In this manuscript, we have studied the microstructure of the axonal cytoskeleton and adopted a b... more In this manuscript, we have studied the microstructure of the axonal cytoskeleton and adopted a bottom-up approach to evaluate the mechanical responses of axons. The cytoskeleton of the axon includes the microtubules (MT), Tau proteins (Tau), neurofilaments (NF), and microfilaments (MF). Although most of the rigidity of the axons is due to the MT, the viscoelastic response of axons comes from the Tau. Early studies have shown that NF and MF do not provide significant elasticity to the overall response of axons. Therefore, the most critical aspect of the mechanical response of axons is the microstructural topology of how MT and Tau are connected and construct the cross-linked network. Using a scanning electron microscope (SEM), the cross-sectional view of the axons revealed that the MTs are organized in a hexagonal array and cross-linked by Tau. Therefore, we have developed a hexagonal Representative Volume Element (RVE) of the axonal microstructure with MT and Tau as fibers. The mat...
Fused filament fabrication (FFF) is one of the most common additive manufacturing techniques, in ... more Fused filament fabrication (FFF) is one of the most common additive manufacturing techniques, in which, continuously extruded semi-molten filaments are deposited in a layer-by-layer manner. The quality of the manufactured part heavily depends on filament-filament contact, filament-filament interfacial adhesion and overall void fraction. In our earlier work, we used a novel fabrication method that applied additional compression to newly deposited filaments using an in situ roller ball. We then studied the effect of in situ compression on the quality of adhesion, and subsequently, on the thermal and mechanical properties of the printed parts. Under an optimized set of experimental conditions, a significant improvement in material toughness and tensile strength was measured. Here, we have developed an integrated theoretical model that predicts the impact of in situ compression rolling on filament-filament contact during deposition. The impact of key parameters associated with the rolling process, such as ball weight, ball temperature and filament temperature on printed part height, void fraction and filament adhesion are studied. Based on the Johnson–Kendall–Roberts (JKR) contact theory and the theory of elasticity, our mathematical model predicts the evolution of filament-to-filament contact width, the corresponding void fraction and part height in a representative volume element of the printed part. Our theoretical predictions are in good agreement with experimental measurements. Later, the theoretical model is used to optimize the filament temperature during the rolling process. Specifically, we find that isothermal contact between filaments results in optimal adhesion. We have concluded that parts fabricated from a system integrated with an in situ preheating and in situ post-rolling would yield 3D printed plastic parts with enhanced mechanical properties suitable for various structural applications.
Typical electronics packages are assembled by integrating various parts on printed circuit boards... more Typical electronics packages are assembled by integrating various parts on printed circuit boards (PCB). Traditional interconnect materials in electronics packages are not suitable for DoD electronics because in many DoD extremely transient conditions, mechanical failures of the whole packages invariably occur due to interconnect junction failures. The long-term objective of the research is to computationally investigate the effect of high strain rate loadings on the thermal and mechanical damage/failure of carbon nanotube reinforced polymer nanocomposites, while retaining their electrical functionality. In pursuit of our research goal, we first seek to obtain the elastic response of the nanocomposites. In particular, carbon nanotubes (CNTs) are dispersed in polymer matrix in a random fashion. In the present study, a two-dimensional network of CNTs spread in polymer matrix is statistically generated using Matlab code. This Representative Volume Element (RVE) is further processed into a Finite Element Model (FEM). Abaqus is employed to evaluate the elastic constants such as Young’s modulus, Poisson’s ratio, and shear modulus for this nanocomposite. Further, Halpin-Tsai equations are used to compare the values obtained from the finite element analysis.
