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Shiv Nadar University

Structures made of variable stiffness (VS) composites possess a rich design space for bistable configurations that demonstrate different values of curvatures and out-of-plane displacements. In this study, several VS composites are investigated which can yield cylindrical bistable shapes similar to those generated from unsymmetrical cross-plies. Such configurations have been found favorable as a component for certain morphing applications. A semi-analytical model based on the Rayleigh-Ritz approach is presented to calculate the thermally induced multistable shapes as well as the snap-through forces particularly taking into account the curvilinear paths of VS composites. A nonlinear finite element analysis is performed to check the accuracy of the semi-analytical method. Cylindrical shapes generated from VS laminates and the corresponding straight fiber cross-ply laminates are analyzed and compared. The snap-through forces are subsequently calculated and compared for different VS laminates and the straight fiber cross-ply. It is observed that certain VS composites have a significant reduction of snap-through forces but with a marginal difference in out-of-plane displacement as compared to the corresponding straight fiber cross-ply laminate.

World University root node

Civil Engineering

(SoE) School of Engineering

Lecture Notes in Applied and Computational Mechanics

Multiscale Modeling of Heterogeneous Structures

Due to their exceptionally high specific strength and specific stiffness, single-and multiwalled carbon nanotubes (SWCNTs and MWCNTs) are widely used as reinforcement in metallic, ceramic and high-performance polymer matrices. The mechanical properties, including stiffness, strength and bending characteristics, of carbon nanotubes (CNTs) are greatly influenced by a number of factors, including CNT diameter, chirality, number of concentric walls in MWCNTs, chemical functionalization, surface defects and so forth. However, the challenges associated with the separation of a single CNT and its experimental characterization lead to less reliable and varying estimations of mechanical properties. This chapter presents a careful overview of the elastic and inelastic mechanical properties of isolated single-and multiwalled carbon nanotubes, determined using tedious experiments and elaborated computational methods and their dependence on nanotube physical/chemical properties.

Carbon Nanotube-Reinforced Polymers: From Nanoscale to Macroscale

Mechanical properties of isolated carbon nanotube

Integration of information, communication and materials technologies into the electricity Smart Grids is the key to sustainable clean energy future. This study focuses on materials innovation and design of power transmission lines. Conventional bi-material power transmission lines consist of aluminum conductors with steel reinforcement. Increase in temperature of transmission lines due to Joule heating limits their current carrying capacity and the efficiency of power transmission. New design of transmission wires proposed in this study consists of carbon nanostructure (CNS)-epoxy composites in a multilayered architecture to enable multifunctional capabilities. Excellent thermal transport properties of CNS-epoxy composites are utilized to optimally dissipate heat from the outer surface of transmission wire in order to maximize its performance. A coupled electrical-thermal finite element (FE) analysis of Aluminum Conductor Steel Reinforced (ACSR) wires is performed and the results are benchmarked with those obtained from relevant IEEE standards. The validated model is then extended to new transmission line composed of aluminum conductor - composite core with CNS-epoxy composite coating (ACCC-CNS). Steady-state coupled FE analyses of ACSR and ACCC-CNS wires indicate that the proposed design of ACCC-CNS wire enables transmission of larger currents than an equivalent ACSR wire for the same amount of conductor material due to reduced operating temperature. The proposed design of power lines would enable development of Smart Grids for more efficient utilization of electricity from renewable sources.

Journal of Cleaner Production

High performance overhead power lines with carbon nanostructures for transmission and distribution of electricity from renewable sources

In order to meet stringent environmental, safety and performance requirements from respective regulatory bodies, various technology-based industries are promoting the use of advanced carbon nanotube (CNT) reinforced lightweight and high strength polymer nanocomposites (PNCs) as a substitute to conventional materials both in structural and non-structural applications. The superior mechanical properties of PNCs made up of CNTs or bundles of CNTs can be attributed to the interfacial interaction between the CNTs and matrix, CNT's morphologies and to their uniform dispersion in the matrix. In PNCs, CNTs physically bond with polymeric matrix at a level where the assumption of continuum level interactions is not applicable. Modeling and prediction of mechanical response and failure behavior of CNTs and their composites becomes a complex task and is dealt with the help of up-scale modeling strategies involving multiple spatial and temporal scales in hierarchical or concurrent manner. Firstly, the article offers an insight into various modeling techniques in studying the mechanical response of CNTs; namely, equivalent continuum approach, quasi-continuum approach and molecular dynamics (MD) simulation. In the subsequent steps, these approaches are combined with analytical and numerical micromechanics models in a multiscale framework to predict the average macroscopic response of PNCs. The review also discusses the implementation aspects of these computational approaches, their current status and associated challenges with a future outlook.

