Vassilis Panoskaltsis | Democritus University of Thrace (original) (raw)

Conference Presentations by Vassilis Panoskaltsis

Research paper thumbnail of International Conference ENERGY in BUILDINGS EinB2018 "ENERGY EFFICIENCY IN HERITAGE BUILDINGS"

Understanding the energy performance of the current stock of buildings is an important step towar... more Understanding the energy performance of the current stock of buildings is an important step toward reaching the Zero Energy Buildings goal.
Buildings use almost 40% of the world’s energy. Old buildings have a specific interest, because when they are restored and reused considerable changes take place concerning their energy performance, because the building envelope is chanced due to restoration interventions and additionally, the use of the building is chanced.
Research shows that there is a great potential for environmental benefit of building reuse. When comparing buildings of equivalent size and function, reusing and retrofitting existing buildings with an average level of energy performance almost always offers environmental savings over demolition and new construction.

Research paper thumbnail of International Conference PRE-XIV Protection and Restoration of the Environment "IMPACT OF THE TUMULUS ON THE STABILITY OF MICROCLIMATE IN UNDERGROUND HERITAGE STRUCTURES"

The tumuli over tombs in north Greece named “Macedonian tombs” were constructed in ancient times ... more The tumuli over tombs in north Greece named “Macedonian tombs” were constructed in ancient times as “far seen signs” of important persons’ burials. Structurally, the tumulus is a big mass of artificial earth, covering monumental tombs which are dated between the 4th and 2nd century B.C. Its construction is the artificial packing of different layers of earth, with different consistency. These layers created a perfect drainage system. This way, the rain water was directed to the periphery of the cone-shaped tumulus and not inside the tombs. Because of the thermal inertia of the surrounding soil, fluctuations of temperature were of less width inside the tombs than outside. The tombs were preserved under stable microclimatic conditions in a very good state.
This study shows that the tumulus is a very important technical achievement for its era, not only due to its great mass -sometimes 12 m. high, but due to its construction and the impact to the protection of the underneath tombs, which are significant heritage structures.

Papers by Vassilis Panoskaltsis

Research paper thumbnail of Shear Rate Effect on Grain Crushing of Sands. A Laboratory Study by the Broomhead Apparatus

COMPDYN Proceedings, Dec 31, 2022

Research paper thumbnail of Sliding element simulating the response of slip surfaces and its application for the prediction of earthquake-induced landslide movement using one-dimensional dynamic analyses

Landslides

Earthquake-induced landslides involve excessive movement of slopes, usually along slip surfaces. ... more Earthquake-induced landslides involve excessive movement of slopes, usually along slip surfaces. This seismic movement of slopes may depend crucially on (a) the soil response along the slip surface, which may include strain softening; (b) the rotation of the sliding mass with displacement towards a gentler configuration; and (c) the dynamic response of the soil profile above the underlying bedrock. Ordinary finite element methods cannot be applied to predict large localized movement along slip surfaces. Even though effects (a)–(c) above have been studied in the bibliography, a cost-effective method for simultaneous simulation to predict the seismic displacement along slip surfaces has not been found in the bibliography. The present work proposes such a cost-effective method. For this purpose, first a new sliding element is introduced which simulates effects (a) and (b) above. For effect (b), a new empirical expression is proposed and validated, while effect (a) is simulated by a previously proposed constitutive model. Then, this element replaces the slip-stick constant resistance element at a previously proposed one-dimensional non-linear dynamic model. A numerical solution of the new model is developed and applied at the well-documented Nikawa landslide. The application illustrated that the method is able to predict the displacement of the landslide, as well as the manner that (i) the stiffness of the soil profile, (ii) the shear stress–displacement response along the slip surface, and (iii) the rotation of the sliding mass affect this displacement.

Research paper thumbnail of Constructing Sustainable Shelters to Safeguard Monuments from Climate Change

ICSD 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Research paper thumbnail of Decompositions of the stress and the rate of deformation tensors for materials undergoing phase transformations

COUPLED V : proceedings of the V International Conference on Computational Methods for Coupled Problems in Science and Engineering :, 2013

An extension of the "Duhamel-Neumann hypothesis" for materials undergoing phase transformations a... more An extension of the "Duhamel-Neumann hypothesis" for materials undergoing phase transformations and for arbitrary magnitudes of strains and rotations is provided.

