| 1. |
- Tojaga, Vedad
(författare)
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On fiber network fracture mechanics and kink band formation in biocomposites
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis summarizes seven appended papers dealing with: (1) The fracture of fibrous materials, e.g., paper and paperboard, toward understanding the upper limits of paper products and eventually optimizing packaging performance in its endeavor to replace plastics with recyclable packaging; (2) The compressive failure of flax fiber composites, a promising eco-friendly alternative to synthetic composite materials, toward understanding the low compressive-compared-to-tensile strength of biocomposites, a design-limiting feature, and ultimately engineer better performing natural fiber composites for sustainable structures. (1) In Paper I, we consider an elastoplastic Timoshenko beam finite element formulation with embedded strong discontinuities in the description of multi-fracturing fibers in fiber networks, a deficiency in previous studies. Seeing that the coupled (monolithic) problem is non-convex, materializing through poor robustness and undesirable material instabilities, we present an alternating minimization (staggered) algorithm for this class of problems and thus retain a positive definite stiffness matrix. In Paper II, we propose a hybrid of monolithic and staggered solution methods for robust and computationally efficient fracture simulations, with an up to 30-fold performance gain compared to the staggered approach in the benchmark exercises. The hybrid method represents a matrix regularization technique that retains a positive definite stiffness matrix while approaching the tangent stiffness matrix of the monolithic problem. In Paper III, we develop a geometrically nonlinear Simo-Reissner beam theory with embedded strong discontinuities based on the method of incompatible modes, capturing the activation of additional fibers during loading. We show that accounting for geometrical nonlinearity in the beam formulation is necessary for direct numerical simulations of fiber networks regardless of the density. (2) In Paper IV, we formulate a multi-scale homogenization framework for layered composite materials, where we model the instantaneous constitutive behavior of the matrix and the fiber separately utilizing a combined Voigt and Reuss approximation, followed by an upscaling to the composite. Advantages include the independence of fiber rotations because it is fully defined in the known initial configuration of the composite. In Paper V, we back-calculate the compressive stress-strain response of the flax fiber from the Impregnated Fiber Bundle Test (IFBT) in compression using the rule of mixtures, necessary input data in the micromechanical description of flax fiber composites. In Paper VI, we formulate hyperelastic models for deformation plasticity into the finite strain range. One application includes mimicking the stress-strain response of the fiber and the matrix in the homogenization of layered composite materials, which we numerically verify against a micromechanical model. In Paper VII, we extend the hyperelastic model to account for fiber damage. We show numerically and experimentally through X-ray Computed Tomography (XCT) and Scanning Electron Microscopy (SEM) that fiber damage plays the utmost role in the compressive failure of flax fiber composites – it is a major determinant of the material’s compressive stress-strain response. The micromechanisms include elementary fiber crushing and intra-technical fiber splitting.
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| 2. |
- Borodulina, Svetlana
(författare)
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Micromechanics of Fiber Networks
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The current trends in papermaking involve, but are not limited to, maintaining the dry strength of paper material at a reduced cost. Since any small changes in the process affect several factors at once, it is difficult to relate the exact impact of these changes promptly. Hence, the detailed models of the network level of a dry sheet have to be studied extensively in order to attain the infinitesimal changes in the final product.In Paper A, we have investigated a relation between micromechanical processes and the stress–strain curve of a dry fiber network during tensile loading. The impact of “non-traditional” bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds, is discussed. In Paper B, we studied the impact of the chemical composition of the fiber cell wall, as well as its geometrical properties, on the fiber mechanical properties using the three-dimensional model of a fiber with helical orientation of microfibrils at a range of different microfibril angles (MFA). In order to accurately characterize the fiber and bond properties inside the network, via statistical distributions, microtomography studies on the handsheets have been carried out. This work is divided into two parts: Paper C, which describes the methods of data acquisition and Paper D, where we discuss the extracted data. Here, all measurements were performed at a fiber level, providing data on the fiber width distribution, width-to-height ratio of isotropically oriented fibers and contact density. In the last paper, we utilize data thus obtained in conjunction with fiber morphology data from Papers C and D to update the network generation algorithm in order to produce more realistic fiber networks. We also successfully verified the models with the help of experimental results from dry sheets tested under uniaxial tensile tests. We carry out numerical simulations on these networks to ascertain the influence of fiber and bond parameters on the network strength properties.
