Novel design and manufacturing of advanced multifunctional structural nanocomposites containing self-healing fibers and graphene sheets with structural health monitoring capabilities

In the first part of this thesis, a direct, one-step tri-axial electrospinning process was used to fabricate multi-walled fibers with a novel architecture. Different healing agents were encapsulated inside the fibers with two separate protective walls. Presence of an extra layer in the fiber structure facilitated the encapsulation of healing agents and extended the efficiency of the healing functionality. We first took a systematical optimization approach to produce tri-axial hollow electrospun fibers with tunable fiber diameters and surface morphology. Next, the effect of tri-axial hollow fibers as a primary reinforcement and co-reinforcement in the presence of glass fibers was scrutinized from a material selection point of view. Furthermore, multi-walled fibers were utilized to encapsulate different healing agents inside the fibers and successful and recurring self-healing ability were achieved while preserving the mechanical properties of the composites. In the second part of this study, three different architectural designs were developed for manufacturing advanced multi-scale reinforced epoxy based composites in which graphene sheets and carbon fibers were utilized as nano- and micro-scale reinforcements, respectively. Graphene/carbon fiber/epoxy composites in various graphene sheet arrangements showed enhancements in in-plane and out of plane mechanical performances. In the hybrid composites, remarkable improvements were observed in the work of fracture by ~55% and the flexural strength by ~51% as well as a notable enhancement on other mechanical properties. In addition, integration of conductive reinforcement in the epoxy matrix enabled us to develop composite structures with high electrical and thermal conductivity, self-heating and de-icing functionalities

Rational design and direct fabrication of multi-walled hollow electrospun fibers with controllable structure and surface properties

Multi-walled hollow fibers with a novel architecture are fabricated through utilizing a direct,one-step tri-axial electrospinning process with a manufacturing methodology which does not require any post-treatments for the removal of core material for creating hollowness in the fiber structure. The hydrophilicity of both inner and outer layers’ solution needs to be dissimilar and carefully controlled for creating a two-walled/layered hollow fiber tructure with a sharp interface. To this end, Hansen solubility parameters are used as n index of layer solution affinity hence allowing for control of diffusion across the layers and the surface porosity whereby an ideal multi-walled hollow electrospun fiber is shown to be producible by tri-axial electrospinning process. Multi-walled hollow electrospun fibers with different inner and outer diameters and different surface morphology are successfully produced by using dissimilar material combinations for inner and outer layers (i.e., hydrophobic polymers as outer layer and hydrophilic polymer as inner layer). Upon using different material combinations for inner and outer layers, it is shown that one may control both the outer and inner diameters of the fiber. The inner layer not only acts as a barrier and thus provides an ease in the encapsulation of functional core materials of interest with different viscosities but also adds stiffness to the fiber. The structure and the surface morphology of fibers are controlled by changing applied voltage, polymer types, polymer concentration, and the evaporation rate of solvents. It is demonstrated that if the vapor pressure of the solvent for a given outer layer polymer is low, the fiber diameter decreases down to 100 nm whereas solvents with higher vapor pressure result in fibers with the outer diameter of up to 1 μm. The influence of electric field strength on the shape of Taylor cone is also monitored during the production process and the manufactured fibers are structurally investigated by relevant surface characterization techniques

Performance comparison of CVD grown carbon nanofiber based on single- and multi-layer graphene oxides in melt-compounded PA6.6 nanocomposites

In the present study, newly design hybrid nanostructures were produced by growing long carbon nanofibers (CNF) on single- and multi-layer graphene oxide (GO) sheets in the presence of catalyst by chemical vapor deposition (CVD). Chemical composition analysis indicated the formation of Fe-C bonds by the deposition of carbon atoms on catalyst surface of Fe2O3 and increasing in C/O atomic ratio confirming CNF growing. These hybrid additives were distributed homogeneously through polyamide 6.6 (PA6.6) chains by high shear thermokinetic mixer in melt phase. Spectroscopic studies showed that the differences in the number of graphene layer in hybrid structures directly affected the crystalline behavior and dispersion state in polymer matrix. Flexural strength and flexural modulus of PA6.6 nanocomposites were improved up to 14.7% and 14% by the integration of 0.5 wt% CNF grown on multi-layer GO, respectively, whereas there was a significant loss in flexural properties of single-layer GO based nanocomposites. Also, the integration of 0.5 wt% multi-layer GO hybrid reinforcement in PA6.6 provided a significant increase in tensile modulus about 24%. Therefore, multi-layer GO with CNF increased the degree of crystallinity in nanocomposites by forming intercalated structure and acted as a nucleating agent causing the improvement in mechanical properties

New hybrid nano additives for thermoplastic compounding: CVD grown carbon fiber on graphene