Research paper thumbnail of Mechanics of Shape Memory Alloy Materials – Constitutive Modeling and Numerical Implications

Shape Memory Alloys - Processing, Characterization and Applications, 2013

Research paper thumbnail of Analysis and Design of Staircases Against Seismic Loadings

Proceedings of the 4th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN 2013), 2014

In this work all types of staircases are studied with respect to: First, earthquake design requir... more In this work all types of staircases are studied with respect to: First, earthquake design requirements and second, their complex interactions with the multistory space frame in which they belong. In the first case the essential influence of the vertical component of the earthquake's acceleration on the structure's performance is examined. In the second case the staircases' behavior is studied, as well as the important role that they play as structural seismic connections, in the response and behavior of space structures. In particular, external staircases, which connect structurally independent multistory systems are studied. In addition, in this work, a multistory external staircase that connects buildings in an area of high seismicity is analyzed. Staircases that present special design challenges due to gravity and earthquake loadings are also investigated. These structures include staircases with a free landing as well as helical cases. This work, beyond its theoretical interest, may prove important to practicing engineers as well, since it contains (almost) all types of staircases.

Research paper thumbnail of Empirical Expression Predicting Seismic Displacement of Sandy Slopes in Greece in Terms of Soil Profile Type Using Non-Linear Dynamic Stick-Slip Analysis

XI International Conference on Structural Dynamics, 2020

Research paper thumbnail of Effect of Rate on Strength and Energy Dissipation of Concrete Materials

The effect of rate of loading on the entire deformation history of concrete materials, including ... more The effect of rate of loading on the entire deformation history of concrete materials, including plastic damage, is considered. Energy dissipation in the linear (viscoelastic) range is described. One-dimensional results are presented and comparison is made with experimental results in tension, at different rates, from quasistatic to impact.

Research paper thumbnail of Rate Dependent Plasticity and Damage for Concrete

Research paper thumbnail of A viscoelastic-plastic-damage model for concrete

Research paper thumbnail of Some Equivalences in the Theory of Linear Viscoelasticity and Their Implications in Modeling and Simulation

Recent Trends in Constitutive Modeling of Advanced Materials, 2000

A class of linear viscoelastic models is examined which accurately represent the response of many... more A class of linear viscoelastic models is examined which accurately represent the response of many materials in a wide frequency range. These include power law and logarithmic type models, such as those of fractional order, the Kuhn model of linear viscoelasticity and generalizations thereof. An equivalence between the fractional element and the generalized Kuhn model is established. A continuous representation of these models is naturally available. It is shown that a discrete rheological representation, in the form of series of springs and dashpots, is also possible. This allows an internal variable formulation, which establishes the thermodynamic admissibility of this class of models, in the sense that they satisfy the dissipation inequality. Introduction of appropriate state variables leads to convolution type equations, which, in the finite deformation case, retain much of the structure of linear viscoelasticity. Numerical implementation of the models is enhanced by the equivale...

Research paper thumbnail of Effect of Soil Profile Characteristics on the Seismic Sliding Displacement of Slopes

8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering Methods in Structural Dynamics and Earthquake Engineering, 2021

In earthquake analyses, usually the seismic displacement is estimated by the sliding-block model,... more In earthquake analyses, usually the seismic displacement is estimated by the sliding-block model, where a rigid block rests on an inclined plane and every time that the applied horizontal acceleration is larger than the critical horizontal acceleration value for relative motion, the block slides. This model is used for the prediction of permanent seismic movement of slopes along a predefined slip surface, by appropriately selecting the equivalent critical and applied acceleration values of the rigid block. However, the input seismic motion is usually specified at the underlying bedrock and the seismic displacement along a slip surface depends not only on the resistance along the slip surface, but also on the dynamic characteristics of the soil both above and below the slip surface. Based on the above, the present work investigates and proposes expressions relating the ratio of the seismic displacement of slopes along a slip surface and the corresponding displacement of the sliding-block model for similar applied acceleration and critical acceleration of the slip surface in terms of characteristics of the soil profile both above and below the slip surface. The proposed expressions were obtained based state-of-theart description of the dynamic response and results of previous dynamic seismic sliding displacement analyses. Parametric analyses using a recently developed numerical code were performed to calibrate and validate the proposed expressions. In these analyses, different acceleration histories measured at rock sites are applied below soil layers with a slip surface with varying critical acceleration value and dynamic properties both above and below the slip surface.