Carbon nanotubes (CNTs) are the filler materials of nanoscale dimensions, which when added to the... more Carbon nanotubes (CNTs) are the filler materials of nanoscale dimensions, which when added to the polymer matrix, form a strong yet light-weight multifunctional composite materials. Before using them effectively and confidently in the aerospace industry, it is important to completely determine their mechanical properties. Previous studies found these properties to be significantly influenced by the waviness of CNTs embedded in the epoxy matrix. In this study, we have developed a unique Python algorithm that is employed to construct the stochastic three-dimensional finite element models, which incorporates the CNT waviness as a variable. Random normal and uniform random distributions are assumed for waviness and orientation angles of the dispersed CNTs, respectively. Four separate finite element models with maximum CNT waviness angles of 0⁰, 25⁰, & 50⁰ are generated for CNT volume fraction of 0.5%. Thereafter, the elastic constants are evaluated and compared for each case. A significant decrease in the properties is observed as the maximum allowable waviness angle of the embedded CNTs in the epoxy is increased. These results suggest that CNT waviness plays a key role in deciding the elastic properties of the CNT based composites.
We report mechanisms to enhance the strength, modulus, and fracture strain of linear low density ... more We report mechanisms to enhance the strength, modulus, and fracture strain of linear low density polyethylene (LDPE) polymer through hybridizing it with nylon 6 as a minor phase and simultaneously reinforcing it with single-walled carbon nanotubes (SWCNTs). Loading of nylon 6 and SWCNTs into LDPE was 25.0 -75% wt% (to understand the effect of minor phase) and 2.0 wt%, respectively. Hybridized polymer nanocomposites were virtually processed using successive simulations including Self-avoiding random walk (SARW) to build polymer network and high temperature molecular dynamics simulation to enable melt mixing. After developing thermodynamically stable hybrid polymer nanocomposite systems, we performed virtual tensile tests using molecular dynamics. The molecular dynamics simulation studies reveal that the interface between polymers and nanotube plays critical role. It is found in study that hybridized LDPE with nylon 6 increase the interfacial strength as well takes more energy to pull out the SWCNT from polymer.
The perineuronal net (PNN) region of the brain’s extracellular matrix (ECM) surrounds the neural ... more The perineuronal net (PNN) region of the brain’s extracellular matrix (ECM) surrounds the neural networks within the brain tissue. The PNN is a protective net-like structure regulating neuronal activity such as neurotransmission, charge balance, and action potential generation. Shock-induced damage of this essential component may lead to neuronal cell death and neurodegenerations. The shock generated during a vehicle accident, fall, or improvised device explosion may produce sufficient energy to damage the structure of the PNN. The goal is to investigate the mechanics of the PNN in reaction to shock loading and to understand the mechanical properties of different PNN components such as glycan, GAG, and protein. In this study, we evaluated the mechanical strength of PNN molecules and the interfacial strength between the PNN components. Afterward, we assessed the PNN molecules’ damage efficiency under various conditions such as shock speed, preexisting bubble, and boundary conditions....
The thermoset epoxy resin Diglycidyl ether of Bisphenol F (EPON 862), crosslinked with the Diethy... more The thermoset epoxy resin Diglycidyl ether of Bisphenol F (EPON 862), crosslinked with the Diethylene Toluene Diamine (DETDA) hardening agent, are utilized as the polymer matrix component in many graphite (carbon fiber) composites. Since it is difficult to experimentally characterize the interfacial region, computational molecular modeling is a necessary tool for understanding the influence of the interfacial molecular structure on bulk-level material properties. The purpose of this research is to evaluate and compare the interfacial shear stress and dipole moment for the pristine carbon fiber composite and the one with the moisture content at the interface. Molecular models are established for Carbon fiber reinforced EPON 862-DETDA polymer with and without the moisture content at the interface. Interatomic interactions are defined by Reactive Force Field (ReaxFF). Material characteristics such as polymer mass-density and dipole moment are investigated near the polymer/fiber interface. It is determined that a region exists near the carbon fiber surface in which the polymer mass density and dipole moment are different than that of the bulk values. It can further be seen that material having larger values of dipole moment in interface region have comparatively lesser values of interfacial shear stress.
