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Papers by Dora Karagiozova
International Journal of Impact Engineering, 2021
Abstract The saturated impulse phenomenon, which may occur in a hollow beam with a circular cross... more Abstract The saturated impulse phenomenon, which may occur in a hollow beam with a circular cross-section, is examined when a rectangular pressure pulse loading is applied uniformly on the upper surface of the beam. A simplified 2DoF model of the dynamic response of a relatively long circular hollow section beam is developed to explore the role of the local and global deformations build up simultaneously during the beam response. In order to simplify the analytical formulation, the elastic deformations of the hollow beam are neglected, and a rigid-plastic method of analysis is applied. Finite element simulations using an elastic, perfectly plastic material model are carried out to verify the proposed analytical model and to reveal the influence of the elasticity on the beam response. The limit of the pressure pulse loading, beyond which saturated impulse is not defined for a hollow beam, is obtained numerically. The responses of hollow beams with lengths between 800 and 1200 mm, radius of 40 mm and several wall thicknesses are analysed. It is observed that the proposed 2DoF model can predict satisfactory the local and global saturated deflections while the saturated pressure pulse duration is underestimated due to the disregard of the stored elastic energy in the beam. It is suggested that the analytically predicted saturated impulse can be used as a lower bound estimate for the saturated impulse for a given beam geometry.
Journal of Aerospace Engineering
A generalised framework of the propagation of compaction waves in layered metal foam materials su... more A generalised framework of the propagation of compaction waves in layered metal foam materials subjected to impact loading is presented based on one-dimensional analytical models. The analysis is focused on layered configurations in which the individual materials exhibit strain hardening in the quasi-static regime of loading. No details of the cellular geometry are considered and it is assumed that the foam is a homogeneous material. A novel approach assuming no pre-defined value of the densification strain is applied. Two mechanisms of compaction are identified during the deformation of the layered materials. The first mechanism occurs when a high velocity impact is suddenly applied to the foam material and a plastic unloading wave starts to propagate. This is a wave of strong discontinuity characterised by discontinuous velocity, stress, strain and density on the wave front while the particles behind the plastic wave front experience elastic or rigid unloading depending on the mat...
Composite Structures, 2020
The present work aims to derive the mechanical properties of a unidirectional (UD) lamina from ex... more The present work aims to derive the mechanical properties of a unidirectional (UD) lamina from experimentally measured properties of non-flat (cylindrical) laminated specimens. The specimens were cut from carbon fiber-epoxy tubes having cross-ply and angle-ply layup configurations. Special tests were designed and applied on these specimens to measure the mechanical properties of the laminate, E x , E y , ν xy, G xy, as well as tensile and shear failure strength. By using the classical laminate theory (CLT), the mechanical properties of UD lamina, E 1 , E 2 , ν 12 and G 12 were expressed explicitly as functions of the measured properties of the crossply laminate and in form of effective mechanical properties of general lamination configuration like the angleply layup. A new shear test fixture was designed in order to obtain the in-plane shear modulus and shear strength of ring-shaped composite specimens. This shear test fixture has proved to be advantageous, especially when one bears in mind that there are very few shear test methods for cylindrical components. The outcomes of this work would be useful for material characterization and quality control when standard flat coupons are not available.
International Journal of Mechanical Sciences, 2017
On the influence of elasticity on the large deflections response of circular plates to uniform qu... more On the influence of elasticity on the large deflections response of circular plates to uniform quasi-static pressure, International
International Journal of Mechanical Sciences, 2016
Abstract Theoretical and numerical analyses are carried out to reveal the role of metal tube rein... more Abstract Theoretical and numerical analyses are carried out to reveal the role of metal tube reinforcement in the enhancement of the energy absorption capacity of foam materials with different densities and strength. A theoretical model is proposed to estimate the strength increase of the reinforcement due to the confined tube, which buckles. Using this model, the contribution of the reinforcement is obtained to the mean quasi-static tube force and also to the total quasi-static average force of reinforced materials comprising different foam and reinforcing components. The proposed model is also used to estimate the efficiency of the reinforced foam materials in the quasi-static loading regime. FE simulations are carried out to verify the theoretical quasi-static model and to examine some essential dynamic effects due to the inertia of the foam and reinforcing tubes on the energy absorption of the material when subjected to impact loading. Attention is paid to the force enhancement at the impacted end and to the dynamic load transfer to the distal end of blocks of reinforced materials.
