A study of the mechanical properties of vapour grown carbon fibres and carbon fibre-thermoplastic composites

As fibras de carbono produzidas na fase de vapor (VGCFs) combinam custos de produção potencialmente baixos com propriedades mecânicas, térmicas e eléctricas favoráveis. Isto torna-as de especial interesse para as aplicações onde as fibras de carbono, baseadas no pitch ou no poliacrilonitrilo (PAN) (designadas por fibras 'convencionais') são demasiado caras e as fibras de vidro não apresentam as propriedades necessárias. O presente projecto de investigação visou três objectivos. Em primeiro lugar, estudar sistematicamente as diferentes morfologias em que as VGCFs podem ser produzidas e avaliar o seu efeito nas propriedades mecânicas. Em segundo lugar, obter conhecimentos sobre a produção de compósitos de VGCF e matriz termoplástica. A determinação das propriedades mecânicas dos compósitos permite avaliar o desempenho das VGCFs como reforço de termoplásticos. Finalmente, pretende-se desenvolver modelos micromecânicos para prever as propriedades mecânicas mais relevantes dos materiais produzidos. Usando estes modelos inversamente, é possí­vel derivar as propriedades das fibras. No caso de VGCFs com diâmetros menores que 1 mm (VGCFs sub-micrométricas), é esta a unica maneira para determinar estas propriedades. Estudaram-se sistemáticamente as diferentes morfologias em que as VGCFs podem ser produzidas e avaliou-se o efeito da forma sobre as propriedades mecânicas das fibras. Concluiu-se que a forma não influencia significativamente o valor do módulo à tracção. No entanto, as fibras com forma diferente de cilindros perfeitos têm uma resistência de ruptura à  tracção mais baixa. Globalmente, o módulo e a resistência à  tracção são significativamente mais baixos do que os das fibras de carbono, ex-pitch ou ex-PAN, comercialmente disponí­veis. Mostrou-se também que o método da fragmentação não pode ser usado para avaliar a qualidade da interface destas fibras em compósitos de matriz polimérica, qualquer que seja a morfologia. Isto deve-se ao tipo de rotura, que é inerente à  estrutura interna das VGCFs. Produziam-se e processaram-se compósitos termoplásticos reforçados com VGCFs submicrométricas usando tecnologias commerciais, sem problemas significativos, sempre que se utilizou o equipamento apropriado. Para avaliar o desempenho das VGCFs, as propriedades dos compósitos foram determinadas e comparadas com as dos reforçados com fibras convencionais. Verificou-se que os compósitos de VGCFs podem ser produzidos com resistência à  ruptura e coeficiente de expansão térmica (CTE) comparáveis, embora com rigidez mais baixa, do que as daqueles compósitos. Usaram-se modelos micromecânicos disponí­veis na literatura e um novo modelo para prever a rigidez, o CTE e a resistência à  ruptura de compósitos reforçados com fibras curtas, a partir das propriedades da fibra e da matriz. Os modelos foram verificados experimentalmente e aplicados inversamente para calcular as propriedades das VGCFs sub-micrométricas. Concluiu-se que as VGCFs têm um CTE aparente mais alto do que o das fibras de carbono ex-PAN e rigidez mais baixa. Embora a resistência à  ruptura das fibras não possa ser calculada, dado que o comprimento da maioria das fibras é inferior ao comprimento crí­tico, a metodologia de modelação inversa permite determinar a resistência ao corte interfacial. Mostra-se que a adesão interfacial entre as VGCFs e a matriz termoplástica é comparável à  das fibras de carbono convencionais. As diferenças de propriedades entre os compósitos de VGCF e os reforçados com fibras de carbono ex-PAN, podem ser atribuí­das à  diferença de propriedades das fibras. Além disso, concluiu-se que a rigidez e o CTE aparentes das VGCFs sub-micrométricas são, pelo menos, tão boas como as das fibras de vidro. Vapour Grown Carbon Fibres (VGCFs) combine potentially low production costs with encouraging mechanical, thermal and electrical properties. This makes them of specific interest for applications where ex-pitch- and ex-polyacrylonitrile (PAN) carbon fibres (designated by 'conventional' fibres) are too expensive, and glass fibres cannot provide the required properties. A research was carried out with three goals. First, to study systematically the different morphologies in which VGCFs can be produced and to evaluate their effect on the mechanical properties. Second, to develop know-how on the production of thermoplastic-VGCF composites. The determination of the mechanical properties of the composites allows the assessment of VGCFs as reinforcements of thermoplastics. Finally, to develop micromechanical models to predict the more relevant mechanical properties of the materials produced. By using these models inversely, it is possible to derive the properties of the fibres. In the case of VGCFs with diameters below 1 mm (submicron VGCFs) this is the only way to determine these properties. The different morphologies in which VGCFs can be grown were studied systematically and the effect of the shape on the mechanical properties of the fibres evaluated. It was concluded that the shape of the VGCFs has a small influence on the value of the tensile modulus. However, fibres with shapes different from perfect cylinders, have a lower tensile strength. Overall, both the tensile modulus and strength were significantly lower than those of commercially available ex-pitch- or ex-PAN carbon fibres. Furthermore, it was shown that the fragmentation method cannot be used to assess the quality