Yes / Designing and manufacturing functional parts with enhanced mechanical property is a major goal of fused filament fabrication (FFF) for polymeric elastomers, which exhibits major advantages in producing such parts with a range of structures. But the unsatisfactory mechanical performance constrains greatly its real application and there is yet no consensus in the mechanical characterization of printed samples. This work takes the nozzle height as the considered factor and tests the tensile property of FFF-printed thermoplastic polyurethane (TPU). Rheological property of the TPU melt, represented here by die swell behavior and shear viscosity, were measured initially to obtain a preliminary assessment of the material suitability and an optimization of melt extrusion conditions for FFF processing. Then correlation between the cross-section profile of deposited bead and the tensile performance of printed sample were evaluated. Both the shape of deposited bead and the bonding strength of two adjacent beads are emphasized when explaining the measured tensile strength. The significance of molecular permeation efficiency at bead-bead interfaces, and bonding-releasing patterns between adjacent beads to the tensile failure of printed objects is discussed. / The support provided by China Scholarship Council (CSC, 201806465028) for Xiang Lin during his academic visit in University of Bradford is acknowledged.

Nanomaterials are becoming increasingly prevalent in final use products as we continue to improve our understanding of their structure and properties and optimize their processing. The useful applications for these materials extend from new drug delivery systems to improved materials for various transport industries and many more. Nanoscale materials which are commonly used include but are not limited to carbon nanotubes, graphene, silica, nanoclays, and cellulose nanocrystals. The literature presented herein aims to investigate structure-process-property relationships of cellulose nanocrystal (CNC) polymer composites. These CNC nanocomposites are unique in that they provide a dynamic mechanical response when exposed to H2O. Currently, these nanocomposite systems are most commonly solvent cast into their final geometry. In order to enable the use of these materials in more commercial processing methods such as extrusion, we must understand their inherent structure-process-property relationships. To do this, we first characterize the influence of temperature and shear orientation on the unique mechanical adaptive response. Next, the melt processability of the nanocomposite was characterized using material extrusion (MatEx) additive manufacturing (AM). Additionally, the diffusion behavior of water within the film, which controls the dynamic mechanical response, was probed to better predict the concentration dependent behavior. Finally, a literature review is presented which outlines the state of the art for melt extrusion AM of fiber filled polymer composite materials and provides insight into how we can further improve mechanical properties through further addition of composite filler materials. The initial focus of the dissertation is on the influence of melt processing CNC thermoplastic urethane (TPU) composites and the resulting impact on the mechanical adaptive response. Dynamic mechanical analysis (DMA) fitted with a submersion clamp was used to measure the mechanical softening of the composite while submerged in water. Small angle x-ray scattering (SAXS) and polarized raman spectroscopy were used to qualify the orientation of the various CNC/TPU composite samples. The results of the orientation measurements show that solvent casting the films orient CNCs into a mostly random state and melt extrusion induces some degree of uniaxial orientation. The DMA results indicate that at the processing conditions tested, the uniaxial orientation and thermal exposure from the melt processing do not significantly impact the mechanical responsiveness of the material. The next objective of this work was to expand upon the aforementioned learnings and determine the CNC composite material processability using MatEx. The ability to process mechanically dynamic CNC/TPU composites with a selective deposition process capable of generating complex geometries may enable new functionality and design freedom. To realize this potential, a two factor (extrusion temperature and extrusion speed) three level (240, 250 and 260 ℃/ 600, 1100 and 1600 mm/min) design of experiments (DOE) was utilized. The resulting printed parts were characterized by DMA to determine their respective mechanical adaptivity. Processing conditions did prove to have a significant impact on the mechanical adaptivity of the printed part. A correlation between applied energy and mechanical adaptivity demonstrates how increasing residence time and temperature can reduce mechanical performance. The shape fixity of the printed parts was calculated to be 80.4% and shape recovery was 44.2%. A 3D prototype part was also produced to demonstrate the unique properties of this material. Although the understanding of the melt processing behavior of these CNC composites had been improved, a stronger understanding of the moisture diffusion behavior within the composite is required to fully realize and control their potential. Therefore, a study was undertaken to capture the diffusion behavior and correlate it to the mechanical responsive mechanism. To do this, a thermogravimetric sorption analysis (TGA-SA) instrument was used to monitor the mass uptake as a function of time exposed to a humid environment. These data were then compared to DMA data collected for the same samples exposed to a similar degree of humidity. All studies were conducted as a function of concentration in order to better elucidate the influence that percolating network structures may have on the resultant properties. Interestingly, the results show how increasing addition of CNCs results in a decrease in the rate of diffusivity, which is counter to what has been commonly hypothesized. It is hypothesized that increasing CNC content restricts the mobility of surrounding amorphous matrix material, thus increasing the resistance for diffusion of a water molecule. However, the rate of mechanical adaptation was found to increase with increasing CNC content, which is believed to be a result of the increased connectivity, enabling further transport of water molecules. / Doctor of Philosophy / Nanomaterials are becoming increasingly prevalent in final use products as we continue to improve our understanding of their structure and properties and optimize their processing. The useful applications for these materials extend from new drug delivery systems to improved materials for various transport industries and many more. The literature presented herein aims to investigate structure-process-property relationships of cellulose nanocrystal (CNC) polymer composites. These CNC nanocomposites are unique in that they provide a unique mechanical response when exposed to water. In order to enable the use of these materials in more commercial processing methods, we must understand their inherent structure-process-property relationships. The following documents multiple aspects of these unique composite materials which enables their commercial viability and scientific versatility.

