Applying classical molecular dynamics simulations, we report the effects of length ( λ) and orien... more Applying classical molecular dynamics simulations, we report the effects of length ( λ) and orientation ( θ) of a line-defect on strength and toughness in defective 2D hexagonal boron nitride. Results reveal the existence of a “transition angle,” θ t = 2.47 °, at which both toughness and strength are insensitive to the finite length of the defect in an infinite domain. For θ θ t, they show the opposite behavior. Examination of the stress-fields shows that θ-dependent variation in stress-localization at the edges of the line-defect and symmetry-breaking of the stress-fields with respect to the defect-axis govern the disparate θ-dependent behavior. For θ θ t, the stress-intensity at the edges is strongly localized at the opposite sides of the line-defect. The stress-intensity increases asymptotically with the increasing defect-length and reduces the strength and toughness of the defective lattice. The stress-localization, however, saturates at a “saturation angle” of around 60 ° for s...
A finite element model of bones with accurate geometry and material properties generated in CAD s... more A finite element model of bones with accurate geometry and material properties generated in CAD softwares are being widely used to make realistic investigations on the mechanical behavior of bone structures. The aim of this study is to create a model of real proximal human femur bone for evaluating the finite element analysis (FEA) and investigate the use of Ti6Al4V and Al2O3/Al FGM for artificial femur. Here, behavior of femur bone is analyzed in ANSYS 13 workbench under physiological load conditions and compared with artificial femur composed of Ti6Al4V and Al2O3/Al FGM. The CAD model was imported in Ansys 13.0 workbench, meshed and analysed in Ansys mechanical APDL workbench under the loading conditions. It was found that both material are suitable for artificial bone material. Human femur with Al2O3/Al FGM showed better mechanical properties and less weight compared to Ti6Al4V. In the biological environment, the demands of biomaterials are challenging. This study will be useful ...
The effect of autofrettage process on thick-walled cylinders has been investigated here. It is ob... more The effect of autofrettage process on thick-walled cylinders has been investigated here. It is observed that flow stress distribution along the cylinders remains same for same k values. Comparison with Zhu & Yang’s model has been also done in determination of optimum elasto-plastic radius, ropt and optimum autofrettage pressure, popt. Equivalent von Mises stress is used as yield criterion. It is observed that percentage of reduction of maximum von Mises stress increases as value of k and working pressure increases. Maximum von Mises stress is minimum for lower working pressure at same value of k and autofrettage pressure. Autofrettage process never starts if autofrettage pressure does not exceed working pressure. It is also observed that two limits of autofrettage pressure Py1 & Py2 are not appropriate. Effect of loading stages on autofrettage process is also investigated. As long as the pressures in first and last stage remains constant, there is no effect of loading stages on auto...
In this study a simple rectangular beam has been analyzed. For simplicity two dimensional plane s... more In this study a simple rectangular beam has been analyzed. For simplicity two dimensional plane strain problem is considered. The beam is fixed at the left and right edges of the bottom surface. Uniformly distributed load is applied along the length of the beam. A new formulation, known as displacement potential function formulation has been used here. Stresses and displacements at different sections along the length and depth of the beam have been plotted and discussed. The curves are found to be in complete conformity with the available literature and physical condition of the beam. The present literature reveals that the fixed connections of the beam are extremely critical regions and have the highest probability to fail.
This paper reveals the existence of a critical separation distance ( d c) beyond which the elasti... more This paper reveals the existence of a critical separation distance ( d c) beyond which the elastic interactions between a pair of monovacancies in graphene or hexagonal boron nitride become inconsequential for the strength and toughness of the defective lattice. This distance is independent of the chirality of the lattice. For any inter-defect distance higher than d c, the lattice behaves mechanically as if there is a single defect. For a distance less than d c, the defect–defect elastic interactions produce distinctive mechanical behavior depending on the orientation ( θ) of the defect pair relative to the loading direction. Both strength and toughness of the lattice containing a pair of “interacting monovacancies (iMVs)” are either higher or smaller than that of the lattice containing a pair of “non-interacting monovacancies (nMVs),” suggesting the existence of a critical orientation angle θ c. For θ < θ c, the smaller the distance between the iMVs, the higher the toughness and strength compared to the lattice containing nMVs, whereas, for θ ≥ θ c, the smaller the separation distance between the iMVs, the smaller the toughness and strength compared to the lattice containing nMVs. The transitional behavior has a negligible dependence on the chirality of the lattice, which indicates that the crystallographic anisotropy has a much weaker influence on toughness and strength compared to the anisotropy induced by the orientation angle itself. These observations underline an important point that the elastic fields emanating from vacancy defects are highly localized and fully contained within a small region of around 1.5 nm radius.