of the interface of these fibres in polymeric matrix composites, irrespective of the morphology. This is due to the failure mode, which is inherent to the inner structure of the VGCFs. The production and processing of submicron VGCF-reinforced thermoplastic composites was done with commercial technologies, without major difficulties, provided the appropriate equipment was used. To evaluate the performance of the fibres, the properties of the composites were determined and compared to those reinforced with conventional ones. It was found that VGCF-composites can be produced with comparable strength and coefficient of thermal expansion (CTE) but with lower stiffness. Micromechanical models available in the literature and a newly developed model were used to predict stiffness, CTE and strength of short fibre reinforced composites from the fibre and matrix properties. The models were validated experimentally and then applied inversely to calculate the submicron VGCFs properties. It was concluded that VGCFs have an apparent CTE that is higher than that of ex-PAN carbon fibres and a lower stiffness. Although the fibre strength could not be calculated, as most of the fibres are well below the critical length, the inverse modelling methodology allows the determination of the interfacial shear strength. It was shown that the interfacial adhesion between VGCFs and the thermoplastic matrix is comparable to that of conventional carbon fibres. The differences in properties between VGCF- and ex-PAN carbon fibre composites, can be attributed to the differences in fibre properties. Furthermore, it was concluded that the apparent stiffness and CTE of submicron VGCFs are, at least, as good as those of glass fibres. Vapour Grown Carbon Fibres (VGCFs) combineren een potentieel lage kostprijs met veelbelovende mechanische, thermische en electrische eigenschappen. Dit maakt hen bijzonder geschikt voor toepassingen waar ex-pitch en ex-polyacrylonitriel (PAN) koolstofvezels (hier ‘conventionele’ vezels genoemd) te duur voor zijn en glasvezels de vereiste eigenschappen niet kunnen bieden. Een onderzoek is uitgevoerd, gericht op drie doelen. Ten eerste het systematisch bestuderen van de verschillende morphologieën waarin VGCFs geproduceerd kunnen worden en hun invloed op de mechanische eigenschappen. Ten tweede het ontwikkelen van kennis op het gebied van de vervaardiging van VGCF-thermoplastische composieten. Door de mechanische eigenschappen van de composieten te bepalen, kan de de rol van VGCFs als versterking voor thermoplasten vastgesteld worden. Tenslotte het ontwikkelen van micromechanische modellen die de relevantere eigenschappen van de geproduceerde materialen kunnen voorspellen. Door deze modellen omgekeerd te gebruiken, kunnen de eigenschappen van de vezels afgeleid worden. Dit is de enige manier om deze eigenschappen te bepalen voor VGCFs met diameters kleiner dan 1 mm (submicron VGCFs). De verschillende morphologieën waarin VGCFs geproduceerd kunnen worden, zijn systematisch bestudeerd en het effect van de vorm van de vezel op de mechanische eigenschappen is geëvalueerd. De vorm van de VGCFs blijkt weinig invloed te hebben op de hoogte van de trekstijfheid. Vezels met een andere dan een perfecte cylinder-vorm, hebben echter een lagere treksterkte. In het algemeen waren zowel de trekstijfheid als de treksterkte van de VGCFs significant lager dan die van commercieel beschikbare ex-pitch of ex-PAN koolstofvezels. Daarnaast is aangetoond dat de fragmentatie-test niet gebruikt kan worden om de kwaliteit van de interface van deze vezel in composieten met een polymeer-matrix te bepalen, ongeacht hun morphologie. Dit komt door hun bezwijkgedrag, dat inherent is aan de interne structuur van de VGCFs. Submicron VGCF-versterkte thermoplastiche composieten zijn zonder noemenswaardige problemen geproduceerd en verwerkt met behulp van commerciele technologieën, onder voorwaarde dat de geschikte apparatuur gebruikt werd. Om de prestaties van de vezels te evalueren, zijn de eigenschappen van de composieten bestudeerd en vergeleken met die van composieten versterkt met conventionele vezels. Het bleek dat VGCF-composieten geproduceerd kunnen worden met een vergelijkbare sterkte en thermische uitzettingscoefficient (CTE) maar met een lagere stijfheid. Micromechanische modellen beschikbaar uit de literatuur en een nieuw ontwikkeld model zijn gebruikt om de stijfheid, CTE en sterkte van korte-vezel versterkte composieten te voorspellen vanuit de vezel- en matrixeigenschappen. De modellen zijn experimenteel gevalideerd en vervolgens omgekeerd toegepast om de submicron VGCF-eigenschappen te berekenen. Geconcludeerd kan worden dat submicron VGCFs een schijnbare CTE hebben die hoger is dan die van ex-PAN koolstofvezels en een lagere stijfheid. Hoewel de sterkte van de vezels niet direct berekend kon worden, omdat de meeste vezels ruim beneden de kritische lengte zijn, maakt invers modelleren wel de afleiding mogelijk van de afschuifsterkte van de interface tussen matrix en vezel. De hechting tussen VGCFs en de thermoplastische matrix blijkt vergelijkbaar met die van conventionele koolstofvezels. De verschillen in eigenschappen tussen VGCF- en ex-PAN koolstofvezel versterkte composieten kunnen worden toegeschreven aan de verschillen in vezeleigenschappen. Daarnaast is geconcludeerd dat de schijnbare stijfheid en CTE van submicron VGCFs zeker zo goed zijn als die van glasvezels.European Economic Community - Human Capital and Mobility Programme (Grant Number CHCRXCT940457)

