In recent years, steel components manufactured through additive manufacturing have increased their popularity and commercial importance and carbon fiber reinforced polymers are widely used in today’s society. Yet there is no composite material combining the properties of steel and carbon fibers. This thesis work is a part of the World Class Material-project at Swerim AB where different methods for manufacturing a steel matrix carbon fiber composite are researched upon. Two methods for additive manufacturing: selective laser melting and electron beam melting were used to evaluate the possibility of creating a steel matrix carbon fiber composite. The experiment with electron beam melting was conducted without powder and the two selective laser melting experiments were conducted with powder using fibers with organic and metal coating respectively. The main aspect evaluated was the survival of carbon fibers during processing. The results showed no intact carbon fibers after processing organically sized fibers in either of the processes. In the selective laser melting experiment with organic coating, big voids were found where the fiber bundles had been placed, showing no infiltration of powder or molten metal into the bundle prior to fiber breakdown. The metal-coated fibers survived partially but showed poor infiltration of matrix material into the carbon fiber bundle. The methods used in this report were not found suitable for the manufacturing of steel matrix carbon fiber composites. / De senaste åren har additiv tillverkning av stålkomponenter blivit alltmer populärt och kolfiberarmerade polymerer används i stor utsträckning i dagens samhälle. Trots det finns det i nuläget inget kompositmaterial som kombinerar stål och kolfiber. Detta kandidatexamensarbete har varit en del av World Class Material-projektet på Swerim AB, där olika metoder för tillverkning av en kolfiberkomposit med stålmatris undersöks. Två additiva tillverkningsmetoder, selective laser melting (SLM) och electron beam melting (EBM), användes för att utvärdera möjligheten att med dessa tillverka en stål-och kolfiberkomposit. Testet i EBM genomfördes utan pulver och i testerna SLM med pulver där fibrerna i ena försöket hade kvar sin organiska ytbeläggning och en metallbeläggning i det andra försöket. Den huvudsakliga aspekten som undersöktes var kolfibrernas förmåga att klara tillverkningsmiljön. Det förekom inga intakta kolfiber efter processen i försöken med EBM och SLM med den organiska ytbeläggningen. Beträffande SLM-försöket, påträffades även stora håligheter där kolfiber befunnit sig, vilket visade på en bristfällig impregnering av fibrerna innan deras nedbrytning. Fibrerna som var belagda med en metall överlevde framställningsprocessen i en större utsträckning, men impregnationen var fortfarande bristfällig. Metoderna som undersöktes i denna rapport var sålunda inte lämpliga för att tillverka en kolfiberkomposit med stålmatris.

Fiber/matrix interface cracking plays an important role in determining the final failureof unidirectional (UD) composites. When subjected to longitudinally tensile loading,fiber/matrix interface debonds originate from fiber breaks or initial defects propagatealong loading direction. Depending on the quality of fiber/matrix interface, debondscould keep growing longitudinally which leads to the degradation of compositestiffness or kink out of interface and connect with neighboring debonds or fiberbreaks that forms a so called critical fracture plane which leads to the final failure ofUD composite. For UD composite subjected to transversely tensile loading, theinitiation, growth and coalesce of arc-shape fiber/matrix interface debonds result inthe formation of macro-size transverse cracks, the propagation and multiplication ofthese transverse cracks, although would not directly lead to the final failure ofcomposite, could cause significant stiffness degradation of composite structures.In the presence thesis, the growth of a fiber/matrix interface debond of a UDcomposite with hexagonal fiber packing under longitudinal and transverse tensileloading was investigated numerically, with the special focus on the influence ofneighboring fibers. In the current study, energy release rate (ERR) is considered as thedriving force for the debond growth and was calculated based on J Integral andVirtual Crack Closure Technique (VCCT) using finite element software ANSSY.Papers A – C in the present thesis deal with the influence of neighboring fibers on theERR of a debond emanating from a fiber break under longitudinal loading condition.In longitudinal loading case, debond growth is mode II dominated. In paper A, anaxisymmetric model consisting 5 concentric cylinders that represent broken fiber withdebond, surrounding matrix, neighboring fibers, surrounding matrix and effectivecomposite was generated. It’s found that there are two stages of debond growth, thefirst stage is when debond length is short, the ERR decreases with increasing debondlength, and the presence of neighboring fibers significantly increase the ERR ofdebond. For relatively long debond, the debond growth is steady when ERR is almostconstant regardless of debond length. In steady state of debond growth, the presenceof neighboring fibers have little effect on the ERR. In papers B and C, a 3-D modelwas generated with broken fiber and its 6 nearest fibers in a hexagonal packed UDcomposite were modelled explicitly, surrounded by the homogenized composite. Based on the obtained results, it’s shown that ERR is varying along debond front, andhas its maximum at the circumferential location where the distance between two fibercenter is the smallest. This indicates that the debond front is not a circle. For steadystate debond, the presence of neighboring fibers have little effect on averaged ERR(averages of ERR along debond front). For short debond, the presences ofneighboring fibers increases the averaged ERR, and that increase is more significantwhen inter-fiber distance is the smallest. Paper D investigates the growth of afiber/matrix debond along fiber circumference under transverse loading. It’s foundthat debond growth in this case is mixed-mode, and both mode I and mode II ERRcomponents increase with increasing debond angle and then decreases. Debondgrowth is mode I dominated for small debond angle and then switch to mode IIdominated. The presence of neighboring fibers have an enhancement effect on debondgrowth up to certain small debond angle and then changes to a protective effect. InPaper E, the interaction between two arc-size debond under transverse loading isinvestigated. It’s found that when two debonds are close to each other, the interactionbetween two debond becomes much stronger, and that interaction leads to the increaseof ERR of each debond significantly, which facilitates further growth for bothdebond.