Electrospinning has been successfully used to develop aligned micro- and nanofiber mats for light... more Electrospinning has been successfully used to develop aligned micro- and nanofiber mats for lightweight reinforcement of composite materials and structures. Electrospinning systems can be oriented either vertically, utilizing gravity-assisted electric field material deposition, or horizontally, utilizing electric field with little effect on gravity. A horizontal apparatus has been designed and fabricated for improved micro- and nano-fiber alignment that provides the possibility of developing better reinforcement materials in the form of aligned fiber mats. These nano fiber mats are stiffer, stronger and have better mechanical, electrical and thermal properties for structural and multi-functional applications. The micro- and nano-fiber mats produced with the set up were scanned under Scanning Electron Microscope (SEM). These images show successful deposition of micro- and nano-fibers with alignment comparable with current electrospinning systems. System enhancement is in work to enhance alignment of deposited fibers through additional electromagnetic controls.
Typical electronics packages are assembled by integrating various parts on printed circuit boards... more Typical electronics packages are assembled by integrating various parts on printed circuit boards (PCB). Traditional interconnect materials in electronics packages are not suitable for DoD electronics because in many DoD extremely transient conditions, mechanical failures of the whole packages invariably occur due to interconnect junction failures. The objective of the research is to computationally investigate the effect of high strain rate loadings on the thermal and mechanical damage/failure of carbon nanotube reinforced polymer nanocomposites. In pursuit of our research goal, we first seek to obtain the elastic properties of the nanocomposites. Properties at interface between CNT/polymer are critical to determine mechanical, electrical, and thermal properties of these nanocomosites. In the present study, we have used reactive force field (ReaxFF) to study the interfacial properties of CNT/EPON 862-DETDA nanocomposite system. Because molecular-level failure events can play a significant role in epoxy mechanical behavior, the ReaxFF can be used as an ideal tool for MD simulations involving crosslinked epoxies. Pull out simulations are performed to characterize the CNT/polymer interfacial interactions. Pull out energy is used to calculate the interfacial shear strength of CNT/polymer nanocomposite.
In blast-induced traumatic brain injury, shock waves (SW) play an important role along with cavit... more In blast-induced traumatic brain injury, shock waves (SW) play an important role along with cavitation phenomena. Due to the lack of reliable and reproducible experimental investigations, we have a limited understanding of the role of cavitation in brain damage. The present study aims to develop an atomistic simulation model to determine the role of shock-induced impulse and different constituents of the brain’s extra-cellular matrix (ECM) on the formation mechanism, stability and collapsing mechanism of nanobubbles in the ECM. The ECM in the brain can be divided into three major types depending on their location behind the blood-brain barrier, namely (a) the basement membrane (basal lamina), (b) the perineuronal nets and (3) the neural interstitial matrix. In this paper, we have studied the interaction of nanobubbles with bio-molecules of the perineuronal nets. We have chosen this zone of the ECM because we are interested to obtain the role of cavitation bubble collapse in neuron d...
The inner part of bones is spongy cell-type three dimensional structure called cancellous bone wh... more The inner part of bones is spongy cell-type three dimensional structure called cancellous bone which is the major contributor in keeping the bone weight very low. Yet, bone can sustain body weight and reasonable impact loads implying bone is a naturally optimized lightweight but strong structure.This study demonstrates a novel approach to manufacture bonelike nanocomposite structure for weight saving structural applications. For this, Silicon Carbide (SiC) nano particles are dispersed in SC-15 epoxy resin (a two part liquid resin that solidifies when mixed together at room temperature). Using our novel 3D+ manufacturing technique we developed lighter but stronger nanocomposite structure. The compression test results showed an improvement in the strength and stiffness of epoxy polymer when a small percentage of SiC nano particles are dispersed in pure epoxy matrix. Using the same manufacturing process, new types of sandwich structures with polyurethane foam core were manufactured and...
Carbon nanotubes (CNTs) are frequently used as the nanoscale filler materials. It has been observ... more Carbon nanotubes (CNTs) are frequently used as the nanoscale filler materials. It has been observed that when they are added to the polymer matrix by a small fraction, they are able to form a strong yet light-weight multifunctional composite materials. Previous studies found these properties to be significantly influenced by the morphology of CNTs embedded in the matrix. In general, long CNTs become wavy and randomly oriented when dispersed in the matrix material. Collectively, they form an interconnected network. Over the past several decades, many theoretical and computational studies have been carried out to capture the mechanics of CNT-reinforced nanocomposites. In most of these models, CNTs are modeled as straight fibers. In addition, a perfect interphase between CNT and epoxy is generally assumed. As such, these models may not capture the realistic morphology of CNT reinforced composites. In the current study, we have developed a method to construct stochastic three-dimensiona...