Cellulose nanomaterials (CNMs) have been widely studied for their potential as sustainable fillers in polymer nanocomposites, optical responsiveness in suspensions and thin films, and their orientation-dependent liquid crystalline behavior in suspensions. Cellulose nanocrystals (CNCs) have seen a particular prominence due to their versatility across a breadth of applications. The unique structure of CNCs, represented as nanoscale rods with a slight twist, provides for their self-assembly into liquid crystalline phases when their concentration is increased and can be used to generate iridescent materials with tunable wavelengths. Further, CNCs are often used as fillers in nanocomposites, due to their high single crystal Young's modulus, achieving vast enhancements in stiffness when incorporated above a critical concentration where a percolating network is formed. The breadth of applications for CNCs strongly depend not only on their crystalline structure, but crucially on the interactions between particles. These interactions are well-known, yet a complete understanding to enable the full exploitation of the properties attainable in CNC-based materials is lacking. The principal emphasis of this dissertation lies in further improving our comprehension of the interactions between CNCs across a variety of applications such that their full potential can be achieved. A review of the current research of CNC-based materials is provided to guide the discussion herein. Interparticle interactions are studied in aqueous suspensions of CNCs in evaporating sessile droplets. This system provides a complex interrelationship between mass, heat, and momentum transport which collectively provide a change in the local CNC concentration as a function of time. CNC interactions can be controlled throughout the evaporation process as a result of these local concentration variations. We implement a novel approach using time-resolved polarized light microscopy to characterize the evolution of these particle interactions via the orientation of CNCs as a function of CNC concentration and droplet volume. Ultimately, boundary interactions at the leading edge of the contact line during evaporation was found to drive a cascade of local CNC interactions resulting in alignment post-deposition. Computational analysis evaluated the influence of evaporation-induced shear flow during evaporation. Orientation was found to be independent of the bulk fluid flow, corroborating the importance of interparticle interactions on the ensuing alignment of CNCs. Characterization of an evaporating droplet of initially liquid crystalline suspension of CNCs verified the simulations which predicted that orientation was not coupled with entrainment. Finally, the multiple modes of orientation showed that local control over CNC properties can be realized through governance of the interactions between CNCs. The interactions of CNCs in polymer nanocomposites were also studied for the development of smart materials which can adapt their properties in response to external stimuli. A well-known example of this phenomena is found when CNCs are introduced as fillers in thermoplastic polyurethanes (TPUs) above a critical concentration required to achieve percolation. The interactions between CNCs in the percolating network provide a strong enhancement to the modulus of these materials. However, these materials soften upon exposure to water following the disruption of inter-CNC hydrogen bonding by the diffusing water molecules, as prevailing theories suggest. CNCs simultaneously enhance water transport into hydrophobic matrices. Thus, a complete understanding of the interrelationship between the mass transport and mechanical performance can facilitate the development of humidity sensing or shape memory materials which operate as a result of the interactions between CNCs inside of a polymer matrix. Despite an increase in the equilibrium water uptake with increasing CNC concentration, a decrease in the apparent diffusivity of water within the nanocomposites was observed as a result of swelling of the bulk polymer. Additionally, we developed a modification to the commonly used percolation model to predict the time-dependent evolution of storage modulus during water-induced softening. We found that the solvent mass transport can be directly coupled to the mechanical integrity of the percolating network of CNCs by evaluating the hydrogen bonding state of the network as a function of time. Finally, a novel nanocomposite filler comprised of CNCs and 2,2,6,6- tetramethylpiperidine 1-oxyl (TEMPO) oxidized cellulose nanofibrils (TOCNFs) was prepared through solution casting to improve the mechanical performance of the individual reinforcements alone. The physical interaction length is increased by incorporating CNMs of different length scales resulting in increased tensile strength and elongation. Further, the morphology, evaluated with polarized light microscopy, atomic force microscopy, and simulated with dissipative particle dynamics, revealed the combined fillers exhibit a cooperative enhancement between CNMs. Characterization of the crystallinity through x-ray diffraction confirmed the interactions occur primarily between the crystalline