Vassilis Panoskaltsis - Academia.edu (original) (raw)

Papers by Vassilis Panoskaltsis

COMPDYN Proceedings, Dec 31, 2022

Large scale landslides have been related to the dramatic decrease of the undrained shear strength... more Large scale landslides have been related to the dramatic decrease of the undrained shear strength of sands along the slip surface. This decrease of shear strength has been attributed to the crushing of sand grains. Landslides may appear to have a wide spectrum of speeds from extremely low (0.3 meters every 5 years) to moderately fast (1.5 meters per month), to extremely rapid. This paper studies the influence of the shear rate on grain crushing of sands, experimentally. It is reported in the literature that even significant changes in shear rate usually play an insignificant role in the shear stress paths of samples, as long as the tests are kept drained. Nevertheless, according to new studies when the changes in shear rates become very large the changes in grain crushing may be of importance. In order to study the effect of shear rate on grain crushing, a new series of tests was performed. The tests were conducted in the Bromhead Ring Shear device at the Soil Mechanics and Foundation Engineering Laboratory of the Department of Civil Engineering at the Demokritos University of Thrace. The specimens used were of the same geometry, from the same materials (with the same density). They were sheared under equal normal stresses at variable shear rates by taking full advantage of the Bromhead apparatus capabilities. These tests were stopped at increasing displacements up to the final point (1m). This way, the study of progress of the grain crushing phenomenon was possible for each speed as well as the comparison of the grain crushing phenomenon for variable speeds.

Landslides, Feb 1, 2021

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.

Acta Mechanica, Jan 5, 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.

COMPDYN Proceedings, 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.

Engineering Transactions, Jan 8, 2015

In this work we examine significant theoretical issues related to the constitutive modelling of a... more In this work we examine significant theoretical issues related to the constitutive modelling of a two-phase shape memory alloy which undergoes large deformations. For this purpose, we propose a new generalized plasticity based model. The model is based on a standard fractions approach and considers a local multiplicative decomposition of the deformation gradient into elastic and inelastic (transformation induced) parts, as its basic kinematic assumption. We also assess the ability of the model in simulating several patterns of the complex behavior of the material in question, by three representative numerical examples. These examples comprise a standard uniaxial tension problem, a torsion problem and an additional problem dealing with non-conventional pseudoelastic response.

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.

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.

This proceedings is a synthesis of the recent advances in pavement mechanics and will be benefici... more This proceedings is a synthesis of the recent advances in pavement mechanics and will be beneficial to engineers working with pavement analysis, design, and performance prediction.

Case Studies in Construction Materials, Jun 1, 2022

Uniaxial compression tests were carried out for 3D printed samples having various types of kinked... more Uniaxial compression tests were carried out for 3D printed samples having various types of kinked fissures by using the rock mechanics servo-controlled testing system. Photo-elastic technique is adopted to characterize and visualize the stress distribution and evolution of 3D printed models subjected to vertical compression. The stress field in the loading process can clearly be captured via a high-speed camera. The results showed that fringes around the kinked fissure tips formed a central symmetrical interference fringe pattern, and failure firstly occurred at interference fringe of highest order. Two failure types i.e. tip-cracking and non-tip-cracking are categorized on the basis of crack propagation pattern of 3D printed samples. Tensile crack propagation of wing cracks is the main form of failure of the antisymmetric kinked fissures, but the inclination of the branch fissures also played a key role on the location of initial fracture. The finite element method was applied to numerically simulate the process of crack propagation. The isochromatic fringe patterns are in good agreement with the experimental investigation. The current work gives an insight for implication of advanced technique to quantify and visualize the distribution of stress field, and provides further understanding of kinked fissure behavior at failure.

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

Springer eBooks, 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.

Acta Mechanica, Nov 19, 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.

EURODYN ..., 2020

The main objective of this research is to study the phenomenon of cracking in the vertical struct... more The main objective of this research is to study the phenomenon of cracking in the vertical structural elements of R/C constructions, in particular the columns and the walls, and more particularly in the extreme regions of the walls, namely the boundary columns. Various parameters of the phenomenon of cracking will be studied, e.g. load influence, tensile strain, etc. It has to be noted the fact that load application is a monotonic axial tensile loading that simulates the strain condition that takes place at the boundary edges of reinforced concrete seismic walls. Specifically, this type of loading simulates the tensile loading that takes place during the first semi-cycle of loading under seismic dynamic events. Experimental research takes place by the construction and use of a group of 4 experimental specimens subjected to different degrees of elongation. This test group examines the tensile parameter and how it affects the cracking. The test specimens in question are all reinforced with the same longitudinal reinforcement ratio (4.02%) and subjected to tensile degrees 10 , 20 , 30 and 50. Significant conclusions are reached on cracking, e.g. its extent, the size of the cracks, their positions, minimum crack width, maximum crack width, average crack width, number of cracks, etc.

Engineering mechanics, 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 ...

