The importance of using natural fiber composites (NFCs) has been addressed as a substitution for synthetic fibers, such as glass and carbon fibers. This substitution contributes significantly to reducing greenhouse gas emissions, aligning with the environmental responsibilities of engineering industries. Wood fiber(WF) is one of the natural fibers (NFs) dominating the market in various businesses. As an excellent alternative to non-renewable sources, the demand for injection-molded applications using natural fiber-reinforced plastics has expanded across various sectors. Despite extensive prior research on the mechanical properties of WFs, there remains a need for a deeper understanding of the connection between fiber orientation and mechanical characteristics. This understanding is essential for developing computational methods aimed at ensuring structural integrity, cost-efficiency, and sustainability in real-world components. This study aims to evaluate coupled injection molding simulation to finite element method with mapping of fiber orientation tensor for a wood fiber composite (WFC). To achieve this, WFC’s mechanical properties and behavior under tensile loading conditions are investigated. The research methodology involves conducting uniaxial tensile testing on dog bone-shaped specimens at different fiber orientations (0 degrees, 45 degrees, and 90 degrees). Experimental data is collected, analyzed, and compared with the obtained results with numerical simulations to validate the accuracy of the models used. Additionally, the aspect ratio and volume fraction of the WFs are measured through both mathematical calculations and image analysis using MATLAB. The main contribution of this study can be summarized in two key observations. Firstly, the investigation of mechanical characteristics across different fiber orientations has revealed distinct patterns. Specimens aligned at 0 degrees exhibit noticeable differences in behavior compared to those at 45 and 90 degrees, highlighting the material's anisotropic nature. Secondly, the comparison between experimental data and computational simulations exhibits the effectiveness of the developed models. The close agreement between the two validates the accuracy of the predictive approach. Moreover, the consistent aspect ratio, volume fraction, and fiber orientation value obtained through both mathematical calculations and image analysis add credibility to the reliability of our measurements. Notably, the comparison with glass fibers (GFs) reveals that WFs exhibit considerably less breakage, highlighting their durability and potential suitability for various applications. / Betydelsen av att använda naturfiberkompositer har behandlats som en ersättning för syntetiska fibrer, såsom glas- och kolbaserade fibrer. Denna substitution bidrar betydligt till att minska utsläpp av växthusgaser och överensstämmer med ingenjörsbranschens miljöansvar. Träfiber är en av de naturfibrer som dominerar marknaden inom olika branscher. Som ett utmärkt alternativ till icke-förnybara källor har efterfrågan på formsprutade applikationer med naturfiberförstärkta plaster ökat inom olika sektorer. Trots omfattande tidigare forskning om träfibrers mekaniska egenskaper finns det fortfarande ett behov av en djupare förståelse för sambandet mellan fiberns orientering och dess mekaniska egenskaper. Denna förståelse är avgörande för att utveckla beräkningsmetoder som syftar till att säkerställa strukturell integritet, kostnadseffektivitet och hållbarhet i komponenter i den verkliga världen. Denna studie syftar till att utvärdera kopplad formsprutningssimulering med ändelementmetod och kartläggning av fibrernas orienteringstensor för en träfiberkomposit. För att uppnå detta undersöks träfiberkompositens mekaniska egenskaper och beteende under dragbelastningsförhållanden. Forskningsmetodiken innefattar genomförande av enaxlig dragprovning på hundbenformade provkroppar vid olika fibrers orientering (0 grader, 45 grader och 90 grader). Experimentella data samlas in, analyseras och jämförs med de erhållna resultaten från numeriska simuleringar för att validera modellernas noggrannhet. Dessutom mäts träfibrernas aspektratio och volymfraktion genom både matematiska beräkningar och bildanalys med hjälp av MATLAB. Huvudbidraget från denna studie kan sammanfattas i två centrala iakttagelser. För det första har undersökningen av mekaniska egenskaper vid olika fibrers orienteringar avslöjat tydliga mönster. Prover som är riktade i 0 grader uppvisar märkbara skillnader i beteende jämfört med de vid 45 och 90 grader, vilket understryker materialets anisotropa natur. För det andra visar jämförelsen mellan experimentella data och beräkningsmässiga simuleringar effektiviteten hos de utvecklade modellerna. Den nära överensstämmelsen mellan de båda validerar noggrannheten i den prediktiva metoden. Dessutom lägger de konsekventa värdena för aspektratio, volymfraktion och fibrernas orientering som erhållits genom både matematiska beräkningar och bildanalys trovärdighet till våra mätningar. Det bör noteras att jämförelsen med glasfiber visar att träfibrer uppvisar betydligt mindre brytning, vilket betonar deras hållbarhet och potentiella lämplighet för olika tillämpningar.