domains of each material. Accordingly, the combination of CNMs resulted in nanocomposite fillers which can be implemented such that the weak interfaces with polymer matrices can be bridged with fillers providing reinforcement over a broader length scale. / Doctor of Philosophy / Cellulose nanocrystals (CNCs) are sustainable and biorenewable nanoparticles derived from cellulose. These materials have been widely studied and are commonly used among a plethora of applications such as in reinforcing fillers in polymer nanocomposites, optically responsive materials that can be used in packaging or anti-counterfeiting technologies, as well as in suspension modifiers for skin care products. These techniques tune the interactions between individual CNCs to modify the behavior of the bulk material. The specific interactions are well-known, yet a complete understanding of the influence of these interactions resulting in the utility of CNC-based materials in various applications is lacking. The principal emphasis of this dissertation lies in further improving our comprehension of the interactions between CNCs across a variety of applications such that their full potential can be achieved. Interactions between CNCs were investigated in three systems comprising of a range of typical use cases for CNC-based materials. The behavior of CNCs was examined in evaporating droplets of aqueous suspensions. These materials exhibited a change in orientation in the final deposit which is dependent on variations in local CNC concentration during drying. These concentration changes describe the relative strength of interactions between CNCs which ultimately influences the final alignment of these materials. Further, these interactions provide a pathway to deposit a controlled orientation of CNCs on a substrate which can be utilized for optically responsive materials or serve as templates for other orientation-dependent materials. CNCs were also incorporated into a thermoplastic polyurethane (TPU) matrix to provide increased stiffness. In these composites, water preferentially interacts with CNCs preventing the nanoparticles from interacting with one another. As water is absorbed, these materials soften as a result of the reduced interactions between CNCs. We investigated the influence of dynamically changing CNC interactions on the mechanical performance of these materials during water absorption and developed an analytical model to describe the observed softening behavior. Finally, CNCs were combined with 2,2,6,6- tetramethylpiperidine 1-oxyl oxidized cellulose nanofibers (TOCNFs) and cast into thin films. The mechanical properties of these differently sized, yet chemically similar, nanoparticles were compared as a function of CNC composition. A cooperative enhancement of the ultimate tensile strength and elongation was observed at low CNC loadings where CNCs and TOCNFs were found to self-organize during casting in a mutually beneficial manner.

A series of RF plasmas was chosen to modify the surface chemical composition of Hercules IM7 carbon fibers. A two-liquid tensiometric method was used to determine the surface energy parameters y(ds) and I(psf) of the fibers. An XPS analysis of air and argon plasma treated fibers indicated a significant surface oxidation of the fibers which translated into low y(ds) values and high I(psf)values. An ammonia plasma was shown to remove an outer layer from the surface of the fibers. It also increased y(ds) compared to as-received fibers without affecting the non-dispersion (IPsf) XPS results indicated that methane and ethylene plasmas deposited a layer of low surface energy hydrocarbon on the fiber surface. A trifluoromethane plasma and a tetrafluoromethane plasma introduced a significant amount of fluorine containing groups in the fiber surface in the form of a fluorinated plasma polymer in the first case and through direct attack of the fiber surface by fluorine atoms in the second case. The surface chemical composition and the surface energy parameters of two series of commercially treated carbon fibers were also determined and compared to the results on IM7 carbon fibers. The adhesion of carbon fibers to polyethersulfone (PES) was measured by using the microbond pull-out test, and compared to the adhesion of the same fibers to an epoxy resin. The load required to debond the microdroplet was used as a measure of the bond strength. The data were also analyzed in terms of interfacial fracture energy accordIng to the model developed by Jiang and Penn (1992). The microbond pull-out test results showed no significant relation between the fiber surface chemical composition or the fiber surface energy, and the adhesion to PES. However, plasmas which have a strong ablative character such as the ammonia and the tetrafluoromethane plasmas did improve the fiber-PES adhesion, when compared to as-received fibers. The study of the fiber-epoxy systems revealed that a chemical effect contributed to the adhesion improvement but to a lesser extent than the "cleaning" effect of the surface treatment. The results support the two part mechanism proposed by Drzal and extend its application to carbon fiber-thermoplastic systems, but in this case the chemical effect is minimal. / Ph. D.