Abstract Tablet computers (PCs) are continuously moving towards light, thin, and small shape fact... more Abstract Tablet computers (PCs) are continuously moving towards light, thin, and small shape factors to achieve an eye appealing design while providing extreme mobility. The increased integration of electronic components with high power density, smaller size, and more compact layout is leading to exceptionally high operating temperatures for tablet PCs. This research focuses on development of an inexpensive and practical solution for thermal management for tablet PCs using phase change materials (PCMs) encapsulated in thin aluminized laminated film. In this paper, the performance of the tablet PC with this PCM thermal energy storage (TES) unit under continuous operation was investigated. PT-37, a commercial PCM from PureTemp, and n-eicosane were used as PCMs, as they have been shown to be safe and relatively inexpensive, and have melting temperatures in the range suitable for thermal management of portable electronics such as tablet PCs. It was found that the insertion of the thin PCM-based TES system within that tablet PC resulted in a reduction of the rate of temperature increase, for both the electronics and the tablet cover, during the transient start-up phase of operation; leading ultimately, to lower operating temperature of the entire tablet. Findings from this research confirm that the implementation of PCM–based TES unit is a viable option for thermal management of tablet PCs.
This paper presents an experimental study of thermal management in a tablet PC under intermittent... more This paper presents an experimental study of thermal management in a tablet PC under intermittent operation, with phase change materials (PCMs) encapsulated in a very thin aluminized laminate film container. It complements a study of the same system under continuous operation that was published in 2018. Two different types of PCMs were used for the experimental work; PT-37 and n-eicosane. A commercially available tablet PC was used as a test subject to ensure representative dimensions and material properties of every intricate part of the real tablet PC. For intermittent operation, the cycle used corresponds to fifteen minutes of operation (heat generation) followed by fifteen minutes of rest, to imitate a regular usage pattern of a tablet PC. It was observed that thin PCM thermal energy storage (TES) units are capable of providing a reduction in the rate of temperature increase during transient operations for both the electronics and the tablet cover. Reduction in peak temperature ...
Abstract This paper presents the implication of crystallographic orientation on toughness and ide... more Abstract This paper presents the implication of crystallographic orientation on toughness and ideal strength in graphene under lattice symmetry-preserving and symmetry-breaking deformations. In symmetry-preserving deformation, both toughness and strength are isotropic, regardless of the chirality of the lattice; whereas, in symmetry-breaking deformation they are strongly anisotropic, even in the presence of vacancy defects. The maximum and minimum of toughness or strength occur for loading along the zigzag direction and the armchair direction, respectively. The anisotropic behavior is governed by a complex interplay among bond-stretching deformation, bond-bending deformation, and the chirality of the lattice. Nevertheless, the condition for crack-nucleation is dictated by the maximum bond-force required for bond rupture, and it is independent of the chiral angle of the lattice or loading direction. At the onset of crack-nucleation a localized nucleation zone is formed, wherein the bonds rupture locally satisfying the maximum bond-force criterion. The nucleation zone acts as the physical origin in triggering the fracture nucleation process, but its presence is undetectable from the macroscopic stress–strain data.
The analysis of two dimensional (2D) flow over NACA 0012 airfoil is validated with NASA Langley R... more The analysis of two dimensional (2D) flow over NACA 0012 airfoil is validated with NASA Langley Research Center validation cases. The k-ω shear stress transport (SST) model is utilized to predict the flow accurately along with turbulence intensities 1% and 5% at velocity inlet and pressure outlet respectively. The computational domain is composed of 120000 structured cells. In order to enclose the boundary layer method the enhancement of the grid near the airfoil is taken care off. This validated simulation technique is further used to analyse aerodynamic characteristics of plain flapped NACA 0012 airfoil subjected to different flap angles and Mach number. The calculation of lift coefficients (CL), drag coefficients (CD) and CL/CD ratio at different operating conditions show that with increasing Mach number (M) CL increases but CD remains somewhat constant. Moreover, a rapid drastic decrease is observed for CL and an abrupt upsurge is observed for Cd with velocity approaching to the...
