Asghar Zajkani - Academia.edu (original) (raw)

Papers by Asghar Zajkani

Research paper thumbnail of Defects Detection at Additive Manufacturing by Convolutional Deep Learning

DergiPark (Istanbul University), May 1, 2024

Additive manufacturing technologies present a wide array of benefits, including the capacity to m... more Additive manufacturing technologies present a wide array of benefits, including the capacity to manufacture components with complex geometric forms, reduced production expenses, minimized material usage, and time efficiency. This research constitutes a significant effort to pinpoint geometric defects and dimensional irregularities as well as surface quality imperfections in the Fused Deposition Modeling process through the development of a deep learning model utilizing multi-scale convolutional neural networks. The proposed methodology encompasses three distinct scales, each capable of identifying defects of varying dimensions. The model underwent extensive hybridizing procedures for precisely training through diverse datasets, and the training process is repeated numerous times until the desired level of accuracy was attained. A sufficiently extensive image datasets are employed to train the models, leading to the precise calibration of the network. As a result, the necessity for prolonged time and intricate computations to identify large-scale defects is eliminated. The highest validation accuracy for defect detection in this study reached 94%.

Research paper thumbnail of The response of viscoelastic composite laminated microplate under low-velocity impact based on nonlocal strain gradient theory for different boundary conditions

Steel and Composite Structures, 2021

Research paper thumbnail of On the stress state-based coupled plasticity – Ductile damage model for aluminum alloys considering the influence of high-rate impulsive preload

International Journal of Impact Engineering, Dec 1, 2020

Abstract The purpose of this study is to produce a ductile damage model for 7075-T6 aluminum allo... more Abstract The purpose of this study is to produce a ductile damage model for 7075-T6 aluminum alloy by correlating the effect of stress - state parameters to the pre-mechanical working exited by an impulsive shock loading. A stress state-based ductile damage model is coupled implicitly with constitutive plasticity framework to anticipate a generalized true fracture locus of the Al-7075-T6. To simulate the constitutive equations, a fully nonlinear finite element procedure is developed to capture the corresponding stress state, hardening, and damage evolutions during the high- velocity impact preload as well as a subsequent progressive static deformation in the tensile tests. The results are calibrated by selected experimental tests representing different stress - states. Also, two conditions are investigated for the stress-strain curves obtained from the uniaxial tests; specimens with the high rate pre-mechanical workings and specimen without pre-mechanical workings. Moreover, by applying two fracture initiation criteria, i.e., the Hosford- Coulomb and Xue models, two types of the fracture locus are predicted for the Al-7075-T6 alloys in terms of the plastic strains and stress - state parameters under the conditions mentioned above. By considering the experimental data, a new evolutionary model of the ductile damage, which is coupled with the J2 plasticity is proposed to relate the initial rate - dependent stress state. By using the fracture criteria, our model may be reliable for a phenomenological scheme of the damage evolution and useful for comparisons with the Xue damage model. Finally, analysis of the fracture surfaces by Scanning Electron Microscope revealed that the fracture surfaces of all samples mainly contained the dimples, which indicates the presence of ductile fracture in all specimens before and after the shock loading.

Research paper thumbnail of A computational investigation for propagation of elasto-viscoplastic zones in the shock loaded circular plates

Engineering Computations, Sep 30, 2014

Purpose– The purpose of this paper is to introduce a computational time dependent modeling to inv... more Purpose– The purpose of this paper is to introduce a computational time dependent modeling to investigate propagation of elastic-viscoplastic zones in the shock wave loaded circular plates.Design/methodology/approach– Constitutive equations are implemented incrementally by the Von-Kármán finite deflection system which is coupled with a mixed strain hardening rule and physical-base viscoplastic models. Time integrations of the equations are done by the return mapping technique through the cutting-plane algorithm. An integrated solution is established by pseudo-spectral collocation methodology. The Chebyshev basis functions are utilized to evaluate the coefficients of displacement fields. Temporal terms are discretized by the Houbolt marching method. Spatial linearizations are accomplished by the quadratic extrapolation technique.Findings– Results of the center point deflections, effective plastic strain and stress (dynamic flow stress) and temperature rise are compared for three features of the Von-Kármán system. Identifying time history of resultant stresses, propagations of the viscoplastic plastic zones are illustrated for two circumstances; with considering strain rate and hardening effects, and without them. Some of modeling and computation aspects are discussed, carefully. When the results are compared with experimental data of shock wave loadings and finite element simulations, good agreements between them are observed.Originality/value– This computational approach makes coupling the structural equations with the physical descriptions of the high rate deformation through step-by-step spectral solution of the constitutive equations.

