Reinforced polymer composites consist of continuous fibers embedded in a polymer matrix. The matrix is usually a thermoplastic or thermosetting resin. When thermosetting matrices are cured during the manufacture of composite parts, residual stresses develop within the part during the manufacture due primarily the thermally and chemically induced volumetric strains imposed on them. This can lead to shape distortions and sometimes weakening of the structure itself.Curing is the manufacturing process in which the thermoset resin is transformed from a liquid to a solid material. The molecular mechanisms involved in this process are quite complex and not well understood. In the macro-level, in addition to volumetric strains, heat is also generated since most thermoset polymerization reactions are exothermic. The mechanical properties of the thermoset also undergo dramatic changes. The material changes from its initial liquid state to a rubbery gel and finally to its vitrified glassy state.In modern day composite manufacturing, to accommodate for the shape distortions caused due to residual stress formation, the mold geometry is compensated. To do this, accurate predictions of the distortion behavior is required via computer simulations. This in turn requires simple mathematical models that can replicate the complex processes that take place during manufacture. One such process that requires attention is the curing of the thermoset. While models exist that assume elastic behavior during cure, they are not accurate throughout the entire cure process. Models based on viscoelastic material during cure offer better prospects in this perspective. However, currently models that are based on full viscoelasticity are either not well defined or are computationally tasking. Viscoelastic materials can be classified further in to thermorheologically simple and complex materials depending on their molecular weights. In layman’s terms, thermorheologically simple materials re those that obey the principles of time-temperature superposition (TTS). TTS requires that all response times (i.e., all relaxation or retardation time), depend equally on temperature. This is expressed by the temperature shift function. Master curves can be then generated extending the time scale beyond the range that could normally be covered in a single experiment. However to fully understand the development of viscoelasticity during cure it is also necessary that the effects of the degree of cure of the thermoset on these times be included in the model definition. This requires defining a cure shift function along with the temperature shift function. In the presented work, an attempt is made to develop a simplified methodology to characterize the viscoelastic material properties during curing. In the first paper, two different methods are investigated in a DMTA instrument to determine the effects of curing on the glassy state of the resin system LY5052/HY5052. A cure shift function was identified in the process. Based on observations it was concluded that the total shift function could be possibly defined as a product of the temperature and cure shift functions. Unique super-master curves were generated as a result. However, these curves showed a dependency of the rubbery modulus on the degree of cure. Hence, in the second paper, the effect of the degree of cure on the rubbery modulus was investigated. Following this a model was reformulated from an existing one and this was used to further simplify the super-master curves.

The polymer composites designed for high-performance applications are mostly based on carbon fiber reinforcement. The two most common precursors used currently for carbon fiber production are poly(acrylonitrile) and pitch (petroleum- or coal- based). As of today, the most promising alternative to these fossil originated raw materials is lignin. Previous research has mainly focused on carbon fiber production from pre-treated hardwood lignin with the addition of different softening agents. Softwood lignin has been considered difficult to melt, and thus, impossible to process by conventional melt spinning processes.The aim of the presented work is to find the way for melt spinning of softwood kraft lignin, by using lignin-derived additives. A method for isolation of kraft lignin was recently developed, making large amounts of high purity lignin available. The thermal properties of this type of lignin make it an interesting candidate for carbon fiber production. During this study, both unfractionated hardwood and softwood kraft lignin were used with addition of their fractionated counterparts, acting as softening agents. The spinning process of the lignin blends was optimized by adapting the processing temperature to the thermal properties of blends of different compositions. Different batches of lignin fibers were produced and characterized with scanning electron microscopy to evaluate the fiber diameter, the surface smoothness, the presence of pores and the shape of fiber cross-section.The fiber batch containing softwood kraft lignin and 10% fractionated hardwood kraft lignin was relatively easy to melt spin, despite the small amount of added fractionated hardwood lignin. Therefore, this batch was further processed into carbon fibers by oxidative stabilisation followed by carbonization in nitrogen atmosphere. X-Ray/Energy Dispersive Spectroscopy confirmed that carbon fibers containing above 90% carbon had been obtained. Mechanical characterization of produced lignin based carbon fibers was carried out. Single fiber tensile tests were performed to evaluate the stiffness and the strength of carbon fibers. In order to determine the properties of the lignin-based CFs, and to estimate the impact of the manufacturing parameters (such as die sizes and winding speeds), fibers of different diameters (≈30, 60 and 90 microns) were made and tested.Carbon fibers are brittle materials and therefore the experimental results (fiber strength) were treated by use of Weibull statistical distribution. Three fiber lengths (10, 20 and 40 mm) for each diameter were tested and strength data was approximated by two-parameter Weibull equation in order to obtain parameters of the strength distribution. The experimental results and predictions based on Weibull statistics showed a good fit.Although strength of the produced fibers is still significantly lower than that of commercially available carbon fibers, this thesis reports the first mechanical characterization of softwood kraft lignin based CFs.The carbon fiber production process differs depending on the raw material used. Most of the studies on lignin have considered hardwood lignin as raw material. As a first step towards a process optimized for softwood kraft lignin based carbon fibers, the stabilization step in the carbon fiber process was developed further.