Research paper thumbnail of Critical Acceleration, Seismic Sliding Displacement and Smart Design of Cantilever Walls

8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering Methods in Structural Dynamics and Earthquake Engineering, 2021

Cantilever walls are a popular type of retaining system. The slip mode of failure of these retain... more Cantilever walls are a popular type of retaining system. The slip mode of failure of these retaining systems under seismic loading has recently been studied. Cantilever walls must be designed in a manner such as to minimize the seismic displacement. The "conventional" slidingblock model is the most commonly used simplified model predicting the seismic displacement of slopes for given earthquake motions. However, in dynamic analyses, usually the seismic motion is applied at the underlying bedrock and the seismic displacement along a slip surfaces depends not only on the seismic response of the slip surface, but also on the dynamic characteristics of the soil profile both above and below the slip surface. The present work proposes a cost-effective method predicting the seismic displacement of cantilever walls, and applies it for their smart design. In order to achieve the paper first proposes a method predicting the critical acceleration value of cantilever walls retaining systems by application of equations predicting the critical acceleration value for gravity walls for the slip mode of failure of these retaining systems. Then the paper proposes a method predicting the seismic displacement of cantilever walls considering the dynamic response of the underlying soil profile by applying a recently developed 1-D 2-body dynamic stick-slip method at the particular geometry of cantilever walls. Finally, based on the above, the paper proposes a cost-effective method for smart design of cantilever walls, and presents a typical example. The work illustrated that the de-amplification effect of the seismic response can be crucial if the soil underlying the wall is soft and deep and in this case a wall with a looser backfill may be preferable.

Research paper thumbnail of On infinitesimal and finite deformations in shape memory alloys

Acta Mechanica, 2018

Two material models for shape memory alloys are developed and are evaluated by comparing and cont... more Two material models for shape memory alloys are developed and are evaluated by comparing and contrasting their predictions in three standard problems involving martensitic transformations. Both models are based on generalized plasticity and comprise a von Mises type of expression for the loading surfaces, a linear evolution law for the material martensite fraction, and a hyperelastic constitutive equation. The first model is an infinitesimal one, based on the usual additive decomposition of the small strain tensor into elastic and inelastic (transformation-induced) parts, while the second is a finite one based on the consistent use of the "physical" (intrinsic material) metric concept. This study reveals that in the first and second problem-uniaxial tension and simple shear-and under small and moderate levels of strain, both models predict almost identical response, while for higher levels of strain the models still predict comparable response, even though their basic kinematical assumptions differ vastly. The third problem, comprising infinitesimal shear with finite rotation, is considered next. In this case, it is shown that while the finite model yields the physically correct response, the infinitesimal model yields completely erroneous results.

Research paper thumbnail of Material covariant constitutive laws for continua with internal structure

Acta Mechanica, 2015

In this work, we study the transformation properties of the local form of the material (referenti... more In this work, we study the transformation properties of the local form of the material (referential) balance of energy equation under the superposition of arbitrary material diffeomorphisms. For this purpose, the tensor analysis on manifolds is utilized. We show that the material balance of energy equation, in general, cannot be invariant; in fact an extra term appears in the transformed balance of energy equation, which is directly related to the work performed by the configurational stresses. By making the fundamental assumption that the body and the ambient space manifolds are always related in the course of deformation and by utilizing the metric concept, we determine this extra term. Building on this, we derive several constitutive equations for the material stress tensor. The compatibility of these constitutive equations with the second law of thermodynamics is evaluated. Finally, we postulate that the material balance of energy equation is covariant, and we study this case in detail, as well.