Aesthetic design using multi-objective evolutionary algorithms

The use of computational methodologies for the optimization of aesthetic parameters is not frequent mainly due to the fact that these parameters are not quantifiable and are subjective. In this work an interactive methodology based on the use of multi-objective optimization algorithms is proposed. This strategy associates the results of different optimization runs considering the existent quantifiable objectives and different sets of boundary conditions concerning the decision variables, as defined by an expert decision maker. The associated results will serve as initial population of solutions for a final optimization run. The idea is that a more global picture of potential ”good” solutions can be found. At the end this will facilitate the work of the expert decision maker since more solutions are available. The method was applied to a case study and the preliminary results obtained showed the potentially of the strategy adopted.One of the authors acknowledges the financial support received by the Portuguese Science Foundation under grant SFRH/BD/44600/ 2008

Development of a new pultrusion equipment to manufacture thermoplastic matrix composite profiles

This paper describes the design and manufacture of a low-cost full scale pultrusion prototype equipment and discusses the production and obtained mechanical properties of polypropylene/glass (GF/PP)reinforced composite bars fabricated by using the prototype equipment. Three different GF/PP pre-impregnated raw-materials, a commercial GF/PP comingled system from Vetrotex, a GF/PP powder coated towpreg [1-3] and, a GF/PP preconsolidated tape (PCT) produced in our laboratories, were used in the production of composite bars that were subsequently submitted to mechanical testing in order to determine the relevant mechanical properties and quantify the consolidation quality. Samples of the different composite profiles were also observed under SEM microscopy(undefined

Dynamic FE model updating using particle swarm optimization method: A methodology to design critical mechanical composite structures

To increase the performance of an industrial cutting machine, this work studied the possibility of replacing its current main steel gantry by a Carbon Fibre Reinforced Polymer (CFRP) composite solution. This component strongly influences the most relevant characteristics of the equipment, namely accuracy and maxima allowed accelerations. The flexibility of composites in terms of number, thickness and orientation of layers and the challenging trade-offs between weight and stiffness motivated the development of an optimisation process. The Particle Swarm Optimisation method (PSO) was used to develop a solution able to ensure higher accelerations and the required accuracy of the equipment, by optimizing continuously the FE model algorithm input and output assessment and updating it. The process resulted in a near optimal solution allowing a 43% weight reduction and an increase of the maximum allowed acceleration in 25%, while ensuring the same accuracy.This work has been supported by FCT – Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020 and the scholarship SFRH/BD/51106/2010

GPGPU-assisted polymer nanocomposite modelling and characterisation

Poster "Advanced Hybrid Materials I I: design and applications" (Simposio P)Development of the hybrid materials with predefined properties by addition of inorganic nanoinclusions to a polymer material constitutes a hard challenge due to significant properties’ variations depending on inclusion’s distribution and interaction. To understand structure-property relations in such materials optical image analysis and numeric modeling are widely used, however matching such data with properties’ measurements for industrial nanocomposites requires a link to be established between experimental and modeling length scales. In this work a computer code was developed to create a model composite structure with a predefined distribution probability of inclusions using NVIDIA CUDA GPGPU approach. The code is capable of randomly populating and analyzing samples of the typical size of microphotographs used for experimental characterization and typical nanoinclusions’ concentrations avoiding unphysical intersections and thus allow correlating the results of both optical characterization and statistical computer modeling. The initial probability distribution can be taken from experimental samples and further varied to investigate the effect of distribution on a desired property. Application to study the effect of carbon nanotubes and carbon nanofibers in a polymer matrix on the composite electrical and mechanical properties is discussed.FEDER - Programa Operacional Factores de Competitividade (COMPETE)Fundação para a Ciência e a Tecnologia (FCT) - CONC-REEQ/443/EEI/2005, PEst-C-FIS/UI607/2011-201

