Adarsh K. Chaurasia

Computational micromechanics analysis of electron-hopping-induced conductive paths and associated macroscale piezoresistive response in carbon nanotube–polymer nanocomposites:

In this study, a computational model is developed using finite-element techniques within a continuum micromechanics framework to capture the effect of electron-hopping-induced conductive paths at the nanoscale which contribute to the macroscale piezoresistive response of the nanocomposite. This is achieved by tracking the position of the nanotubes under applied deformations and modifying the conductivity of the intertube region depending on the relative proximity of individual pairs of nanotubes. The formation and disruption of the electron-hopping pathways are highly dependent on intertube distances and under deformations can result in microstructural rearrangements in terms of electrostatic properties leading to transitions in material symmetries and component magnitudes of the effective electrostatic properties. Thus, in order to capture the complexities of changing inhomogeneous nanoscale electrostatic behavior, where analytical Eshelby’s approaches cannot be used, a computational micromechanics model is needed. The effective conductivity and piezoresistive strain tensor coefficients are evaluated using volume-averaged energy equivalencies for aligned CNT–polymer nanocomposites in the transverse direction exploring different volume fractions of CNTs in the polymer and the maximum electron-hopping range. The impact of the electron-hopping mechanism on the effective piezoresistive response is studied through the macroscale effective gauge factors under different loading conditions. The effective piezoresistive strain coefficients and macroscale effective gauge factors are observed to be nonlinear with applied macroscale strain and are highly dependent on the type of boundary conditions. The effective macroscale gauge factors observed in the current study have magnitudes comparable to experimental observations reported in the literature with higher gauge factors observed closer to the percolation threshold.

Modeling of mesoscale dispersion effect on the piezoresistivity of carbon nanotube-polymer nanocomposites via 3D computational multiscale micromechanics methods

In uniaxial tension and compression experiments, carbon nanotube (CNT)-polymer nanocomposites have demonstrated exceptional mechanical and coupled electrostatic properties in the form of piezoresistivity. In order to better understand the correlation of the piezoresistive response with the CNT dispersion at the mesoscale, a 3D computational multiscale micromechanics model based on finite element analysis is constructed to predict the effective macroscale piezoresistive response of CNT/polymer nanocomposites. The key factors that may contribute to the overall piezoresistive response, i.e. the nanoscale electrical tunneling effect, the inherent CNT piezoresistivity and the CNT mesoscale network effect are incorporated in the model based on a 3D multiscale mechanical–electrostatic coupled code. The results not only explain how different nanoscale mechanisms influence the overall macroscale piezoresistive response through the mesoscale CNT network, but also give reason and provide bounds for the wide range of gauge factors found in the literature offering insight regarding how control of the mesoscale CNT networks can be used to tailor nanocomposite piezoresistive response.

Computational micromechanics analysis of electron hopping and interfacial damage induced piezoresistive response in carbon nanotube-polymer nanocomposites

Carbon nanotube (CNT)-polymer nanocomposites have been observed to exhibit an effective macroscale piezoresistive response, i.e., change in macroscale resistivity when subjected to applied deformation. The macroscale piezoresistive response of CNT-polymer nanocomposites leads to deformation/strain sensing capabilities. It is believed that the nanoscale phenomenon of electron hopping is the major driving force behind the observed macroscale piezoresistivity of such nanocomposites. Additionally, CNT-polymer nanocomposites provide damage sensing capabilities because of local changes in electron hopping pathways at the nanoscale because of initiation/evolution of damage. The primary focus of the current work is to explore the effect of interfacial separation and damage at the nanoscale CNT-polymer interface on the effective macroscale piezoresistive response. Interfacial separation and damage are allowed to evolve at the CNT-polymer interface through coupled electromechanical cohesive zones, within a finite element based computational micromechanics framework, resulting in electron hopping based current density across the separated CNT-polymer interface. The macroscale effective material properties and gauge factors are evaluated using micromechanics techniques based on electrostatic energy equivalence. The impact of the electron hopping mechanism, nanoscale interface separation and damage evolution on the effective nanocomposite electrostatic and piezoresistive response is studied in comparison with the perfectly bonded interface. The effective electrostatic/piezoresistive response for the perfectly bonded interface is obtained based on a computational micromechanics model developed in the authors? earlier work. It is observed that the macroscale effective gauge factors are highly sensitive to strain induced formation/disruption of electron hopping pathways, interface separation and the initiation/evolution of interfacial damage.

Computational micromechanics analysis of electron hopping induced piezoresistive response in carbon nanotube-polymer nanocomposites

In this study, a multiscale micromechanics based computational model is developed to capture the effect of electron hopping induced formation/disruption of conductive paths at the nanoscale which govern the effective macroscale conductive and piezoresistive response of the CNT-polymer nanocomposite. The local nanoscale conductivity at the nanoscale is allowed to evolve by tracking the position of the nanotubes under applied deformations and modifying the conductivity of the intertube region depending on the relative proximity of individual pairs of nanotubes. The formation and disruption of the high conductivity bands are highly dependent on the applied strain and yield odd microstructural symmetries for the effective electrical properties, emphasizing on the need of a computational micromechanics model. The effect of local CNT volume fractions and maximum electron hopping range on the effective macroscale gauge factors is studied for different cases of microstructure morphology i.e. CNT-polymer nanocomposites uniformly dispersed aligned CNTs and aligned/randomly oriented microscale bundles composed of aligned CNTs. Random orientation of macroscale bundles is taken into account using micromechanics based techniques by averaging over a discrete number of orientations of microscale bundles in a consistent manner. The results presented herein indicate that the effective gauge factors follow a strain dependent nonlinear response with asymmetry on application of tensile/compressive strains. In addition, the effective gauge factors were observed to be smaller for a microstructure with aligned microscale bundles as compared to randomly oriented microscale bundle. The gauge factors obtained from the current work are of comparable magnitude with those reported in the literature based on experimental investigation indicating that electron hopping could be the dominant mechanism governing the macroscale piezoresistive response of CNT-polymer nanocomposites.