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This thesis presents systematic numerical study of stresses in the adhesive of a single-lap joint subjected to various loading scenarios (mechanical and thermal loading). The main objective of this work is to improve understanding of the main material and geometrical parameters determining performance of adhesive joint for the future analysis of failure initiation and development in these structures. The first part of the thesis deals with development of a 3D model as well as 2D model, optimized with respect to the computational efficiency by use of novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates). The model takes into account the nonlinearity of materials (adherend and adhesive) with geometrical nonlinearity also accounted for. The parameters of geometry of the joint are normalized with respect to the dimensions of adhesive (e.g. thickness) thus making analysis of results more general and applicable to wide range of different joints. Optimal geometry of the single-lap joint is selected based on results of the parametric analysis by using peel and shear stress distributions in the adhesive layer as a criteria and it allows separation of edge and end effects. Three different types of single lap joint with similar and dissimilar (hybrid) materials are considered: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). The influence of the abovementioned parameters is carefully examined by analyzing peel and shear stress distributions in the adhesive layer. Discussion and conclusions with respect to the magnitude of the stress concentration at the ends of the joint overlap as well as overall level of stresses within overlap are presented. Recommendations concerning use of nonlinear material model are given. The rest of the work is related to the various methods of manufacturing of joint (curing) and application of thermo-mechanical loading suitable to these scenarios. The appropriate sequences of application of thermal and mechanical loads for the analysis of the residual thermal stresses developed due to manufacturing of joints at elevated temperature required to cure polymer (adhesive/composite) are proposed. It is shown that the most common approach used in many studies of simple superposition of thermal and mechanical stresses works well only for linear materials and produces wrong results if material is non-linear. The model and simulation technique presented in the current thesis rectifies this issue and accurate stress distributions are obtained. Based on the analysis of these stress distributions the following conclusions can be made: joint processing at elevated temperature causes high stresses inside the adhesive layer; the residual thermal stresses will reduce the peel stress concentration at the ends of overlap joint and the shear stress within the overlap, moreover, this effect is more pronounced for the case of the one-step joint manufacturing in comparison with two-step processing technique. This study has generated a lot of results for better understand of behavior of adhesive joints and it will help in design of stronger, more durable adhesive single-lap joints in the future.

Godkänd; 1996; 20071115 (joffe)

Flax fibers, along with a number of other natural fibers, are being considered as an environmentally friendly 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. Stiffness and strength under uniaxial tension of flax fiber composites with thermoset and thermoplastic polymer matrices are considered. The applicability of rule of mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is evaluated. / <p>Godkänd; 2006; 20061206 (pafi)</p>