Actin and spectrin are important constituents of axonal cytoskeleton. Periodic actin-spectrin str... more Actin and spectrin are important constituents of axonal cytoskeleton. Periodic actin-spectrin structures are found in dendrites, initial segment of axon, and main axon. Actin-spectrin periodicity has been hypothesized to be manipulating the axon stability and mechanical behavior. Several experimental and computational studies have been performed focusing on the mechanical behavior of actin, spectrin, and actin-spectrin network. However, most of the actin studies focus on typical long F-actin and do not provide quantitative comparison between the mechanical behavior of short and long actin filaments. Also, most of the spectrin studies focus on erythrocytic spectrin and do not shed light on the behavior of structurally different axonal spectrin. Only a few studies have highlighted forced unfolding of axonal spectrin which are relevant to brain injury scenario. A comprehensive, strain rate dependent mechanical study is still absent in the literature. Moreover, the current opinions regarding periodic actin-spectrin network structure in axon are disputed due to conflicting results on actin ring organizationas argued by recent super-resolution microscopy studies. This review summarizes the ongoing limitations in this regard and provides insights on possible approaches to address them. This study will invoke further investigation into relevant high strain rate response of actin, spectrin, and actin-spectrin networkshedding light into brain pathology scenario such as traumatic brain injury (TBI).
Cavitation in soft biomaterials occurs at higher tensile pressure than pure water. A bubble needs... more Cavitation in soft biomaterials occurs at higher tensile pressure than pure water. A bubble needs to overcome the surface energy and the strain energy contribution from the random fiber network, which is the source of the extra tensile pressure.
As a major cytoskeleton element of the axon, the breaking of microtubules (MTs) has been consider... more As a major cytoskeleton element of the axon, the breaking of microtubules (MTs) has been considered as a major cause of the axon degeneration. High strain rate loading is considered as one of the key factors in microtubule breaking. Due to the small size of microtubule, the real-time behavior of microtubule breaking is hard to capture. This study employs fully-atomistic molecular dynamics (MD) simulation to determine the failure modes of microtubule under different loadings conditions such as, unidirectional stretching, bending and hydrostatic expansion. For each loading conditions, MT is subjected to extreme high strain rate (108–109 s−1) loading. We argue that such level of high strain rate may be realized during cavitation bubble implosion. For each loading type, we have determined the critical energy for MT rupture. The associated rupture mechanisms are also discussed. We observed that the stretching has the lowest energy barrier to break the MT at the nanosecond time scale. Mor...
Silicon Carbide (SiC) exhibits excellent mechanical, thermal and electrical properties. Low fract... more Silicon Carbide (SiC) exhibits excellent mechanical, thermal and electrical properties. Low fracture toughness is one of the limiting properties of SiC that hinders its widespread applications. Recent studies suggest that controlled alteration of local micro-structure may lead to dislocation nucleation in certain SiC poly-types. Here, we report classical molecular dynamics simulations results to demonstrate mechanical behavior of a new type SiC-based ''C enriched" ceramics where certain Si atoms are substituted by C atoms. We studied four different systems with different fraction of ''C" enrichment, namely 10%, 20%, 30% and 40%. Thermodynamic viability of such novel micro-structures have been studied recently (Adnan and Ferdous, 2015). Significant effects of ''C" enrichment on the tensile and shear properties of the new ceramic materials have been observed. Compared to pure SiC, the tensile strengths of enriched systems always increase but to a different extent depending on the amount of enrichment. Shear strengths, however, tend to decrease with increase in carbon enrichment until enrichment is 40%. The elastic constants C 11 and C 44 were measured and both increase significantly with carbon enrichment. The area under the stress-strain tensile and shear curves have been estimated to assess the tensile and shear toughness properties and it has been found that both types of toughness increase significantly with carbon enrichment.