Research paper thumbnail of Integration Algorithm for Frictional Materials Including Plasticity, Damage and Rate Effects

Anisotropy and Localization of Plastic Deformation, 1991

A new unified model for the description of frictional materials was developed [1], whose basic id... more A new unified model for the description of frictional materials was developed [1], whose basic idea is to fully describe the viscoelastic-plastic-damage behavior of frictional materials, with a representation of the rate effects on the entire stress-strain curve. This paper presents a time-integration algorithm for the model in the case of biaxial compression.

Research paper thumbnail of Erratum to: Invariance in non-isothermal generalized plasticity

Research paper thumbnail of Gradient Dependent Fatigue Limit Criterion

High-Cycle Metal Fatigue, 1999

Experimental evidence has shown that the fatigue limit of metallic cylindrical specimens in fully... more Experimental evidence has shown that the fatigue limit of metallic cylindrical specimens in fully reversed bending is significantly higher than the respective limit in fully reversed tension-compression. The higher values of the bending fatigue limits observed have to be attributed to the benign influence of the gradient of the bending normal stress on the fatigue strength of the metal. Although many approaches for modelling the gradient effect under uniaxial normal cyclic stress have already been tried, attempts to model the very same problem under multiaxial cyclic stress systems are scarce. The present paper starts re-analyzing existing experimental results under cyclic normal stress (i.e. bending, tension-compression) and under cyclic shear stress (i.e. torsion). This closer examination shows that, although the fatigue srength at very high lives is strongly affected by the gradient of the normal stress in bending tests, it remains insensitive to variations of the gradient of the shear stress in torsion tests. Based on these observations, a gradient dependent multiaxial high-cycle fatigue criterion function of the stress invariants is formulated.

Research paper thumbnail of International Conference ENERGY in BUILDINGS EinB2018 "ENERGY EFFICIENCY IN HERITAGE BUILDINGS"

Understanding the energy performance of the current stock of buildings is an important step towar... more Understanding the energy performance of the current stock of buildings is an important step toward reaching the Zero Energy Buildings goal.
Buildings use almost 40% of the world’s energy. Old buildings have a specific interest, because when they are restored and reused considerable changes take place concerning their energy performance, because the building envelope is chanced due to restoration interventions and additionally, the use of the building is chanced.
Research shows that there is a great potential for environmental benefit of building reuse. When comparing buildings of equivalent size and function, reusing and retrofitting existing buildings with an average level of energy performance almost always offers environmental savings over demolition and new construction.

Research paper thumbnail of International Conference PRE-XIV Protection and Restoration of the Environment "IMPACT OF THE TUMULUS ON THE STABILITY OF MICROCLIMATE IN UNDERGROUND HERITAGE STRUCTURES"

The tumuli over tombs in north Greece named “Macedonian tombs” were constructed in ancient times ... more The tumuli over tombs in north Greece named “Macedonian tombs” were constructed in ancient times as “far seen signs” of important persons’ burials. Structurally, the tumulus is a big mass of artificial earth, covering monumental tombs which are dated between the 4th and 2nd century B.C. Its construction is the artificial packing of different layers of earth, with different consistency. These layers created a perfect drainage system. This way, the rain water was directed to the periphery of the cone-shaped tumulus and not inside the tombs. Because of the thermal inertia of the surrounding soil, fluctuations of temperature were of less width inside the tombs than outside. The tombs were preserved under stable microclimatic conditions in a very good state.
This study shows that the tumulus is a very important technical achievement for its era, not only due to its great mass -sometimes 12 m. high, but due to its construction and the impact to the protection of the underneath tombs, which are significant heritage structures.

Research paper thumbnail of Shear Rate Effect on Grain Crushing of Sands. A Laboratory Study by the Broomhead Apparatus

COMPDYN Proceedings, Dec 31, 2022

Research paper thumbnail of Sliding element simulating the response of slip surfaces and its application for the prediction of earthquake-induced landslide movement using one-dimensional dynamic analyses

