| 1. |
- Molavitabrizi, Danial
(author)
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Linear and Nonlinear Mechanics of Lattice Materials : Computational Modelling and Experiments
- 2023
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Doctoral thesis (other academic/artistic)abstract
- Lattice materials are artificial materials made of repeating unit-cells. The internal architecture of these materials can be engineered for a specific application such as energy absorption, heat transfer, or acoustic damping. The advancements in additive manufacturing have enabled the design and fabrication of lattice materials with complex geometries, but the lack of understanding about their mechanical performance has limited their application. This thesis investigates the mechanics of lattice materials via numerical simulations and mechanical tests. We start with development of a discrete homogenization scheme for the elastic analysis of lattice materials with arbitrary level of complexity. Next, the method is extended to a continuum elastoplastic homogenization and accompanied with the theory of critical distances to assess the low cycle fatigue behaviour of lattice materials. Following that, the model is coupled with continuum damage mechanics to mimic the fracture initiation of the material unit-cell under quasi-static loads. The proposed model is calibrated using tensile tests, leading to a defect-informed numerical model that accommodates the manufacturing imperfections. Following this, an environmentally assisted failure known as hydrogen embrittlement is studied by incorporating hydrogen failure mechanisms into elastoplastic homogenization model. In the next step, numerical simulations and compression tests are employed to analyze the mechanical coupling and elastic anisotropy in the so-called non-regular tetrahedron lattice. The results show a good correspondence but the periodicity assumption in computational homogenization –mimicking an infinite cell number–should be considered when comparing numerical results with the data obtained from real samples. To capture the size effect, as the final step, we develop a homogenization scheme based on strain-gradient elasticity. The model is verified using numerical and experimental three-point bending tests and has shown to be more precise compared to classical elasticity. The methodologies proposed in this thesis are generic and can be used as guidelines for design of micro-architectured materials.
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| 2. |
- Pakkam Gabriel, Vivek Richards, 1995-
(author)
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Transverse cracking in cross-ply composites during static and fatigue loading at different temperatures
- 2024
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Doctoral thesis (other academic/artistic)abstract
- Polymer composite laminates are preferred in many load bearing applications for its tailorable mechanical properties while offering light-weight solution, corrosive resistance etc. Hence, polymer composites are attractive material choice for aircraft manufacturers to reduce weight and emissions. However, one of the challenges existing in composite laminates is accumulation of damage before final failure, that reduces mechanical properties of the composite laminates during service life. Hence it is crucial to develop a reliable model to predict damage and consequently mechanical properties degradation. The thesis focuses on transverse/intralaminar cracks, that are the first form of damage to appear in off-axis layers of composite laminates when subjected to tensile load and they increase in number with increase in load. Transverse crack growth in numbers was analyzed in terms of transverse crack density (= number of cracks / observed length) growth. Appended papers present methodologies developed using statistical transverse failure stress distribution approach to predict the transverse crack density growth when composite laminates subjected to quasi-static tensile and tension-tension fatigue loading at different temperatures. For that purpose, continuous fibers reinforced polymer composite cross-ply laminates containing different material systems were manufactured, and damage growth was studied in 90-layer in coupon scale specimens. In static tests, the crack density growth in specimens were analyzed against the thermo-mechanical transverse stress in the 90-layer. Distribution of transverse failure stress to initiate a crack along the transverse direction of the layer has been defined using 2 parameter Weibull distribution model. Paper 1 presents, methodology to predict crack density growth, using probability of failure stress distribution (based on Weibull model) in Monte Carlo simulation along with the developed stress distribution model between cracks, in specimens tested at room temperature (RT). The crack density was well predicted in both non-interactive and interactive crack density region using improved Weibull parameter determination routine. The presented Weibull model was extended to address the effect of iso-thermal heat treatment and test temperature in Paper 