Research paper thumbnail of Numerical modeling of a body vessel for dynamic study of a nano cylindrical shell carrying fluid and a moving nanoparticle

Engineering Analysis with Boundary Elements

Research paper thumbnail of Constitutive Modeling of the Elasto-Viscoplastic Deformation a Semi-Crystalline Polymer at Different Thermal Regimes

Research paper thumbnail of A self - Consistent higher-order strain gradient plasticity analysis of micropolar dual-phase crystalline FGM made of Titanium-boride/ Titanium

Research paper thumbnail of Mechanical properties of top neck mollusks shell nano composite in different environmental conditions

Advances in materials research, Sep 1, 2018

Research paper thumbnail of A pseudospectral contact mechanics simulation of microindentation in two slip systems activated single crystalline material by using a homographic conformal mapping

Engineering Analysis with Boundary Elements

Research paper thumbnail of On the Hybrid Modeling of Phenomenological Damage Evolution in Low Carbon Steels During Equal Channel Angular Extrusion Process

Metals and Materials International, 2021

Low carbon steel is widely used in industry due to its excellent elastoplastic behavior with the ... more Low carbon steel is widely used in industry due to its excellent elastoplastic behavior with the mechanism of damage accumulation during severe plastic deformation processes which lead to increase strength resistance. In this study, the strain locus of the failure is predicted based on the stress triaxiality and Lode angle-dependent damage model by considering damage evolution during the process. Therefore, by performing various tests for several stress states and modeling the load paths related to damage accumulations, the coefficients of the modified Mohr–Coulomb damage model and Johnson–Cook are determined. According to results achieved among the test with good accuracy, the predictor locus of the failure strain is illustrated. Then, using the determined coefficients, the steel damage evolution is investigated during the process of Equal Channel Angular Extrusion (ECAE). It is shown that the damage accumulation during the ECAE process in a die with respectively internal and external angles of 90 and 30 degrees grows from the region closer to the internal angle as reaches a maximum value in central regions. Also, the amount of damage in vicinity of the outer angle is extremely reduced. According to experiments, the steel does resist fracture events after the ECAE process.

Research paper thumbnail of An analytical modeling for springback prediction during U-bending process of advanced high-strength steels based on anisotropic nonlinear kinematic hardening model

The International Journal of Advanced Manufacturing Technology, 2016

An undesirable factor that affects the dimensional precision and final shape of metallic parts pr... more An undesirable factor that affects the dimensional precision and final shape of metallic parts produced in cold forming processes is springback phenomenon. In this paper, an analytical model is introduced to predict springback in U-shaped bending process of DP780 dual-phase steel sheet. It is based on the Hill48 yielding criterion and plane strain condition. In this model, the effect of forming history, the sheet thinning, and the motion of the neutral surface on the springback of U-shaped bending process is taken into account. The anisotropic nonlinear kinematic hardening model (ANK) is used to consider the impact of complex deformation, including stretching, bending and reverse bending. This model is able to investigate the Bauschinger effect, transient behavior and permanent softening. This model is used for the Numisheet2011 benchmark U-shaped bending problem. The effect of the sheet holder force, the coefficient of friction, thickness, material anisotropy, and hardening parameters on the sheet springback is studied. It can be seen that analytical model which presented in this paper has desirable accuracy in the springback prediction, and results are close to experimental data.

Research paper thumbnail of Predictability of Social-anamnestic Variables on Receptive Vocabulary and Cognitive Functioning of the Elderly Population

Materia Socio Medica, 2016

Research paper thumbnail of Thermal effect on dynamics of thin and thick composite laminated microbeams by modified couple stress theory for different boundary conditions