Natural fiber composites, (NFC) are defined in this work as a group of materials where at least the fibers originates from renewable and CO2 neutral resources. NFC consists of a polymer matrix and a natural fiber. The fibers which originate from wood or plants can replace non-renewable fibers or fillers or simply replace part of the plastic. If plastics from renewable resources are used, NFC is a 100% renewable material. Even though there is very large variety of fibers, matrices and manufacturing techniques used to produce NFC, these materials are often separated as its own material class. However, the variety of constituents and processing methods result in completely different materials with very diverse properties. NFC could thus be suitable for an extremely wide area of applications. We believe that it is important to distinguish different types of NFC and classify them based on matrix (thermoplastic or thermoset), fiber (long or short/orientation) and manufacturing techniques. For instance compression molded composites are very different from injection molded materials. Therefore it is important to find the limits of their performance in connection to the processing parameters. The focus of this work is on the compounding and injection molding techniques. Although extensive research has been done on injection molded NFC, this is one area where the natural fibers still have not made a market breakthrough. We believe that the reason for the limited use of natural fiber compound in injection molded products is partly due to uncertainties about the influence of different constituents on the final properties and lack of defined framework for product design and manufacturing in order to optimize the material and assure consistent quality. Although deep knowledge about these materials have been accumulated among producers and researchers in this area, guidelines or simple rules of thumb for NFC development and processing are quite hard to find in literature. Thus, in order to make natural fiber compounds a more interesting alternative for the injection molding industry, this work is focused on finding limitations on important properties and giving general guidelines for material optimization and processing of natural fiber composites. / <p>Godkänd; 2007; 20070523 (ysko)</p>

A Boundary Element analysis of micromechanical elastic fields in the single-fiber fragmentation test is presented in this thesis. The work carried out is roughly divided in two main tasks: the development of the BE code and the numerical simulation of the single-fiber fragmentation test. The numerical study is primarily concerned with the analysis of the initiation and growth of a debond crack along the fiber-matrix interface in the single fiber fragmentation test, although different configurations in which the crack propagates through the matrix have also been considered. The asymptotic behavior of the near-tip singular elastic solutions in the fiber cracks, the interface cracks and the matrix cracks are studied. Additionally, asymptotic behavior of the Energy Release Rate for a wide range of debond lengths is analyzed. Firstly, the numerical analysis is performed in the framework of the two linear elastic models of interface cracks, open model and frictionless contact model, and a discussion of their adequacy based on the numerical results presented is given. Finally, a frictional contact model is employed to elucidate the influence that the friction between the debond crack faces may have in the near-tip singular elastic solutions and crack propagation. Therefore, a Boundary Element code has been developed which allows the elastic analysis of axially symmetric bodies to be carried out, permitting the definition of multiple solids bonded or in contact, taking into account the residual stresses developed during the curing of the samples and allowing non conforming meshes to be used in the interfaces and contact zones. Moreover, a novel extremely accurate integration technique has been developed to allow the near-tip singular elastic solutions to be precisely obtained. / <p>Godkänd; 2007; 20071107 (ysko)</p>

