Strain mapping at the micro-scale in hierarchical polymer composites with aligned carbon nanotube grafted fibers

For the first time, micro-scale digital image correlation (μDIC) is investigated for measurement of strain fields in hierarchical fiber-reinforced composites. The methodology is developed on an exemplary alumina fiber/epoxy composite laminate with aligned carbon nanotubes (A-CNTs) grown on fibers. Utilizing environmental scanning electron microscopy and nano-scale random speckle patterns, sufficient precision is achieved to detect the influence of the A-CNTs on the deformation field around the fibers. Debonded regions at the fiber/matrix interface with openings as small as 35 nm could be detected. μDIC could identify the propagation of the debonded region based on the non-linear increase of the opening. The image correlation uncertainty in the displacement analysis is estimated to be below 5 nm. The experimental results are validated by computational analysis performed on the region of interest. For this, an advanced model with two scales of reinforcement (microscopic fibers and nanotubes) and boundary conditions taken from the experiment is used. As verified by the model, A-CNTs are found to constrain matrix deformation in their longitudinal direction. Keywords: Digital image correlation (DIC); Scanning electron microscopy (SEM); Carbon nanotubes; Mechanical properties; Finite element analysis (FEA

Micro and Nano structured Hierarchical Carbon Fibre Composites

Hierarchically structured fibre reinforced poly mer (FRP) composites are a new generation of struc tural materials with high potential for tailored d esign. The increased degree of freedom in material selection and design is the main advantage o f these materials over conventional composites. Ho wever, the available literature still lacks a comp rehensive study on the structure-property relation ship and the interactions between the constit uents in these composite materials. This wor k aims at exploring various aspects of new hi erarchically structured carbon fibre polymer compo sites (CFRP) with the goal of understanding the in terplay between the different components in r elation to their mechanical properties and fractur e. Multiwall carbon nanotubes (CNTs) and a ph ase separating thermoplastic modifier (polyoxymeth ylene (POM)) are the main structural elements ¨used to form the microstructure of the studied ma terials. The principal approach adopted in t his work is to establish an initial understanding¨ of the structure-property relationship in binary ( POM/epoxy or CNT/epoxy) and ternary (POM/CNT/epoxy ) bulk resin blends. This involves a study of the¨ phase morphology, dynamic mechanical properties, a nd fracture toughness of various types of bulk res in blends with different compositions. At the next ¨step, fiber reinforced composites based on the pr eviously studied matrix blends are produced a nd characterized. Considering the challenges invol ved in the processing of the POM modified matrix b lends into the corresponding composites at high te mperature, a new manufacturing technique based on¨ resin transfer molding is developed and furth er optimized. The knowledge acquired during the st udy of the matrix blends is employed to expla in morphological observations as well as fracture¨ properties and damage behaviour monitored during q uasi-static tensile loading of the produced lamina tes. A correlation between the microstructure and phase morphology of the matrix and the proper ties of the laminates is established. It is shown that phase separated POM particles are able to enhance fracture toughness of bulk e poxy resins, as long as the particulate morph ology is dominant. Fracture toughness and ultimate ¨mechanical properties of the bulk resin materials ¨are shown to be highly sensitive to the phas e morphology of the phase-separated blend. Th erefore, any external factors affecting phase sepa ration of the thermoplastic modified blends can dr astically influence resulting properties. For ¨instance, it is illustrated that the presence of¨ fiber reinforcement changes the phase morphology o f the matrix and, hence, affects the transfer ¨of the toughness improvements of the bulk re sin to the laminates. Inclusion of CNTs is a lso shown to influence the phase morphology of the¨bulk resin as well as the microstructure of the r esulting hierarchical laminates. It is shown¨ that CNTs limit mass diffusion during phase separa tion of the thermoplastic phase, causing redu ction in the particle size of the resulting thermo plastic particles. This in turn affects the fractu re toughness of the bulk resin blends and damage d evelopment in the resulting CFRP laminate. In this part of the work, a new approach for incorpo ration of CNTs in the POM modified matrices is int roduced, in which the phase separating thermoplast ic particles are surrounded by clusters of CNT agg lomerates.Abstract v Contents xiii List of Figures xix List of Tables xxix 1 General introduction 1 1.1 Motivationandaim ........................ 1 1.2 Micro/nano structure - Materials selection . . . . . . . 1.2.1 Nanoscale..................... 1.2.2 Microscale..................... 1.3 Thesisoutline....................... 2 State of the art 2.1 High performance CFRP epoxy composites . . . . . . 