Landslides

Earthquake-induced landslides involve excessive movement of slopes, usually along slip surfaces. ... more Earthquake-induced landslides involve excessive movement of slopes, usually along slip surfaces. This seismic movement of slopes may depend crucially on (a) the soil response along the slip surface, which may include strain softening; (b) the rotation of the sliding mass with displacement towards a gentler configuration; and (c) the dynamic response of the soil profile above the underlying bedrock. Ordinary finite element methods cannot be applied to predict large localized movement along slip surfaces. Even though effects (a)–(c) above have been studied in the bibliography, a cost-effective method for simultaneous simulation to predict the seismic displacement along slip surfaces has not been found in the bibliography. The present work proposes such a cost-effective method. For this purpose, first a new sliding element is introduced which simulates effects (a) and (b) above. For effect (b), a new empirical expression is proposed and validated, while effect (a) is simulated by a previously proposed constitutive model. Then, this element replaces the slip-stick constant resistance element at a previously proposed one-dimensional non-linear dynamic model. A numerical solution of the new model is developed and applied at the well-documented Nikawa landslide. The application illustrated that the method is able to predict the displacement of the landslide, as well as the manner that (i) the stiffness of the soil profile, (ii) the shear stress–displacement response along the slip surface, and (iii) the rotation of the sliding mass affect this displacement.

Research paper thumbnail of Constructing Sustainable Shelters to Safeguard Monuments from Climate Change

ICSD 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Research paper thumbnail of Decompositions of the stress and the rate of deformation tensors for materials undergoing phase transformations

COUPLED V : proceedings of the V International Conference on Computational Methods for Coupled Problems in Science and Engineering :, 2013

An extension of the "Duhamel-Neumann hypothesis" for materials undergoing phase transformations a... more An extension of the "Duhamel-Neumann hypothesis" for materials undergoing phase transformations and for arbitrary magnitudes of strains and rotations is provided.

Research paper thumbnail of Mechanics of Shape Memory Alloy Materials – Constitutive Modeling and Numerical Implications

Shape Memory Alloys - Processing, Characterization and Applications, 2013

Research paper thumbnail of Analysis and Design of Staircases Against Seismic Loadings

Proceedings of the 4th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN 2013), 2014

In this work all types of staircases are studied with respect to: First, earthquake design requir... more In this work all types of staircases are studied with respect to: First, earthquake design requirements and second, their complex interactions with the multistory space frame in which they belong. In the first case the essential influence of the vertical component of the earthquake's acceleration on the structure's performance is examined. In the second case the staircases' behavior is studied, as well as the important role that they play as structural seismic connections, in the response and behavior of space structures. In particular, external staircases, which connect structurally independent multistory systems are studied. In addition, in this work, a multistory external staircase that connects buildings in an area of high seismicity is analyzed. Staircases that present special design challenges due to gravity and earthquake loadings are also investigated. These structures include staircases with a free landing as well as helical cases. This work, beyond its theoretical interest, may prove important to practicing engineers as well, since it contains (almost) all types of staircases.

Research paper thumbnail of Empirical Expression Predicting Seismic Displacement of Sandy Slopes in Greece in Terms of Soil Profile Type Using Non-Linear Dynamic Stick-Slip Analysis

XI International Conference on Structural Dynamics, 2020

Research paper thumbnail of Effect of Rate on Strength and Energy Dissipation of Concrete Materials

The effect of rate of loading on the entire deformation history of concrete materials, including ... more The effect of rate of loading on the entire deformation history of concrete materials, including plastic damage, is considered. Energy dissipation in the linear (viscoelastic) range is described. One-dimensional results are presented and comparison is made with experimental results in tension, at different rates, from quasistatic to impact.

Research paper thumbnail of Rate Dependent Plasticity and Damage for Concrete

Research paper thumbnail of A viscoelastic-plastic-damage model for concrete

Research paper thumbnail of Some Equivalences in the Theory of Linear Viscoelasticity and Their Implications in Modeling and Simulation

Recent Trends in Constitutive Modeling of Advanced Materials, 2000

A class of linear viscoelastic models is examined which accurately represent the response of many... more A class of linear viscoelastic models is examined which accurately represent the response of many materials in a wide frequency range. These include power law and logarithmic type models, such as those of fractional order, the Kuhn model of linear viscoelasticity and generalizations thereof. An equivalence between the fractional element and the generalized Kuhn model is established. A continuous representation of these models is naturally available. It is shown that a discrete rheological representation, in the form of series of springs and dashpots, is also possible. This allows an internal variable formulation, which establishes the thermodynamic admissibility of this class of models, in the sense that they satisfy the dissipation inequality. Introduction of appropriate state variables leads to convolution type equations, which, in the finite deformation case, retain much of the structure of linear viscoelasticity. Numerical implementation of the models is enhanced by the equivale...