Fiber reinforced polymer composite laminates make up more than 50% of modern aircrafts. Such composite laminates are exposed to various environmental and in-service thermo-mechanical load conditions. Transverse/intralaminar cracking is usually the first form of damage appears in a composite laminate and they tend to increase in number during the service life. The growth in number of these cracks significantly degrades the thermo-elastic properties of the composite laminate and eventually leads to final failure. Thus, it is important to predict the crack density (number of cracks per unit length) growth in both non-interactive crack density region and interactive crack density region and its effect in thermo-elastic properties degradation. Non-interactive crack density region is the region where the cracks are far apart and stress perturbation between cracks do not overlap. Interactive crack density region is where the cracks are close to each other and stress perturbation between cracks overlaps and affects the formation of new cracks. In this study, transverse cracks in thick Glass Fiber Epoxy (GF/EP) cross-ply composite laminates under quasi-static tensile loading and tension-tension fatigue loading have been analyzed and predicted. In the first paper attached here, increase in number of transverse cracks in GF/EP cross-ply laminates under quasi-static tensile loading at room temperature (RT) are analyzed using 2 material systems. The failure stress distribution in 90° plies of the laminates is defined by Weibull distribution and the Weibull parameters are determined from crack density versus applied thermo-mechanical transverse stress in 90° layer (σTCLT) data points within the non-interactive crack density region. The crack density growth is then predicted versus the σTCLT and applied mechanical strain in the laminate from the determined Weibull parameters using Monte Carlo method and the stress distribution models between adjacent cracks. The predicted results using the novel stress distribution model introduced here were in good agreement with the non-interactive and interactive crack density regions of test results. The importance of using the Monte Carlo method and novel stress distribution model to predict the whole crack density region have been emphasized in the article, in addition to that it also redefined the interval of non-interactive crack density region.  The second paper expands the concept from the first paper, to address the tension-tension fatigue loading at RT. It deals with the crack density analysis and prediction in [0/90]s GF/EP laminate under fatigue loading at RT. The fatigue tests were performed at 3 maximum stress levels. Here the Weibull parameters were determined from the data points within the non-interactive crack density region in quasi-static and fatigue loading. From the determined Weibull parameters of each stress level and using Monte Carlo method and the novel stress distribution model, the crack density versus the number of fatigue cycles were predicted and in good agreement with the fatigue test results at the respective stress level. The intention here was to use Weibull parameters of one stress level to predict crack density at arbitrary stress levels. Based on it, the predicted results were not sufficiently good and suggested to revisit the Weibull parameter determination by performing fatigue tests at two stress levels.  In the attached paper 3, new methodology on crack density growth simulation and Weibull parameter determination in tension-tension fatigue loading has been developed. In the newly developed methodology, in detailed fatigue tests are performed at one maximum stress level to obtain all data points and at higher stress level to obtain one data point that is a crack density data point at certain number of cycles to determine Weibull parameters. Using the determined Weibull parameters from non-interactive crack density region, the whole crack density region was successfully predicted for other stress levels.