Development of reinforced composite sandwich panels based on 3D fabrics

relative new type of sandwich material was investigated, based on 3D knitted sandwich fabric preforms. Due to the integrally interlacement of both sandwich-fabric the skins by the connection yarns – core debonding resistance of panels and structures based on the perform is very high. [1]. In this work the mechanical performance of sandwich composite panels based on sandwich knitted fabrics is presented and discussed. Different 3D sandwich knitted fabric performs have been produced varying the thickness and the interlacement pattern. Composite panels using these performs have been produced using vacuum infusion technique. Panel thicknesses of 8, 15 and 25 mm, using two resin types – polyester and epoxy – have been produced. Materials thus obtained have been tested in tensile, bending and impact. The results obtained are presented, discussed and compared to models. Various samples of 3D sandwich spacer fabrics using vacuum infusion process have been produced in order to study the impregnation process. The dimensional properties investigated for non-impregnated core structures include cross-threads density, areal mass, yarns linear density, etc. Results obtained show that the mechanical performances vary according to the type of 3D knitted sandwich perform and the type of resin used

Design, manufacturing and testing of a rotorcraft access panel door from recycled carbon fiber reinforced polyphenylenesulfide

An integrally-stiffened access panel for a rotorcraft is selected for detail design, testing and actual flight to demonstrate a novel recycling route for thermoplastic composites. The design, development and validation followed the ‘Building Block approach’. The used material is post-industrial carbon fiber reinforced polyphenylene sulfide waste. This material originates from thermoplastic components of the very same rotorcraft as the panel will be mounted on, improving traceability, logistics and fixing supply and demand. Material data have been gathered from mechanical tests and used to predict the panels strength and stiffness. A critical design detail was selected and tested for validation. This section was included in a manufacturing demo, along with other integrated design features, enabling testing the processability. The final panel design was successfully produced and tested on component level. The re-manufacturing process includes simultaneously applied heat and low-shear mixing, followed by compression molding in an isothermal mold. This offers the possibility to retain long fibers and therefore high mechanical properties at short cycle times. In comparison to the current carbon/epoxy solution, the resulting product is lighter, significantly more cost-effective and made of recycled material (fiber and matrix). The prototype panel is targeted for flight testing on the rotorcraft in 2019.Dutch Organization of Applied Research – SIA, projeto SIA-RAAK 2014-01-72PR

Redesign of machine component in polymeric matrix composite towards increased productivity

This work is focused in the maximization of the acceleration a 2D Industrial Laser Cutting Machine (ILCM). The changes to be implemented are centered in the replacement of a metallic critical component: the gantry. This component largely influences precision and maximum acceleration. Finite Elements Analysis was performed to the current metallic part. From this analysis the maximum allowed deformations were established. A replacement composite component capable of an equally valid behavior was designed in carbon fiber. To establish the maximum increase in acceleration that does not lead to precision losses, the working conditions were simulated and the acceleration to which the component was subjected to was varied. The variation of the thickness of layers with different orientations and locations in the part allowed for the understanding of how the mass varies along with the maximum possible acceleration. This analysis, asides with considering the maximum force allowed by the linear motor that is responsible by the gantry motion, establishes the limit in terms of maximum acceleration of the machine. An increase of 22% in the maximum acceleration while maintaining the precision is possible due to the higher specific rigidity of composite materials and the use of an optimization heuristic

Finite elements versus experimental for a CFRP structure

The work presented herein is part of a project focused on the optimization of a carbon fibre reinforced epoxy matrix (CFRP) structure. The implemented process resorts to finite elements modelling in order to evaluate the performance of the analysed structure. To validate the finite elements model at the basis of the optimization process a scale model prototype of the composite structure was built and tested under similar loading conditions. The experimental results determined were then compared with those obtained from simulations of a built numerical model depicting the experimental set up and taken into account the mechanical and geometrical properties of the composite part, the used accessories, the interface between parts and production constraints.Portuguese Foundation for Science and Technology under project UID/CTM/50025/2013 and scholarship SFRH / BD / 51106 / 201

Redesign of an industrial laser cutting machine’s gantry in composite material

This work is focused in the design stage of a composite structure intended to replace a metallic critical component in a 2D Industrial Laser Cutting Machine (ILCM). The component is the gantry, largely responsible for most of the ILCM’s characteristics. These include precision and maximum acceleration, which are critical. The dimensioning of the component is initially performed based on analytical models, but latter stages use the numerical capabilities of Finite Elements Method. In the end it is possible to take advantage of the higher specific rigidity of composite materials to increase the maximum acceleration that the machine allows for while maintaining the precision.(undefined