In this manuscript, we have studied the microstructure of the axonal cytoskeleton and adopted a b... more In this manuscript, we have studied the microstructure of the axonal cytoskeleton and adopted a bottom-up approach to evaluate the mechanical responses of axons. The cytoskeleton of the axon includes the microtubules (MT), Tau proteins (Tau), neurofilaments (NF), and microfilaments (MF). Although most of the rigidity of the axons is due to the MT, the viscoelastic response of axons comes from the Tau. Early studies have shown that NF and MF do not provide significant elasticity to the overall response of axons. Therefore, the most critical aspect of the mechanical response of axons is the microstructural topology of how MT and Tau are connected and construct the cross-linked network. Using a scanning electron microscope (SEM), the cross-sectional view of the axons revealed that the MTs are organized in a hexagonal array and cross-linked by Tau. Therefore, we have developed a hexagonal Representative Volume Element (RVE) of the axonal microstructure with MT and Tau as fibers. The mat...
Fused filament fabrication (FFF) is one of the most common additive manufacturing techniques, in ... more Fused filament fabrication (FFF) is one of the most common additive manufacturing techniques, in which, continuously extruded semi-molten filaments are deposited in a layer-by-layer manner. The quality of the manufactured part heavily depends on filament-filament contact, filament-filament interfacial adhesion and overall void fraction. In our earlier work, we used a novel fabrication method that applied additional compression to newly deposited filaments using an in situ roller ball. We then studied the effect of in situ compression on the quality of adhesion, and subsequently, on the thermal and mechanical properties of the printed parts. Under an optimized set of experimental conditions, a significant improvement in material toughness and tensile strength was measured. Here, we have developed an integrated theoretical model that predicts the impact of in situ compression rolling on filament-filament contact during deposition. The impact of key parameters associated with the rolling process, such as ball weight, ball temperature and filament temperature on printed part height, void fraction and filament adhesion are studied. Based on the Johnson–Kendall–Roberts (JKR) contact theory and the theory of elasticity, our mathematical model predicts the evolution of filament-to-filament contact width, the corresponding void fraction and part height in a representative volume element of the printed part. Our theoretical predictions are in good agreement with experimental measurements. Later, the theoretical model is used to optimize the filament temperature during the rolling process. Specifically, we find that isothermal contact between filaments results in optimal adhesion. We have concluded that parts fabricated from a system integrated with an in situ preheating and in situ post-rolling would yield 3D printed plastic parts with enhanced mechanical properties suitable for various structural applications.
Typical electronics packages are assembled by integrating various parts on printed circuit boards... more Typical electronics packages are assembled by integrating various parts on printed circuit boards (PCB). Traditional interconnect materials in electronics packages are not suitable for DoD electronics because in many DoD extremely transient conditions, mechanical failures of the whole packages invariably occur due to interconnect junction failures. The long-term objective of the research is to computationally investigate the effect of high strain rate loadings on the thermal and mechanical damage/failure of carbon nanotube reinforced polymer nanocomposites, while retaining their electrical functionality. In pursuit of our research goal, we first seek to obtain the elastic response of the nanocomposites. In particular, carbon nanotubes (CNTs) are dispersed in polymer matrix in a random fashion. In the present study, a two-dimensional network of CNTs spread in polymer matrix is statistically generated using Matlab code. This Representative Volume Element (RVE) is further processed into a Finite Element Model (FEM). Abaqus is employed to evaluate the elastic constants such as Young’s modulus, Poisson’s ratio, and shear modulus for this nanocomposite. Further, Halpin-Tsai equations are used to compare the values obtained from the finite element analysis.