In this project, the possibility of using thermography as a non-destructive testing tool in the manufacturing process of boats and to assess the quality of end-of-life composite structures has been explored. To do so, a literature surview regarding the current applications of thermography and the techniques currently used for quality control in the boats industry has been conducted. Additionally, the set-up of the thermal camera has been improved for the testing and measurements on several composite parts have been performed. Here, the resulting images were analysed regarding different features important for the intended new application, such as existence of delamination, bonding quality, and fibre orientations. The technique appears to be well suited to evaluate the bonding quality in case of glass fibre composite plates and to detect delaminations and other defects in such. Regarding the fibre orientation more studies are needed to judge the practicality.

The use of wind tunnel models for aerodynamic research is nowadays indispensable to aviation progress in the last years as aircrafts have become more complex. Wind tunnel model design and manufacturing has adopted many different processes and materials such as the use of a five-axis CNC; making this process a relatively long and expensive one. Composite materials offer a good trade-off between ease and cost of manufacturing compared to the more traditional methods, especially for in-house-built prototypes. This volume covers the different phases from design to manufacturing of a wind tunnel model for the MK18 conceptual blended wing-body UAV designed by KTH Green Raven Project students.The model is a down-scaled 1.5 meter span version with a belly-mounted two-strut support. The main structural requirements for the model are to withstand the aerodynamic loads obtained via CFD simulations. A mechanical interface for the support structure connection was designed. Carbon fiber reinforcement with an epoxy resin matrix was selected as the constituents for the airframe skins. A finite element model of the design was developed by using Abaqus to verify the overall structural behavior and stability. The manufacturing strategy of the airframe skins involved producing lightweight fiberglass molds out of CNC milled MDF male patterns and using vacuum infusion process to obtain the final carbon fiber parts. The internal structure members were manufactured out of different materials and processes from water-jet cutting of aluminum profiles to 3D-printed plastic components. The FEA study results showed that the model withstands the maximum loads with a high safety factor and a wing-tip deflection of less than 2\% of half the wingspan. The manufacturing of the molds turned out to be longer and more complicated than expected, but with overall good results. The composite skins came out with good mechanical and surface quality. The total weight of the model resulted in approximately 4.5 kg. Pressure taps were positioned and installed on the model skins. Their respective tubes routed in CAD to visualize the networking for manufacturing. This ensured proper placement to balance ease of installation with meaningful data collection. / Användningen av vindtunnelmodeller för aerodynamisk forskning är idag oumbärlig för flygets framsteg eftersom flygplan de senaste åren har blivit mer komplexa. Vindtunnelmodelldesign och konstruktion har använder många olika tillverkningsmetoder och material såsom femaxlig CNC; vilket gör processen relativt långsam och dyr. Kompositmaterial ger en bra avvägning mellan enkelhet och tillverkningskostnad jämfört med de mer traditionella metoderna, särskilt för egenbyggda prototyper. Denna rapport behandlar faserna från design till tillverkning av en vindtunnelmodell för en konceptuell blended wing-body UAV, MK18, konstruerad av KTH Green Raven Project­studenter. Modellen är en nedskalad version med 1,5 meter spännvidd som monteras på ett bukmonterat, tvådelat stöd. De viktigaste kraven på modellen är att kunna motstå de aerodynamiska belastningarna som beräknats via CFD­simuleringar. Den interna strukturen i modellen utformades för att integrera anslutningen med stödstrukturen. Kolfiber tillsammans med en epoxihartsmatris valdes som beståndsdelar för flygplanets skal. En finit elementmodell av designen utvecklades med hjälp av Abaqus FEA för att verifiera det övergripande strukturella beteendet och stabiliteten. Tillverkningsstrategin för flygplansskalet innebar att man tillverkade lätta glasfiberformar på CNC­frästa MDF­hanformar och använde en vakuuminfusionsprocess (VIP) för att erhålla de slutliga kolfiberdelarna. De inre strukturdelarna tillverkades av olika material och processer från bearbetning av aluminiumprofiler till 3D­utskrivna plastkomponenter. FEA­studieresultaten visade att modellen tål de maximala belastningarna med en hög säkerhetsfaktor och uppvisar en utböjning vid vingspetsarna på mindre än 2% av halva spännvidden. Tillverkningen av formarna visade sig ta längre tid och vara mer komplicerad än väntat, men gav övergripande goda resultat. Kompositskalet visade sig ha god mekanisk ytskvalitet. Modellens totala vikt blev under 5 kg. Hål för tryckmätning placerades också på modellens skal och rören drogs i en CADmodell för att visualisera nätverket för tillverkning. Detta säkerställde korrekt placering för att balansera enkelhet i installationen med meningsfull datainsamling.