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| 3. |
- Gimåker, Magnus, 1980-
(författare)
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Influence of fibre modification on moisture sorption and the mechanical properties of paper
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fibre modification might be a way to improve the performance of paper, to increase its cost competiveness and enable new paper-based products to be developed. Therefore, the influence of fibre modification (with polyelectrolytes or by fibre cross-linking) on the mechanical properties of special importance for packaging paper grades was studied. Creep deformation under varying humidity conditions (i.e. mechano-sorptive creep) is of outmost importance for the stacking life of paper-based boxes. The influence on creep behaviour of adsorbing polyallylamine (a cationic polyelectrolyte) to fibre surfaces or throughout the fibre walls was studied. Adsorption to fibre surfaces reduced the creep at constant humidity. The mechano-sorptive creep was not however influenced. The use of polyelectrolytes did not thus appear to be a feasible strategy for reducing mechano-sorptive creep. Polyelectrolytes can however be efficient in improving other mechanical properties. The use of multilayers consisting of polyallylamine (PAH) and polyacrylic acid (PAA) was for example shown to significantly increase the strength of paper with much less densification and build-up of residual stress than is obtained by beating. Cross-linking by oxidation with periodate radically decreased the mechano-sorptive creep of sheets made from the oxidised fibres. The basic mechanism behind the reduction in mechano-sorptive with cross-linking was found to be that the cross-linking slowed down the moisture sorption kinetics. A lower sorption rate led to smaller moisture content variations during the mechano-sorptive creep testing, and thus less sorption-induced swelling and stress concentrations at fibre/fibre joints. However, for cross-linking to be a practical way to reduce creep, the large problem of embrittlement must be solved. The shear strength of couched sheets was measured to study the interaction between the sheets at different solids content. The shear strength was low until a solids content of approximately 60−70% was reached, which suggests that interactions important for the strength at complete dryness start to develop at this solids content. The effect of different fibre modifications and additives on how the fibres interact during the consolidation process is not always well understood. The method of shear strength determination could in the future be applied to modified fibres to hopefully increase the understanding of how different modifications influence the fibre/fibre interactions. A deeper understanding might reduce the time for the development of new and improved fibre modifications.
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| 4. |
- Linvill, Eric
(författare)
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3-D Forming of Paper Materials
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper materials have a long history of use as a packaging material, although traditional paper-based packaging is limited in its shape, complexity, and design. In order to better understand the deformation and failure mechanisms during 3-D forming, two experimental studies of paper materials have been conducted. Furthermore, constitutive modeling combined with explicit finite element modeling have been validated against numerous experimental setups and utilized to develop further understanding of 3-D forming processes.Two experimental studies were necessary to further investigate and model the 3-D formability of paper materials. The combined effect of moisture and temperature on the uniaxial mechanical properties of paper was investigated, providing new insights into how moisture and temperature affect both the elastic and plastic properties of paper materials. Furthermore, the in-plane, biaxial yield and failure surfaces were experimentally investigated in both stress and strain space, which gave an operating window for 3-D forming processes as well as input parameters for the constitutive models.The constitutive modeling of paper materials and explicit finite element modeling were directed towards two 3-D forming processes: deep drawing and hydroforming. The constitutive models were calibrated and validated against simple (typically uniaxial) mechanical tests, and the explicit finite element models (which utilize the developed constitutive models) were validated against 3-D forming experiments. Hand-made papers with fibers partially oxidized to dialcohol cellulose, which has greater extensibility than typical paper materials, was furthermore characterized, modeled, and 3-D formed as a demonstration of the potential of modified paper fiber products for 3-D forming applications.