Carbon nanotubes (CNTs) are the filler materials of nanoscale dimensions, which when added to the... more Carbon nanotubes (CNTs) are the filler materials of nanoscale dimensions, which when added to the polymer matrix, form a strong yet light-weight multifunctional composite materials. Before using them effectively and confidently in the aerospace industry, it is important to completely determine their mechanical properties. Previous studies found these properties to be significantly influenced by the waviness of CNTs embedded in the epoxy matrix. In this study, we have developed a unique Python algorithm that is employed to construct the stochastic three-dimensional finite element models, which incorporates the CNT waviness as a variable. Random normal and uniform random distributions are assumed for waviness and orientation angles of the dispersed CNTs, respectively. Four separate finite element models with maximum CNT waviness angles of 0⁰, 25⁰, & 50⁰ are generated for CNT volume fraction of 0.5%. Thereafter, the elastic constants are evaluated and compared for each case. A significant decrease in the properties is observed as the maximum allowable waviness angle of the embedded CNTs in the epoxy is increased. These results suggest that CNT waviness plays a key role in deciding the elastic properties of the CNT based composites.
We report mechanisms to enhance the strength, modulus, and fracture strain of linear low density ... more We report mechanisms to enhance the strength, modulus, and fracture strain of linear low density polyethylene (LDPE) polymer through hybridizing it with nylon 6 as a minor phase and simultaneously reinforcing it with single-walled carbon nanotubes (SWCNTs). Loading of nylon 6 and SWCNTs into LDPE was 25.0 -75% wt% (to understand the effect of minor phase) and 2.0 wt%, respectively. Hybridized polymer nanocomposites were virtually processed using successive simulations including Self-avoiding random walk (SARW) to build polymer network and high temperature molecular dynamics simulation to enable melt mixing. After developing thermodynamically stable hybrid polymer nanocomposite systems, we performed virtual tensile tests using molecular dynamics. The molecular dynamics simulation studies reveal that the interface between polymers and nanotube plays critical role. It is found in study that hybridized LDPE with nylon 6 increase the interfacial strength as well takes more energy to pull out the SWCNT from polymer.
The perineuronal net (PNN) region of the brain’s extracellular matrix (ECM) surrounds the neural ... more The perineuronal net (PNN) region of the brain’s extracellular matrix (ECM) surrounds the neural networks within the brain tissue. The PNN is a protective net-like structure regulating neuronal activity such as neurotransmission, charge balance, and action potential generation. Shock-induced damage of this essential component may lead to neuronal cell death and neurodegenerations. The shock generated during a vehicle accident, fall, or improvised device explosion may produce sufficient energy to damage the structure of the PNN. The goal is to investigate the mechanics of the PNN in reaction to shock loading and to understand the mechanical properties of different PNN components such as glycan, GAG, and protein. In this study, we evaluated the mechanical strength of PNN molecules and the interfacial strength between the PNN components. Afterward, we assessed the PNN molecules’ damage efficiency under various conditions such as shock speed, preexisting bubble, and boundary conditions....
The thermoset epoxy resin Diglycidyl ether of Bisphenol F (EPON 862), crosslinked with the Diethy... more The thermoset epoxy resin Diglycidyl ether of Bisphenol F (EPON 862), crosslinked with the Diethylene Toluene Diamine (DETDA) hardening agent, are utilized as the polymer matrix component in many graphite (carbon fiber) composites. Since it is difficult to experimentally characterize the interfacial region, computational molecular modeling is a necessary tool for understanding the influence of the interfacial molecular structure on bulk-level material properties. The purpose of this research is to evaluate and compare the interfacial shear stress and dipole moment for the pristine carbon fiber composite and the one with the moisture content at the interface. Molecular models are established for Carbon fiber reinforced EPON 862-DETDA polymer with and without the moisture content at the interface. Interatomic interactions are defined by Reactive Force Field (ReaxFF). Material characteristics such as polymer mass-density and dipole moment are investigated near the polymer/fiber interface. It is determined that a region exists near the carbon fiber surface in which the polymer mass density and dipole moment are different than that of the bulk values. It can further be seen that material having larger values of dipole moment in interface region have comparatively lesser values of interfacial shear stress.