4. It was observed that both heat treatment and elevated test temperatures, in general, resulted in reduction of transverse cracking resistance. The effect of heat treatment and test temperature on transverse cracking was modelled as Weibull scale parameter dependency using polynomial expression. The developed model was validated against laminates with same material system but with different layups and fiber content.Fatigue tests were performed at different maximum stress levels and at RT and 150℃. Crack density growth was analyzed against number of fatigue cycles. The observed decrease in resistance to transverse cracking with every cycle of load was interpreted as monotonic decrease of Weibull scale parameter. Simple power function with respect to number of cycles was proposed to decrease the scale parameter. Paper 2 presents, fatigue test results at RT and methodology to predict crack density growth in different fatigue stress levels. The methodology, using maximum local transverse stress in a fatigue cycle in Weibull model and the Weibull parameters determined at a reference fatigue stress level, was limited in ability to predict the crack density growth at other stress levels. It was then found that the crack density growth not only depends on maximum local stress in a fatigue cycle, but also on the local stress ratio in the 90-layer, presented in Paper 3. Wherein, an equivalent stress was introduced to replace maximum local stress in Weibull model by addressing the combined effect of maximum local stress in a cycle and also the local stress ratio. Equivalent stress model was validated across different layups and fiber content with same material system. Paper 5 presents fatigue test results at different stress levels and temperatures. It was found that in fatigue tests at 150℃, in spite of lower thermal stress, crack density growth was more rapid than for RT fatigue tests. Methodology to predict crack density growth in 150℃ fatigue tests by combining the analytical model with the equivalent stress and with enhanced test temperature effect has been presented.
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| 3. |
- Almgren, Karin M., 1980-
(author)
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Wood-fibre composites : Stress transfer and hygroexpansion
- 2010
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Doctoral thesis (other academic/artistic)abstract
- Wood fibres is a type of natural fibres suitable for composite applications. The abundance of wood in Swedish forests makes wood-fibre composites a new and interesting application for the Swedish pulp and paper industry. For large scale production of composites reinforced by wood fibres to be realized, the mechanical properties of the materials have to be optimized. Furthermore, the negative effects of moisture, such as softening, creep and degradation, have to be limited. A better understanding of how design parameters such as choice of fibres and matrix material, fibre modifications and fibre orientation distribution affect the properties of the resulting composite material would help the development of wood-fibre composites. In this thesis, focus has been on the fibre-matrix interface, wood-fibre hygroexpansion and resulting mechanical properties of the composite. The importance of an efficient fibre-matrix interface for composite properties is well known, but the determination of interface properties in wood-fibre composites is difficult due to the miniscule dimensions of the fibres. This is a problem also when hygroexpansion of wood fibres is investigated. Instead of tedious single-fibre tests, more straightforward, macroscopic approaches are suggested. Halpin-Tsai’s micromechanical models and laminate analogy were used to attain efficient interface characteristics of a wood-fibre composite. When Halpin-Tsai’s model was replaced by Hashin’s concentric cylinder assembly model, a value of an interface parameter could be derived from dynamic mechanical analysis. A micromechanical model developed by Hashin was used also to identify the coefficient of hygroexpansion of wood fibres. Measurements of thickness swelling of wood-fibre composites were performed. Back-calculation through laminate analogy and the micromechanical model made it possible to estimate the wood-fibre coefficient of hygroexpansion. Through these back-calculation procedures, information of fibre and interface properties can be gained for ranking of e.g. fibre types and modifications. Dynamic FT-IR (Fourier Transform Infrared) spectroscopy was investigated as a tool for interface characterization at the molecular level. The effects of relative humidity in the test chamber on the IR spectra were studied. The elastic response of the matrix material increased relative to the motion of the reinforcing cellulose backbone. This could be understood as a stress transfer from fibres to matrix when moisture was introduced to the system, e.g. as a consequence of reduced interface efficiency in the moist environment. The method is still qualitative and further development is potentially very useful to measure stress redistribution on the molecular level.