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| 5. |
- Esteves, Claudia, 1989-
(författare)
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Pulp strength enhancement by oxygen delignification
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Oxygen delignification is widely used in the pulp and paper industry as a part of delignification process between the kraft cook and bleaching. However, its potential has not been fully utilized. Rather than an intermediate process between cooking and bleaching, oxygen delignification is a strong oxidizing agent with powerful effects on the pulp properties. In this work, the hypothesis that oxygen delignification has the potential to improve the pulp mechanical properties was investigated. Several pulps were produced by either kraft cooking or kraft cooking combined with a subsequent oxygen delignification stage to a similar kappa number and their properties were analyzed and compared. This methodology assessed the real oxidative potential of oxygen on the final fiber properties. Total fiber charge, pulp mechanical properties, fiber morphology, swellability and fiber nanostructure, were studied.The major part of this research investigated the relationship between the carboxylic acid groups, seen as total fiber charge, and the mechanical strength of the paper. The total fiber charge was evaluated by conductometric titration and correlated with the pulp swellability and mechanical properties. It was demonstrated that oxygen delignification could significantly increase the charge content and the swelling of the pulp when an extended oxygen delignification (i.e, higher delignification degree) was used. In addition, the tensile index of the sheets increased when the fiber charge after oxygen delignification was sufficiently high. The swelling of the different pulps was investigated by Schopper-Riegler degree (SR°), water retention value (WRV) and fiber saturation point (FSP). It was determined that the higher the fiber charge, the higher the swelling ability, regardless of the lignin content. High alkali impregnation was utilized in this study due to its potential to increase cooking yield. The yield was compared to kraft pulp cooked with standard and high alkali impregnation, followed by oxygen delignification and bleaching. It was observed that the increase in yield was preserved in both unit processes, i.e., after oxygen delignification and after bleaching. During this work, pulp properties such as fiber morphology and fiber nanostructure were also important properties that were studied following each unit process and refining step. Oxygen-delignified pulps presented higher fiber deformation when compared to the kraft-cooked pulps. However, even with higher fiber deformation, oxygen-delignified pulps showed higher mechanical strength, contradicting previous reports that claimed lower pulp strength for oxygen-delignified pulps, due to fiber deformation. Additionally, it was found that fiber deformation tends to increase with PFI-refining for kraft-cooked pulps, while for oxygen and bleached pulps it tends to decrease. Fiber nanostructure was additionally studied by X-ray scattering, and the results obtained from pulp delignification by kraft and kraft followed by oxygen delignification were compared. This thesis highlights the benefits of increasing fiber charge by performing an extended oxygen delignification after a reduced kraft cooking. The results indicate that when oxygen-delignified pulps achieve 80 % higher fiber charge than kraft-cooked pulps at a similar kappa number, the pulp tensile index can be improved by up to 18 %. The oxidation reactions that occur during the oxygen delignification lead to a significant increase in the carboxylic acid groups in the fibers which increases the fiber's swelling ability and improves the refining process efficiency. The combination of those effects results in a higher tensile index and lower refining energy required. However, to obtain mechanical improvement, the oxygen delignification must be sufficiently long (extended). Therefore, it is believed that an extended oxygen delignification will yield a more uniform distribution of the charged groups in the fibers, which will increase the fiber swelling and fiber flexibility leading to a more efficient refining process and stronger fiber bonding structure in the paper.
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| 6. |
- Gard Timmerfors, Jessica, 1989-
(författare)
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Wood chips for kraft and sulfite pulping : evaluation of novel forest-industrial drum-chipping technology
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wood chipping and the supply of high-quality wood chips are of critical importance for most forest-industrial processes. The quality of wood chips affects product yield, product quality, and processability. Wood chips from a novel type of forest-industrial drum chipper, with a large drum and specially designed wood-chip channels, were evaluated with regard to wood chips for the Kraft and sulfite processes. Wood chips from a full-scale demonstration version of the drum chipper and from a conventional disc chipper at a Kraft mill were compared. The average bulk density and the fractions of oversized and overthick wood chips were similar, but the demonstration drum chipper produced 51% more large accept chips, 11% more total accept chips, and 74% less pin chips and fines. A pilot-scale drum chipper based on the new technology was used to produce short wood chips designed for acidic processes. When the drum velocity was 30-34 m/s and the average wood-chip length 21-22 mm, the fraction of pin chips and fines was 4.2% and the fraction of total accept was 89-90%. When the average wood-chip length was decreased to 17 mm, the fraction of pin chips and fines increased to 8.5% and the fraction of total accept decreased to 80-82%. The pilot drum chipper was used to investigate the influence of using different tree species (aspen, birch, pine, and spruce), processing of wood with different moisture content, and frozen wood. For hardwood (aspen and birch), the fraction of total accept reached ~90% when the average wood chip length was 17 mm. The pilot drum chipper was also used to generate wood chips of heartwood of pine for a comparison of 15 sulfite-process reaction conditions that differed with regard to impregnation and cooking procedures. The analyses included absorption of liquid in a specially designed impregnation reactor, pulp yield, reject, viscosity, kappa number, brightness, fiber properties, and chemical composition as determined using compositional analysis based on two-step hydrolysis with sulfuric acid and pyrolysis-gas chromatography/mass spectrometry. The results reveal in detail how the individual wood constituents were affected by the different treatments, and demonstrate the benefits of using a pressurized impregnation step prior to sulfite cooking.