International Journal of Impact Engineering, 2016
Abstract This article presents the results of an investigation into the response of partially con... more Abstract This article presents the results of an investigation into the response of partially confined hollow stainless steel cylinders to internal air-blast loading. Numerical, theoretical and experimental analyses were performed to gain insight into the factors governing the deformation processes. Numerically, LS-DYNA was used to simulate the response of the steel single-walled cylinders and compare it to the response of various equivalent mass sandwich-walled cylinders with different mass distributions through the wall thickness. It was established that sandwich-walled cylinders with low density foam core outperform the single-walled cylinder while the higher density foam core sandwich-walled cylinders respond by larger maximum deflections compared to the single-walled equivalent. A theoretical model was developed for the deformation of a sandwich-walled ring configuration and used to analyse and interpret the process of the dynamic foam compaction and stress transmission to the outer wall of a sandwich-walled cylinder. The results from the numerical parametric study and theoretical analysis are used further to design a sandwich-walled cylinder which would outperform the equivalent mass single-walled steel tube. Experimentally, the blast loading was generated by detonating spheres of plastic explosive at the mid-point of the centre line of the cylinders. Partial confinement was created by closing one end of the cylinder and leaving the other end free to vent to air.
Encyclopedia of Continuum Mechanics, 2018
International Journal of Mechanical Sciences, 1993
This paper discusses and illustrates the influence of transverse shear on the response of a thick... more This paper discusses and illustrates the influence of transverse shear on the response of a thick elastic-plastic beam when it is subjected to a pulse load. The pseudo-normal mode superposition method is used for solving the equations of motion. Numerical results are obtained for simply supported and clamped beams subjected to a pulse load of finite duration.
Marine Structures, 2017
In this paper, a series of experiments are performed in order to examine the response of clamped ... more In this paper, a series of experiments are performed in order to examine the response of clamped steel plates loaded quasi-statically at their centre by a rigid rectangular indenter. Four types of boundary conditions, which offer varying levels of the supports flexibility, are investigated in the finite element modelling study. The numerical simulations showed that the predicted permanent deflections of the plate are very sensitive to the way in which the supports are modeled. Simplified boundary conditions are formulated in order to account for the actual support constraint. The deformation modes for plates with different aspect ratios are analyzed and a new mode, which is observed in plates with relatively large aspect ratio, is defined within the rigid-plastic mechanism analysis framework. It was shown that the numerical simulations can predict accurately the permanent deflections and deformation modes when appropriate boundary conditions are applied. The influences of the aspect ratio and plate thickness, on both the boundary conditions definition and in-plane displacements are discussed from further numerical studies.
IMPACT ENGINEERING: Fundamentals, Experiments and Nonlinear Finite Elements, 2020
Latin American Journal of Solids and Structures, 2015
The International Conference on Applied Mechanics and Mechanical Engineering
In order to study the dynamic effects on the snap-through behaviour of an elastic spherical shell... more In order to study the dynamic effects on the snap-through behaviour of an elastic spherical shell under normal impact, ping-pong balls were accelerated by projectiles fired from an airgun and impinged onto a rigid plate with the velocity ranging 10-50m/s. Apart from the forcedisplacement relationship, a particular attention was paid to the evolution of the contact zone between the ball and the plate, as recorded by a high-speed digital camera. As a result, the impact duration, the maximum contact diameter, and the contact diameter at snap-through buckling under different impact velocities were obtained. An axi-symmetric finite element model is generated and the dynamic simulation is conducted using ABAQUS/Explicit. Based on the experiments and simulations, a theoretical model is proposed, which simplifies the deformed shape by piecewise constant-curvature regions, but captures the major features of the deformation process of a thin-walled spherical shell, such as the onset of the snapthrough buckling, the evolution of the contact zone, etc. By taking into account the inertia effects in the deformed regions and minimizing the total work done, the evolution of the deformed shape of the shell is revealed. The theoretical predictions for the contact force and contact diameter are in good agreement, while it is concluded that the local inertia is mainly responsible for the difference between the dynamic deformation behavior and the quasi-static one.