Shape Memory Alloys - Processing, Characterization and Applications, 2013

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.

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.

COMPDYN Proceedings, Dec 31, 2022

Large scale landslides have been related to the dramatic decrease of the undrained shear strength... more Large scale landslides have been related to the dramatic decrease of the undrained shear strength of sands along the slip surface. This decrease of shear strength has been attributed to the crushing of sand grains. Landslides may appear to have a wide spectrum of speeds from extremely low (0.3 meters every 5 years) to moderately fast (1.5 meters per month), to extremely rapid. This paper studies the influence of the shear rate on grain crushing of sands, experimentally. It is reported in the literature that even significant changes in shear rate usually play an insignificant role in the shear stress paths of samples, as long as the tests are kept drained. Nevertheless, according to new studies when the changes in shear rates become very large the changes in grain crushing may be of importance. In order to study the effect of shear rate on grain crushing, a new series of tests was performed. The tests were conducted in the Bromhead Ring Shear device at the Soil Mechanics and Foundation Engineering Laboratory of the Department of Civil Engineering at the Demokritos University of Thrace. The specimens used were of the same geometry, from the same materials (with the same density). They were sheared under equal normal stresses at variable shear rates by taking full advantage of the Bromhead apparatus capabilities. These tests were stopped at increasing displacements up to the final point (1m). This way, the study of progress of the grain crushing phenomenon was possible for each speed as well as the comparison of the grain crushing phenomenon for variable speeds.

Landslides, Feb 1, 2021

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.

Acta Mechanica, Jan 5, 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.

COMPDYN Proceedings, 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.

Engineering Transactions, Jan 8, 2015

In this work we examine significant theoretical issues related to the constitutive modelling of a... more In this work we examine significant theoretical issues related to the constitutive modelling of a two-phase shape memory alloy which undergoes large deformations. For this purpose, we propose a new generalized plasticity based model. The model is based on a standard fractions approach and considers a local multiplicative decomposition of the deformation gradient into elastic and inelastic (transformation induced) parts, as its basic kinematic assumption. We also assess the ability of the model in simulating several patterns of the complex behavior of the material in question, by three representative numerical examples. These examples comprise a standard uniaxial tension problem, a torsion problem and an additional problem dealing with non-conventional pseudoelastic response.

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.

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.

This proceedings is a synthesis of the recent advances in pavement mechanics and will be benefici... more This proceedings is a synthesis of the recent advances in pavement mechanics and will be beneficial to engineers working with pavement analysis, design, and performance prediction.

Case Studies in Construction Materials, Jun 1, 2022

Uniaxial compression tests were carried out for 3D printed samples having various types of kinked... more Uniaxial compression tests were carried out for 3D printed samples having various types of kinked fissures by using the rock mechanics servo-controlled testing system. Photo-elastic technique is adopted to characterize and visualize the stress distribution and evolution of 3D printed models subjected to vertical compression. The stress field in the loading process can clearly be captured via a high-speed camera. The results showed that fringes around the kinked fissure tips formed a central symmetrical interference fringe pattern, and failure firstly occurred at interference fringe of highest order. Two failure types i.e. tip-cracking and non-tip-cracking are categorized on the basis of crack propagation pattern of 3D printed samples. Tensile crack propagation of wing cracks is the main form of failure of the antisymmetric kinked fissures, but the inclination of the branch fissures also played a key role on the location of initial fracture. The finite element method was applied to numerically simulate the process of crack propagation. The isochromatic fringe patterns are in good agreement with the experimental investigation. The current work gives an insight for implication of advanced technique to quantify and visualize the distribution of stress field, and provides further understanding of kinked fissure behavior at failure.

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

Springer eBooks, 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.

Acta Mechanica, Nov 19, 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.

EURODYN ..., 2020

The main objective of this research is to study the phenomenon of cracking in the vertical struct... more The main objective of this research is to study the phenomenon of cracking in the vertical structural elements of R/C constructions, in particular the columns and the walls, and more particularly in the extreme regions of the walls, namely the boundary columns. Various parameters of the phenomenon of cracking will be studied, e.g. load influence, tensile strain, etc. It has to be noted the fact that load application is a monotonic axial tensile loading that simulates the strain condition that takes place at the boundary edges of reinforced concrete seismic walls. Specifically, this type of loading simulates the tensile loading that takes place during the first semi-cycle of loading under seismic dynamic events. Experimental research takes place by the construction and use of a group of 4 experimental specimens subjected to different degrees of elongation. This test group examines the tensile parameter and how it affects the cracking. The test specimens in question are all reinforced with the same longitudinal reinforcement ratio (4.02%) and subjected to tensile degrees 10 , 20 , 30 and 50. Significant conclusions are reached on cracking, e.g. its extent, the size of the cracks, their positions, minimum crack width, maximum crack width, average crack width, number of cracks, etc.

Engineering mechanics, 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 ...

Shape Memory Alloys - Processing, Characterization and Applications, 2013

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.

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.