Hierarchical nanoreinforced composites: Computational analysis of damage mechanisms

The potential of hierarchical composites with secondary nanoreinforcement is discussed and analysed on the basis of the computational modelling. The concept of nanostructuring of interfaces as an important reserve of the improvement of the composite properties is discussed. The influence of distribution, shape, orientation of nanoparticles (carbon nanotube, graphene) in unidirectional polymer matrix composites on the strength and damage resistance of the composites is studied in computational studies. The possible directions of the improvement of nanoreinforced composites by controlling shapes, localization and other parameters of nanoreinforcements are reviewed

MODELLING AND OPTIMIZATION OF THE MECHANICAL PROPERTIES OF POLYMER NANOCOMPOSITES

The research activity here reported spans different areas of the nanocomposite material science, giving a contribution toward the advancement in its state of the art. The study of the effects of the filler distribution on the mechanical properties of nanomodified polymers is a major research topic which is carried out. Different computational and theoretical approaches have been developed, considering statistics, finite element analyses and micromechanics. Several nanomodified epoxy resins have been manufactured along with glass fibre reinforced laminates with nanomodified matrix. The effects of the nanomodification on the mechanical properties of these composites have been studied, in order to validate predictive models and supply technical data to material designers. The results obtained so far highlight the importance, in this class of materials, of the study of interactions at the nanoscale between the nanoreinforcement and the matrix. The thesis is articulated in an introduction, followed by two sections, dedicated to the experimental activity and the modelling one, and by an appendix. The introduction presents a brief overview on nanocomposites giving a primer to a reader devoid of prior experience with this class of materials. The experimental activity section is constituted by four chapters. The first one contains the results of mechanical tests performed on a nanoparticle reinforced epoxy, alongside an analysis on the processing parameters employed in the composite preparation. The enhancement in the nanocomposite fracture toughness is compared with a theoretical model, showing a satisfactory agreement. The second chapter reports the study of the effect of the testing temperature on the fracture toughness of the same material. The analysis highlights the fundamental importance of the processing parameters and of the testing temperature on the nanomodification effects. The third chapter considers a different aspect of the nanomodification: a nanoplatelet reinforced epoxy is used to prepare notched specimens and the strength analysis of the notched component is carried out. The fourth chapter reports the research activity performed on the study of the matrix nanomodification of glass fibre reinforced laminates. The effects on the matrix dominated mechanical properties and the feasibility in the production of a laminate with antibacterial bulk properties are analysed. The modelling section is constituted by three chapters. The first one contains a comparison between two approaches for the estimation of the elastic properties of a nanocomposite material, considering explicitly the presence of an interphase surrounding the nanoreinforcements. The first approach implements a two-step micromechanical model, while the second one is based on a finite element analysis. The second chapter reports the research activity carried out on the generation of representative volume elements of nanoparticle reinforced materials. A statistically based algorithm is implemented to minimize the volume element size while retaining its representativeness. The study of the interphase extent and of the overall elastic modulus, through finite element analyses, is reported. The third chapter extends these considerations to the generation of volume elements for nanoplatelet reinforced materials. The random sequential absorption approach is implemented and its hidden effects on the filler distribution highlighted. A new version of the algorithm is proposed to remove these unwanted behaviours. The appendix section is dedicated to the implementation activity of approaches which have not been completed yet. The results obtained on the use of molecular dynamics in the simulation of polymers and nanomodified polymers are reported. While a detailed procedure for the simulation of epoxy resins is listed, the approach for the study of nanoplatelet-epoxy interactions is still in progress.L'attività di ricerca riportata nella presente tesi riguarda diverse aree della scienza dei materiali nanocompositi e mira