The effect of autofrettage process on thick-walled cylinders has been investigated here. It is ob... more The effect of autofrettage process on thick-walled cylinders has been investigated here. It is observed that flow stress distribution along the cylinders remains same for same k values. Comparison with Zhu & Yang's model has been also done in determination of optimum elasto-plastic radius, r opt and optimum autofrettage pressure, p opt. Equivalent von Mises stress is used as yield criterion. It is observed that percentage of reduction of maximum von Mises stress increases as value of k and working pressure increases. Maximum von Mises stress is minimum for lower working pressure at same value of k and autofrettage pressure. Autofrettage process never starts if autofrettage pressure does not exceed working pressure. It is also observed that two limits of autofrettage pressure Py1 & Py2 are not appropriate. Effect of loading stages on autofrettage process is also investigated. As long as the pressures in first and last stage remains constant, there is no effect of loading stages o...
Thermal flow simulation can be used to study the fluid flow and heat transfer for a wide variety ... more Thermal flow simulation can be used to study the fluid flow and heat transfer for a wide variety of engineering equipment. Flow simulations with the advent of computer architectures with superfast processing capabilities are rapidly emerging as an attractive alternative to conventional thermal flow analysis which is either too restrictive or expensive. In thermodynamic applications, increase of thermal efficiency of heat exchangers (i.e. radiators, cooling towers, condensers, intercoolers) is essential for compact design and improving whole cycle efficiency thus improving economic viability of the system. This paper outlines the process taken to optimize the geometry of conventional heat exchanger. Models were drawn into Solidworks and a computational domain was created. Solidworks Thermal Simulation was used to iterate toward a converged solution with the goal of obtaining a better efficiency of the heat exchanger. The results are analyzed and compared between two differently desig...
Applying classical molecular dynamics simulations, we report the effects of length ( λ) and orien... more Applying classical molecular dynamics simulations, we report the effects of length ( λ) and orientation ( θ) of a line-defect on strength and toughness in defective 2D hexagonal boron nitride. Results reveal the existence of a “transition angle,” θ t = 2.47 °, at which both toughness and strength are insensitive to the finite length of the defect in an infinite domain. For θ θ t, they show the opposite behavior. Examination of the stress-fields shows that θ-dependent variation in stress-localization at the edges of the line-defect and symmetry-breaking of the stress-fields with respect to the defect-axis govern the disparate θ-dependent behavior. For θ θ t, the stress-intensity at the edges is strongly localized at the opposite sides of the line-defect. The stress-intensity increases asymptotically with the increasing defect-length and reduces the strength and toughness of the defective lattice. The stress-localization, however, saturates at a “saturation angle” of around 60 ° for s...
A finite element model of bones with accurate geometry and material properties generated in CAD s... more A finite element model of bones with accurate geometry and material properties generated in CAD softwares are being widely used to make realistic investigations on the mechanical behavior of bone structures. The aim of this study is to create a model of real proximal human femur bone for evaluating the finite element analysis (FEA) and investigate the use of Ti6Al4V and Al2O3/Al FGM for artificial femur. Here, behavior of femur bone is analyzed in ANSYS 13 workbench under physiological load conditions and compared with artificial femur composed of Ti6Al4V and Al2O3/Al FGM. The CAD model was imported in Ansys 13.0 workbench, meshed and analysed in Ansys mechanical APDL workbench under the loading conditions. It was found that both material are suitable for artificial bone material. Human femur with Al2O3/Al FGM showed better mechanical properties and less weight compared to Ti6Al4V. In the biological environment, the demands of biomaterials are challenging. This study will be useful ...