In recent times, there has been an increased interest in replacing conventional metals and synthetic fiber composites in various automobile parts with natural fiber polymer composites. These natural fiber composites offer benefits over conventional materials in terms of reduction in weight, lower cost, comparable high specific properties such as specific stiffness and strength, low abrasion property, and availability of these materials. Environmetal concerns and the need to make eco-friendly materials also encouraged the use of natural fiber polymer composites (NFPC) for various automotive applications, as their usage reduces the emission of harmful pollutants, thereby providing a safer and cleaner environment. The reduction in weight of these materials also helps to reduce fuel consumption in cars. Therefore, this study focused on the investigation and characterization (mechanical, thermal, and tribological) of hemp fibers reinforced polypropylene made with virgin (Biolite) and recycled (Revo) composites, consisting of different fiber loadings from 10% to 30% for possible automotive applications. The result showed an improvement in the compressive properties of the composite as hemp fiber loading increases, with the recycled composite exhibiting a better compression performance in comparison to virgin composites. Revo 41 had the best compressive strength  and a 35% higher value than Biolite 2 of the same hemp fiber loading. However, there was a decrease in the fracture toughness of the composites as hemp fiber loading increases. This was more significant at about 27% from Biolite 1 to Biloite 2, Revo 41 experienced a slight improvement of 15% in fracture toughness with 20% hemp fiber loading. Revo 35 composite which had more degradation steps showed the best thermal performance at low temperature region with a degradation temperature of 276°C, while Biolite 1 at 444°C exhibited the best thermal stability at high temperature degradation. In comparing the tribological behavior of pure polypropylene with the composites, the composites exhibited a better tribological performance as there was a slight decrease in the coefficient of friction and wear rates of composites as the hemp fiber loading increases. Revo 41 of 20% hemp fiber fiber loading had the best wear performance. Considering its strength and slightly better tribological performance, Revo 41 is considered the best performing composite for automotive applications in comparison to the other composites.