In the present thesis, the sources of linear/non-linear viscoelastic and viscoplastic behaviour in polymer composite materials are under study. The significance of this work is related to the nature of all composite materials. All polymer composites tend to indicate a time-dependent behaviour. This behaviour can be either linear or nonlinear. No matter what it is, is very important to be taken into account in the analysis, since it is related to strain rate effects, microdamage induced to the structure of the composite and/or irreversible plastic strains.This microdamage is usually caused due to the application of high stresses or high strain. For that reason additional stiffness degradation experiments were performed. In these tests, samples were subjected to high stress levels. Such high stress levels are also responsible for irreversible phenomena that were mentioned before. Then, a material model was used to study the viscoelastic and viscoplastic behaviour. This model assumes that the viscoelastic and viscoplastic responses may be decoupled; the micro-damage influenced viscoelastic strain response can be separated from viscoplastic response which is also affected by damage. In this thesis, three materials were studied, each one corresponding to a submitted/published scientific article. The first paper entitled "Time dependent nonlinear behaviour of recycled PolyPropylene (rPP) in high tensile stress loading" studied the behaviour of recycled polypropylene and recycled polypropylene with the addition of Maleic Anhydride grafted PolyPropylene (MAPP). The time dependent response was decomposed into nonlinear viscoelastic and viscoplastic parts and each of them was quantified. It was found that the elastic properties did not degrade due to high loading. The addition of MAPP did not change the mechanical properties of the rPP. Then the material model was applied and the involved parameters were identified.In the second article, entitled "Mechanical properties of a recycled carbon fibre reinforced MAPP modified polypropylene composite", the previously studied rPP/MAPP matrix was used to form a composite by using recycled carbon fibres. It was found that in creep tests, the time and stress dependence of viscoplastic strains follows a power law, which makes the determination of the parameters in the viscoplasticity model relatively simple. What is more, the viscoelastic response of the composite was found to be linear in the investigated stress domain. The material model was validated in constant stress rate tensile tests. Finally, in the third article, entitled "The sources of inelastic behaviour of GF/VE NCF [45/-45]s laminates" a glass fibre non-crimp fabric laminate was studied. The viscoelastic and viscoplastic material model parameters were calculated and it was found that the material indicates no linear region. This fact was also attributed to the fibre orientation. Loading the fibres in an off-axis direction caused shear stresses, which are responsible for microdamage (related to the fibre-matrix interface and intralaminal cracks) which is considered to be an important source of non-linearity.

In this thesis, experimental investigations and theoretical studies on the stochastic matrix cracking evolution under static loading in composite laminates with manufacturing defects are presented. The presented work demonstrates a methodology that accounts for the statistically distributed defects in damage mechanics models for the assessment of the integrity of composites and for the structural design of composites.The experimental study deals with the mechanisms of the stochastic process for the multiplication of cracks in laminates. The defects introduced by the manufacturing processes are found to have significant effect on the matrix cracking evolution. Influenced by the distributed defects, the initiation and multiplication of cracks evolve in a stochastic way.Based on the experimental investigations, a statistical model for the matrix cracking evolution is developed. Simulations based on the model yield accurate predictions compared to the experimental data. The parameters of the assumed Weibull distribution of the static strength are estimated from the experimental crack density data. The estimated Weibull distribution provides an efficient basis to characterize the manufacturing quality of composite laminates. Compared to deterministic approaches, this approach provides comprehensive information on the strength property of composite laminates.

