No description available.

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)

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 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 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.Using FEM, we assume linear elastic material with ideal crack geometry. Fiber bridging over the crack surface is possible which can affect COD and CSD. The only correct way to validate these assumptions is through experiments.The main objective of the fourth and the fifth paper is to measure these parameters for different laminate lay-ups in this way providing models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edge of a [90/0]s and [903/0]s carbon fiber/epoxy laminates specimens with multiple intralaminar cracks in the surface layer is studied. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). / Godkänd; 2013; 20130828 (mohlou); Tillkännagivande disputation 2013-09-11 Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Mohamed Sahbi Loukil Ämne: Polymera konstruktionsmaterial/Polymeric Composite Materials Avhandling: Experimental and Numerical Studies of Intralaminar Cracking in High Performance Composites Opponent: Professor Constantinos Soutis, School of Mechanical, Aerospace & Civil Engineering, University of Manchester, UK Ordförande: Professor Janis Varna, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag den 4 oktober 2013, kl 09.00 Plats: E231, Luleå tekniska universitet

The polymeric composite materials are in high demand by industries where light and strong materials are required. Although manmade fiber (e.g. glass, carbon, aramid fibers) are most often used to reinforce polymers, natural fibers due to their environmental friendliness and sustainability have been also considered. Natural fiber composites have shown to have great potential as a substitute for conventional glass fiber materials. However, bio-based composites exhibit highly non-linear behavior, besides they are very sensitive to elevated moisture and temperature. Therefore, careful design and optimization of composite properties defined by constituents, composition and internal structure is needed to meet requirements of real-life applications. This can be done by using accurate models that can take into account factors responsible for inelastic behavior of these materials. The initial part of this thesis is dealing with development of phenomenological approach to predict inelastic behavior of composites in tension. Viscoelasticity and viscoplasticity was analyzed in short term creep tests and modulus degradation in stiffness degradation tests. Schapery’s model for viscoelasticity and Zapa’s model for viscoplasticity was used to characterize nonlinearity. This method was then validated on short, randomly oriented fiber composites with different cellulosic fibers (flax, viscose) and bio-polymers (PLA, Lignin). The elastic modulus, tensile stress-strain curves and failure were analyzed at different humidity and temperature levels. Results showed high sensitivity to moisture and temperature and highly non-linear behavior of these materials. Modeling showed good agreement between experimental data and simulations.Since there is need for simulations of strain controlled tests, this model was rewritten in inverted incremental form. Simulations of stress-strain curves showed, that predictions are more accurate, when characterization of viscoelastic and viscoplastic parameters was done at stresses close to failure. However, due to creep rapture it was not always possible to characterize material at high stresses and in this case viscoelastic functions have to be extrapolated. The stress-strain curves can be then used to further adjust extrapolation of model parameters.The model developed in the first part of the thesis proved to be capable of predicting behavior of short fiber composites with good accuracy. However, in order to carry out simulations input parameters have to be experimentally obtained and it has to be done for every composite that is studied. The second part of this thesis is dedicated to development of constitutive model which uses parameters of constituents to predict behavior of material with any composition. This model then is applied on semi-structural natural fiber composites consisting of bio-based resins reinforced with continuous cellulosic fibers. Mechanical properties of different bio-based thermoset resins and regenerated cellulose fibers have been analyzed. Results showed comparable properties of bio-based and synthetic epoxy resins, even at elevated humidity levels, but high scattering of properties from sample to sample. They also showed that bio-based resin exhibit limited non-linearity whereas regenerated cellulose fiber is highly non-linear.In order to avoid large scatter typical for bio-based materials and improve accuracy of the model, methodology for parameter identification for viscoplastic model with use of only one sample has been suggested.The objective here is to simulate strain controlled tests and the most convenient way to do it is with Schapery’s strain formulation model. The parameters for such model can be obtained from relaxation tests, where viscoelastic strain is kept constant but due to presence of viscoplastic strain component such experiments are difficult to perform. Instead, constituents exhibiting viscoplastic behavior have been characterized in creep and viscoelastic parameters for Schapery’s strain formulation are obtained from simulations of relaxation tests with inverted incremental model. Then these parameters are used to simulate behavior of composite subjected to iso-strain conditions. / Godkänd; 2015; 20150315 (livroz); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Liva Pupure Ämne: Polymera konstruktionsmaterial/Polymeric Composite Materials Avhandling: Non-linear Model Applied on Composites Exhibiting Inelastic Behavior: Development and Validation Opponent: Industrial Assistant professor Maciej Wysocki, Chalmers tekniska högskola, Göteborg/Scientific Coordinator Leader Swerea SICOMP, Mölndal Ordförande: Professor Roberts Joffe, Avdelningen för materialvetenskap, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet, Luleå Tid: Fredag 17 april kl 10.00 Plats: E246, Luleå tekniska universitet