Applied Physics A, 2016

Abstract In this article, thermal effect on free vibration behavior of composite laminated microb... more Abstract In this article, thermal effect on free vibration behavior of composite laminated microbeams based on the modified couple stress theory is presented. The proposed anisotropic model is developed by using a variational formulation. The governing equations and boundary conditions are obtained based on a modified couple stress theory and using the principle of minimum potential energy and considering different beam theories, i.e., Euler–Bernoulli, Timoshenko and Reddy beam theories. Unlike the classical beam theories, this model contains a material length scale parameter and can capture the size effect. Free vibration of a simply supported beam is solved by utilizing Fourier series. In addition, the fundamental frequency is achieved by using the generalized differential quadrature method for four types of cross-ply laminations with clamped–clamped, clamped–hinged and hinged–hinged boundary conditions for different beam theories. For investigating different parameters including temperature changes, material length scale parameter, beam thickness, some numerical results on different cross-ply laminated beams are presented. The fundamental frequency of different thin and thick beam theories is investigated by increasing slenderness ratio and thermal loads. The results prove that the modified couple stress theory increases the natural frequency under the thermal effects for free vibration of composite laminated microbeams.

Research paper thumbnail of Effective rigid perfectly plastic models to predict deflection and residual velocity of post local failure motion of impulsively loaded circular plates

International Journal of Physical Sciences, Mar 30, 2013

Two deformation models are proposed for clamped circular plates undergoing pulse loadings. By the... more Two deformation models are proposed for clamped circular plates undergoing pulse loadings. By these models, behaviors of plate are studied effectively for the situations before and after local failure. In the first model, it is assumed that impulsive load is uniformly distributed and final deformation is of a spherical dome shape. In order to analyze this model, it assumed that under the shock wave, the mechanism of deformation is represented by a multi-peripheral stationary hinge scheme. In the second model, the final shape is considered to have a conical shape represented by a single peripheral plastic moving hinge. In this part, an alternate deformation model is proposed and the final shape is induced by transverse and radial motion of the plastic hinge. For each model, the deformation and motion after severance of the plate (post local failure) will be analyzed. Calculated plastic energies dissipated in deformation process, energy absorbed in boundaries during failure, residual kinetic energy and velocity after local failure are evaluated and discussed. Computed results show good agreement between our approaches and experimental data; better than that obtained with other models.

Research paper thumbnail of A computational investigation for propagation of elasto-viscoplastic zones in the shock loaded circular plates

Engineering Computations, 2014

Purpose – The purpose of this paper is to introduce a computational time dependent modeling to in... more Purpose – The purpose of this paper is to introduce a computational time dependent modeling to investigate propagation of elastic-viscoplastic zones in the shock wave loaded circular plates. Design/methodology/approach – Constitutive equations are implemented incrementally by the Von-Kármán finite deflection system which is coupled with a mixed strain hardening rule and physical-base viscoplastic models. Time integrations of the equations are done by the return mapping technique through the cutting-plane algorithm. An integrated solution is established by pseudo-spectral collocation methodology. The Chebyshev basis functions are utilized to evaluate the coefficients of displacement fields. Temporal terms are discretized by the Houbolt marching method. Spatial linearizations are accomplished by the quadratic extrapolation technique. Findings – Results of the center point deflections, effective plastic strain and stress (dynamic flow stress) and temperature rise are compared for three...

Research paper thumbnail of Analytical modelling of high-rate elasto-viscoplastic deformation of circular plates subjected to impulsive loads using pseudo-spectral collocation method

The Journal of Strain Analysis for Engineering Design, 2012

ABSTRACT An analytical methodology is developed to study dynamic elasto-viscoplastic behaviour of... more ABSTRACT An analytical methodology is developed to study dynamic elasto-viscoplastic behaviour of moderately thick circular plates subjected to high-intensity impulsive loads, comprehensively. First, incremental kinematic formulations are derived based on the first-order shear deformation theory to take into account viscous damping and rotary inertia. Geometrical and material non-linearities are applied by the complete von Kármán system and a mixed strain hardening law coupled with a physically based viscoplastic model, respectively. A semi-implicit scheme of return-mapping is employed by the cutting-plane algorithm to obtain effective plastic strains apart from satisfying the consistency condition. The subsequent part is devoted to the transformation of this boundary value problem into an initial value problem, to evaluate displacement fields. Spatial and temporal discretizations are carried out by the Chebyshev pseudo-spectral collocation method and Houbolt time-marching scheme, respectively. This transformation has been handled in the compact matrix forms to stabilize the solution and to make it more convenient. Influence of impulsive load and other parameters on plate deflections, effective strain and stress, temperature rise and stresses are considered. Ultimately, good accuracy is achieved through comparison between results and existing experimental data and finite element simulation from the literature. In addition, some challengeable aspects for the modelling are discussed.