International Journal of Solids and Structures, 2015
Abstract Theoretical analysis of the propagation of stress waves in cellular solids with non-unif... more Abstract Theoretical analysis of the propagation of stress waves in cellular solids with non-uniform density, and consequently strength, is carried out to deepen the understanding of their dynamic compaction due to impact loading. Materials with continuously varying density in the direction of loading are considered when analysing the response to two types of loading conditions: an impact of a stationary cellular block by a rigid mass and an impact of a cellular block on a rigid wall. It is assumed that the local stress–strain characteristics of the graded materials exhibit strain hardening. The plastic strain field in the deformed graded cellular solid is sought here as a function of the impact velocity and material properties when using the Hugoniot material representation. It is shown that the initial density variation with respect to the boundaries of a finite thickness block has a significant effect on the history of the stresses and strains during the compaction process. FE models using ABAQUS are constructed and numerical simulations are carried out to verify the predictions of the theoretical analysis. Attention is paid to the energy absorption capacity of the materials depending on their initial density distribution when comparing their dynamic responses with the response of equivalent mass cellular block with uniform density. Significant advantages in using density graded cellular solids in finite thickness layers are not identified for the analysed loading rate.
International Journal of Impact Engineering
International Journal of Mechanical Sciences
International Journal of Impact Engineering, 2021
Abstract The saturated impulse phenomenon, which may occur in a hollow beam with a circular cross... more Abstract The saturated impulse phenomenon, which may occur in a hollow beam with a circular cross-section, is examined when a rectangular pressure pulse loading is applied uniformly on the upper surface of the beam. A simplified 2DoF model of the dynamic response of a relatively long circular hollow section beam is developed to explore the role of the local and global deformations build up simultaneously during the beam response. In order to simplify the analytical formulation, the elastic deformations of the hollow beam are neglected, and a rigid-plastic method of analysis is applied. Finite element simulations using an elastic, perfectly plastic material model are carried out to verify the proposed analytical model and to reveal the influence of the elasticity on the beam response. The limit of the pressure pulse loading, beyond which saturated impulse is not defined for a hollow beam, is obtained numerically. The responses of hollow beams with lengths between 800 and 1200 mm, radius of 40 mm and several wall thicknesses are analysed. It is observed that the proposed 2DoF model can predict satisfactory the local and global saturated deflections while the saturated pressure pulse duration is underestimated due to the disregard of the stored elastic energy in the beam. It is suggested that the analytically predicted saturated impulse can be used as a lower bound estimate for the saturated impulse for a given beam geometry.
Journal of Aerospace Engineering
A generalised framework of the propagation of compaction waves in layered metal foam materials su... more A generalised framework of the propagation of compaction waves in layered metal foam materials subjected to impact loading is presented based on one-dimensional analytical models. The analysis is focused on layered configurations in which the individual materials exhibit strain hardening in the quasi-static regime of loading. No details of the cellular geometry are considered and it is assumed that the foam is a homogeneous material. A novel approach assuming no pre-defined value of the densification strain is applied. Two mechanisms of compaction are identified during the deformation of the layered materials. The first mechanism occurs when a high velocity impact is suddenly applied to the foam material and a plastic unloading wave starts to propagate. This is a wave of strong discontinuity characterised by discontinuous velocity, stress, strain and density on the wave front while the particles behind the plastic wave front experience elastic or rigid unloading depending on the mat...