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| 7. |
- Linvill, Eric, 1989-
(författare)
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Development of Finite Element Models for 3-D Forming Processes of Paper and Paperboard
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper materials have a long history of use in packaging products, although traditional paper-based packaging is limited in its shape and design. In order to enable more advanced paper-based packaging, various 3-D forming processes for paper materials have been studied. Since 3-D forming processes typically include the application of moisture and/or temperature, the effects of moisture and temperature on the mechanical response of paper have also been investigated.In Paper A, an experimental study of the combined effects of moisture and temperature on the uniaxial mechanical properties of paper was conducted. These experiments provided new insights into how moisture and temperature affect both the elastic and plastic properties of paper materials. These experiments also provided the framework from which the effects of moisture and temperature were modelled in Paper C.In Paper B, an explicit finite element model of the paperboard deep-drawing process was developed. An orthotropic material model with in-plane quadrant hardening was developed and verified for paper. The simulation results matched the trends from experimental deep-drawing up to when micro-scale wrinkling occured. Since most experimental failures occur prior to wrinkling, this model provided quantitative understanding of failure in the paperboard deep-drawing process.In Paper C, an explicit finite element model of paper hydroforming, utilizing the same material model for paper materials as in Paper B, was developed and verified. The simulation results matched well with experimental results, and a parametric study with the finite element model produced quantitative understanding of the hydroforming process for paper materials. Additionally, drying was identified as an important phenomenon for determining the extent of formability of paper materials.
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| 8. |
- Magnusson, Mikael S.
(författare)
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Interfibre Joint Strength under Mixed Modes of Loading
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The load carrying capacity of interbre joints are one of the key entities for build-up of strength inpaper materials. In order to gain insight in how to tailor the macroscopic properties of such materialsby chemical and/or mechanical treatments at a microscopic level, direct measurement of individualbre{bre crosses are typically performed. However, the state of loading in the interbre joint, intesting of individual bre{bre crosses, is in general very complex and an increased understandingfor how to evaluate the mechanical properties of interbre joints is desirable. In Paper A, amethod for manufacturing and measuring the strength of isolated interbre joints is presented. Themethod is applied to investigate the strength of bre{bre crosses at two dierent modes of loading.Also, an investigation on the manufacturing conditions is presented. The strength distribution ofindividually prepared bre{bre crosses is characterized and it was found that the median strengthin a peeling type of loading was about 20% compared to samples tested in the conventional shearingtype of loading. In Paper B, a procedure for evaluating interbre joint strength measurementsin terms of resultant forces and moments in the interbre joint region is presented. The methodis applied to investigate the state of loading in bre{bre crosses tested in peeling and shearing,respectively. It is shown that for a typical interbre joint strength test, the load components otherthan shear, cannot in general be neglected and is strongly dependent on the structural geometry ofthe bre{bre crosses. In Paper C, four distinctly dierent load cases; peeling, shearing, tearingand a biaxial type of loading was tested mechanically and evaluated numerically in order to gainmore information on how interbre joints behave in dierent modes of loading. In Paper D, thein uence of a chemical additive on the interbre joint strength is investigated on the microscopic(joint) scale and correlated to the eect previously observed on the macroscopic (sheet) scale. Xraymicrotomography and image analysis was used to understand structural changes in the brousnetwork in terms of the number of interbre joints as well as the average interbre joint contact area.The results showed that the median interbre joint strength increased by 18% upon adsorption, andthat the polyelectrolyte increased the number of contacts between the bres as well as an increasedarea of contact. In Paper E, the damage behaviour of individual interbre joints is analyzed. Froman extensive number of mechanical tests, the typical damage behaviour is identied and a failurecriterion is used to study the in uence of failure properties to give indications on how to tailor thematerial to optimize the joint strength.