Research paper thumbnail of Incremental integrated modeling of dynamic viscoplastic responses of the annular sector plates exposed to shock wave loading

The Journal of Strain Analysis for Engineering Design, 2013

An incremental integrated modeling is presented to obtain high-rate dynamic viscoplastic behavior... more An incremental integrated modeling is presented to obtain high-rate dynamic viscoplastic behavior of annular sector plates. The large amplitude shock loads are imparted uniformly over the plate’s surface. Using the first-order shear deformation theory including the nonlinear Von-Kármán system, incremental differential equations are derived for nonaxisymmetric motion of the plate. A combined strain hardening law, in conjunction with special physical-based viscoplastic models, is applied to consider the material nonlinearity. Evaluation of the viscoplastic constitutive equations is accomplished by a semi-implicit scheme of the return mapping technique. An efficient algorithm is applied by the cutting-plane iteration to enforce plasticity admissibility during evolution of yield surface. A pseudo-spectral collocation methodology is implemented based on the Chebyshev polynomials in order to calculate displacement fields stepwise. Velocity and inertia terms are discrete by the Houbolt mar...

Research paper thumbnail of Mathematical Modeling of Large-Amplitude Dynamic-Plastic Behavior of Circular Plates Subjected to Impulsive Loads

Journal of Mechanics, 2010

ABSTRACTIn this paper, an analysis of the large-amplitude dynamic-plastic behavior of the circula... more ABSTRACTIn this paper, an analysis of the large-amplitude dynamic-plastic behavior of the circular plates with a rigid perfectly plastic material is presented. The plate is subjected to a short-time high-intensity impulsive load uniformly distributed over the surface. Modeling is complemented by using specific convex yield criteria. Corresponding to boundary conditions of the plate, it can be deformed through more than one mechanism, so, the mathematical formulation is based on the principle of calculus of variations in which the transverse displacement fields are assumed as a combination of appropriate paths. Based on the upper bound approach, the different terms of kinetic and consumed plastic energies likewise the applied impulse energy derived to produce an energy functional with unknown coefficients which is minimized through the displacement path. Finally, calculating the constants maximum residual deflection and strain distribution are obtained. Results of present model show ...

Research paper thumbnail of On the Dependency of Ductile Damage Evolution to Stress State with Shock Loading Pre-Mechanical Working in 7075-T6 Aluminum Alloy

International Journal of Applied Mechanics, 2016

In this paper, the evolution of a ductile damage in the 7075-T6 aluminum alloy is considered base... more In this paper, the evolution of a ductile damage in the 7075-T6 aluminum alloy is considered based on stress state parameters with a special focus on pre-mechanical working dependency. Uniaxial stress–strain curves are investigated experimentally for two conditions; specimens with shock loaded pre-mechanical working and without it. This kind of loading is applied in order to find out impulsive pressure effects of damage variation procedure. Some experiments are done to take different stress states. Applying two fracture initiations criteria, i.e., Hosford–Coulomb and Xue models, two types of fracture locus of Al-7075-T6 are predicted in terms of plastic strains and stress state parameters under above conditions. By considering experimental data, a new ductile damage evolution model is proposed among plastic behavior. It is introduced by explicating an uncoupled plasticity and related to initial rate dependent stress state. By using both fracture models, our damage evolution model is...

Research paper thumbnail of A New Approach for Stress State - Dependent Flow Localization Failure Bounded Through Ductile Damage in Dynamically Loaded Sheets

Journal of Solid Mechanics, 2020

In this paper, a new approach is proposed for stress state - dependent flow localization in bifur... more In this paper, a new approach is proposed for stress state - dependent flow localization in bifurcation failure model bounded through ductile damage in dynamically loaded sheets. Onset of localized necking is considered in phenomenological way for different strain rates to draw the forming limit diagram (FLD). Using a strain metal hardening exponent in the Vertex theory related to the strain rate helps investigate rate- dependent metal forming limits. Besides, the paper utilizes the model of ductile damage as a function of strain condition, stress states (triaxiality and Lode parameters), and the symbols of stiffness strain to predict the onset of the necking. It is worth noting that updated level of elasticity modulus in the plastic deforming is attributed as an essential index for the ductile damage measuring. According to original formulations, a UMAT subroutine is developed in the finite element simulation by ABAQUS code to analyze and connect the related constitutive models. Re...