2.2 Damage initiation and growth in epoxy based CFRPs 2.3 Fracturemodesandfracturetoughness. . . . . . . . . ..... 4 ..... 4 ..... 4 ..... 6 9 ..... 9 ..... 10 . . . . . 13 2.3.1 Fractureenergy(G)approach............... 14 2.3.2 Locallizedstressfield(K)approach. . . . . . . . . . . . 15 xiii xiv CONTENTS 3 2.3.3 Plasticdeformationzone.................. 16 2.3.4 Thicknessconsiderations.................. 16 2.4 Toughening of epoxy resins and their composites . . . . . . . . 18 2.4.1 Thermoplasticmodifiedepoxyresins . . . . . . . . . . . 20 2.4.2 Reaction induced phase separation (RIPS) . . . . . . . . 21 2.4.3 Fracturetoughnessmechanisms. . . . . . . . . . . . . . 24 2.4.4 Polyoxymethylene (POM) as a toughening component . 28 2.5 Carbonnanotubemodifiedepoxyresins . . . . . . . . . . . . . 30 2.5.1 CNTsandtoughnessofepoxyresins . . . . . . . . . . . 31 2.5.2 CNTs in fiber reinforced epoxy composites . . . . . . . . 32 2.5.2.1 FRP composites based on CNT modified epoxy matrices...................... 33 2.5.2.2 FRP composites based on CNT modified fibers 35 2.6 Hybridepoxyresins ........................ 37 2.6.1 Hybridepoxymatrices................... 37 2.6.2 FRP composites based on hybrid epoxy matrices . . . . 39 2.7 Conclusions............................. 39 Binary matrix systems 41 3.1 TP/epoxyblends.......................... 41 3.1.1 Experimental........................ 42 3.1.1.1 Materials ..................... 42 3.1.1.2 Preparationoftheblends............ 43 3.1.1.3 Characterization techniques . . . . . . . . . . . 43 3.1.2 Resultsanddiscussion................... 46 3.1.2.1 Phasemorphology................ 51 3.1.2.2 Dynamic mechanical properties . . . . . . . . . 53 3.1.2.3 Tensileproperties ................ 53 CONTENTS xv 3.1.2.4 Mode I fracture toughness analysis . . . . . . . 55 3.1.3 Conclusions......................... 61 3.2 CNT/epoxyblends......................... 62 3.2.1 Introduction ........................ 62 3.2.2 Experimental........................ 63 3.2.2.1 Materials ..................... 63 3.2.2.2 Characterizationmethods. . . . . . . . . . . . 65 3.2.3 Resultsanddiscussion................... 67 3.2.3.1 Masterbatch(stage1).............. 67 3.2.3.2 Diluted masterbatch (stage 2) . . . . . . . . . 69 3.2.3.3 Transition from liquid to solid (stage 3) . . . . 71 3.2.3.4 Solid(cured) nanocomposite (stage4) . . . . . . 74 3.2.4 Conclusions......................... 76 4 Ternary matrix systems 79 4.1 Experimental ............................ 80 4.1.1 Materials .......................... 80 4.1.2 Samplepreparation .................... 80 4.1.2.1 TernaryECNT/POMblends . . . . . . . . . . 80 4.1.2.2 TernaryE/POMCNT .............. 81 4.1.3 Characterizationtechniques................ 82 4.1.3.1 DMAmeasurements............... 82 4.1.3.2 ModeIfracturetoughness . . . . . . . . . . . 82 4.1.3.3 Microscopic characterization . . . . . . . . . . 83 4.2 Resultsanddiscussion ....................... 83 4.2.1 Morphologyandphasebehaviour . . . . . . . . . . . . . 83 4.2.1.1 ECNT/POMternaryblends . . . . . . . . . . 83 xvi CONTENTS 5 Fiber reinforced polymer composites based on multiphase matrices 93 5.1 CFRP laminates based on binary POM/epoxy matrices . . . . 93 5.1.1 Experimental........................ 93 5.1.1.1 Materials ..................... 93 5.1.1.2 Preparationoftheblends............ 94 5.1.1.3 Manufacturing of CFRP laminates . . . . . . . 94 5.1.2 TestingandCharacterization ............... 100 5.1.2.1 Tensiletesting .................. 100 5.1.2.2 Mode I interlaminar fracture toughness . . . . 106 5.1.2.3 Microscopic characterization . . . . . . . . . . 106 5.1.3 Resultsanddiscussion................... 107 5.1.3.1 Morphological observations . . . . . . . . . . . 107 5.1.3.2 Tensile properties in fiber direction . . . . . . 110 5.1.3.3 Tensile properties in bias ± 45° direction . . . 121 5.1.3.4 Mode I interlaminar fracture toughness results 123 5.2 CFRPlaminatesbasedonternarymatrices . . . . . . . . . . . 127 5.2.1 Morphologicalobservations ................ 127 5.2.2 Tensilepropertiesinfiberdirection . . . . . . . . . . . . 128 5.2.3 Conclusions......................... 135 Conclusions and recommendations for future work 137 6.1 Phase behaviour and fracture toughness of the bulk resin blends 138 6.1.1 POMmodifiedbinarymatrices .............. 138 6 4.2.2 4.2.1.2 E/POMCNTternaryblends . . . . . . . . . . 85 Dynamicmechanicalproperties.............. 87 ModeIfracturetoughnessanalysis . . . . . . . . . . . . 88 4.2.3 4.3 Conclusions............................. 91 CONTENTS xvii 6.1.2 CNTmodifiedbinarymatrices .............. 139 6.1.3 CNT/POM modified ternary matrices . . . . . . . . . . 139 6.2 CFRP composite laminates based on modified matrices . . . . 140 6.2.1 CFRP laminates based on POM modified binary matrices 140 6.2.2 CFRP laminates based on CNT/POM modified ternary matrices........................... 140 6.3 Recommendationsforfuturework ................ 142 A Preparation and characterization of unidirectional carbon fibre composites 145 B Micro-scale strain mapping of FRP composites 150 C POM-modified CFRP laminates based on CNT grafted (fuzzy) carbon fabrics 152 D POM particles deformed during fracture (particle crack tearing) 157 E Design sketches 159 Bibliography 169 Curriculum Vitae 189nrpages: 188status: publishe