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| 9. |
- Magnusson, Mikael S., 1984-
(författare)
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Testing and Evaluation of Interfibre Joint Strength under Mixed-Mode Loading
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)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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| 10. |
- Marin, Gustav
(författare)
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Impact of paperboard deformation and damage mechanisms on packaging performance
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper-based materials, such as paperboard, are commonly used as packaging materials. In addition to the fact that paper is renewable, there are also many other benefits of paperboard. From a mechanical point of view, paperboard has a high bending stiffness compared to its relatively low weight and high foldability, which are properties of significance in the design of packages. However, a distinct drawback of paperboard is its significant sensitivity to moisture. The moisture reduces the mechanical properties of the paperboard and consequently reduces the performance of the package. This thesis addresses the impact of paperboard deformation and damage mechanisms on packaging performance, with the characterization of the material properties as a starting point. Initially, the relations between moisture and different mechanical properties on a continuum material level were investigated. Then, experimental testing and finite element (FE) simulations were applied to evaluate these relations at the packaging design level.In Paper A, a material characterization was performed on five commercial paperboards with different basis weights, from the same producer. Five types of mechanical tests to characterize the paperboards material properties were performed:In-plane tensile test,Out-of-plane tensile test,Short-span Compression Test (SCT),Two-point folding,Double-notch shear test.All tests were performed at several levels of relative humidity (RH). Linear relations between the mechanical properties normalized with their respective value at 50 % RH and moisture ratio were found. Paper B examined whether the linear relationships discovered in Paper A are also valid for other paperboard series. Therefore, this study investigated 15 paperboards from four producers at the same RH levels as in Paper A. The paperboards were chosen to be different in furnish and construction, where four recycled boards were included. Here, the in-plane stiffnesses, strengths and SCT values were evaluated as a function of moisture. When the investigated paperboards’ moisture ratios were also normalized, all paperboards followed a linear master curve between normalized mechanical property and normalized moisture ratio. Additionally, a bilinear elastic-plastic in‑plane model was developed to predict the stress-strain relation of an arbitrary paperboard at an arbitrary moisture level without requiring mechanical testing except at standard conditions (50% RH, 23 °C).In Paper C, the master curve developed in Paper B was used to estimate material input parameters for simulating a Box Compression Test (BCT) at different moisture levels by using an orthotropic material model with a stress-based failure criterion, i.e., a relatively simple material model with few input parameters. The result showed that it was possible to accurately predict the load-compression curve of a BCT when accounting for moisture. Furthermore, it was concluded that modeling the creases’ mechanical properties is vital for capturing the stiffness response of the package. Here, a measurable approach for reducing the creases’ mechanical properties was suggested, based on a folding test to obtain the relative creasing strength (RCS) and a short-span tensile test to obtain the relative tensile strength (RTS). It should be emphasized that the model does not include any fitting parameters. All input data is based on measured values. Due to the importance of creases, the RCS and RTS ratios were investigated further in Paper D. When evaluated against normative shear strength during creasing, the RCS and RTS values together formed a creasing window, where the RTS values corresponded to in-plane cracks (upper limit) and the RCS values corresponded to delamination damage (lower limit). It was observed that both the lower and upper limits exhibit linear relations as functions of shear strain. Since creases have an evident effect on the packaging performance from a stacking point of view, it was interesting to investigate a load case exposing the package to shear. Therefore, an additional load case was investigated in Paper E: torsion of paperboard packages, where the experimental data was accurately predicted. Additionally, the effect of bending stiffness was investigated by developing two FE models. Model 1 (used in Paper C) treated the paperboard as a homogeneous material, and Model 2 considered the paperboard a three-ply laminate structure. No significant effect was noted, and it was concluded that the strength has a more significant effect on the BCT than the bending stiffness. It should also be mentioned that there were no problems with cracks when the paperboards were creased and mounted to packages used in Papers C and E. This correlates to the creasing window developed in Paper D since the creasing depth used for the packages is located within the creasing window.To conclude, the primary procedure in this thesis is developing an easy-to-use model with few material parameters that demonstrably can predict the load-deformation curves for two different load cases. The purpose of the model is not to be used for the precise prediction of failure loads but to gain knowledge about damage mechanisms during the testing procedures. A clear advantage of this approach is that the model can be used to either change the package’s geometry or perform a parametric study on the ingoing material parameters. This can also be varied for each ply separately, which helps converters and paperboard producers. It has also been shown that the model can account for different moisture levels if the master curve developed within this thesis is applied. Finally, it should be emphasized again that the model does not include any fitting parameters. All input data is based on measured values.
