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

The vehicles of the future must have less negative environmental impact during their use phase than the vehicles of today in order to avoid major climate change on earth. Consequently electric vehicles are currently under development with the purpose of reducing CO2 emissions when the vehicle is driven. There are also efforts put in to reducing the weight of vehicles in order to reduce the demand for energy to drive them. One important aspect of weight reduction is that new materials and technologies are developed. Plastic materials have low a density and can therefore be used to reduce the weight of vehicle components and with composite materials there is further potential for weight reduction. Self-reinforced thermoplastic composite materials are materials in which both reinforcement and matrix are thermoplastic materials and thanks to their low density and relatively good mechanical properties, these materials may be used for weight reduction of vehicle components.   The aim of this thesis is to study selected process parameters for component manufacturing with self-reinforced poly(ethylene terephthalate) (SrPET) in order to increase knowledge and thereby advance the field of self-reinforced PET composites. This thesis shows that stretching the material in the manufacturing process increases the mechanical performance of the material due to increased orientation of the amorphous phase in the PET reinforcement. However, stretching introduces stresses in the material that give rise to negative shape distortions in the formed component. The degree of stretching during forming must therefore be controlled in order to achieve a robust serial production. The concept of a SrPET component over-moulded for integration of stiffeners and attachments has been evaluated in a life-cycle-assessment. This evaluation shows that the component weight can be reduced compared to technology currently in use and thereby contribute to increased sustainability of transport. / <p>QC 20171215</p>

Fiber composites are today widely used in different load carrying structures. The main reason is their high stiffness and strength to weight ratio. Due to these specific properties they are frequently used in car, marine and space industry. The damage that develops in such composites is generally much more complex in comparison with other conventional structural materials such as steel and aluminium. In those materials, one particular flaw might lead to failure. In laminated composites, many different kinds of defects can be obtained without leading to ultimate failure. If the composite laminates under life cycle undergo complex combinations of thermal and mechanical loading, it might lead to microdamage accumulation in plies. The first mode of damage is usually intralaminar cracking with the crack plane transverse to the laminate middle-plane, crossing the whole width of the specimen. The density of cracks in a ply depends on layer orientation with respect to mechanical loads, temperature change, number of cycles in fatigue, laminate lay-up, ply thickness and certainly material fracture toughness. Many papers have been written on this subject, covering a broad range from micromechanics based to continuum damage mechanics based models. Most of the research has, however, been focused on cross-ply laminates which are excellent for academic studies of phenomena but are seldom used in practical applications. Laminates with a general lay-up containing cracks in several layers of different orientation is, therefore, a challenge for any constitutive model. In paper A an attempt to derive the constitutive relationships for a damaged laminate is presented in the framework of the laminate theory. The most advantage is the transparency of derivations and simplicity of application. Stiffness, compliance matrices and thermal expansion coefficients of an arbitrary symmetric laminate with damage in certain layers are presented in an explicit form. Derivation of constitutive relationships is following the same routes as in classical laminate theory. As an input from homogenization theory the relationships between volume averaged and boundary surface averaged quantities is used. The differences between undamaged and damaged laminate case are indicated in each step of derivation. The damaged laminate stiffness and thermal expansion coefficient matrices are calculated from the undamaged laminate matrices multiplying it by a matrix which differs from the identity matrix by terms dependent on crack density in layers, stiffness matrix and orientation of these layers and includes a crack face displacement related matrix. The normalized COD (crack opening displacement) and crack face sliding are considered as dependent on the position of the cracked layer (outside or inside cracks) and on the constraint of the surrounding layers in terms of their stiffness and thickness. These dependences are analyzed using FEM calculated crack opening displacement profiles in generalized plane strain formulation and presenting the results in form of power laws. In a special case of balanced laminates with cracks in 90-layer only, expressions for thermo-elastic properties are presented in an explicit and compact form. In the paper B the sliding effect of a [Sm,90n]s laminate with transverse cracks in 90-layer is studied in order to determine the normalized average crack face sliding displacement. It is needed if a cross-ply laminate is subjected to shear loading or if a general laminate with cracks in other layer than 0- and 90-layer is subjected to any kind of loading. The normalized average crack face sliding displacement is approximated by a power law in the same way as the normalized average crack face opening displacement in paper A and used for in the constitutive equations in order to predict the in-plane shear modulus for the damaged laminate. A FE-model is constructed in order to calculate the shear modulus of the damaged laminate and to find the sliding displacement. / Godkänd; 2003; 20070215 (ysko)