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| 4. |
- Andersson, Rasmus
(author)
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Evaluation of two hydrocyclone designs for pulp fractionation
- 2010
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Licentiate thesis (other academic/artistic)abstract
- The process conditions and fractionation efficiency of two hydrocyclone designs, a novel and a conventional conical design, were evaluated. The novel design comprised a modified inlet section, where the pulp suspension had to pass a narrow ring-shaped opening, and a very compact fractionation zone. The influence of feed concentration and fine fraction mass ratio was studied. The trials were performed with never-dried, unrefined bleached chemical softwood pulp. Fractionation efficiency was evaluated in terms of change of surface roughness of handsheets made out of the fractions and the feed pulp respectively.The fractionation efficiency increased considerably with decreasing fine fraction mass ratio, especially at higher feed concentrations. This finding prompted a hypothesis on the existence of a radial gradient in the composition of the suspension inside the novel hydrocyclone. Using the novel hydrocyclone in a feed-forward fractionation system would therefore prove to be more favourable as a larger total fine fraction of better properties can be obtained. A three-stage feed-forward fractionation system was evaluated in laboratory scale. Here, it was indeed possible to extract fine fractions with improved surface properties in each of the three consecutive stages. All three fine fractions had about the same surface roughness.The fractionation performance of the novel design was benchmarked against that of a conventional, best available technology (BAT) design. In terms of fractionation efficiency, the BAT design performed better. However, the fractions produced with the novel hydrocyclone had a much smaller difference in concentration, implying a much less pronounced enrichment of fines in the fine fraction. It is unclear, to what extent the lower share of latewood fibres and the increased fines content, respectively, contributed to the improved surface roughness of the fine fractions. However, it is clear that the lower enrichment of fines in the novel hydrocyclone makes it easier to install it in industrial applications without a need for auxiliary equipment to redistribute large water flows.
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| 5. |
- Magnusson, Mikael S., 1984-
(author)
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Testing and Evaluation of Interfibre Joint Strength under Mixed-Mode Loading
- 2013
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Licentiate thesis (other academic/artistic)abstract
- The failure properties of interfibre bonds are the key for the build-up of strength in fibrous materials such as paper and paperboard. In order to tailor the properties of such materials by chemical or mechanical treatments and to learn how such modifications influence the properties at a microscopic level, direct measurement of individual fibre--fibre crosses are typically performed. However, the state of loading in the interfibre joint, in testing of individual fibre--fibre crosses, is in general very complex and a greater understanding for how to evaluate the mechanical properties of interfibre joints is desirable.In Paper A, a method for manufacturing multiple fibre--fibre cross specimens and a procedure for testing interfibre joints at different modes of loading is presented. The method is applied to investigate the strength of fibre-fibre crosses with different geometry and at two principally different modes of loading. Also, an investigation on the influence of drying pressure, the drying method as well as a comparison of pulp fibres from two different degrees of refining is presented. The force at rupture is scaled in terms of different geometric parameters; nominal overlap area, length and width of the joint region. It is shown that neither of the methods of scaling unambiguously reduced the coefficient of variation of the mean strength and that the force at rupture in a peeling type of loading was about 20% of the ones tested in the conventional shearing type of loading.In Paper B, a procedure for evaluating interfibre joint strength measurements in terms of resultant forces and moments at rupture is presented. The method is applied to investigate the state of loading in fibre-fibre crosses tested in two principally different modes of loading. It is shown that for a typical interfibre joint test, the modes of loading other than pure shear, cannot in general be neglected and is strongly dependent on the structural geometry of the fibre-fibre crosses. Also, the stress state in the interface centroid was estimated in order to quantify how the mode of loading influence the amount of normal stresses that develop in relation to the amount of shear stresses in the interfibre joint.