Research paper thumbnail of Effect of Soil Profile Characteristics on the Seismic Sliding Displacement of Slopes

8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering Methods in Structural Dynamics and Earthquake Engineering, 2021

In earthquake analyses, usually the seismic displacement is estimated by the sliding-block model,... more In earthquake analyses, usually the seismic displacement is estimated by the sliding-block model, where a rigid block rests on an inclined plane and every time that the applied horizontal acceleration is larger than the critical horizontal acceleration value for relative motion, the block slides. This model is used for the prediction of permanent seismic movement of slopes along a predefined slip surface, by appropriately selecting the equivalent critical and applied acceleration values of the rigid block. However, the input seismic motion is usually specified at the underlying bedrock and the seismic displacement along a slip surface depends not only on the resistance along the slip surface, but also on the dynamic characteristics of the soil both above and below the slip surface. Based on the above, the present work investigates and proposes expressions relating the ratio of the seismic displacement of slopes along a slip surface and the corresponding displacement of the sliding-block model for similar applied acceleration and critical acceleration of the slip surface in terms of characteristics of the soil profile both above and below the slip surface. The proposed expressions were obtained based state-of-theart description of the dynamic response and results of previous dynamic seismic sliding displacement analyses. Parametric analyses using a recently developed numerical code were performed to calibrate and validate the proposed expressions. In these analyses, different acceleration histories measured at rock sites are applied below soil layers with a slip surface with varying critical acceleration value and dynamic properties both above and below the slip surface.

Research paper thumbnail of Critical Acceleration, Seismic Sliding Displacement and Smart Design of Cantilever Walls

8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering Methods in Structural Dynamics and Earthquake Engineering, 2021

Cantilever walls are a popular type of retaining system. The slip mode of failure of these retain... more Cantilever walls are a popular type of retaining system. The slip mode of failure of these retaining systems under seismic loading has recently been studied. Cantilever walls must be designed in a manner such as to minimize the seismic displacement. The "conventional" slidingblock model is the most commonly used simplified model predicting the seismic displacement of slopes for given earthquake motions. However, in dynamic analyses, usually the seismic motion is applied at the underlying bedrock and the seismic displacement along a slip surfaces depends not only on the seismic response of the slip surface, but also on the dynamic characteristics of the soil profile both above and below the slip surface. The present work proposes a cost-effective method predicting the seismic displacement of cantilever walls, and applies it for their smart design. In order to achieve the paper first proposes a method predicting the critical acceleration value of cantilever walls retaining systems by application of equations predicting the critical acceleration value for gravity walls for the slip mode of failure of these retaining systems. Then the paper proposes a method predicting the seismic displacement of cantilever walls considering the dynamic response of the underlying soil profile by applying a recently developed 1-D 2-body dynamic stick-slip method at the particular geometry of cantilever walls. Finally, based on the above, the paper proposes a cost-effective method for smart design of cantilever walls, and presents a typical example. The work illustrated that the de-amplification effect of the seismic response can be crucial if the soil underlying the wall is soft and deep and in this case a wall with a looser backfill may be preferable.

Research paper thumbnail of On infinitesimal and finite deformations in shape memory alloys

Acta Mechanica, 2018

Two material models for shape memory alloys are developed and are evaluated by comparing and cont... more Two material models for shape memory alloys are developed and are evaluated by comparing and contrasting their predictions in three standard problems involving martensitic transformations. Both models are based on generalized plasticity and comprise a von Mises type of expression for the loading surfaces, a linear evolution law for the material martensite fraction, and a hyperelastic constitutive equation. The first model is an infinitesimal one, based on the usual additive decomposition of the small strain tensor into elastic and inelastic (transformation-induced) parts, while the second is a finite one based on the consistent use of the "physical" (intrinsic material) metric concept. This study reveals that in the first and second problem-uniaxial tension and simple shear-and under small and moderate levels of strain, both models predict almost identical response, while for higher levels of strain the models still predict comparable response, even though their basic kinematical assumptions differ vastly. The third problem, comprising infinitesimal shear with finite rotation, is considered next. In this case, it is shown that while the finite model yields the physically correct response, the infinitesimal model yields completely erroneous results.