Morphology and fracture behavior of POM modified epoxy matrices and their carbon fiber composites

© 2015 Elsevier Ltd. In the present study, polyoxymethylene (POM), a crystallizable thermoplastic, is investigated for toughening of an epoxy resin and a carbon fiber reinforced composite produced from it. Depending on the blend composition POM forms different morphologies during the curing reaction of epoxy (particulate, co-continuous or phase-inverted). A multi-fold increase in the fracture toughness of the neat resin is obtained for particulate morphology. Fractographic investigation of fracture surfaces reveals a number of energy dissipation mechanisms, including particle crack bridging, matrix shear yielding, and plastic void growth following particle debonding. The presence of fibers is found to severely disturb the phase separation process and to change the resulting morphology in the fiber reinforced composite. With it the interlaminar fracture behavior of the composite is compromised.publisher: Elsevier articletitle: Morphology and fracture behavior of POM modified epoxy matrices and their carbon fiber composites journaltitle: Composites Science and Technology articlelink: http://dx.doi.org/10.1016/j.compscitech.2015.01.017 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved.status: publishe

The effect of CNT positioning on morphology and fracture toughness of multiphase epoxy nano-composites

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Full-field strain measurements at the micro-scale in fiber-reinforced composites using digital image correlation

© 2016 Elsevier Ltd. Few studies have been conducted to monitor inter-fiber deformations in fiber-reinforced composites. In the present work, we demonstrate full-field strain measurements in the composites at the micro-scale, using digital image correlation (DIC). The study is performed on a unidirectional glass fiber reinforced composite loaded in transverse three-point bending inside an environmental scanning electron microscope. A nano-scale random speckle pattern of high quality is created. Validity of the measured fields is assessed against results of a finite element (FE) model with boundary conditions retrieved from the experiment. A good agreement is found between the DIC-measured and FE-predicted results. The precise recognition of very small-scale strain concentrations requires enhancement of the correlation process and removal of microscopy imperfections. The investigated methodology shows promise for real-time deformation measurements in composites at the micro-scale.publisher: Elsevier articletitle: Full-field strain measurements at the micro-scale in fiber-reinforced composites using digital image correlation journaltitle: Composite Structures articlelink: http://dx.doi.org/10.1016/j.compstruct.2015.12.020 content_type: article copyright: Copyright © 2016 Elsevier Ltd. All rights reserved.status: publishe

Micro-scale strain mapping in nano-engineered fiber-reinforced composites

Full-field strain measurement in materials that exhibit mechanical heterogeneity at the micro-scale are of great interest. This is particularly important in the emerging field of nano-engineered fiber-reinforced composites, where the effects of nano-modifications are as-yet uncharacterized. Formerly, the authors proposed a methodology to analyze deformations in conventional laminated composites at the micro-scale via digital image correlation (DIC). In the present study, this technique is applied to capture strain heterogeneity in microfiber-reinforced laminated composites that are additionally modified with aligned carbon nanotubes (CNTs), grown radially on microfibers as aligned forests. The composite is loaded in transverse three-point bending inside an environmental scanning electron microscope. The paper describes acquisition of 2D displacement and strain maps, and investigates the effect of CNTs on the deformation of these hierarchical architectures. As a prerequisite for micro-DIC, a high-quality nano-scale random speckle pattern of alumina particles is deposited on the surface. A finite element model of the micro- and nano-structure geometry, with boundary conditions obtained from microscopy and DIC analysis. A good correlation between experimental and modeling results was obtained, indicating that the micro-scale DIC is a promising technique to study deformation in nano-engineered composites.status: publishe