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| 6. |
- Tavares da Costa, Marcus Vinícius, 1989-
(author)
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Mechanical characterization and modelling of the fracture behaviour of thin brittle coatings on polymer films
- 2020
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Doctoral thesis (other academic/artistic)abstract
- Thin brittle coatings with thickness of a few nanometres deposited on flexible polymer substrates form an interesting material combination for food packaging and flexible electronics. This combination may enhance the barrier performance in carton packages. In flexible electronics, coatings could conduct electricity despite their small thickness. However, cracking is a concern when brittle coatings are subjected to tension and bending in the manufacturing process or in final products with curved shapes. Therefore, the fracture behavior of potential coatings for such applications was studied by a combined use of experimental, analytical and finite element methods.In the experimental work, the input parameters for nanometre-scale models were quantified. These parameters represent properties of the coating and of the interface between substrate and coating. The Young’s moduli of the coatings were estimated by use of three independent experimental methods: nanoindentation, buckling of coatings, and atomic force microscopy (AFM). The advantages and drawbacks of these methods were addressed for each material system. Then, multiple cracking of coatings subjected to uniaxial tensile loading was examined in situ by scanning electron microscopy using high magnification and resolution. The strain fields of the coated substrates were measured up to large tensile deformations by use of digital image correlation. Statistical micromechanical models were adapted and used for the multiple cracking behaviour of the coatings. Adhesive and cohesive properties, such as interfacial shear strength and coating strength, were determined. The multiple cracking behaviour was investigated also by using linear fracture mechanics and finite elements models to obtain energy release rates. The interfacial energy release rates were quantified also from local delamination of the coatings due transverse Poisson contraction of the substrates loaded in tension. Nanoindentation turned out to be a useful technique for measuring the Young’s modulus of coatings. Suitable indentation depth was found to be around 10% of the coating thickness. AFM was found to be the best method because it was found to minimize the viscous effect of the polymer. Because of delamination, buckling of coatings was not suitable for estimation of the Young’s modulus. However, ensuing ridged cracks were suitable to estimate interfacial energy release rates. The progressive crack density was examined in detail, and in high magnification details of the cracks were observed, such as the crack-opening displacements needed for prediction of barrier properties. Measured material properties on nanometre are useful for industrial implementation of coating technology and selecting coating materials for sustainable product design.
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| 7. |
- Vorobyev, Alexey, 1985-
(author)
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Static and time-dependent mechanical behaviour of preserved archaeological wood : Case studies of the seventeenth century warship Vasa
- 2017
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Doctoral thesis (other academic/artistic)abstract
- Wooden objects have been widely used in the history of humanity and play an important role in our cultural heritage. The preservation of such objects is of great importance and can be a challenging task. This thesis investigates the static and time-dependent mechanical behaviour of archaeological oak wood from the Vasa warship. Characterisation of mechanical properties is necessary for the formulation of a numerical model to design an improved support structure. The ship was impregnated with polyethylene glycol (PEG) for dimensional stabilisation. All elastic engineering constants of the Vasa oak have been identified and compared with those of recent oak by means of the static and dynamic testing. The experiments were done on samples with cubic geometry, which allowed obtaining all elastic constants from a single sample. The usage of cubic samples with orthotropic mechanical properties during compressive experiments was validated with finite-element simulations. The Young's moduli of the Vasa oak in all orthotropic directions were smaller than those for the recent oak. The shear moduli of Vasa oak was determined and verified with the resonant ultrasound spectroscopy. The time-dependent mechanical behaviour of the Vasa oak has been studied. Creep studies were performed in uniaxial compression on the cubic samples in all orthotropic directions. The samples loaded in the longitudinal direction were subjected to different stress levels. A stress level below 15% of the yield stress in the longitudinal direction did not result in non-linear creep with increasing creep rates within the time frame of the tests. The results of the studies in radial and tangential directions showed that creep was dominated by the effect of annual fluctuations in relative humidity and temperature. The weight changes based on annual fluctuations of relative humidity were measured for Vasa oak and recent oak. The Vasa oak showed higher variations due to an increased hygroscopicity which is the result of the impregnation with PEG. In conceiving a full-scale finite-element model of Vasa ship, not only the stress-strain relations of the material but also those of the structural joints are needed. Since the in-situ measurement of joints is not an option, a replica of a section of the ship hull was built and tested mechanically. The load-induced displacements were measured using 3D laser scanning which proved to have advantages to conventional point displacement measurements. The mechanical characteristics of the Vasa oak and joint information presented in this work can be used as input for a finite-element model of the Vasa ship for simulation of static and time-dependent behaviour on a larger scale.