The concept of employing engineering thermoplastics as toughness modifiers for Bismaleimide resins was utilized to improve the fracture toughness properties of these important materials, which have applications as matrix resins for high performance composites. Modifier molecular weight, end group functionality, backbone structure and weight percent incorporation were all studied with respect to their influence on K_lc, fracture toughness properties. Increases in fracture toughness were created with thermoplastic oligomers without sacrificing high temperature properties and desirable hot-melt processing conditions. Investigations were also made to study the morphological features that develop within these modified thermosets and their resistance to specific environments. In addition, unidirectional carbon fiber composites were prepared and their mode I and II strain energy release rates measured. Respectable increases in the interlaminar fracture toughness were obtained, 15 and 20 percent by weight loadings of maleimide terminated polysulfone modifiers yielded G_lc values of 489±25 and 734±10 J/m² respectively, a substantial improvement over the control value of 359±17 J/m². Laminates were prepared using carbon fibers that had been investigated in terms of their surface energies using Inverse Gas Chromatography. It was illustrated how this technique could distinguish between the acid-base properties of fibers possessing different degrees of proprietary surface treatments. Fiber composites containing both contrasting and subtle changes at the fiber-matrix interphase were prepared and their mechanical properties evaluated using a variety of test methods. Dramatic increases in laminate properties were measured for composites possessing contrasting interphases. Furthermore, the mode II fracture toughness test was sensitive to interphase differences; however, the mode I fracture toughness test was not. Specimens subjected to the new Continuous Ball Indentation test method (meso-indentation) were compared with single fiber micro-indentation test results. Differences were detected in composites prepared using untreated and surface treated fibers. The new method was also sensitive to changes in matrix ductility. Certain anomalies that were noted to be surprising from micro-indentation measurements were not present in the meso-indentation test results. These observations brought to light certain limitations found within the micro-indentation test, but further supported the new test method as a potential technique for fiber-matrix interphase evaluation. / Ph. D.

Injection Molding The effects of partial miscibility on the mechanical properties and morphology of thermotropic liquid crystalline polymer blends were investigated in this part of the work. Blends of an immiscible (Vectra A900) and partially miscible (HX1000) thermotropic liquid crystalline polymer (TLCP) with a polyetherimide (PEI) were used in the investigations. The blends were injection molded into mini-tensile bars and rectangular plaques, and their mechanical properties were evaluated. Interfacial, rheological, and morphological properties along with molecular orientation analysis were carried out in order to explain the mechanical properties of the blends. Mechanical tests showed that both the tensile and flexural modulus deviate positively from the law of mixtures. However, for the PEI/HX1000 system the deviation from the law of mixtures appeared at lower TLCP concentrations compared to the PEI/Vectra A900 system. It was found that the tensile modulus correlated well with the structure developed during injection molding. Morphological tests show that finer higher aspect ratio TLCP fibers developed in the PEI/HX1000 system relative to the PEI/Vectra A system. In addition, both blends showed a maximum in the tensile modulus at 90 wt% TLCP. Rheological tests indicated that for TLCP-rich compositions, a higher viscosity was observed for the blends in comparison to the neat TLCPs. Therefore, due to a greater viscosity, higher magnitudes of stresses, consequently inducing a higher degree of molecular orientation, were experienced by the blends relative to the neat TLCPs. Although partial miscibility seemed to affect more strongly the stiffness of the in situ composite, the ultimate properties of the TLCP strongly dominated the ultimate properties of the PEI/TLCP composite. Mechanical tests showed that the ultimate properties of Vectra A were at least twice those of HX1000. Consequently, for TLCP-rich compositions, higher values of toughness, elongation at break and tensile strength were observed for PEI/Vectra A blends compared to PEI/HX1000 blends. The study presented here seems to suggest that the selection of a TLCP to reinforce a polymeric matrix is not only dependent upon