Nano additives have unique characteristics widely used in high technology applications due to their ultrahigh mechanical and thermal properties. They are not preferred in price sensitive sectors especially in automotive applications because of their high cost. On the other hand, there is a growing interest to use graphene as a reinforcing agent in composite production. At this point, graphene platelet (GNP) produced from the recycle source was used as a template for carbon nanofiber production by using chemical vapor deposition (CVD) technique to overcome commercialization harrier. This bicomponent and novel structure is a good candidate to be used as a reinforcing agent in compound formulations. This produced hybrid additive was dispersed in thennoplastic resin by thennokinetic mixer to get homogeneous dispersion and provide strong interfacial interactions. In the current work, the outstanding properties of graphene with carbon fibers were combined into one type structure. With the further research, the number of graphene layer were adjusted in this hybrid structure to bring a new insight in graphene and its composite applications. After the fabrication of graphene and carbon fiber-based reinforcements with different graphene sources, mechanically and thermally improved Polyamide 6.6 were developed at very low loadings by a thermokinetic high shear mixer. This developed technology will utilize an innovation to produce advanced thermoplastic prepregs including graphene and its hybrid additives with high mechanical properties and increased recycling degree by decreasing manufacturing costs

Co-Bonded Hybrid Thermoplastic-Thermoset Composite Interphase: Process-Microstructure-Property Correlation

Co-bonding is an effective joining method for fiber-reinforced composites in which a prefabricated part bonds with a thermoset resin during the curing process. Manufacturing of co-bonded thermoset-thermoplastic hybrid composites is a challenging task due to the complexities of the interdiffusion of reactive thermoset resin and thermoplastic polymer at the interface between two plies. Herein, the interphase properties of co-bonded acrylonitrile butadiene styrene thermoplastic to unsaturated polyester thermoset are investigated for different processing conditions. The effect of processing temperature on the cure kinetics and interdiffusion kinetics are studied experimentally. The interphase thickness and microstructure are linked to the chemo-rheological properties of the materials. The interdiffusion mechanisms are explored and models are developed to predict the interphase thickness and microstructure for various process conditions. The temperature-dependent diffusivities were estimated by incorporating an inverse diffusion model. The mechanical response of interphases was analyzed by the Vickers microhardness test and was correlated to the processing condition and microstructure. It was observed that processing temperature has significant effect on the interdiffusion process and, consequently, on the interphase thickness, its microstructure and mechanical performanc

Combatting rain erosion of offshore wind turbine blades by co-bonded thermoplastic-thermoset hybrid composites

An integrated thermoplastic-thermoset hybrid leading edge protection system is developed based on the co-bonding process. Co-bonding is a joining method in which a prefabricated part joints with a thermoset composite during the curing process. In such a multi-material hybrid design, the reliability of the bonding between the prefabricated protection layer and the main body of the blade is of crucial importance to prevent any delamination failures. Nevertheless, the adhesion of prefabricated thermoplastics to the thermoset remains a challenge as the interphase between two dissimilar materials is prone to form defects and irregularities. Such interface defects may lead to early failure and reduced structural integrity of the components. Therefore, the focus of this study is on achieving a strong, and reliable bonding between the prefabricated thermoplastic leading edge protection system and thermoset main body of the blade. In this study, the effect of processing temperature on the interphase quality and thickness during the co-bonding process is investigated. Next, mechanical characteristics and microstructure of the interphases are examined by Vickers microhardness tests. The effect of processing condition on the fracture toughness of structure is examined by climbing drum peel tests (CDP). Finally, fractography investigations are used to provide an understanding of failure mechanisms and its correlations with interphase morphology and microstructure

Characterization of interdiffusion mechanisms during co-bonding of unsaturated polyester resin to thermoplastics with different thermodynamic affinities

Co-bonding of thermoplastics to thermosets remains as a challenge in hybrid composites manufacturing. The interphase of the joint is controlled by the thermodynamic affinity and the processing condition. Thus, the main objective of this study is to describe the interphase formation mechanisms between a thermoset resin and thermoplastics. Therefore, a methodology is developed to predict, control, and analyse the interphase morphology experimentally while modelling approaches are used to assess the microstructure. Hansen solubility parameters are used to select thermoplastics with different thermodynamic affinities to the unsaturated polyester resin as the thermoset. The resin uptake experiments are conducted to identify the kinetics of diffusion and empirical models are recognised to predict the swelling at various processing temperatures. Next, a coarse grain lattice model is established which couples the resin uptake and microscopic observation to extract information on the microstructure of interphase through predicting volume fraction of resin at the interphase

Manufacturing of multilayer graphene oxide/poly(ethylene terephthalate) nanocomposites with tunable crystallinity, chain orientations and thermal transitions

Thermally exfoliated graphene oxide (TEGO) reinforced polyethylene terephthalate (PET) nanocomposites with controlled crystallinity, chain conformations and thermo-mechanical properties were produced with very low TEGO weight fractions by a twin-screw compounding extruder. Tensile modulus was found to increase by 52% by the addition of 1 wt% TEGO. This significant increase in mechanical properties of PET nanocomposites was explained by well intercalation of PET chains through multi-layer graphene sheets and complete coverage of graphene surface by electrostatic interactions. An increase in the ratio of gauche and trans conformations in PET chains indicated that PET nanocomposites became more crystalline by increasing TEGO amount. Transmission electron microscopy observations showed the favorable interaction between TEGO sheets and PET matrix facilitating the dispersion and flattening of graphene sheets into polymeric matrix during elongation. The integration of 1 wt% TEGO sheets into PET matrix enhanced heat distortion temperature from 71 °C for neat specimen upto 91.6 °C at the constant stress of 0.45 MPa, and increased Vicat softening point from 76 °C upto 95 °C. Therefore, the failures of PET considerably reduced by improving short-term heat resistance and its softening properties between glass transition temperature and melting temperature by the incorporation of TEGO sheets