Composite Structures, 2020
The present work aims to derive the mechanical properties of a unidirectional (UD) lamina from ex... more The present work aims to derive the mechanical properties of a unidirectional (UD) lamina from experimentally measured properties of non-flat (cylindrical) laminated specimens. The specimens were cut from carbon fiber-epoxy tubes having cross-ply and angle-ply layup configurations. Special tests were designed and applied on these specimens to measure the mechanical properties of the laminate, E x , E y , ν xy, G xy, as well as tensile and shear failure strength. By using the classical laminate theory (CLT), the mechanical properties of UD lamina, E 1 , E 2 , ν 12 and G 12 were expressed explicitly as functions of the measured properties of the crossply laminate and in form of effective mechanical properties of general lamination configuration like the angleply layup. A new shear test fixture was designed in order to obtain the in-plane shear modulus and shear strength of ring-shaped composite specimens. This shear test fixture has proved to be advantageous, especially when one bears in mind that there are very few shear test methods for cylindrical components. The outcomes of this work would be useful for material characterization and quality control when standard flat coupons are not available.
International Journal of Mechanical Sciences, 2017
On the influence of elasticity on the large deflections response of circular plates to uniform qu... more On the influence of elasticity on the large deflections response of circular plates to uniform quasi-static pressure, International
International Journal of Mechanical Sciences, 2016
Abstract Theoretical and numerical analyses are carried out to reveal the role of metal tube rein... more Abstract Theoretical and numerical analyses are carried out to reveal the role of metal tube reinforcement in the enhancement of the energy absorption capacity of foam materials with different densities and strength. A theoretical model is proposed to estimate the strength increase of the reinforcement due to the confined tube, which buckles. Using this model, the contribution of the reinforcement is obtained to the mean quasi-static tube force and also to the total quasi-static average force of reinforced materials comprising different foam and reinforcing components. The proposed model is also used to estimate the efficiency of the reinforced foam materials in the quasi-static loading regime. FE simulations are carried out to verify the theoretical quasi-static model and to examine some essential dynamic effects due to the inertia of the foam and reinforcing tubes on the energy absorption of the material when subjected to impact loading. Attention is paid to the force enhancement at the impacted end and to the dynamic load transfer to the distal end of blocks of reinforced materials.
International Journal of Impact Engineering, 2016
Abstract This article presents the results of an investigation into the response of partially con... more Abstract This article presents the results of an investigation into the response of partially confined hollow stainless steel cylinders to internal air-blast loading. Numerical, theoretical and experimental analyses were performed to gain insight into the factors governing the deformation processes. Numerically, LS-DYNA was used to simulate the response of the steel single-walled cylinders and compare it to the response of various equivalent mass sandwich-walled cylinders with different mass distributions through the wall thickness. It was established that sandwich-walled cylinders with low density foam core outperform the single-walled cylinder while the higher density foam core sandwich-walled cylinders respond by larger maximum deflections compared to the single-walled equivalent. A theoretical model was developed for the deformation of a sandwich-walled ring configuration and used to analyse and interpret the process of the dynamic foam compaction and stress transmission to the outer wall of a sandwich-walled cylinder. The results from the numerical parametric study and theoretical analysis are used further to design a sandwich-walled cylinder which would outperform the equivalent mass single-walled steel tube. Experimentally, the blast loading was generated by detonating spheres of plastic explosive at the mid-point of the centre line of the cylinders. Partial confinement was created by closing one end of the cylinder and leaving the other end free to vent to air.