Research paper thumbnail of Material covariant constitutive laws for continua with internal structure

Acta Mechanica, 2015

In this work, we study the transformation properties of the local form of the material (referenti... more In this work, we study the transformation properties of the local form of the material (referential) balance of energy equation under the superposition of arbitrary material diffeomorphisms. For this purpose, the tensor analysis on manifolds is utilized. We show that the material balance of energy equation, in general, cannot be invariant; in fact an extra term appears in the transformed balance of energy equation, which is directly related to the work performed by the configurational stresses. By making the fundamental assumption that the body and the ambient space manifolds are always related in the course of deformation and by utilizing the metric concept, we determine this extra term. Building on this, we derive several constitutive equations for the material stress tensor. The compatibility of these constitutive equations with the second law of thermodynamics is evaluated. Finally, we postulate that the material balance of energy equation is covariant, and we study this case in detail, as well.

Research paper thumbnail of Integration Algorithm for Frictional Materials Including Plasticity, Damage and Rate Effects

Anisotropy and Localization of Plastic Deformation, 1991

A new unified model for the description of frictional materials was developed [1], whose basic id... more A new unified model for the description of frictional materials was developed [1], whose basic idea is to fully describe the viscoelastic-plastic-damage behavior of frictional materials, with a representation of the rate effects on the entire stress-strain curve. This paper presents a time-integration algorithm for the model in the case of biaxial compression.

Research paper thumbnail of Erratum to: Invariance in non-isothermal generalized plasticity

Research paper thumbnail of Gradient Dependent Fatigue Limit Criterion

High-Cycle Metal Fatigue, 1999

Experimental evidence has shown that the fatigue limit of metallic cylindrical specimens in fully... more Experimental evidence has shown that the fatigue limit of metallic cylindrical specimens in fully reversed bending is significantly higher than the respective limit in fully reversed tension-compression. The higher values of the bending fatigue limits observed have to be attributed to the benign influence of the gradient of the bending normal stress on the fatigue strength of the metal. Although many approaches for modelling the gradient effect under uniaxial normal cyclic stress have already been tried, attempts to model the very same problem under multiaxial cyclic stress systems are scarce. The present paper starts re-analyzing existing experimental results under cyclic normal stress (i.e. bending, tension-compression) and under cyclic shear stress (i.e. torsion). This closer examination shows that, although the fatigue srength at very high lives is strongly affected by the gradient of the normal stress in bending tests, it remains insensitive to variations of the gradient of the shear stress in torsion tests. Based on these observations, a gradient dependent multiaxial high-cycle fatigue criterion function of the stress invariants is formulated.

Research paper thumbnail of A geometric theory of plasticity

A new geometric formulation of rate-independent generalized plasticity is presented. The formulat... more A new geometric formulation of rate-independent generalized plasticity is presented. The formulation relies crucially on the consideration of the physical (referential) metric as a primary internal variable and does not invoke any decomposition of the kinematical quantities into elastic and plastic parts. On the basis of a purely geometrical argument the transition to classical plasticity is demonstrated. The covariant balance of energy is systematically employed for the derivation of the mechanical state equations. It is shown that unlike the case of finite elasticity, in finite plasticity, the covariant balance of energy does not yield the Doyle-Ericksen formula, unless a further assumption is made. As an application, a new material model is developed and is tested numerically for the solution of several problems of large scale plastic flow.

Research paper thumbnail of Energy dissipation in concrete materials due to viscoelastic and damage mechanisms

ASCE, NEW YORK, NY,(USA) …, 1996

Energy dissipation in concrete materials due to viscoelastic and damage mechanisms. Vassilis P Pa... more Energy dissipation in concrete materials due to viscoelastic and damage mechanisms. Vassilis P Panoskaltsis, Saurabh Bahuguna, Dimitris Soldatos ASCE, NEW YORK, NY,(USA). 2, 857-860, 1996. In this paper, the main mechanisms ...