The effect of autofrettage process on thick-walled cylinders has been investigated here. It is ob... more The effect of autofrettage process on thick-walled cylinders has been investigated here. It is observed that flow stress distribution along the cylinders remains same for same k values. Comparison with Zhu & Yang’s model has been also done in determination of optimum elasto-plastic radius, ropt and optimum autofrettage pressure, popt. Equivalent von Mises stress is used as yield criterion. It is observed that percentage of reduction of maximum von Mises stress increases as value of k and working pressure increases. Maximum von Mises stress is minimum for lower working pressure at same value of k and autofrettage pressure. Autofrettage process never starts if autofrettage pressure does not exceed working pressure. It is also observed that two limits of autofrettage pressure Py1 & Py2 are not appropriate. Effect of loading stages on autofrettage process is also investigated. As long as the pressures in first and last stage remains constant, there is no effect of loading stages on auto...
In this study a simple rectangular beam has been analyzed. For simplicity two dimensional plane s... more In this study a simple rectangular beam has been analyzed. For simplicity two dimensional plane strain problem is considered. The beam is fixed at the left and right edges of the bottom surface. Uniformly distributed load is applied along the length of the beam. A new formulation, known as displacement potential function formulation has been used here. Stresses and displacements at different sections along the length and depth of the beam have been plotted and discussed. The curves are found to be in complete conformity with the available literature and physical condition of the beam. The present literature reveals that the fixed connections of the beam are extremely critical regions and have the highest probability to fail.
This paper reveals the existence of a critical separation distance ( d c) beyond which the elasti... more This paper reveals the existence of a critical separation distance ( d c) beyond which the elastic interactions between a pair of monovacancies in graphene or hexagonal boron nitride become inconsequential for the strength and toughness of the defective lattice. This distance is independent of the chirality of the lattice. For any inter-defect distance higher than d c, the lattice behaves mechanically as if there is a single defect. For a distance less than d c, the defect–defect elastic interactions produce distinctive mechanical behavior depending on the orientation ( θ) of the defect pair relative to the loading direction. Both strength and toughness of the lattice containing a pair of “interacting monovacancies (iMVs)” are either higher or smaller than that of the lattice containing a pair of “non-interacting monovacancies (nMVs),” suggesting the existence of a critical orientation angle θ c. For θ < θ c, the smaller the distance between the iMVs, the higher the toughness and strength compared to the lattice containing nMVs, whereas, for θ ≥ θ c, the smaller the separation distance between the iMVs, the smaller the toughness and strength compared to the lattice containing nMVs. The transitional behavior has a negligible dependence on the chirality of the lattice, which indicates that the crystallographic anisotropy has a much weaker influence on toughness and strength compared to the anisotropy induced by the orientation angle itself. These observations underline an important point that the elastic fields emanating from vacancy defects are highly localized and fully contained within a small region of around 1.5 nm radius.
Abstract Tablet computers (PCs) are continuously moving towards light, thin, and small shape fact... more Abstract Tablet computers (PCs) are continuously moving towards light, thin, and small shape factors to achieve an eye appealing design while providing extreme mobility. The increased integration of electronic components with high power density, smaller size, and more compact layout is leading to exceptionally high operating temperatures for tablet PCs. This research focuses on development of an inexpensive and practical solution for thermal management for tablet PCs using phase change materials (PCMs) encapsulated in thin aluminized laminated film. In this paper, the performance of the tablet PC with this PCM thermal energy storage (TES) unit under continuous operation was investigated. PT-37, a commercial PCM from PureTemp, and n-eicosane were used as PCMs, as they have been shown to be safe and relatively inexpensive, and have melting temperatures in the range suitable for thermal management of portable electronics such as tablet PCs. It was found that the insertion of the thin PCM-based TES system within that tablet PC resulted in a reduction of the rate of temperature increase, for both the electronics and the tablet cover, during the transient start-up phase of operation; leading ultimately, to lower operating temperature of the entire tablet. Findings from this research confirm that the implementation of PCM–based TES unit is a viable option for thermal management of tablet PCs.
This paper presents an experimental study of thermal management in a tablet PC under intermittent... more This paper presents an experimental study of thermal management in a tablet PC under intermittent operation, with phase change materials (PCMs) encapsulated in a very thin aluminized laminate film container. It complements a study of the same system under continuous operation that was published in 2018. Two different types of PCMs were used for the experimental work; PT-37 and n-eicosane. A commercially available tablet PC was used as a test subject to ensure representative dimensions and material properties of every intricate part of the real tablet PC. For intermittent operation, the cycle used corresponds to fifteen minutes of operation (heat generation) followed by fifteen minutes of rest, to imitate a regular usage pattern of a tablet PC. It was observed that thin PCM thermal energy storage (TES) units are capable of providing a reduction in the rate of temperature increase during transient operations for both the electronics and the tablet cover. Reduction in peak temperature ...