whether partial miscibility or compatibility between the TLCP and matrix polymer exist, but also on the mechanical properties of the TLCP. Shearfree Elongational Deformation The effects of uniaxial, planar and biaxial deformations on the morphology and mechanical properties of bends of a polyetherimide with thermotropic liquid crystalline polymers were investigated in this part of the work. Extruded sheets and molded plaques of PEI/Vectra A and PEI/HX1000 blends were used in the studies. In the case of injection molded plaques, in which the initial morphology was that of fibers and droplets, the direction of the applied deformation relative to the initial direction of the TLCP fibrils was an important factor in affecting the resultant morphology and corresponding mechanical properties of the blends. If the direction of the applied uniaxial deformation was parallel to the initial fiber direction, the deformation tended to increase the average aspect ratio of the TLCP fibers and mechanical properties were enhanced along the direction of deformation. However, if the deformation was applied transverse to the initial fiber direction, the fibers tended to follow the deformation and a 90° rotation was observed. In terms of mechanical properties, an increase in the transverse direction properties accompanied by a reduction in the flow direction properties followed the realignment of the fibers. In addition, equal flow and transverse mechanical properties appeared at 0.5 units of transverse uniaxial strain. Planar deformation led to the spreading of the fibers in the plane of deformation and a ribbon-like morphological structure developed. However, at comparative magnitudes of planar strains, transverse planar compression tends to promote a greater spreading of the fibers relative to planar compression applied parallel to the initial direction of the fibers. In addition, planar stretching applied in a direction perpendicular to the initial direction of the TLCP fibers was effective in reducing the mechanical anisotropy of the molded plaques. Samples showing equal flow and transverse properties were obtained when planar strains greater than 0.5 units were applied in a direction perpendicular to the initial direction of the fibers. In the case of extruded sheets, in which the initial morphology was that of drops, it appeared that partial miscibility was an important factor in affecting the final morphology of the sheet. For the immiscible PEI/Vectra A system, longer and more stable TLCP fibrils were found compared to PEI/HX1000 system. It is believed that, due to lower interfacial tension, stress induced fiber breakup occurred during stretching of the PEI/HX1000 blend. Thermoforming of In Situ Composites The use of in situ thermoplastic composites based on blends of a polyetherimide with an amorphous and a semicrystalline liquid crystalline polymer in the thermoforming process was explored in this part of the work. Injection molded and extruded samples, in which the initial morphology of the dispersed TLCP phase was either in the form of fibers or droplets, were subjected to thermoforming. It was found that in the case where the initial morphology of the dispersed TLCP phase was that of droplets, the elongational stresses generated during forming were capable of deforming the TLCP phase into fibers, and the aspect ratio of the fibers was increased with depth of draw. However, when the initial morphology of the the TLCP phase was in the form of fibers, then the relative alignment of the fibers with respect to the forming direction was an important factor in affecting the final structure of the TLCP phase in the formed tray. When the fibers were aligned parallel to the forming direction, the elongational strains generated during forming tended to further increase the aspect ratio of the fibers. In the case where the initial TLCP fibers were aligned transversely to the forming direction, the fibers tended to spread into a ribbon-like structure after forming. Pre-stretching of the samples prior to thermoforming tended to contribute to an increase in the aspect ratio of the TLCP fibers. As a result, an enhancement in the deflection resistance of the prestretched/formed samples was observed. In situ thermoplastic composites seemed to be advantageous compared to glass reinforced thermoplastics in thermoforming applications. The elongational stresses generated during forming tended to deform the TLCP phase into a specific morphology. Depending on the relative direction of the deformation, either fibers or a ribbon-like structure may be developed. This is in contrast to glass reinforced PEI, in where breakage of the glass fibers occurred upon forming, which may contribute to a reduction in the mechanical performance of glass reinforced materials. / Ph. D.