Electrospinning has been successfully used to develop aligned micro- and nanofiber mats for light... more Electrospinning has been successfully used to develop aligned micro- and nanofiber mats for lightweight reinforcement of composite materials and structures. Electrospinning systems can be oriented either vertically, utilizing gravity-assisted electric field material deposition, or horizontally, utilizing electric field with little effect on gravity. A horizontal apparatus has been designed and fabricated for improved micro- and nano-fiber alignment that provides the possibility of developing better reinforcement materials in the form of aligned fiber mats. These nano fiber mats are stiffer, stronger and have better mechanical, electrical and thermal properties for structural and multi-functional applications. The micro- and nano-fiber mats produced with the set up were scanned under Scanning Electron Microscope (SEM). These images show successful deposition of micro- and nano-fibers with alignment comparable with current electrospinning systems. System enhancement is in work to enhance alignment of deposited fibers through additional electromagnetic controls.
Typical electronics packages are assembled by integrating various parts on printed circuit boards... more Typical electronics packages are assembled by integrating various parts on printed circuit boards (PCB). Traditional interconnect materials in electronics packages are not suitable for DoD electronics because in many DoD extremely transient conditions, mechanical failures of the whole packages invariably occur due to interconnect junction failures. The objective of the research is to computationally investigate the effect of high strain rate loadings on the thermal and mechanical damage/failure of carbon nanotube reinforced polymer nanocomposites. In pursuit of our research goal, we first seek to obtain the elastic properties of the nanocomposites. Properties at interface between CNT/polymer are critical to determine mechanical, electrical, and thermal properties of these nanocomosites. In the present study, we have used reactive force field (ReaxFF) to study the interfacial properties of CNT/EPON 862-DETDA nanocomposite system. Because molecular-level failure events can play a significant role in epoxy mechanical behavior, the ReaxFF can be used as an ideal tool for MD simulations involving crosslinked epoxies. Pull out simulations are performed to characterize the CNT/polymer interfacial interactions. Pull out energy is used to calculate the interfacial shear strength of CNT/polymer nanocomposite.
In blast-induced traumatic brain injury, shock waves (SW) play an important role along with cavit... more In blast-induced traumatic brain injury, shock waves (SW) play an important role along with cavitation phenomena. Due to the lack of reliable and reproducible experimental investigations, we have a limited understanding of the role of cavitation in brain damage. The present study aims to develop an atomistic simulation model to determine the role of shock-induced impulse and different constituents of the brain’s extra-cellular matrix (ECM) on the formation mechanism, stability and collapsing mechanism of nanobubbles in the ECM. The ECM in the brain can be divided into three major types depending on their location behind the blood-brain barrier, namely (a) the basement membrane (basal lamina), (b) the perineuronal nets and (3) the neural interstitial matrix. In this paper, we have studied the interaction of nanobubbles with bio-molecules of the perineuronal nets. We have chosen this zone of the ECM because we are interested to obtain the role of cavitation bubble collapse in neuron d...
The inner part of bones is spongy cell-type three dimensional structure called cancellous bone wh... more The inner part of bones is spongy cell-type three dimensional structure called cancellous bone which is the major contributor in keeping the bone weight very low. Yet, bone can sustain body weight and reasonable impact loads implying bone is a naturally optimized lightweight but strong structure.This study demonstrates a novel approach to manufacture bonelike nanocomposite structure for weight saving structural applications. For this, Silicon Carbide (SiC) nano particles are dispersed in SC-15 epoxy resin (a two part liquid resin that solidifies when mixed together at room temperature). Using our novel 3D+ manufacturing technique we developed lighter but stronger nanocomposite structure. The compression test results showed an improvement in the strength and stiffness of epoxy polymer when a small percentage of SiC nano particles are dispersed in pure epoxy matrix. Using the same manufacturing process, new types of sandwich structures with polyurethane foam core were manufactured and...
Carbon nanotubes (CNTs) are frequently used as the nanoscale filler materials. It has been observ... more Carbon nanotubes (CNTs) are frequently used as the nanoscale filler materials. It has been observed that when they are added to the polymer matrix by a small fraction, they are able to form a strong yet light-weight multifunctional composite materials. Previous studies found these properties to be significantly influenced by the morphology of CNTs embedded in the matrix. In general, long CNTs become wavy and randomly oriented when dispersed in the matrix material. Collectively, they form an interconnected network. Over the past several decades, many theoretical and computational studies have been carried out to capture the mechanics of CNT-reinforced nanocomposites. In most of these models, CNTs are modeled as straight fibers. In addition, a perfect interphase between CNT and epoxy is generally assumed. As such, these models may not capture the realistic morphology of CNT reinforced composites. In the current study, we have developed a method to construct stochastic three-dimensiona...