Research paper thumbnail of Defects Detection at Additive Manufacturing by Convolutional Deep Learning

DergiPark (Istanbul University), May 1, 2024

Additive manufacturing technologies present a wide array of benefits, including the capacity to m... more Additive manufacturing technologies present a wide array of benefits, including the capacity to manufacture components with complex geometric forms, reduced production expenses, minimized material usage, and time efficiency. This research constitutes a significant effort to pinpoint geometric defects and dimensional irregularities as well as surface quality imperfections in the Fused Deposition Modeling process through the development of a deep learning model utilizing multi-scale convolutional neural networks. The proposed methodology encompasses three distinct scales, each capable of identifying defects of varying dimensions. The model underwent extensive hybridizing procedures for precisely training through diverse datasets, and the training process is repeated numerous times until the desired level of accuracy was attained. A sufficiently extensive image datasets are employed to train the models, leading to the precise calibration of the network. As a result, the necessity for prolonged time and intricate computations to identify large-scale defects is eliminated. The highest validation accuracy for defect detection in this study reached 94%.

Research paper thumbnail of The response of viscoelastic composite laminated microplate under low-velocity impact based on nonlocal strain gradient theory for different boundary conditions

Steel and Composite Structures, 2021

Research paper thumbnail of On the stress state-based coupled plasticity – Ductile damage model for aluminum alloys considering the influence of high-rate impulsive preload

International Journal of Impact Engineering, Dec 1, 2020

Abstract The purpose of this study is to produce a ductile damage model for 7075-T6 aluminum allo... more Abstract The purpose of this study is to produce a ductile damage model for 7075-T6 aluminum alloy by correlating the effect of stress - state parameters to the pre-mechanical working exited by an impulsive shock loading. A stress state-based ductile damage model is coupled implicitly with constitutive plasticity framework to anticipate a generalized true fracture locus of the Al-7075-T6. To simulate the constitutive equations, a fully nonlinear finite element procedure is developed to capture the corresponding stress state, hardening, and damage evolutions during the high- velocity impact preload as well as a subsequent progressive static deformation in the tensile tests. The results are calibrated by selected experimental tests representing different stress - states. Also, two conditions are investigated for the stress-strain curves obtained from the uniaxial tests; specimens with the high rate pre-mechanical workings and specimen without pre-mechanical workings. Moreover, by applying two fracture initiation criteria, i.e., the Hosford- Coulomb and Xue models, two types of the fracture locus are predicted for the Al-7075-T6 alloys in terms of the plastic strains and stress - state parameters under the conditions mentioned above. By considering the experimental data, a new evolutionary model of the ductile damage, which is coupled with the J2 plasticity is proposed to relate the initial rate - dependent stress state. By using the fracture criteria, our model may be reliable for a phenomenological scheme of the damage evolution and useful for comparisons with the Xue damage model. Finally, analysis of the fracture surfaces by Scanning Electron Microscope revealed that the fracture surfaces of all samples mainly contained the dimples, which indicates the presence of ductile fracture in all specimens before and after the shock loading.

Research paper thumbnail of A computational investigation for propagation of elasto-viscoplastic zones in the shock loaded circular plates

Engineering Computations, Sep 30, 2014

Purpose– The purpose of this paper is to introduce a computational time dependent modeling to inv... more Purpose– The purpose of this paper is to introduce a computational time dependent modeling to investigate propagation of elastic-viscoplastic zones in the shock wave loaded circular plates.Design/methodology/approach– Constitutive equations are implemented incrementally by the Von-Kármán finite deflection system which is coupled with a mixed strain hardening rule and physical-base viscoplastic models. Time integrations of the equations are done by the return mapping technique through the cutting-plane algorithm. An integrated solution is established by pseudo-spectral collocation methodology. The Chebyshev basis functions are utilized to evaluate the coefficients of displacement fields. Temporal terms are discretized by the Houbolt marching method. Spatial linearizations are accomplished by the quadratic extrapolation technique.Findings– Results of the center point deflections, effective plastic strain and stress (dynamic flow stress) and temperature rise are compared for three features of the Von-Kármán system. Identifying time history of resultant stresses, propagations of the viscoplastic plastic zones are illustrated for two circumstances; with considering strain rate and hardening effects, and without them. Some of modeling and computation aspects are discussed, carefully. When the results are compared with experimental data of shock wave loadings and finite element simulations, good agreements between them are observed.Originality/value– This computational approach makes coupling the structural equations with the physical descriptions of the high rate deformation through step-by-step spectral solution of the constitutive equations.