Composite durability can be influenced by varying the properties of the fiber/matrix interphase region. One method to modifY the properties of this interphase is through the application of a sizing to the carbon fiber. Recent work at Virginia Tech has shown that polymer-modified interphases can lead to increases by as much as two orders of magnitude in notched fatigue lifetime. In the present work, an apparatus was constructed to uniformly coat carbon fiber tow with water-soluble and dispersible polymers. Few such devices have been developed for use in academic settings because of the processing complexities presented by the aqueous coating system. Due to the high surface tensions of the aqueous solutions, fiber clumping and heterogeneous sizing deposition were major bottlenecks. Our novel process utilizes high tensions, high spreading, and low line speeds to accomplish the sizing step. Each processing independent variable can be continuously monitored and controlled which allowed for statistical correlation to the sizing level and uniformity. The sizing process was shown to satisfy three criteria for quality. 1. The average sizing level or weight percent on the final fiber can be readily controlled to achieve typical target values. 2. Filament clumping as a result of cohesion between corresponding filaments is kept to a minimum. 3. The sizing process must produce fiber with a consistent level of polymer sizing. In addition, characterization techniques for the sized fiber were developed. Pyrolysis in a high temperature nitrogen furnace was developed as a precise technique to ascertain the quantitative sizing level on the carbon fiber. SEM and ESCA were utilized to determine fiber clumping and sizing homogeneity. The sizing process along with the statistical process model and the characterization techniques allow for the precise development of optimal interphase materials that are tailored to meet the performance requirements of the composite consumer. / Master of Science

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Thermoplastic composites (TPCs) have shown significant advantages over thermosetting composites. They have only been put into use recently and global knowledge in TPCs is often proprietary, therefore a study into the application, processing and properties is of importance. The aim of the study is to contribute knowledge in TPCs for South African industry and academic institutions. This thesis studies continuous fibre reinforced thermoplastics (CFRTPs), focussing on the autoclave processing of commingled CFRTPs. A literature study provided background knowledge to CFRTPs regarding processing techniques and mechanics. Flexural testing and impact testing were performed on a variety of CFRTPs and thermosetting composites (TSCs). These tests were performed to further understand CFRTPs as well as to compare CFRTPs and TSCs. The flexural testing revealed that CFRTPs have comparable strength and stiffness to the TSCs that were tested. They also revealed that pre-consolidated sheets showed better and more consistent properties than sheets made from commingled fabric. The impact testing revealed that the tested CFRTPs and TSCs had similar impact resistance even though thermoplastic composites are supposed to be more impact resistant. The tests also showed that thick unreinforced thermoplastics had much higher impact resistance than the reinforced materials. Manufacturing experiments were performed to establish sound processing methods of CFRTPs. It was realised here that the high temperatures required to process the materials require specific processing consumables and tooling. The experiments began by processing flat panels in a convection oven with vacuum bagging techniques. They then progressed to autoclave processing of parts with complex geometry. An airline seat backrest was chosen as the case study in the application of CFRTPs. This application requires structural strength and stiffness and also has strict fire, smoke, toxicity and heat release (FSTH) requirements. Its geometry was sufficiently complex to demonstrate the use of commingled CFRTP material. Backrests were made from both CFRTPs and TSCs so that a comparison could be made between the two types. The backrest was modelled using finite element methods (FEM) to determine an adequate lay-up. This lay-up was then used for both the CFRTP and TSC backrests to ensure similarity between