of anxiety and depression, whereas tests of mediation demonstrated that disengagement coping mediated the impact of psychological abuse on distress. Thus, findings support the hypothesis that coping responses are influenced by violence itself and underline the dysfunctional nature of disengagement coping among victims.
The Benefits of a Culturally Specific Intimate Partner Violence Intervention for African American Survivors
Authors: Gillum Tameka L.
Published: July 2008

In light of evidence and theorization of culturally specific factors contributing to intimate partner violence (IPV) within African American relationships and the Eurocentric approach many mainstream agencies take to service delivery, researchers have indicated a need for culturally appropriate IPV interventions for African American survivors to adequately address the issue of IPV within this community. The purpose of the current study was to qualitatively investigate how helpful a culturally specific IPV program, which targets the African American community, has been to African American female survivors. Results suggest that this culturally specific agency is successfully meeting the needs of these survivors.
Using Therapeutic Sound With Progressive Audiologic Tinnitus Management
Authors: Henry James A. , Zaugg Tara L. , Myers Paula J. , Schechter Martin A.
Published: July 2008

Management of tinnitus generally involves educational counseling, stress reduction, and/or the use of therapeutic sound. This article focuses on therapeutic sound, which can involve three objectives: (a) producing a sense of relief from tinnitus-associated stress (using soothing sound); (b) passively diverting attention away from tinnitus by reducing contrast between tinnitus and the acoustic environment (using background sound); and (c) actively diverting attention away from tinnitus (using interesting sound). Each of these goals can be accomplished using three different types of sound—broadly categorized as environmental sound, music, and speech—resulting in nine combinations of uses of sound and types of sound to manage tinnitus. The authors explain the uses and types of sound, how they can be combined, and how the different combinations are used with Progressive Audiologic Tinnitus Management. They also describe how sound is used with other sound-based methods of tinnitus management (Tinnitus Masking, Tinnitus Retraining Therapy, and Neuromonics).
Factor Structure of the Restricted Academic Situation Scale
Authors: Karama Sherif , Ben Amor Leila , Grizenko Natalie , Ciampi Antonio , Mbekou Valentin , Ter-Stepanian Marina , Lageix Philippe , Baron Chantal , Schwartz George , Joober Ridha
Published: July 2008

Background: To study the factor structure of the Restricted Academic Situation Scale (RASS), a psychometric tool used to assess behavior in children with ADHD, 117 boys and 21 girls meeting Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) criteria for ADHD and aged between 6 and 12 years were recruited. Assessments were carried out before and 65 min after the administration of either a placebo or 0.25 mg/kg of methylphenidate. Placebo and methylphenidate were each administered according to a double-blind placebo-controlled crossover design. Results: Principal component analysis, followed by a confirmatory maximum likelihood factor analysis, revealed two main factors for the RASS, task disengagement (TD) and motor activation (MA). Only TD was inversely correlated with age, indicating that TD and MA may be differentially modulated during development. Conclusions: MA and TD are distinct traits of ADHD. It may be important to consider them separately when conducting studies on ADHD. (J. of Att. Dis. 2009; 12(5) 442-448)