Research paper thumbnail of Numerical modeling of a body vessel for dynamic study of a nano cylindrical shell carrying fluid and a moving nanoparticle

Engineering Analysis with Boundary Elements

Research paper thumbnail of Constitutive Modeling of the Elasto-Viscoplastic Deformation a Semi-Crystalline Polymer at Different Thermal Regimes

Research paper thumbnail of A self - Consistent higher-order strain gradient plasticity analysis of micropolar dual-phase crystalline FGM made of Titanium-boride/ Titanium

Research paper thumbnail of Mechanical properties of top neck mollusks shell nano composite in different environmental conditions

Advances in materials research, Sep 1, 2018

Research paper thumbnail of A pseudospectral contact mechanics simulation of microindentation in two slip systems activated single crystalline material by using a homographic conformal mapping

Engineering Analysis with Boundary Elements

Research paper thumbnail of On the Hybrid Modeling of Phenomenological Damage Evolution in Low Carbon Steels During Equal Channel Angular Extrusion Process

Metals and Materials International, 2021

Low carbon steel is widely used in industry due to its excellent elastoplastic behavior with the ... more Low carbon steel is widely used in industry due to its excellent elastoplastic behavior with the mechanism of damage accumulation during severe plastic deformation processes which lead to increase strength resistance. In this study, the strain locus of the failure is predicted based on the stress triaxiality and Lode angle-dependent damage model by considering damage evolution during the process. Therefore, by performing various tests for several stress states and modeling the load paths related to damage accumulations, the coefficients of the modified Mohr–Coulomb damage model and Johnson–Cook are determined. According to results achieved among the test with good accuracy, the predictor locus of the failure strain is illustrated. Then, using the determined coefficients, the steel damage evolution is investigated during the process of Equal Channel Angular Extrusion (ECAE). It is shown that the damage accumulation during the ECAE process in a die with respectively internal and external angles of 90 and 30 degrees grows from the region closer to the internal angle as reaches a maximum value in central regions. Also, the amount of damage in vicinity of the outer angle is extremely reduced. According to experiments, the steel does resist fracture events after the ECAE process.

Research paper thumbnail of An analytical modeling for springback prediction during U-bending process of advanced high-strength steels based on anisotropic nonlinear kinematic hardening model

The International Journal of Advanced Manufacturing Technology, 2016

An undesirable factor that affects the dimensional precision and final shape of metallic parts pr... more An undesirable factor that affects the dimensional precision and final shape of metallic parts produced in cold forming processes is springback phenomenon. In this paper, an analytical model is introduced to predict springback in U-shaped bending process of DP780 dual-phase steel sheet. It is based on the Hill48 yielding criterion and plane strain condition. In this model, the effect of forming history, the sheet thinning, and the motion of the neutral surface on the springback of U-shaped bending process is taken into account. The anisotropic nonlinear kinematic hardening model (ANK) is used to consider the impact of complex deformation, including stretching, bending and reverse bending. This model is able to investigate the Bauschinger effect, transient behavior and permanent softening. This model is used for the Numisheet2011 benchmark U-shaped bending problem. The effect of the sheet holder force, the coefficient of friction, thickness, material anisotropy, and hardening parameters on the sheet springback is studied. It can be seen that analytical model which presented in this paper has desirable accuracy in the springback prediction, and results are close to experimental data.

Research paper thumbnail of Predictability of Social-anamnestic Variables on Receptive Vocabulary and Cognitive Functioning of the Elderly Population

Materia Socio Medica, 2016

Research paper thumbnail of Thermal effect on dynamics of thin and thick composite laminated microbeams by modified couple stress theory for different boundary conditions