the backrests of both materials. LPET (modified polyethylene terephthalate) was the chosen thermoplastic matrix as it was more attainable than PPS (polyphenylene sulphide) CFRTPs. The backrests of both materials were manufactured in an autoclave with a vacuum bag method and then assembled using adhesives and bonding jigs. Testing revealed that the stiffness and mass of the CFRTP backrests were very similar to the epoxy backrests. This implies that commingled CFRTPs can replace the use of TSCs in similar applications. A basic cost comparison was also performed to compare the manufacture of CFRTP backrests to TSC backrests. Further work is needed to optimise processing time of these materials to make them more competitive with TSCs. The processing time of commingled materials will probably never be as quick as that of press formed pre-consolidated sheets. Their ability to be formed into more complex parts does however make their use advantageous. / AFRIKAANSE OPSOMMING: Termoplastiese saamgestelde materiale (Engels: thermoplastic composites (TPCs)) toon beduidende voordele bo termoverhardbare saamgestelde materiale. Hulle word eers sedert onlangs benut en algemene kennis in TPCs is dikwels patentregtelik, dus is ’n studie van die aanwending, prosessering en eienskappe daarvan van belang. Die doel van hierdie studie is om ’n bydrae te lewer tot die kennis van TPCs vir die Suid-Afrikaanse industrie en akademiese instellings. Hierdie tesis ondersoek kontinue veselversterkte termoplastieke (Engels: continuous fibre reinforced thermoplastics (CFRTPs)) en fokus op die outoklaafprosessering van vermengde (Engels: commingled) CFRTPs. ’n Literatuurstudie het die agtergrondkennis rakende die prosesseringstegnieke en meganika van CFRTPs verskaf. Buigtoetsing en impaktoetsing is op ’n verskeidenheid CFRTPs en termoverhardbare saamgestelde materiale (Engels: thermosetting composites (TSCs)) uitgevoer. Hierdie toetse is uitgevoer om CFRTPs beter te verstaan asook om CFRTPs en TSCs te vergelyk. Die buigtoetsing het onthul dat CFRTPs ooreenstemmende sterkte en styfheid het as die TSCs wat getoets is. Dit het ook getoon dat vooraf-gekonsolideerde plate beter en meer konsekwente eienskappe getoon het as plate wat van vermengde materiaal gemaak is. Die impaktoetsing het onthul dat die CFRTPs en TSCs wat getoets is soortgelyke impakweerstand gehad het, selfs al is termoplastiese saamgestelde materiale veronderstel om meer impakweerstand te toon. Die toetse het ook getoon dat dik onversterkte termoplastieke veel hoër impakweerstand gehad het as die versterkte materiale. Vervaardigingseksperimente is uitgevoer om betroubare prosesseringsmetodes vir CFRTPs vas te stel. Daar is besef dat die hoër temperature wat vereis word om die materiale te prosesseer ook spesifieke prosesseringsverbruiksware en -gereedskap benodig. Die eksperimente het begin met die prosessering van reguit panele in ’n konveksie-oond met vakuumsaktegnieke. Daar is toe aanbeweeg na die outoklaafprosessering van onderdele met komplekse geometrie. Die rugleuning van ’n vliegtuigsitplek is gekies as die gevallestudie in die gebruik van CFRTPs. Hierdie toepassing vereis strukturele sterkte en styfheid en is ook onderhewig aan streng vereistes t.o.v. brand, rook, toksisiteit en hittevrystellimg (Engels FSTH). Die geometrie daarvan was kompleks genoeg om die gebruik van vermengde CFRTP-materiaal te demonstreer. Rugleunings is gemaak van beide CFRTPs en TSCs sodat ’n vergelyking tussen die twee tipes gemaak kon word. Die rugleuning is gemodelleer deur eindige element metodes (EEM) te gebruik om ’n aanvaarbare oplegging te bepaal. Hierdie oplegging is toe gebruik vir beide die CFRTP en TSC rugleunings om die gelykvormigheid tussen die rugleunings van beide materiale te verseker. LPET (Engels: modified polyethylene terephthalate) was die gekose termoplastiese matriks aangesien dit meer verkrygbaar was as PPS (Engels: polyphenylene sulphide) CFRTPs. Die rugleunings van beide materiale is vervaardig in ’n outoklaaf met ’n vakuumsakmetode en toe geintegreer deur die gebruik van kleefstowwe en setmate. Toetsing het getoon dat die styfheid en massa van die CFRTP rugleunings baie soortgelyk was aan die epoksie rugleunings. Dit impliseer dat vermengde CFRTP die plek van TSCs in soortgelyke gebruike kan inneem. ’n Basiese kostevergelyking is ook gedoen om die vervaardiging van CFRTP-rugleunings teenoor TSC-rugleunings te vergelyk.