a dare un contributo nell'avanzamento del suo stato dell'arte. Uno degli ambiti principali di indagine è stato inerente allo studio degli effetti della distribuzione dei rinforzi sulle proprietà meccaniche esibite dai polimeri nanomodificati. Diversi approcci, computazionali e teorici, sono stati sviluppati, sfruttando considerazioni statistiche, l'analisi agli elementi finiti e la modellazione micromeccanica. E' stata eseguita la nanomodificazione di diverse resine epossidiche e la produzione di laminati in fibra di vetro con matrici nanomodificate. Sono quindi stati studiati gli effetti della nanomodificazione sulle proprietà meccaniche di questi compositi, al fine di validare modelli previsionali e fornire dati ai progettisti di materiali. I risultati finora conseguiti evidenziano l'importanza che, in questa classe di materiali, ricopre lo studio delle interazioni alla nanoscala che si sviluppano tra i nanorinforzi e la matrice. La tesi è articolata in una introduzione, seguita da due sezioni dedicate all'attività sperimentale e alla modellazione, e da una appendice a concludere. La sezione inerente l'attività sperimentale è costituita da quattro capitoli. Il primo contiene i risultati dei test sperimentali eseguiti su resine epossidiche nanomodificate, volti ad indagarne le proprietà meccaniche includendo l'effetto dei parametri di processo. E' inoltre riportato il confronto tra l'incremento della tenacità a frattura misurato sperimentalmente e le previsioni di un modello teorico, evidenziando una soddisfacente congruenza dei risultati. Il secondo capitolo riporta lo studio degli effetti della temperatura sulla tenacità a frattura dello stesso materiale. Tale analisi rimarca la fondamentale importanza dei parametri di processo e della temperatura di prova sugli effetti della nanomodificazione. Il terzo capitolo considera un diverso aspetto della nanomodificazione, ovvero la resistenza di campioni intagliati, prodotti con resine epossidiche rinforzate con nanoplatelets. Il quarto capitolo riporta l'attività di ricerca portata avanti sull'impiego di matrici nanomodificate in laminati rinforzati in fibra di vetro. Oggetto di analisi sono le proprietà meccaniche dipendenti dalla matrice e la possibilità di produrre laminati con proprietà antibatteriche. La sezione inerente l'attività di modellazione è invece costituita da tre capitoli. Il primo contiene un confronto tra due approcci volti alla stima delle proprietà elastiche di un materiale nanocomposito, considerando in modo esplicito la presenza di una interfase che circonda i nanorinforzi. Il primo degli approcci implementati è un modello micromeccanico in due passi, mentre il secondo è basato su una analisi agli elementi finiti. Il secondo capitolo riporta l'attività di ricerca inerente la creazione di volumi rappresentativi di materiali rinforzati da nanoparticelle. Tale studio si basa sull'impiego di un algoritmo in grado di considerare aspetti statistici volti alla riduzione della dimensione del volume di controllo, pur mantenendone la rappresentatività. Sono quindi riportate le analisi sull'estensione dell'interfase e sulle proprietà elastiche globali del composito, valutate tramite analisi agli elementi finiti. Il terzo capitolo estende i concetti esposti nel precedente per la generazione di volumi rappresentativi di materiali rinforzati con nanoplatelets. Un approccio basato sull'algoritmo di random sequential absorption è stato implementato, evidenziando gli effetti che questo comporta sulla distribuzione dei rinforzi. E' stata quindi proposta una nuova versione di questo algoritmo, capace di rimuovere queste conseguenze indesiderate. L'appendice è dedicata all'esposizione dell'attività svolta nell'implementazione di approcci che però non sono ancora stati completati. In essa sono riportati i risultati finora conseguiti sull'uso della dinamica molecolare nella simulazione di polimeri e nanocompositi. In particolare è elencata una procedura dettagliata per la simulazione di resine epossidiche, e la prima parte di un approccio per lo studio di resine epossidiche rinforzate con nanoclay

THE EFFECT OF MODELLING NANOPLATELET AND NANOTUBE CURVATURE ON THE REPRESENTATIVITY OF NANOCOMPOSITE VOLUME ELEMENTS

This article presents a methodology to model curved CNTs and Graphene nanoplatelets for generating nanocomposite volume elements. The focus is placed on comparing the effect of curvature on isolated reinforcements and on cubic VEs where the interparticle distance is of interest, such as in the case of detecting percolation. The results support the idea that curvature reduces the 3D span of both reinforcements in the same way, whereas percolation appears affected only in the case of CNTs. However, for GNPs the difficulties in generating Volume Elements with a sufficiently high volume fraction could support only preliminary considerations