Magnetic measurements are useful tools for quality control of cemented carbides. Previous work at Sandvik Mining and Rock Technology has shown that the coercivity increases with increased cooling rate during sintering for a specific grade.  This study aims to investigate why the coercivity changes with the cooling rate and if this is true for other cemented carbide grades as well. Three different cemented carbide grades were sintered with different cooling rates and evaluated with coercivity, Cobalt-magnetic saturation and hardness measurements, and with microscopy and Electron Backscatter Diffraction analysis. It was found that the coercivity increased with increasing cooling rates for the previously studied grade, but not for the two other grades. It was expected that the increased coercivity would indicate a decrease in WC grain size, but the results showed that the WC grain size of the fastest and slowest cooling rate were the same. However, a change in size of the Co areas between the WC grains was found. The fast cooled sample showed smaller Co areas than the slow cooled sample. These Co/WC grain boundaries increase the coercivity. An increased fraction of hcp-Co/fcc-Co was also found for the fast cooled material which also increases the coercivity. No relationship between the hardness and the coercivity or the cooling rate was found.   The contiguity for the different grades was also calculated.  No significant difference in contiguity between the different cooling rates of each material was found but the contiguity values between the different materials differed.  This is probably mainly dependent on the different binder contents of the materials. / Magnetiska egenskaper är en viktig del av kvalitetskontrollen av hårdmetaller.  Tidigare forskning hos Sandvik Mining and Rock Technology har visat att koerciviteten hos en hårdmetallsort ökar med ökad kylhastighet vid sintring.  Målet med den här studien är att undersöka varför koerciviteten ökar med kylhastigheten och om detta även gäller andra hårdmetallsorter.  Tre olika hårdmetallsorter sintrades med olika kylhastigheter undersöktes med mätningar av koercivitet, magnetisk mättnat i koboltfasen och hårdhet,  samt med mikroskopi och Electron Backscatter Diffraction analys. Det  visade  sig  att  koerciviteten  ökade  med  ökad  kylhastighet  för  hårdmetallsorten  som  studerats tidigare, men inte för de andra två sorterna.  En ökad koercivitet förväntas visa på en minskad WC-kornstorlek, men resultaten visar att det långsamt kylda och snabbkylda provet hade samma WC-kornstorlek.  Däremot upptäcktes en ändring av storleken på Co-områdena mellan WC-kornen.  Det snabbkylda provet visade en mindre storlek av Co-områdena jämfört med det långsamt kylda provet. Fler Co/WC-korngränser i det snabbkylda provet leder till en ökad koercivitet.  Fraktionen av hcp-Co/fcc-Co-korngränser ökade också för det snabbkylda provet vilket också ökar koerciviteten. Inget samband mellan hårdheten och koerciviteten eller kylhastigheten upptäcktes.  Beräkning av kontiguiteten för de olika sorterna genomfördes också. Ingen signifikant skillnad i koercivitet mellan de olika kylhastigheterna för the olika sorterna hittades,  men kontiguiteten mellan de olika sorterna varierade. Det beror mest troligt på att dom olika sorterna har olika mängd matrismaterial.

Cemented carbide powder production is the first step in the manufacturing of cemented carbide inserts.The quality of the powder affects the successive process steps in the production of the cemented carbide inserts. The powder is produced by spray drying of a slurry. The slurry consists of polymer, water, ethanol, and dry components. The operating conditions of the spray dryer have been studied greatly to optimize the powder properties but less is known about the influence of the slurry on the powder. This work examines the effect of slurry composition on the cemented carbide powder properties. The work is necessary to predict optimum slurry composition to produce good quality cemented carbide powders. To characterise the powders, flowability, density, particle morphology and hollowness of the powder granules were measured for different slurry compositions. No direct correlation was observed between slurry viscosity and the powder properties but a change in the amount of raw material and organic additives in the slurry affected various powder properties. An optimum slurry composition was obtained which can produce better quality of cemented carbide powder. Additionally, it was found that an increase in slurry viscosity can hinder the spray drying process. / Tillverkning av hårdmetallpulver är det första steget i tillverkningen av hårdmetallinsatser. Pulverkvaliteten påverkar de successiva processstegen vid tillverkningen av hårdmetallinsatserna. Pulvret framställs genom spraytorkning av en uppslamning. Uppslamning består av polymer, vatten, etanol och torra komponenter. Driftförhållandena för spraytork har studerats mycket för att optimera pulveregenskaperna, men mindre är känt om påverkan av uppslamningen på pulvret. Detta arbete undersöker effekten av uppslamningskomposition på egenskaperna för hårdmetallpulver. Arbetet är nödvändigt för att förutsäga optimal uppslamningskomposition för att producera hårdmetallpulver av god kvalitet. För att karakterisera pulvren mättes flytbarhet, densitet, partikelmorfologi och hålighet hos pulvergranulerna för olika uppslamningskompositioner. Ingen direkt korrelation observerades mellan uppslamningsviskositet och pulveregenskaperna men en förändring i mängden råmaterial och organiska tillsatser i uppslamningen påverkade olika pulveregenskaper. En optimal uppslamningskomposition erhölls som kan ge bättre kvalitet på hårdmetallpulver. Dessutom fann man att en ökning av uppslamningsviskositeten kan hindra spraytorkningsprocessen.