Les composites à matrice polymère sont de plus en plus utilisés dans le secteur automobile. Afin de remplir les conditions exigeantes du cahier des charges vis-à-vis des conditions de mise en service, les pièces en composite doivent maintenir leur forme géométrique sous des conditions thermo-mécaniques parfois extrêmes. Par exemple, un assemblage de hayon composite est soumis à des contraintes mécaniques élevées associées à des variations de température importantes lors des essais de validation du cahier des charges. Les enjeux de la thèse sont axés sur l’aide à la conception dans le domaine quasi-statique de pièces industrielles injectées en thermoplastique renforcé de fibres discontinues. L’amélioration des outils numériques doit permettre un dimensionnement virtuel optimal de ces pièces en anticipant les variations rencontrées en service et les distorsions résiduelles résultantes de chargements thermo-mécaniques. Cette démarche s’appuie sur la connaissance du comportement thermo-mécanique du matériau de l’étude, celui du renfort de hayon, un polypropylène renforcé à 40% en masse de fibres de verre discontinues, et sur la modélisation du comportement de ce type de matériau. / Discontinuous fibers reinforced thermoplastic materials have been widely used for several years in the automotive industry. These parts must resist demanding service life conditions and must meet thermo- mechanical specifications. Indeed, structural automotive spare parts have to endure high temperatures, like a few tens of degrees Celsius, for a long duration, at least a few hours. As an example, a structural part of tailgate is subject to high mechanical loading, associated to strong temperature variations, during the validation test, regarding specifications. The purpose of this work is to improve the design of complex industrial parts, like the tailgate in quasi-static domain, by relying on numerical simulation. One of the challenges related to the use of such material, is to have a reliable virtual design of industrial parts by predicting the geometrical variations during service life conditions, and residual strain. Therefore, it is necessary to characterize and to model the thermo-mechanical behavior of the tailgate material, a polypropylene matrix reinforced with discontinuous glass fibers, with a given mass fraction of 40%

L'objet de cette thèse est de développer un matériau polymère élastomère permettant d'assurer la liaison et d'absorber les déformations imposées entre deux matériaux composites à matrice thermoplastique. Différents élastomères thermoplastiques ont été identifiés et leur comportement thermomécanique a été étudié en comparaison d’un élastomère thermodurcissable utilisé jusqu'alors. Les polyuréthanes thermoplastiques (TPU) sont apparus comme une alternative intéressante. Nous en avons étudié la chimie et la microstructure ainsi que les propriétés mécaniques en traction et le comportement lors de sollicitations successives multiples qui mettent en évidence l’effet Mullins. L’effet Mullins permet en particulier de quantifier les déformations plastiques irréversibles qui sont liées au caractère thermoplastique des matériaux étudiés. Il a été constaté que ces déformations irréversibles sont comparables à celles observables dans le cas de l’élastomère thermodurcissable conventionnel. Nous nous sommes aussi intéressés au lien existant entre les propriétés thermiques et mécaniques finales du matériau aux différentes échelles, (de l’échelle moléculaire à l’échelle macroscopique notamment). En effet, les TPU sont des matériaux nanostructurés qui peuvent présenter une pseudo-cristallinité et une nano-séparation de phase susceptible d’évoluer avec l’histoire thermique du matériau. Nous avons ainsi cherché à établir un lien entre la structure du matériau à diverses échelles, son procédé de mise en œuvre et les propriétés physiques résultantes. Différentes stratégies ont été explorées pour optimiser le comportement thermomécanique soit par l'incorporation de nano/micro charges ou soit par l'addition d'une faible quantité de liquides ioniques dans la matrice polymère. / The aim of this thesis is the development of an elastomer material able to constitute the junction of two thermoplastic elastomers pieces and to absorb the deformations on these two parts. Various thermoplastic elastomers were identified and there thermomechanical behaviour was studied and compared to the thermoset elastomer presently used. Thermoplastic polyurethanes (TPU) appeared as suitable matches? Their chemical structure, microstructure were studied as well as their mechanical properties in tensile strength and their behaviour toward multiple solicitations that highlights the Mullins effect. The Mullins Effect allows the quantification of irreversible plastic deformations which are induced by the thermoplasticity of the studied materials. It was observed that the irreversible deformations are close to the one of ther thermoset elastomer. We also focused on the existing link between the thermal and mechanical final properties at different scales, (from the molecular level to the macroscopic scale). In fact, the TPU are nanostructured materials that can contain pseudo-crystallinity and nano-phase separation that migth evolve with thermal history. This way, we tried to establish a link between the multi-scaled structure of the materials, it's processing and the resulting physical properties. Various strategies were explored to optimize the thermomechanical behaviour, either by adding nano/micro fillers or by the addition ok small amounts of ionic liquids in the polymer matrix.