Abstract This paper presents the implication of crystallographic orientation on toughness and ide... more Abstract This paper presents the implication of crystallographic orientation on toughness and ideal strength in graphene under lattice symmetry-preserving and symmetry-breaking deformations. In symmetry-preserving deformation, both toughness and strength are isotropic, regardless of the chirality of the lattice; whereas, in symmetry-breaking deformation they are strongly anisotropic, even in the presence of vacancy defects. The maximum and minimum of toughness or strength occur for loading along the zigzag direction and the armchair direction, respectively. The anisotropic behavior is governed by a complex interplay among bond-stretching deformation, bond-bending deformation, and the chirality of the lattice. Nevertheless, the condition for crack-nucleation is dictated by the maximum bond-force required for bond rupture, and it is independent of the chiral angle of the lattice or loading direction. At the onset of crack-nucleation a localized nucleation zone is formed, wherein the bonds rupture locally satisfying the maximum bond-force criterion. The nucleation zone acts as the physical origin in triggering the fracture nucleation process, but its presence is undetectable from the macroscopic stress–strain data.
The analysis of two dimensional (2D) flow over NACA 0012 airfoil is validated with NASA Langley R... more The analysis of two dimensional (2D) flow over NACA 0012 airfoil is validated with NASA Langley Research Center validation cases. The k-ω shear stress transport (SST) model is utilized to predict the flow accurately along with turbulence intensities 1% and 5% at velocity inlet and pressure outlet respectively. The computational domain is composed of 120000 structured cells. In order to enclose the boundary layer method the enhancement of the grid near the airfoil is taken care off. This validated simulation technique is further used to analyse aerodynamic characteristics of plain flapped NACA 0012 airfoil subjected to different flap angles and Mach number. The calculation of lift coefficients (CL), drag coefficients (CD) and CL/CD ratio at different operating conditions show that with increasing Mach number (M) CL increases but CD remains somewhat constant. Moreover, a rapid drastic decrease is observed for CL and an abrupt upsurge is observed for Cd with velocity approaching to the...
The effect of autofrettage process on thick-walled cylinders has been investigated here. It is ob... more The effect of autofrettage process on thick-walled cylinders has been investigated here. It is observed that flow stress distribution along the cylinders remains same for same k values. Comparison with Zhu & Yang's model has been also done in determination of optimum elasto-plastic radius, r opt and optimum autofrettage pressure, p opt. Equivalent von Mises stress is used as yield criterion. It is observed that percentage of reduction of maximum von Mises stress increases as value of k and working pressure increases. Maximum von Mises stress is minimum for lower working pressure at same value of k and autofrettage pressure. Autofrettage process never starts if autofrettage pressure does not exceed working pressure. It is also observed that two limits of autofrettage pressure Py1 & Py2 are not appropriate. Effect of loading stages on autofrettage process is also investigated. As long as the pressures in first and last stage remains constant, there is no effect of loading stages o...
Thermal flow simulation can be used to study the fluid flow and heat transfer for a wide variety ... more Thermal flow simulation can be used to study the fluid flow and heat transfer for a wide variety of engineering equipment. Flow simulations with the advent of computer architectures with superfast processing capabilities are rapidly emerging as an attractive alternative to conventional thermal flow analysis which is either too restrictive or expensive. In thermodynamic applications, increase of thermal efficiency of heat exchangers (i.e. radiators, cooling towers, condensers, intercoolers) is essential for compact design and improving whole cycle efficiency thus improving economic viability of the system. This paper outlines the process taken to optimize the geometry of conventional heat exchanger. Models were drawn into Solidworks and a computational domain was created. Solidworks Thermal Simulation was used to iterate toward a converged solution with the goal of obtaining a better efficiency of the heat exchanger. The results are analyzed and compared between two differently desig...
Uploads
Papers by Tousif Ahmed