Cellulose is an environmentally friendly material which is obtainable in vast quantities, since it is present in every plant. Cellulosic fibers are commercially found in two forms: natural (flax, hemp, cotton, sisal, wood, etc.) and regenerated cellulose fibers (RCF). The biodegradability, the morphological and mechanical properties make these fibers a good alternative to the synthetic reinforcement (e.g. glass fibers). However, as all other cellulosic fibers these materials also have similar drawbacks, such as sensitivity to moisture and poor adhesion with polymers. The first part of this work concerned a heterogeneous modification of cellulose nanocrystals (CNC) by using esterification and amidification to attach long aliphatic chains. Long-chain aliphatic acid chlorides and amines were used as grafting reagents. Surface grafting with acyl chains was confirmed by Fourier-transform infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy. It was found that the degree of substitution (DS) of the surface is highly dependent on the method of modification. The contact angle measurement showed that after modification, the surface of CNC was found to be hydrophobic. The second part was devoted to modification of RCF by CNC using Isocyanatopropyl triethoxysilane as coupling agent. Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy and X-ray diffraction analysis were performed to verify the degree of modification. The mechanical properties of the unmodified and modified fibers were analyzed using fiber bundle tensile static and loading–unloading tests. To show the effect of cellulose whiskers grafting on the Cordenka fibers, epoxy based composites were manufactured and tensile tests done on transverse uni-directional specimens. It was found that the mechanical properties were significantly increased by fiber modification and addition of the nano-phase into composite reinforced with micro- sized fibers.

Lightweight materials with high stiffness and damage tolerance are requested for aerospace, marine and automotive industries. Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection. In the last years new types of composites; Non-crimp-fabric (NCF) reinforced composites, where the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; have made a break-through and have been widely used.As its names indicates, NCF composites consist of layers with ideally straight fiber bundles oriented in different directions, knitted by secondary yarn and separated by resin. This technique of dry preforms impregnated by resin infusion or RTM combine a perfect placement of reinforcement with easy, cheap and automated manufacturing. It produces a composite that can be formed easily in complex shapes, with improvement in damage tolerance as well as the out-of-plane fracture toughness. However, the stitching distorts and crimps the fiber bundles, which leads to large out-of-plane waviness. This deviation affects the mechanical properties of NCF composites. The bundle crimps reduces the stiffness and causes incorrect predictions of the laminate elastic properties employing assumption of the classical laminate theory (CLT).In the present study, the fiber tow waviness is assumed as sinusoidal and the undulation effect on the stiffness reduction is analyzed using Finite Element Method (FEM). The waviness parameters i.e. wavelength and amplitude as well as geometrical parameters like bundle thickness are used in modeling the elastic properties of the representative volume element of the waved structure using meso-scale FEM analysis.The possibility of applying CLT for cross-ply NCF composite stiffness determination is approved, by replacing the curved structure by idealized straight one using effective stiffness for the 0⁰- and the 90⁰- layers. The cross-ply NCF stiffness reduction is dominated by the stiffness reduction of the 0⁰-layer. The 0⁰-layer effective stiffness can be determined either by modeling a single curved tow subjected to distributed load, to reproduce its interaction with the neighboring layers, together with symmetry boundary conditions, or using a master curve approach, where a knock down factor is introduced to characterize the stiffness reduction and analytical expression is suggested. This expressions allows for determination of knock down factor for any given wavelength and amplitude of the waviness.

The use of lightweight materials in structural applications is ever increasing. Today, lightweight engineering materials are needed to realise greener, safer and more competitive products. A route to achieve this could be to combine more than one primary function in a material or component to create multi-functionality, thus reducing the number of components and ultimately the overall weight. This thesis presents an approach towards realising novel multi-functional polymer composites. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. In papers I and II, capacitors have been manufactured using different papers and polymer films as dielectric separator employing carbon fibre/epoxy pre-pregs as structural electrodes. Plasma treatment was used as a route for improved epoxy/polymer film adhesion. The manufactured materials were evaluated for mechanical performance by ILSS and tearing tests and electrical performance by measuring capacitance and dielectric breakdown voltage. In paper III the concept was extended in a parametric study using the most promising approach with a polymer film as dielectric separator. Three thicknesses of PET (50, 75 and 125 µm) were used as dielectric separator with carbon fibre/epoxy pre-pregs as structural electrodes. PET was chosen due to availability in different thicknesses as well as the frequent use in ordinary capacitors making it a suitable candidate. As in paper I and II, plasma treatment was used to improve the PET/epoxy adhesion. The capacitor materials were evaluated for mechanical performance by tensile tests and ILSS and for electrical performance by measuring capacitance and dielectric breakdown voltage. The multifunctional materials shows good potential for replacing steel and other materials with lower specific mechanical properties but cannot match the high specific mechanical performance of mono-functional materials. Both mechanical and electrical performance could have large benefits from developing new separator materials adapted for use in multifunctional applications and could be an interesting field for extended research.