Progress in Aerospace Sciences

Modeling of carbon nanotubes and carbon nanotube-polymer composites

Epoxy matrix reinforced with conventional microscale short carbon fibers (SCFs) and carbon nanotubes (CNTs) form a hybrid material system where the characteristic length scales of SCFs and CNTs differ by multiple orders of magnitude. Several recent studies show that the addition of CNTs into a non-conducting polymer matrix improves both structural performance such as modulus, strength and fracture toughness and functional response such as electrical and thermal conductivities of the resulting nano-composite. In this study, a physics-based hierarchical multiscale modeling approach is presented to calculate the effective electrical conductivity of SCF-CNT-polymer hybrid composites. A dual step procedure is adopted to couple the effects of nano- and micro-scale so as to estimate the effective electrical properties of the composite. First, CNTs are dispersed into the non-conducting polymer matrix to obtain an electrically conductive CNT-epoxy composite. The effective electrical conductivity of CNT-epoxy composite is modeled using a physics-based formulation for both randomly distributed and vertically aligned cases of CNTs and the results are verified with the measured data available in the literature. In the second step, SCFs are randomly distributed in the CNT-epoxy composite and the effective electrical conductivity of the resulting SCF-CNT-epoxy hybrid composite is estimated using a micromechanics based self-consistent approach considering SCFs as microscopic inhomogeneities.

Materials and Design

Multiscale modeling of effective electrical conductivity of short carbon fiber-carbon nanotube-polymer matrix hybrid composites

Design of high-performance power lines with advanced materials is indispensable to effectively eliminate losses in electrical power transmission and distribution (T&D) lines. In this study, aluminum conductor composite core with carbon nanostructure (ACCC-CNS) coating in a multilayered architecture is considered as a novel design alternative to conventional aluminum conductor steel-reinforced (ACSR) transmission line. In the multiphysics approach presented herein, first, electrothermal finite element analysis (FEA) of the ACSR line is performed to obtain its steady-state temperature for a given current. Subsequently, the sag of the ACSR line due to self-weight and thermal expansion is determined by performing thermostructural analysis employing an analytical model. The results are then verified with those obtained from the FEA of the ACSR line. The electrothermal finite element (FE) model and the thermostructural analytical model are then extended to the ACCC-CNS line. The results indicate that the ACCC-CNS line has higher current-carrying capacity (CCC) and lower sag compared to those of the ACSR line. Motivated by the improved performance of the ACCC-CNS line, a systematic parametric study is conducted in order to determine the optimum ampacity, core diameter and span length. The findings of this study would provide insights into the optimal design of high-performance overhead power lines.

Journal of Engineering Materials and Technology, Transactions of the ASME

High-Ampacity Overhead Power Lines with Carbon Nanostructure-Epoxy Composites

20th International Conference on Composite Materials

Coupled thermal-electrical analysis of overhead electric transmission wires made up of carbon nanostructured (CNS) composites

In this study, two-dimensional (plane-stress and plane-strain) theoretical models are presented for the stress analysis of adhesively bonded single-lap composite joints subjected to thermal, mechanical loads and a combination thereof. The joints consist of similar/dissimilar orthotropic or isotropic adherends and an isotropic adhesive interlayer. The governing diffierential equations of the problem are obtained using a variational method which minimizes the complementary strain energy in the bonded assembly. In this formulation, through-thickness variation of shear and peel stresses in the interlayer is considered. All the traction free boundary conditions are exactly satisfied. Peel and shear stresses obtained from plane-strain analytical models considering mono-modulus adhesive interlayer are in good agreement with the finite element predictions. A systematic parametric study is conducted to identify an ideal set of geometrical and material properties for the optimal design of joints.

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference

A variational approach for stress analysis of single-lap composite joints under mechanical and thermal loads

The long-term performance of any novel material in its working environment is an intriguing area of research. The combined effect of the moisture content (relative humidity) and temperature of the environment degrades the mechanical properties of polymer composites. In the first part of this manuscript, polyurea–polyhedral oligomeric silsesquioxane (POSS) nanocomposites are subjected to continuous hygrothermal cycling. The effects of prolonged hygrothermal aging on the mechanical performance of the composites are evaluated in terms of the change in tensile strength and elongation at failure. Aged composites are subjected to FTIR spectroscopy to study the chemical changes resulting from the hydrolysis of polyurea. In the second part, attempts are made to study the effect of the addition of POSS nanoreinforcement on the mechanical properties of the resulting composite materials using molecular dynamics (MD) simulations. Results obtained here show poor agreement between MD simulation results and results obtained using dynamic mechanical analysis studies. However, qualitatively, both simulation and experimental results exhibit similar effects due to addition of POSS nanoparticles on the mechanical properties of polyurea–POSS nanocomposites.