Applied Physics A, 2016

Abstract In this article, thermal effect on free vibration behavior of composite laminated microb... more Abstract In this article, thermal effect on free vibration behavior of composite laminated microbeams based on the modified couple stress theory is presented. The proposed anisotropic model is developed by using a variational formulation. The governing equations and boundary conditions are obtained based on a modified couple stress theory and using the principle of minimum potential energy and considering different beam theories, i.e., Euler–Bernoulli, Timoshenko and Reddy beam theories. Unlike the classical beam theories, this model contains a material length scale parameter and can capture the size effect. Free vibration of a simply supported beam is solved by utilizing Fourier series. In addition, the fundamental frequency is achieved by using the generalized differential quadrature method for four types of cross-ply laminations with clamped–clamped, clamped–hinged and hinged–hinged boundary conditions for different beam theories. For investigating different parameters including temperature changes, material length scale parameter, beam thickness, some numerical results on different cross-ply laminated beams are presented. The fundamental frequency of different thin and thick beam theories is investigated by increasing slenderness ratio and thermal loads. The results prove that the modified couple stress theory increases the natural frequency under the thermal effects for free vibration of composite laminated microbeams.

Research paper thumbnail of Effective rigid perfectly plastic models to predict deflection and residual velocity of post local failure motion of impulsively loaded circular plates

International Journal of Physical Sciences, Mar 30, 2013

Two deformation models are proposed for clamped circular plates undergoing pulse loadings. By the... more Two deformation models are proposed for clamped circular plates undergoing pulse loadings. By these models, behaviors of plate are studied effectively for the situations before and after local failure. In the first model, it is assumed that impulsive load is uniformly distributed and final deformation is of a spherical dome shape. In order to analyze this model, it assumed that under the shock wave, the mechanism of deformation is represented by a multi-peripheral stationary hinge scheme. In the second model, the final shape is considered to have a conical shape represented by a single peripheral plastic moving hinge. In this part, an alternate deformation model is proposed and the final shape is induced by transverse and radial motion of the plastic hinge. For each model, the deformation and motion after severance of the plate (post local failure) will be analyzed. Calculated plastic energies dissipated in deformation process, energy absorbed in boundaries during failure, residual kinetic energy and velocity after local failure are evaluated and discussed. Computed results show good agreement between our approaches and experimental data; better than that obtained with other models.

Research paper thumbnail of A computational investigation for propagation of elasto-viscoplastic zones in the shock loaded circular plates

Engineering Computations, 2014

Purpose – The purpose of this paper is to introduce a computational time dependent modeling to in... more Purpose – The purpose of this paper is to introduce a computational time dependent modeling to investigate propagation of elastic-viscoplastic zones in the shock wave loaded circular plates. Design/methodology/approach – Constitutive equations are implemented incrementally by the Von-Kármán finite deflection system which is coupled with a mixed strain hardening rule and physical-base viscoplastic models. Time integrations of the equations are done by the return mapping technique through the cutting-plane algorithm. An integrated solution is established by pseudo-spectral collocation methodology. The Chebyshev basis functions are utilized to evaluate the coefficients of displacement fields. Temporal terms are discretized by the Houbolt marching method. Spatial linearizations are accomplished by the quadratic extrapolation technique. Findings – Results of the center point deflections, effective plastic strain and stress (dynamic flow stress) and temperature rise are compared for three...

Research paper thumbnail of Analytical modelling of high-rate elasto-viscoplastic deformation of circular plates subjected to impulsive loads using pseudo-spectral collocation method

The Journal of Strain Analysis for Engineering Design, 2012

ABSTRACT An analytical methodology is developed to study dynamic elasto-viscoplastic behaviour of... more ABSTRACT An analytical methodology is developed to study dynamic elasto-viscoplastic behaviour of moderately thick circular plates subjected to high-intensity impulsive loads, comprehensively. First, incremental kinematic formulations are derived based on the first-order shear deformation theory to take into account viscous damping and rotary inertia. Geometrical and material non-linearities are applied by the complete von Kármán system and a mixed strain hardening law coupled with a physically based viscoplastic model, respectively. A semi-implicit scheme of return-mapping is employed by the cutting-plane algorithm to obtain effective plastic strains apart from satisfying the consistency condition. The subsequent part is devoted to the transformation of this boundary value problem into an initial value problem, to evaluate displacement fields. Spatial and temporal discretizations are carried out by the Chebyshev pseudo-spectral collocation method and Houbolt time-marching scheme, respectively. This transformation has been handled in the compact matrix forms to stabilize the solution and to make it more convenient. Influence of impulsive load and other parameters on plate deflections, effective strain and stress, temperature rise and stresses are considered. Ultimately, good accuracy is achieved through comparison between results and existing experimental data and finite element simulation from the literature. In addition, some challengeable aspects for the modelling are discussed.