Encyclopedia of Continuum Mechanics, 2018
International Journal of Mechanical Sciences, 1993
This paper discusses and illustrates the influence of transverse shear on the response of a thick... more This paper discusses and illustrates the influence of transverse shear on the response of a thick elastic-plastic beam when it is subjected to a pulse load. The pseudo-normal mode superposition method is used for solving the equations of motion. Numerical results are obtained for simply supported and clamped beams subjected to a pulse load of finite duration.
Marine Structures, 2017
In this paper, a series of experiments are performed in order to examine the response of clamped ... more In this paper, a series of experiments are performed in order to examine the response of clamped steel plates loaded quasi-statically at their centre by a rigid rectangular indenter. Four types of boundary conditions, which offer varying levels of the supports flexibility, are investigated in the finite element modelling study. The numerical simulations showed that the predicted permanent deflections of the plate are very sensitive to the way in which the supports are modeled. Simplified boundary conditions are formulated in order to account for the actual support constraint. The deformation modes for plates with different aspect ratios are analyzed and a new mode, which is observed in plates with relatively large aspect ratio, is defined within the rigid-plastic mechanism analysis framework. It was shown that the numerical simulations can predict accurately the permanent deflections and deformation modes when appropriate boundary conditions are applied. The influences of the aspect ratio and plate thickness, on both the boundary conditions definition and in-plane displacements are discussed from further numerical studies.
IMPACT ENGINEERING: Fundamentals, Experiments and Nonlinear Finite Elements, 2020
Latin American Journal of Solids and Structures, 2015
The International Conference on Applied Mechanics and Mechanical Engineering
In order to study the dynamic effects on the snap-through behaviour of an elastic spherical shell... more In order to study the dynamic effects on the snap-through behaviour of an elastic spherical shell under normal impact, ping-pong balls were accelerated by projectiles fired from an airgun and impinged onto a rigid plate with the velocity ranging 10-50m/s. Apart from the forcedisplacement relationship, a particular attention was paid to the evolution of the contact zone between the ball and the plate, as recorded by a high-speed digital camera. As a result, the impact duration, the maximum contact diameter, and the contact diameter at snap-through buckling under different impact velocities were obtained. An axi-symmetric finite element model is generated and the dynamic simulation is conducted using ABAQUS/Explicit. Based on the experiments and simulations, a theoretical model is proposed, which simplifies the deformed shape by piecewise constant-curvature regions, but captures the major features of the deformation process of a thin-walled spherical shell, such as the onset of the snapthrough buckling, the evolution of the contact zone, etc. By taking into account the inertia effects in the deformed regions and minimizing the total work done, the evolution of the deformed shape of the shell is revealed. The theoretical predictions for the contact force and contact diameter are in good agreement, while it is concluded that the local inertia is mainly responsible for the difference between the dynamic deformation behavior and the quasi-static one.
International Journal of Solids and Structures, 2015
Abstract Theoretical analysis of the propagation of stress waves in cellular solids with non-unif... more Abstract Theoretical analysis of the propagation of stress waves in cellular solids with non-uniform density, and consequently strength, is carried out to deepen the understanding of their dynamic compaction due to impact loading. Materials with continuously varying density in the direction of loading are considered when analysing the response to two types of loading conditions: an impact of a stationary cellular block by a rigid mass and an impact of a cellular block on a rigid wall. It is assumed that the local stress–strain characteristics of the graded materials exhibit strain hardening. The plastic strain field in the deformed graded cellular solid is sought here as a function of the impact velocity and material properties when using the Hugoniot material representation. It is shown that the initial density variation with respect to the boundaries of a finite thickness block has a significant effect on the history of the stresses and strains during the compaction process. FE models using ABAQUS are constructed and numerical simulations are carried out to verify the predictions of the theoretical analysis. Attention is paid to the energy absorption capacity of the materials depending on their initial density distribution when comparing their dynamic responses with the response of equivalent mass cellular block with uniform density. Significant advantages in using density graded cellular solids in finite thickness layers are not identified for the analysed loading rate.
International Journal of Impact Engineering
International Journal of Mechanical Sciences