Characterisation of flax fibres and flax fibre composite -being cellulose based sources of materials-
Başlık çevirisi mevcut değil.
- Tez No: 400004
- Danışmanlar: BENT F. SØRENSEN, BO MADSEN
- Tez Türü: Doktora
- Konular: Enerji, Energy
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2012
- Dil: İngilizce
- Üniversite: Technical University of Denmark
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 168
Özet
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Özet (Çeviri)
Cellulosic fibres, like wood and plant fibres, have the potential for use as load-bearing constituentsin composite materials due to their attractive properties such as high stiffness-to-weight ratio thatmakes cellulosic fibre composites ideal for many structural applications. There is thus a growinginterest among composite manufacturers for such low-cost and low-weight cellulosic fibrecomposites. In addition, wood and plant fibre based composites with thermoplastic polymericmatrices are recyclable, and they are cost attractive alternatives to oil based fibre reinforcedpolymer composites that currently have the largest market share for composite applications.However, the most critical limitation in the use of cellulosic fibre composites for structuralapplications is the lack of well described fibre properties, in particular, the tensile strength. This isdue to variations in fibre morphology, fibre processing conditions, and applied test methods. Otherlimitations such as dimensional instability and low fibre-matrix adhesion have already beenintensively investigated, and solutions have been found for many commercial applications.Therefore, a better understanding of the mechanical performance of these fibres, and with a focuson increasing their strength will make it possible for them to reach their full potential asreinforcement in composites. The present PhD study deals with several important subjects related tothe use of flax fibres in composites. The emphasis is on the relationship between the complexmicrostructure and the tensile properties of flax fibres and their composites, based on textile flaxyarn and a thermoplastic polymeric matrix.Single flax fibres were isolated from flax fibre bundles which have been processed in two differentsteps of natural treatments (retting) and mechanical treatments (scutching and hackling).Microscopic observations of the defects formed in the fibres and their fracture surfaces after tensiletesting show that large fracture areas are formed in a complex way due to defects in the fibre cellwall, and due to anisotropy of the internal cell wall structures. This is in contrast to the crackgrowth in brittle ceramic and glass fibres. Moreover, two typical stress-strain curves (linear andnon-linear) measured for the flax fibres were found to be correlated with the amount of defectedregion in the fibres. The defects are induced in larger numbers and larger sizes during processing ofthe fibres, and this is found to be correlated with a decrease in tensile strength of the fibres. It isfound that processing reduces the tensile strength from average values of 1450 MPa for naturallyprocessed single fibres to 810 MPa for mechanically processed single fibres.The large variation in tensile properties of flax fibres leads to an examination of the effect ofdefects and applied test methods. The fibres show a large coefficient of variation (CV) in the range20-60% in general for all measured tensile properties. One reason for these relative large variationscan be attributed to the assumption of a circular cross sectional area of the fibres. On average, theseresults in a 39% lower tensile strength than when the true fibre cross sectional area is used, andmoreover, the variable aspect ratio of the cross section of fibres significantly affects the variation ofthe results. Also, the large variation in properties is likely to be attributed to the distribution ofdefects along the fibres since the large defects lead to low mechanical properties, whereas smallerdefects result in less reduced mechanical properties.On the level of composites, the effect of consolidation pressure on the tensile properties of flaxfibre composites was investigated. A porosity corrected rule of mixtures model, and a volumetriccomposition model for composites were used to model the experimental data. Flax fibre yarns andthermoplastic low-melting temperature polyethylene terephthalate (LPET) filaments were alignedin assemblies of different fibre weight fractions in the range 0.24 to 0.83 to manufactureunidirectional composites using two different consolidation pressures of 1.67 and 4.10 MPa. Themaximum attainable fibre volume fraction is found to be 47% for the low pressure composites,whereas it is found to be 60% for the high pressure composites. The stiffness of the flax fibre/LPETcomposites is measured to be in the range 16 to 33 GPa depending on the volumetric compositionof the composites. The high pressure composites are found to have superior tensile properties incomparison with the low pressure composites. The tensile strength (mean ± std. dev.) of the lowpressure composites was found to be 183±7 MPa while that of the high pressure composites wasfound to be 209±6 MPa at a fibre volume fraction of 22%. The effect of fibre correlated porosityand structural porosity in the composites is found to be highly important for the volumetriccomposition and tensile behaviour of the composites. The total porosity is measured in the range2.4 to 32%, and it is found to be increased dramatically when the fibre weight fraction is increasedabove a transition value, as predicted by the volumetric composition model. This leads furthermoreto a scatter in the experimental data of stiffness at high fibre weight fractions. The qualitativeanalysis of the composite cross sections by microscopy also shows that the low and high pressurecomposites have a similar microstructure at low fibre weight fractions. However, when the fibrecontent is increased, a difference in porosity content can be observed from the composite crosssections.The nominal tensile strength of the unidirectional flax fibre/LPET composites is measured in therange 180 to 340 MPa. However, in many cases, the tensile strength determined of unidirectionalcomposites is not valid due to the fact that failure does not occur in the gauge section. It is actuallycommon that unidirectional composites fail close to the grips, and they then split along thespecimen in the tensile direction. Traditionally, the problem has been approached by the use oflocal reinforcement of the specimen in the gripping areas, the so-called tabs, but the problem hasnot been efficiently solved in practice. A key problem is that the stress state at the end of the tab canbe singular, leading to premature failure of the tensile specimen. In the present study, thedependence of the order of the stress singularity at the vertex of dissimilar isotropic and orthotropicmaterials is investigated in terms of the elastic mismatches between the specimen and the tabmaterials, and the tab angle. Finite element modelling is performed to analyse the situation of astress singularity. The results are aimed at creating a better specimen/tab design to accomplishfailure in the gauge section of the tensile specimens, and thereby determine the true tensile strengthof the materials. It is found that the stress singularity in the tab wedge is reduced with a decreasedtab angle and with a decreased stiffness of the tab material. A simple criterion is proposed for theassessment of the severity of the stress singularity. In practice, gauge section failures should beachievable by selecting a test specimen design based on combinations of a stiff material in the tabsection combined with a soft material (eg. epoxy adhesive) at the wedge end of the tab, forming awedge. The wedge tip should have a small wedge angle in the range 5° and 10° depending on thestiffness ratio.The conclusion of the PhD study is that flax fibres are an important source of cellulosic fibres.When the appropriate composite processing methods and the accurate test methods are used, flaxfibre composites are demonstrated to be promising material candidates for structural applications asan attractive alternative to synthetic fibre composites.
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