In der vorliegenden Arbeit wird die kationische Polymerisation neuer Furanmonomere beschrieben, die zu anorganisch-organischen Nanokompositen führt. Die kationische Polymerisation des Tetrafurfuryloxysilans steht dabei im Vordergrund. Ausgehend von den synthetisierten Kompositen wird die Herstellung von anorganischen Oxiden durch thermische Oxidation und von Kohlenstoffmaterialien durch thermische Behandlung in Schutzgasatmosphäre beschrieben. Die Charakterisierung der Komposite, Oxide und Kohlenstoffmaterialien erfolgt mittels Festkörper-NMR-Spektroskopie, Röntgenbeugung und Elektronenmikroskopie. Das Prinzip der neu entdeckten Zwillingspolymerisation wird vorgestellt und anhand verschiedener Beispiele auf seine weitere Anwendbarkeit zur Synthese anorganisch-organischer Kompositmaterialien überprüft.

The following thesis work was performed by Tobias Bäckman between 2018-01-15 – 2018-06-01 on behalf of Deva Mecaneyes. Deva Mecaneyes had identified a need and a possible application area for additive manufacturing based on the Markforged Mark Two 3D-printer which they had purchased. However, many question marks remained regarding how the materials from the printer would behave. How to design against this manufacturing method and which applications that could be beneficial for the company. At the start of the project it was identified that Deva Mecaneyes main limitations for not implementing 3D-manufacturing more extensive in their product development process was partly the lack of experience of additive manufacturing methods but mainly due to the lack of reliable material data for the printed parts. Based on this, three research questions were formulated. These research questions discuss how to replace traditional manufacturing methods, which material relationships are possible to identify, and which factors should be considered when designing against additive manufacturing. The bulk of the work thus consisted of producing material data for materials that are compatible with Markforged mark Two. This was done based on ASTM standards that treat tensile tests, bending tests and fatigue tests. Two already existing products from Deva Mecaneyes in the field of lifting gear for manufacturing industries were selected with the purpose to redesign these products to be manufactured with the Mark Two 3D-printer instead. In this way, an alternative way for these applications could be showed by replacing the traditional manufacturing methods with additive manufacturing methods. The reconstructed lifting gear was also manufactured to be verified against the produced material data but also to demonstrate improvement or deterioration against the existing lifting gear. The measurable goals for the project were to prove cost and time reduction by at least 50% by replacing the traditional manufacturing methods with additive manufacturing methods while maintaining the same reliability. The result demonstrated two redesigned lifting gears with many improvement areas. A great result that stood out was a cost reduction of approximately 80% and 90% respectively. Several material relationships have been identified during the work and new experiences regarding printed details have arisen. The author believes that the work, with addition to the accomplished goals, has laid a good ground to begin to understand the materials more and more and thus a good beginning to obtaining a reability from 3D-printed details. Which is a decisive factor to begin replacing the traditional manufacturing methods. / Följande examensarbete är utfört av Tobias Bäckman mellan 2018-01-15 – 2018-06-01 på uppdrag av företaget Deva Mecaneyes. Deva Mecaneyes hade identifierat ett behov och ett möjligt användningsområde för additiv tillverkning baserat på en 3D-skrivare av modellen Markforged mark Two som de köpt in. Dock kvarstod det många frågetecken hur materialen i de utskrivna detaljerna beter sig, hur man skall konstruera mot denna tillverkningsmetod samt vilka tillämpningsområden som skulle kunna vara fördelaktiga. Vid uppstart av projektet identifierades de största begränsningarna till varför Deva Mecaneys inte implementerar 3D- utskrifter mer