Composite materials are playing an essential role in construction industries, automobile industries, and mechanical industries which have better physical properties than original materials. Hybrix material is a kind of Composite sandwich material which have better properties like lightweight, durable, eco-friendly than original material. In an attempt to prove the quality in finite element analysis Hybrix material, creating a numerical tool to simulate the metal forming conditions. Spring back effect and residual stress are also taken into account in the method.

In several European countries, dental composites are replacing mercury-containing amalgams as the most common restorative materials. The problems with dental composites are they can induce pain for the patient, fracture of the tooth, gap between the tooth and the filling what will induce secondary caries. The main reason is residual stresses. The factors affecting residual stresses are known; it is Young's modulus, volume changes, relaxation, geometry, but their importance is unknown. A model approach has been chosen in order to determine what are the main factors. An experimental set-up, shown in the second paper, to measure residual stresses has been made based on the bimetallic experiment. The dental composite is cured on an aluminum substrate and two strain gages register the bending of the substrate. From this experiment, the residual stresses in the composite could be determined. The modeling, treated in the second paper of this thesis, can be divided in sub-models. The first one is the cure kinetics in order to obtain the degree of cure. From this sub-model, the volume change and the Young's modulus can be determined. From the two last sub-models and the geometry, the stresses can be calculated. The chemical shrinkage was considered as linearly dependent on the degree of conversion. A simple pseudo-autocatalytic model was used for the cure kinetics. In order to do the calculation the change of modulus as a function of degree of cure has to be model. The viscoelastic properties of pure resin samples light-cure at different degree of conversion were determined using dynamic mechanical analysis and time-temperature superposition. The viscoelastic Young's modulus has been represented by a discrete exponential series and it has been observed that time-cure superposition works, what means that the weight factors do not depend on the degree of cure. Only the relaxed modulus, the unrelaxed modulus, and the principal relaxation time (time at which the relaxation spectrum has its maximum) depend on the degree of cure. A linear relation was found between the logarithm of these parameters and the degree of cure. An elastic Young's modulus model was done by taking the same expression as for viscoelasticity, but replacing the time by 1s what corresponds to the change of the 1Hz modulus as a function of degree of cure. The calculations were done for 2D-constraint geometry like for the bimetallic experiment. Finite difference was used for the calculations. The changes of physical properties as a function of degree of cure were done on pure resin, but we need the changes for different filler content. This is the reason why the first paper deals with micro-mechanical models to predict the effect of filler on the Young's modulus and the chemical shrinkage. The Young's modulus is well described by the upper bound of Hashin's sphere model. Whereas the chemical shrinkage is well described by a modified Rosen and Hashin's model that was developed for the thermal coefficient expansion. Since this two models work well they were used for the calculation of the composite chemical shrinkage and the calculation of relaxed and unrelaxed Young's modulus considering that the weight factors and relaxation time were the same as for the resin. Two isothermal models have been done: one elastic and one viscoelastic. The viscoelastic model gives stresses that are 15% lower than elastic case. The viscoelastic model gives good results at the beginning compare to the experimental data, but after it overestimates a lot the stresses. There are two main reasons. First the modeling of shrinkage is inadequate, it is believed that the shrinkage decreases near vitrification so the linear relation do not hold and induce an overestimation of stresses. The second factor is the fact that there is an exotherm from room temperature to 55ºC in the case of pure resin, so the isothermal conditions are not fulfilled. This study shows us the validity of time-cure superposition. It also demonstrates that the modeling of shrinkage should be done more carefully and that the non-isothermal conditions should certainly be taken into account. The results of this thesis are presented in following papers: P. Lingois and L. Berglund, "Modeling Young's modulus and volume shrinkage of dental composites" P. Lingois, L. Berglund, A. Mafezzoli, and A. Greco, "Chemically induced residual stresses in dental composites" / Godkänd; 2000; 20070318 (ysko)