The macroscopic failure of composite laminates subjected to tensile increasing load is preceded by initiation and evolution of several microdamage modes. The most common damage mode and the one examined in this thesis is intralaminar cracking in layers. Due to this kind of microdamage the laminate undergoes stiffness reduction when loaded in tension. For example, the elastic modulus in the loading direction and the corresponding Poisson’s ratio will decrease. The degradation of the elastic properties of these materials is caused by reduced stress in the damaged layer which is mainly due to two parameters: crack opening displacement (COD) and crack sliding displacement (CSD). At fixed applied load these parameters depend on the properties of the damaged and surrounding layers, on layer orientation and on thickness. When the number of cracks per unit length is high (high crack density in the layer) the COD and CSD are reduced because of to crack interaction. The main objective of the first paper is to investigate the effect of crack interaction on COD using FEM and to describe the identified dependence on crack density in a simple and accurate form by introducing an interaction function dependent on crack density. This interaction function together with COD of non-interactive crack gives accurate predictions of the damaged laminate thermo-elastic properties. The application of this function to more complex laminate lay-ups is demonstrated. All these calculations are performed assuming that cracks are equidistant. However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In the second paper, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis the COD and CSD are calculated using FEM, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution we obtain lower bond to elastic modulus. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction. In the third paper, the thermo-elastic constants of damaged laminates were calculated using shear lag models and variational models in a general calculation approach (GLOB-LOC) for symmetric laminates with transverse cracks in 90° layer. The comparison of these two models with FEM was presented for cross-ply and quasi-isotropic laminates.

This study is an initial step within the on-going project on development of high performance bio-based composites with improved mechanical (fatigue) and environmental (elevated humidity and temperature) durability. In the presented thesis the performance of cellulosic fibers (flax and regenerated cellulose), bio-based resins (Tribest, EpoBioX, Palapreg, and Envirez) and their composites under exposure to elevated humidity has been studied. Composites reinforcement was in a form of fiber rovings and fabrics to manufacture uni-directional and cross-ply laminates. Water absorption experiments were performed at different humidity levels to measure apparent diffusion coefficient and moisture content at saturation. Effect of chemical treatment (alkali and silane) on fibers as protection against moisture was also subjected to study. The comparison of results for pristine resins and composites showed that primarily cellulosic reinforcement is responsible for moisture uptake in composites. However, fiber treatment did not improve moisture resistance in composites significantly. Mechanical testing was carried out in order to estimate the influence of humidity on behavior of these materials. Results were compared with data for glass fiber and epoxy, as reference materials. The results indicated that some of the bio-based resins and composites with these polymers performed very well and have comparable properties with composites of synthetic epoxy, even at elevated humidity.

The development and application of bio-based composite materials have been frequently studied. Most of the work is done on quasi-static performance of these materials. However, these composites are highly non-linear therefore there is need for investigation of their viscelastic and viscoplastic behavior. This thesis is dealing with characterization and modeling of behavior of bio-based composite. The effect of temperature and relative humidity on mechanical behavior of natural fiber reinforced bio-based matrix composites subjected to tensile loading was investigated. Composites with different natural fibers (flax, viscose) and bio-based matrices (PLA, Lignin) were studied. Elastic modulus, the nonlinear tensile stress-strain curves and failure were analyzed showing that all materials are temperature sensitive. The nonlinearity was evaluated by studying modulus degradation as well as development of viscoelastic and viscoplastic strains as a function of applied load. The time-dependent phenomena were investigated in short term creep and strain recovery tests at several high stress levels. These tests demonstrated significantly higher viscoplastic strain in lignin than PLA based composites. Both, viscoelastic and viscoplastic strains are larger at higher relative humidity. The observed nonlinearity was attributed to microdamage, viscoelastic and viscoplastic response suggesting Schapery’s type of model for viscoelasticilty and Zapas’ model for viscoplasticity. PLA and lignin based flax fiber composites have been analyzed in order to obtain parameters needed for model. It was found that in PLA based composites after loading at stress levels below the maximum possible the elastic modulus is not affected and, therefore, damage does not need to be included in the material model. The modulus reduction in lignin based composites in tension starts before the maximum in stress-strain curve is reached and it can be as large as 50%. With increasing relative humidity these effects are slightly magnified. It appears that there is no region of linear viscoelasticity for PLA based composites. Nonlinear elasticity, viscoelasticity and viscoplasticity are equally responsible for observed nonlinearity in tensile tests.