omfattande i deras konstruktionsarbete som delvis den bristande erfarenheten av additiva tillverkningsmetoder, men främst på grund av avsaknaden av trovärdig materialdata och beräkningsunderlag att tillämpa för fysiska 3D-utskrivna detaljer. Utifrån detta formulerades tre stycken forskningsfrågor. Dessa forskningsfrågor behandlar hur man skulle kunna ersätta traditionella tillverkningsmetoder, vilka materialsamband som är möjliga att identifiera samt vilka faktorer som bör tas hänsyn till vid konstruktion mot additiva tillverkningsmetoder.Huvuddelen av arbetet har därmed bestått av att producera materialdata för materialen som är kompatibla med Markforged Mark Two. Detta har skett baserat på ASTM-standarder som behandlar dragprover, böjprover och utmattningsprover.Två befintliga produkter från Deva Mecaneyes inom området lyftredskap valdes sedan ut för att omdesignas mot additiva tillverkningsmetoder. På så vis redovisas en alternativ väg att gå genom att ersätta de traditionella tillverkningsmetoderna med additiva tillverkningsmetoder. De omkonstruerade lyftredskapen tillverkades även för att dels verifieras mot den framtagna materialdatan men även för att redogöra förbättring alternativt försämring mot de befintliga lyftredskapen.De mätbara målen för arbetet var att kunna påvisa kostnads och tidsreducering med 50% genom att byta ut de traditionella tillverkningsmetoderna mot additiva tillverkningsmetoder. Resultatet påvisade två omkonstruerade lyftredskap med många förbättringsområden. Där framförallt en kostnadsreducering på cirka 80% respektive 90% identifierades.Under arbetets gång har ett antal materialsamband kunnat identifieras och nya erfarenheter angående utskrivna detaljer från Markforged Mark Two har uppstått. Författaren anser att arbetet, utöver de uppfyllda målen, har lagt en god grund till att börja förstå materialen mer och mer och därmed en god början till att börja erhålla en tillförlitlighet hos 3D-utskrivna detaljer som är en avgörande faktor till att börja ersätta vissa av de traditionella tillverkningsmetoderna.

Flax fibers, along with a number of other natural fibers, are being considered as an environmentally friendlier alternative of synthetic fibers in fiber-reinforced polymer composites. A common feature of natural fibers is a much higher variability of mechanical properties. This necessitates study of the flax fiber strength distribution and efficient experimental methods for its determination. Elementary flax fibers of different gauge lengths are tested by single fiber tension in order to obtain the stress-strain response and strength and failure strain distributions. The applicability of single fiber fragmentation test for flax fiber failure strain and strength characterization is considered. It is shown that fiber fragmentation test can be used to determine the fiber length effect on mean fiber strength and limit strain. The effect of mechanical damage in the form of kink bands and of diameter variability on the strength of elementary flax fibers is considered. Stiffness and strength under uniaxial tension of flax fiber composites with thermoset and thermoplastic polymer matrices are studied. The applicability of rule of mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers are evaluated. Both the quasi-static and time dependent mechanical properties of flax fiber/thermoplastic starch based composites are analyzed. The effect of temperature and relative humidity is investigated. It is found that microdamage accumulation in this type of composites is not significant. Results show that the composite elastic modulus and failure stress are linearly related to the maximum stress reached by the matrix in tensile tests. Simple material models are suggested to account for the observed nonlinear viscoelasticity and viscoplasticity. / Godkänd; 2009; 20091029 (edgspa); DISPUTATION Ämnesområde: Polymera konstruktionsmaterial/Polymeric Composite Materials Opponent: Docent Kristofer Gamstedt, Kungliga Tekniska Högskolan, Stockholm Ordförande: Docent Roberts Joffe, Luleå tekniska universitet Tid: Onsdag den 9 december 2009, kl 10.00 Plats: E 231, Luleå tekniska universitet