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| 8. |
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| 9. |
- Bengtsson, Rhodel
(author)
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Creep aspects of softwood from the cell-wall level to structures
- 2023
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Doctoral thesis (other academic/artistic)abstract
- This thesis addresses the intricate mechanical behaviour of natural materials, with a particular focus on wood. Despite millennia of use, understanding the mechanical behaviour of wood materials remains challenging due to their complex microstructures. For instance, they exhibit variations in properties among samples, nonlinear behaviour under elevated loads, and are sensitive to alterations in moisture content.Wood and related natural biobased materials hold immense potential due to their renewability, cost-effectiveness, eco-friendliness, and ease of use in sustainable construction. Wood boasts remarkable stiffness and strength along its primary axis, surpassing many man-made materials in strength-to-weight ratios. However, its anisotropic and heterogeneous nature gives rise to challenges, necessitating the consideration of multiple parameters for accurate characterization to be used in design.Wood is intrinsically heterogeneous, leading to considerable variations in local stresses and deformations during loading. To address these microstructural effects on macroscopically measurable phenomena, mathematical homogenization methods, established since the 1970s, have found applications in material mechanics, including both fibre composites and wood.In recent years, there has been a growing focus on the viscoelastic behaviour of composites and timber structures, given their increased long-term use in load-carrying applications. While numerous investigations have explored the relationship between the microstructure of wood and its elastic properties, few studies have explored the connection between microstructure and viscoelastic properties.The thesis focuses on the static and, more notably, on the time-dependent mechanical properties of wood, bridging the gap from cell-wall creep to structures. It includes experiments and numerical work, culminating in the development of a material model suitable for orthotropic materials like wood. The multiscale model establishes a link between microstructural parameters and macroscopic properties, potentially applicable to various softwood species. Given the lack of shear creep data in the literature, the thesis introduces straightforward methods to characterize shear creep properties, addressing a significant knowledge gap.Furthermore, the thesis progresses from material-level experiments to higher length scales, demonstrating how the results can be applied to larger wooden structures, such as the tower for a counter-rotating axis tilted turbine. While these results require further validation in the absence of experimental data for wooden wind turbine structures, they offer useful insights into simulating creep behaviour in such applications.In conclusion, this thesis highlights the multifaceted nature of a natural material like wood, its mechanical challenges, and the promising research avenues for comprehensive understanding and practical use. The outcome provides contributions to the efficient utilization of wood in load-carrying structures and underlines the importance of ongoing research in this field.
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| 10. |
- Brandberg, August, 1990-
(author)
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Insights in paper and paperboard performance by fiber network micromechanics
- 2019
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Licentiate thesis (other academic/artistic)abstract
- Fiber networks are ubiquitous due to their low cost and high ratio of mechanical performance to weight. Fiber networks made of cellulose fibers from trees are used as information carriers (paper) and as packaging (board). Often the ideal product is both mechanically sturdy and possible to print on. This thesis investigates the underlying reasons for the mechanical performance of paper and board through the discretization and direct simulation of every fiber in the network.In Paper A the effect of fiber-fiber bond geometry on sheet stiffness is investigated. Many packaging products seek to maximize the bending stiffness by employing stiff outer layers and a bulkier layer in the middle. In bulky sheets, the fibers are frequently uncollapsed resulting in a more compliant bonded segment. Because all the loads in the network are transferred via the bonds, such compliance can cause unexpectedly large decreases in mechanical performance. Although many models have been presented which aim to predict the tensile stiffness of a sheet, these predictions tend to overestimate the resulting stiffness. One reason is that the bonds are generally considered rigid. By finite element simulations, we demonstrated the effect of the lumina configuration on the stiffness of the bonded segment on the scale of single fiber-to-fiber bonds, and that the average state of the fiber lumen has a marked effect on the macroscopic response of fiber networks when the network is bulky, has few bonds, or has a low grammage.Compression strength is central in many industrial applications. In paper B we recreated the short span compression test in a simulation setting. The networks considered are fully three-dimensional and have a grammage of 80 to 400 gsm, which is the industrially relevant range. By modeling compression strength at the level of individual fibers and bonds, we showed that fiber level buckling or bifurcation phenomena are unlikely to appear at the loads at which the macroscopic sheet fails.In paper C, we developed a micromechanical model to study the creation of curl in paper sheets subjected to a moisture gradient through the sheet. A moisture gradient is always created during the printing process, which may lead to out-of-plane dimensional instability. We showed that the swelling anisotropy of individual fibers bonded at non-parallel angles causes an additional contribution to the curl observed on the sheet level.
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