Sensing Interfacial Damage Initiation, Evolution and Accumulation in Carbon Nanotube-Polymer Nanocomposites Under Cyclic Loading: A Computational Micromechanics Approach

The current 2-scale computational multiscale micromechanics based exploration of sensing capabilities in carbon nanotube (CNT)-polymer nanocomposites focuses on the macroscale piezoresistive response when the nanocomposite is subjected to cyclic loading conditions. It has been shown that electron hopping at the nanoscale is the primary mechanism behind the observed macroscale piezoresistivity for such nanocomposites. A novel continuum description of the non-continuum electron hopping effect used in the current work enables the use of multiscale continuum micromechanics based approaches to study nanocomposite piezoresistivity. The focus of the current work is on the interfacial separation/damage initiation, evolution and accumulation when subjected to cyclic loading. Interfacial separation/damage is allowed at the nanoscale CNT-polymer interface using electromechanical cohesive zones to account for electron hopping across the separated interface. The mechanical response of the CNT-polymer interface is obtained in terms of normal/tangential traction-separation behavior from atomistic scale Molecular Dynamics based models. The coupled electrostatic response is based on evolving interfacial resistance through the electron hopping induced current density across the separated interface. Such coupled electromechanical description allowing for current density across the separated interface in addition to normal/tangential tractions is unique in its implementation to the best of the authors knowledge. It is observed that the effective macroscale piezoresistive response obtained from the current modeling framework captures interfacial separation/damage state and shows sensitivity to damage accumulation over several cycles. Such exploration of the governing physical mechanisms starting at the smallest scale of influence and transitioning into macroscale effective properties provides key insights into the multiscale problem.Copyright

Computational modeling and experimental characterization of macroscale piezoresistivity in aligned carbon nanotube and fuzzy fiber nanocomposites

In this study, a multiscale computational micromechanics based approach is developed to study the effect of applied strains on the effective macroscale piezoresistivity of carbon nanotube (CNT)-polymer and fuzzy fiber-polymer nanocomposites. The computational models developed in this study allow for electron hopping and inherent CNT piezoresistivity at the nanoscale in addition to interfacial damage at the CNT-polymer interface. The CNT-polymer nanocomposite is studied at the nanoscale allowing for interfacial damage at the CNT-polymer interface using electromechanical cohesive zones. For fuzzy fiberpolymer nanocomposites, a 3-scale computational model is developed allowing for concurrent coupling of the microscale and nanoscale. The electromechanical boundary value problem is solved using finite elements at each of the scales and the effective electrostatic properties are obtained by using electrostatic energy equivalence. The effective electrostatic properties are used to evaluate the relative change in effective resistivity and the macroscale effective gauge factors for the nanocomposites. In addition, the piezoresistive response of aligned CNT-polymer and fuzzy fiber-polymer nanocomposites is investigated experimentally. The results obtained from the computational models are compared to the experimentally observed change in resistance with applied strains and associated gauge factors.

Computational Micromechanics Analysis of Damage Induced Piezoresistivity in Carbon Nanotube-Polymer Nanocomposites Under Cyclic Loading Conditions

The current 2-scale computational multiscale micromechanics based exploration of sensing capabilities in carbon nanotube (CNT) polymer nanocomposites focuses on the macroscale piezoresistive response when the nanocomposite undergoes damage. It has been shown that electron hopping at the nanoscale is the primary mechanism behind the observed macroscale piezoresistivity for such nanocomposites. A novel continuum description of the non-continuum electron hopping effect used in the current work enables the use of multiscale continuum micromechanics based approaches to study nanocomposite piezoresistivity. The current work aims at exploring the effect of nanoscale interfacial damage and local matrix damage on the effective properties of the nanocomposites. The interfacial damage in CNT-polymer nanocomposites is modeled through electromechanical cohesive zones and the local polymer matrix damage is modeled through continuum damage mechanics modeling. The effect of each of these damage mechanisms is studied independently under monotonic and cyclic loading conditions to differentiate between the different damage evolution paths and to explore the evolution of associated effective electrostatic and piezoresistive response.

Multiscale Modeling of Effective Piezoresistivity in Nanocomposite Bounded Explosives

The current work aims to explore the effective material properties of polymer bonded explosive (PBX) materials where the polymer medium is reinforced with carbon nanotubes (CNT). In the current work, the effective properties of nanocomposite bound polymer explosives (NCBX) are evaluated using micromechanics based 2-scale hierarchal multiscale models connecting the CNT-Polymer nanocomposite scale (nanoscale) to the grain structure scale (microscale). At the microscale, the binding polymer nanocomposite medium is modeled as cohesive zones between adjacent explosive grains. The cohesive laws for the polymer nanocomposite binder phase are developed based on the effective response of the CNT-polymer nanocomposite at the nanoscale. Effective piezoresistive properties of NCBX materials are evaluated through hierarchal multiscale modeling where the nanoscale effective response of the nanocomposites is evaluated offline using finite elements based micromechanics modeling. The effective properties obtained from homogenization of the nanoscale response are then used to construct microstructure informed cohesive laws which are embedded into the microscale grain boundary problem at the grain interfaces. The results show a significant piezoresistive response when the microscale boundary value problem is subjected to different types of applied deformation.