Polymer composite materials are in wide-spread use in the transportation industry. In aerospace industry the use these materials are established while in automotive industry the interest is increasing. The attention of automotive industry is to a great deal focused on various kinds of molded composites such as glass mat reinforced thermoplastics (GMT) and sheet molding compound (SMC). Their interest is to a large extent driven by the possibility to manufacture components of complex geometry in a cost- efficient process with these materials. An increasing number of car and truck manufacturers are using SMC for external panels such as trunk covers, hoods, roofs and spoilers. A property of obvious importance for an external car- or truck-panel is its capacity to withstand impact. In this context, improved understanding of crack growth and toughening mechanisms of the material is of great interest. A major part of the work presented in this thesis is driven by an interest to increase the understanding of how material composition and microstructure of short fiber composites influence their overall fracture behaviour. In materials such as metals and unreinforced polymers, linear elastic fracture mechanics (LEFM) is widely used, often with great success, both in design and in development of new materials. Unfortunately, problems arise when LEFM is applied to short fiber composites. This is due to the large process zone that develops ahead of a crack in these materials. The fundamental assumption of LEFM, that the damage zone at the tip of the crack is small compared to crack length, is often violated in experiments. The presented thesis considers a different approach, in which the damage ahead of a crack tip is described by a bridging-law. By considering the bridging-law as the major failure property of the material, a coupling between mechanisms acting on a microscale and the macroscopic failure behaviour can be established. No such information can be obtained using a LEFM approach where the material behaviour is described in terms of a single value, the fracture toughness. Bridging-laws for three different short fiber composites are experimentally determined and presented in the first paper of the thesis. A matter of key importance for future work in this field is that there are methods available for experimental verification of the suggested fiber bridging approach. Optical strain field measurement methods are therefore very useful. The thesis contains a pilot study to evaluate the use of two recently developed optical methods , Stereoscopic Digital Speckle Photography (Stereo-DSP) and combined DSP-DSPI (Digital Speckle Pattern Interferometry), for measurements of fracture behaviour of notched short fiber composites. We found Stereo-DSP to be a versatile technique that can be used when knowledge of overall displacement fields is required. The combined technique can with advantage be used when detailed information about large deformation at small areas is of interest, e.g. the complex fiber bridging interaction at the crack tip of a short fiber composite. The last paper in the thesis presents a study where the influence of fiber surface treatment on transverse cracking in cross-ply laminates was investigated. In the case of tubes and pressure vessels, the formation of transverse cracks ultimately leads to leakage since cracks connect and form a path through the wall. In the presented study, our ambition was to investigate the influence of film former polymer on transverse cracking properties of cross-ply laminates.Both onset of transverse cracking and tendency for multiple crack development were strongly affected by the different film formers. The strong film former effect was proposed to be due to a combination of improved interfacial adhesion and the plasticizing effect from the film former on the interphase region. The thesis is composed by the following papers: Patrik Fernberg, Lars Berglund, Bridging law and toughness characterisation of CSM and SMC composite, to be submitted. Angelica Andersson, Patrik Fernberg, Mikael Sjödahl, Optical methods to study fracture of notched glass mat composites. Proceedings of the International Conference on trends in Optical Nondestructive Testing, Lugano, Switzerland, May 3-6, 2000 (in press). Patrik Fernberg, Lars Berglund, Effects of glass fiber size composition (film former type) on transverse cracking in cross-ply laminates. Accepted for publication in Composites, Part A. / Godkänd; 2000; 20070318 (ysko)

Godkänd; 1996; 20080328 (ysko)

Godkänd; 1995; 20080330 (ysko)

Godkänd; 1995; 20080330 (ysko)