Actin and spectrin are important constituents of axonal cytoskeleton. Periodic actin-spectrin str... more Actin and spectrin are important constituents of axonal cytoskeleton. Periodic actin-spectrin structures are found in dendrites, initial segment of axon, and main axon. Actin-spectrin periodicity has been hypothesized to be manipulating the axon stability and mechanical behavior. Several experimental and computational studies have been performed focusing on the mechanical behavior of actin, spectrin, and actin-spectrin network. However, most of the actin studies focus on typical long F-actin and do not provide quantitative comparison between the mechanical behavior of short and long actin filaments. Also, most of the spectrin studies focus on erythrocytic spectrin and do not shed light on the behavior of structurally different axonal spectrin. Only a few studies have highlighted forced unfolding of axonal spectrin which are relevant to brain injury scenario. A comprehensive, strain rate dependent mechanical study is still absent in the literature. Moreover, the current opinions regarding periodic actin-spectrin network structure in axon are disputed due to conflicting results on actin ring organizationas argued by recent super-resolution microscopy studies. This review summarizes the ongoing limitations in this regard and provides insights on possible approaches to address them. This study will invoke further investigation into relevant high strain rate response of actin, spectrin, and actin-spectrin networkshedding light into brain pathology scenario such as traumatic brain injury (TBI).
Cavitation in soft biomaterials occurs at higher tensile pressure than pure water. A bubble needs... more Cavitation in soft biomaterials occurs at higher tensile pressure than pure water. A bubble needs to overcome the surface energy and the strain energy contribution from the random fiber network, which is the source of the extra tensile pressure.
As a major cytoskeleton element of the axon, the breaking of microtubules (MTs) has been consider... more As a major cytoskeleton element of the axon, the breaking of microtubules (MTs) has been considered as a major cause of the axon degeneration. High strain rate loading is considered as one of the key factors in microtubule breaking. Due to the small size of microtubule, the real-time behavior of microtubule breaking is hard to capture. This study employs fully-atomistic molecular dynamics (MD) simulation to determine the failure modes of microtubule under different loadings conditions such as, unidirectional stretching, bending and hydrostatic expansion. For each loading conditions, MT is subjected to extreme high strain rate (108–109 s−1) loading. We argue that such level of high strain rate may be realized during cavitation bubble implosion. For each loading type, we have determined the critical energy for MT rupture. The associated rupture mechanisms are also discussed. We observed that the stretching has the lowest energy barrier to break the MT at the nanosecond time scale. Mor...
Silicon Carbide (SiC) exhibits excellent mechanical, thermal and electrical properties. Low fract... more Silicon Carbide (SiC) exhibits excellent mechanical, thermal and electrical properties. Low fracture toughness is one of the limiting properties of SiC that hinders its widespread applications. Recent studies suggest that controlled alteration of local micro-structure may lead to dislocation nucleation in certain SiC poly-types. Here, we report classical molecular dynamics simulations results to demonstrate mechanical behavior of a new type SiC-based ''C enriched" ceramics where certain Si atoms are substituted by C atoms. We studied four different systems with different fraction of ''C" enrichment, namely 10%, 20%, 30% and 40%. Thermodynamic viability of such novel micro-structures have been studied recently (Adnan and Ferdous, 2015). Significant effects of ''C" enrichment on the tensile and shear properties of the new ceramic materials have been observed. Compared to pure SiC, the tensile strengths of enriched systems always increase but to a different extent depending on the amount of enrichment. Shear strengths, however, tend to decrease with increase in carbon enrichment until enrichment is 40%. The elastic constants C 11 and C 44 were measured and both increase significantly with carbon enrichment. The area under the stress-strain tensile and shear curves have been estimated to assess the tensile and shear toughness properties and it has been found that both types of toughness increase significantly with carbon enrichment.
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Papers by Ashfaq Adnan