Research paper thumbnail of Incremental integrated modeling of dynamic viscoplastic responses of the annular sector plates exposed to shock wave loading

The Journal of Strain Analysis for Engineering Design, 2013

An incremental integrated modeling is presented to obtain high-rate dynamic viscoplastic behavior... more An incremental integrated modeling is presented to obtain high-rate dynamic viscoplastic behavior of annular sector plates. The large amplitude shock loads are imparted uniformly over the plate’s surface. Using the first-order shear deformation theory including the nonlinear Von-Kármán system, incremental differential equations are derived for nonaxisymmetric motion of the plate. A combined strain hardening law, in conjunction with special physical-based viscoplastic models, is applied to consider the material nonlinearity. Evaluation of the viscoplastic constitutive equations is accomplished by a semi-implicit scheme of the return mapping technique. An efficient algorithm is applied by the cutting-plane iteration to enforce plasticity admissibility during evolution of yield surface. A pseudo-spectral collocation methodology is implemented based on the Chebyshev polynomials in order to calculate displacement fields stepwise. Velocity and inertia terms are discrete by the Houbolt mar...

Research paper thumbnail of Mathematical Modeling of Large-Amplitude Dynamic-Plastic Behavior of Circular Plates Subjected to Impulsive Loads

Journal of Mechanics, 2010

ABSTRACTIn this paper, an analysis of the large-amplitude dynamic-plastic behavior of the circula... more ABSTRACTIn this paper, an analysis of the large-amplitude dynamic-plastic behavior of the circular plates with a rigid perfectly plastic material is presented. The plate is subjected to a short-time high-intensity impulsive load uniformly distributed over the surface. Modeling is complemented by using specific convex yield criteria. Corresponding to boundary conditions of the plate, it can be deformed through more than one mechanism, so, the mathematical formulation is based on the principle of calculus of variations in which the transverse displacement fields are assumed as a combination of appropriate paths. Based on the upper bound approach, the different terms of kinetic and consumed plastic energies likewise the applied impulse energy derived to produce an energy functional with unknown coefficients which is minimized through the displacement path. Finally, calculating the constants maximum residual deflection and strain distribution are obtained. Results of present model show ...

Research paper thumbnail of On the Dependency of Ductile Damage Evolution to Stress State with Shock Loading Pre-Mechanical Working in 7075-T6 Aluminum Alloy

International Journal of Applied Mechanics, 2016

In this paper, the evolution of a ductile damage in the 7075-T6 aluminum alloy is considered base... more In this paper, the evolution of a ductile damage in the 7075-T6 aluminum alloy is considered based on stress state parameters with a special focus on pre-mechanical working dependency. Uniaxial stress–strain curves are investigated experimentally for two conditions; specimens with shock loaded pre-mechanical working and without it. This kind of loading is applied in order to find out impulsive pressure effects of damage variation procedure. Some experiments are done to take different stress states. Applying two fracture initiations criteria, i.e., Hosford–Coulomb and Xue models, two types of fracture locus of Al-7075-T6 are predicted in terms of plastic strains and stress state parameters under above conditions. By considering experimental data, a new ductile damage evolution model is proposed among plastic behavior. It is introduced by explicating an uncoupled plasticity and related to initial rate dependent stress state. By using both fracture models, our damage evolution model is...

Research paper thumbnail of A New Approach for Stress State - Dependent Flow Localization Failure Bounded Through Ductile Damage in Dynamically Loaded Sheets

Journal of Solid Mechanics, 2020

In this paper, a new approach is proposed for stress state - dependent flow localization in bifur... more In this paper, a new approach is proposed for stress state - dependent flow localization in bifurcation failure model bounded through ductile damage in dynamically loaded sheets. Onset of localized necking is considered in phenomenological way for different strain rates to draw the forming limit diagram (FLD). Using a strain metal hardening exponent in the Vertex theory related to the strain rate helps investigate rate- dependent metal forming limits. Besides, the paper utilizes the model of ductile damage as a function of strain condition, stress states (triaxiality and Lode parameters), and the symbols of stiffness strain to predict the onset of the necking. It is worth noting that updated level of elasticity modulus in the plastic deforming is attributed as an essential index for the ductile damage measuring. According to original formulations, a UMAT subroutine is developed in the finite element simulation by ABAQUS code to analyze and connect the related constitutive models. Re...