Nanomorphology of graphene and CNT reinforced polymer and its effect on damage: Micromechanical numerical study

The effect of morphology, shapes and distribution of nanoscale carbon reinforcement in polymers on their strength and damage resistance is studied using computational micromechanical modeling. A new software and approach were developed for the automatic generation of finite element unit cell models of nanocomposites with inclusions of arbitrary and complex shapes. The effect of curved, zigzagged, snakelike shapes of real carbon nanotubes, as well as re-stacking of graphene on the damage evolution was studied in the computational experiments based on the developed code. The potential of hybrid (carbon nanotubes and graphene) nanoscale reinforcement was studied with view on its effect of damage resistance. It was demonstrated that idealized, cylinder like models of carbon nanotubes in polymers lead to an underestimation of the stress concentration and damage likelihood in the nanocomposites. The main damage mechanisms in CNT reinforced polymers are debonding and pull-out/fiber bridging, while in graphene reinforced polymers the main role is played by crack deviation and stack splitting, with following micro-crack merging. The potential of hybrid (carbon nanotubes and graphene) nanoscale reinforcement was studied with view on its effect of damage resistance. (C) 2016 Elsevier Ltd. All rights reserved

Percolation in Carbon Nanotube-Reinforced Polymers for Strain-Sensing Applications: Computational Investigation on Carbon Nanotube Distribution, Curvature, and Aggregation

The present article investigates the possibility of simulating the electrical conductivity of carbon nanotube-reinforced polymer composites by numerical methods. Periodic representative volume elements are generated by randomly distributing perfectly conductive reinforcements in an insulating matrix and are used to assemble an electrical network representative of the nanocomposite, where the nanotube–nanotube contacts are considered equivalent resistors modeled by means of Simmons’ equation. A comparison of the results with experimental data from the literature supports the conclusion that a random distribution of reinforcements is not suitable for simulating this class of materials since percolation thresholds and conductivity trends are different, with experimental percolation taking place before the expectations. Including nanotube curvature does not solve the issue, since it hinders percolation even further. In agreement with experimental observations, the investigation suggests that a suitable approach requires the inclusion of aggregation during the volume element generation to reduce the volume fraction required to reach percolation. Some solutions available in the literature to generate properly representative volume elements are thus listed. Concerning strain sensing, the results suggest that representative volume elements generated with random distributions overestimate the strain sensitivity of the actual composites

Recent Advances in the Analytical Stress Field Solutions for Radiused Notches in Orthotropic Solids

The main aim of this work is to provide a brief overview of the analytical solutions available to describe the in-plane and out-of-plane stress fields in orthotropic solids with radiused notches. To this end, initially, a brief summary on the bases of complex potentials for orthotropic elasticity is presented, with reference to plane stress or strain and antiplane shear problems. Subsequently, the attention is moved to the relevant expressions for the notch stress fields, considering elliptical holes, symmetric hyperbolic notches, parabolic notches (blunt cracks), and radiused V-notches. Eventually, examples of applications are presented, comparing the presented analytical solutions with the results from numerical analyses carried out on relevant cases

Modelling the thermoelectric behaviour of composite laminates in the presence of transverse cracks

Composite materials, based on carbon fibres and/or nano-modified polymers, are charac-terised by a thermoelectric coupling. Indeed, an electric potential drop is generated in the presence of a thermal gradient (Seebeck effect), which makes them suitable for thermo-electric applications.In this work, novel analytical models are presented for the calculation of the apparent in-plane electric, thermal and thermoelectric properties of composite cross-ply laminates in the presence of transverse cracks. The aim of this analysis is to understand the variation in the apparent laminate properties if damage takes place in the form of transverse cracks, this being typically the first damage mode occurring in composite laminates under static and cyclic tensile loadings.(c) 2022 Elsevier Inc. All rights reserved