Journal of Polymers and the Environment

Molecular Dynamics Simulations and Hygrothermal Aging of Polyurea–POSS Nanocomposites

Physical, mechanical and chemical aging of polyurea nanocomposites

This manuscript presents a new multiscale framework for the analysis of failure of thin heterogeneous structures. The new framework is based on the asymptotic homogenization method with multiple spatial scales, which provides a rigorous mathematical basis for bridging the microscopic scales associated with the periodic microstructure and thickness and the macroscopic scale associated with the in-plane dimensions of the macrostructure. The proposed approach generalizes the Caillerie-Kohn-Vogelius elastostatic heterogeneous plate theory for failure analysis when subjected to static and dynamic loads. Inelastic fields are represented using the eigendeformation concept. A computationally efficient n-partition computational homogenization model is developed for simulation of large scale structural systems without significantly compromising on the solution accuracy. The proposed model is verified against direct 3D finite element simulations and experimental observations under static and dynamic loads. {\textcopyright} 2010 The Author(s).

Fulltext

A multiscale failure model for analysis of thin heterogeneous plates

We present a new multiscale model for failure analysis of thin composite structures. The present model is based on the asymptotic homogenization method with multiple spatial scales. The proposed approach is a generalization of the elastostatic Caillerie-Kohn-Vogelius theory to study the failure mechanics of thin composite plates under static and dynamic loads. We employ a model reduction methodology based on the eigendeformation concept to minimize the computational burden. An efficient n-partition computational homogenization model is developed for simulation of large scale structural systems without significantly compromising on the solution accuracy. The proposed model is verified against direct 3-D finite element simulations.

American Society for Composites - 23rd Technical Conference of the American Society for Composites 2008

A multiscale failure model for thin heterogeneous plates

44th Annual Technical Meeting of Society of Engineering Science

A new multiscale approach to failure analysis of composite plates and shells

9th United States National Congress on Computational Mechanics

Eigendeformation-based reduced order homogenization

Many experimental and theoretical studies have been performed on single and multi-wall nanotubes. In particular, deformation modes and overall tube stiffness have been examined. This problem will be even more challenging in the case of nanocomposites in which carbon nanotubes are dispersed randomly in a polymeric (i.e. polystyrene, epoxy etc.) matrix. A critical issue for nanotechnology is the ability to understand, model and simulate the behavior of the small structures and to make the connection between structure properties and functions. Most nanosystems are too small for direct measurements but too large to be described by current rigorous first principles in theoretical and computational methods. They exhibit too many statistical ensembles. The vital role of molecular modeling in this field is to enable engineering design at the component and systems level and to set objectives that could guide laboratory efforts of the physical implications. An important component in molecular mechanics calculations of the nanostructure of materials is the description of the forces between individual atoms which are characterized by a force field. In this paper, the stiffness matrix of a single wall carbon nanotube (SWCNT) of (7, 0) chirality is calculated using molecular dynamics simulation under various combinations of loads. Both the magnitude and direction of load are varied on SWCNT and its composites. When dispersed in the polymeric matrix, it was observed that SWCNTs tend to cluster in bundles. Bundles of 7, 9 and 19 SWCNT (7, 0) are modeled using molecular dynamics and the stiffness matrix is calculated for all the three cases. The polymeric matrix is simulated and its mechanical properties are calculated. SWCNT-polymer nanocomposite is then modeled using a single wall carbon nanotube embedded in a polymeric matrix. Also, the mechanical properties of the interface for the same nanocomposite are calculated. In this case, the density of the SWCNT-polymer nanocomposite in the vicinity of SWCNT is maintained to match the experimentally observed density of the same kind of nanocomposite.

American Society for Composites - 21st Technical Conference of the American Society for Composites 2006

Molecular dynamics simulation of SWCNT - Polymer nanocomposite and its constituents

Over the years many researchers have contributed to develop an understanding of moisture absorption behavior of different fiber reinforced composite systems under the influence of various internal factors (fiber volume fraction, fiber orientation, size of fiber reinforcement, moisture absorption nature of fiber and type of fiber etc.) and external factors (relative humidity, temperature of environment and the state of stress experienced by composites etc). In the present research the extent of degradation in flexural properties as a function of amount of absorbed moisture present in pultruded composite material is studied. Further, these results were compared with the effects of water and air freeze thaw on flexural properties of pre-conditioned (150 °F, 24 hours) and non-conditioned samples with and without prior absorbed moisture. For this study, one freeze thaw cycle was comprised of 8 hours of freezing and 16 hours of thawing. This ensured the complete freezing and thawing of the composite material during freeze thaw cycling. A set of samples with and without absorbed moisture were also subjected to room temperature aging. Statistical analysis of results obtained after water and air freeze thaw and room temperature aging was performed in accordance with MIL Handbook - 17. Results of this study indicate that the effect of amount of absorbed moisture on flexural strength is much greater than that of freeze thaw in water or air. However, as a result of drying during room temperature aging, samples with absorbed moisture are able to regain a significant portion of their flexural strength, which was lost due to ingress of moisture. Samples with higher moisture content (0.96%) also regain their strength as a result of drying in a vacuum oven.