Damage in composites reduces its performance and durability and thus its usefulness. The common subject in all papers presented in the licentiate thesis is distributed microdamage, and the materials of interest are a Hemp/Lignin natural composite and glass/carbon fiber reinforced plastics composites. The focus is on how the damage affects the performance in terms of creep strain and stiffness. In Paper A a nonlinear viscoelastic viscoplastic model of a Hemp/Lignin composite is generalized by including stiffness reduction, and thus the degree of microdamage, in the composite (when loaded in the axial direction). Schapery's model is used to model the nonlinear viscoelasticity whereas the viscoplastic strain is described by a nonlinear function presented by Zapas and Crissman. In order to include stiffness reduction due to damage, Schapery's model is modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. The model successfully describes the main features for the investigated material and shows good accuracy within the considered stress range. In Paper B the stiffness reduction of a unidirectional (UD) composite containing fiber breaks with partial interface debonding is analyzed. The analysis is performed by studying how the average crack opening displacement (COD) depends on fiber and matrix properties, fiber content and debond length. The COD is normalized with respect to the size of the fiber crack and to the far field stress in the fiber. In contrast to other performed analysis an analytical relationship is developed which links the entire stiffness matrix of the damaged UD composite with the COD and the crack sliding displacement (CSD). However, the CSD is excluded from the analysis since it is found by parametric inspection that it does not affect the longitudinal stiffness. Some trends regarding the COD dependence on the different properties can be extracted from available approximate analytical stress transfer models. To obtain more reliable results, in the current analysis these dependences are extracted from extensive FEM based parametric analysis performed on a model consisting of three concentric cylinders: a) broken fiber; b) matrix cylinder around it; c) large effective composite cylinder surrounding them. This model is used since it is more adequate than unit cell models considering only fiber and matrix. The cracks, which are only in the fibers, are distributed in such a way that they are non-interactive. It is shown that the parameters that affect the COD the most are the ratio of the longitudinal fiber modulus and matrix modulus, the fiber content and the debond length. These relationships are described by simple fitting functions which excellently fit the numerical results. These simple functions are merged into one relationship describing the COD's dependence on the relevant parameters. Simulations performed for carbon and glass fiber polymer composites show that the relative longitudinal stiffness reduction in the carbon fiber composite is slightly larger than in the glass fiber composite. This trend holds for all considered debond lengths and is related to higher longitudinal fiber and matrix modulus ratio in the carbon fiber composite leading to larger crack openings and larger stress perturbation zones. It is shown that the stiffness reduction depends on the debond length. In Paper C the analysis performed in Paper B is continued by studying how the COD is affected when the cracks are interactive. It is shown that the effect on the COD in the glass fiber composite is negligible. However, the effect on the COD in the carbon fiber composite is significant. This difference is related to higher longitudinal fiber and matrix modulus ratio for the carbon fiber composite. In Paper D the same model is used to analyse the strain energy release rate related to the debond crack growth along the fiber. The energy release rate is calculated using the virtual crack closure technique applied to displacement and stress field in the vicinity of the debond crack tip calculated using refined FE model. It is shown that the energy release rate is larger for very short debonds. It reduces to a constant value indicating a stable debond crack growth after its initiation. It is shown that the strain energy release rate in the plateau region also can be calculated using a simple analytical model based on the self-similar crack growth assumption. When the stress state perturbations related to debonds at both fiber ends start to interact, the energy release rate decreases. In a future work the obtained relationships for the energy release rate will be incorporated in a microdamage evolution model describing the statistics of fiber breaks and debond growth in fatigue loading conditions. / <p>Godkänd; 2007; 20071128 (ysko)</p>

Due to environmental concerns the interest in use of renewable and recyclable materials has dramatically increased over the recent years. Wood and other lignocellulosic fiber reinforced polymers have large potential as structural materials due to the high specific stiffness, high specific strength and high aspect ratio of the fibers. Composites made from wood fiber mats from paper production are also interesting from an economical point of view. In present time the limited use of cellulosic fiber composites in structural design is predominantly associated with disadvantages such as dimensional instability in humid environments, lack of well defined fiber properties and the fibers low ability to adhere to common matrix materials for efficient stress transfer. A better understanding of dimensional stability and both long term and short term mechanical performance of cellulose fiber composites is necessary if these materials are to reach their full potential. The objective of the work presented in this thesis is twofold: (i) to present material models and suitable data reduction methodology with the ambition to characterize these materials very complex time dependent behavior (Paper A and B) and (ii) to develop micromechanical models that can be used in parametric studies of fiber properties and their influence on composite properties (Paper C-E). In Paper A the nonlinear viscoelastic behavior of flax/polypropylene composites was characterized using different forms of the creep compliance. The viscoplastic behavior was described using a nonlinear function with respect to time and stress. In Paper B hemp/lignin composites were characterized in terms of nonlinear viscoelastic behavior using Prony series form of creep compliance. The viscoplastic behavior was described using the same nonlinear function as in Paper A. The presented material model also included a stiffness degradation function based on previous strain history. An incremental form of the constitutive model was used to simulate the material behavior in loading and unloading ramps and validated through experiments. In Paper C the effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness was analyzed. An analytical model for an N-phased concentric cylinder assembly with orthotropic properties of constituents was developed and used. The model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. In Paper D the same concentric cylinder model was used and extended to include also free hygroexpansion terms in the elastic stress-strain relationship. The hygroelastic properties on three levels were calculated. Using material data for the wood polymers available from literature the swelling characteristics on the (i) ultrastructural level, i.e. the microfibril unit cell was determined; (ii) the hygroexpansion coefficients of the fiber cell wall layers were determined and finally (iii) the hygroexpansion coefficients of an aligned wood fiber composite were calculated. In Paper E the influence of helical fiber structure on composite properties was evaluated. The fibers helical structure leads to an extension-twist coupling and thus a free fiber will deform axially and also rotate upon loading in longitudinal fiber axis direction. Within the composite the fiber rotation will be restricted however. Therefore, the decision was to compare the elastic properties in two extreme cases on both fiber- and composite level: (i) free rotation and (ii) no rotation of the layers in the cylinder assembly. / Godkänd; 2007; 20080219 (evan)