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| 11. |
- Alzweighi, Mossab
(författare)
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Modelling Fiber Network Materials:Micromechanics, Constitutive Behaviour and AI
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on understanding the mechanical behavior of fiber-based materials by utilizing various modeling approaches. Particular emphasis is placed on their structural variability, anisotropic properties, and damage behavior. Furthermore, the study explores moisture diffusion phenomena within these materials, leveraging machine learning techniques. The research employs a blend of multiscale modeling, experimental investigation, machine learning, and continuum modeling to enhance the predictive capabilities for modelling fiber-based materials.In Paper I, the work investigates the impact of stochastic variations in the structural properties of thin fiber networks on their mechanical performance. A multiscale approach that includes modeling, numerical simulation, and experimental measurements is proposed to assess this relationship. The research also considers the influence of drying conditions during production on fiber properties. The study finds that spatial variability in density has a significant impact on local strain fields, while fiber orientation angle with respect to drying restraints is a key influencer of the mechanical response. In Paper II, the research delves into the investigation of anisotropic properties and pressure sensitivity of fiber network materials. It draws a comparison between the Hoffman yield criterion and the Xia model, which are widely utilized for simulating the mechanical response in fiber-based materials. The study performs a detailed analysis of these models under bi-axial loading conditions, assessing their numerical stability and calibration flexibility. Further supporting the research community, the paper provides open-source access to the user material implementations of both models and introduces a calibration tool specifically for the Xia model, thereby promoting ease of usage and facilitating further research in this domain. In Paper III a novel thermodynamically consistent continuum damage model for fiber-based materials is introduced. Through the integration of elastoplasticity and damage mechanisms, the model employs non-quadratic surfaces comprised of multi sub-surfaces, augmented with an enhanced gradient damage approach. The model’s capability is demonstrated by predicting the nonlinear mechanical behavior under in-plane loading. This study provides valuable insights into the damage behavior of fiber-based materials, showcasing a range of failure modes from brittle-like to ductile. In Paper IV, the study examines moisture penetration in fiber-based materials and the resultant out-of-plane deformation, known as curl deformation, using a combination of traditional experiments, machine learning techniques, and continuum modeling. The paper compares the effectiveness of two machine learning models, a Feedforward Neural Network (FNN) and a Recurrent Neural Network (RNN), in predicting the gradient of the moisture profile history. The study finds that the RNN model, which accounts for temporal dependencies, provides superior accuracy. The predicted gradient moisture profile enables simulating the curl response, offering a deeper understanding of the relationship between moisture penetration and paper curling.
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| 12. |
- Brandberg, August, 1990-
(författare)
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Insights in paper and paperboard performance by fiber network micromechanics
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)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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| 13. |
- Eriksson, Daniel, 1987-
(författare)
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Getting to grips with cartons : Interactions of carbonboard packages with an artificial finger
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Packaging is an important part of most products in our modern world. It produces waste, but it also enables products to reach consumers safely and efficiently. Hence, the proper design of packaging is becoming increasingly important. Historically, cartonboard packages were designed for box compression strength. While this remains important, there are other types of loads that are important to consider. One such type of load arises from manual handling. As packages as moved and used, consumers need to exert forces on the package. These forces deform and can damage the package.Understanding these interactions can be challenging. By developing a method for quantifying the deformation due to manual handling, it becomes possible to measure and compare a redesigned package with the original to see if the performance has changed. This can aid packaging designers, but it can also be used for product control. The converting process is complex and deviations from specification can be introduced at many points along the production process.In this work, a method for quantifying interactions similar to those in manual handling is presented and evaluated. The method is then applied to study the effect of position and material properties on the mechanics of the interaction. The method is shown to have low variability and be robust to modifications in packaging and experimental design. It was seen that increasing the size of packages from 82 mm to 98 mm corresponded to decreasing the grammage by 10-20%. The method also showed the stiffening effect of corners and flaps, suggesting that the strategic placement of these design elements could help maintain the desired mechanical properties of the package at the point of interaction, provided the most likely point can be predicted.
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| 14. |
- Hagman, Anton, 1984-
(författare)
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Influence of inhomogeneities on the tensile and compressive mechanical properties of paperboard
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The in-plane properties of paperboard have always been of interest to paper scientists. Tensile properties are crucial when the board is fed through converting machines at high speeds. Compressive properties are essential in the later use. Inhomogeneities affect both the compressive and tensile properties. For the tensile properties, it is the inherent heterogeneity of the paperboard that might cause problems for the board-maker. Varying material properties, through the thickness of the paperboard, are on the other hand used to achieve high bending stiffness with low fiber usage. It is of interest to know how this practice affects the local compressive properties. Papers A and B aims to address this, while C, D and E focus on in-plane heterogeneities. Paper A investigates the mechanism that causes failure in the short span compression test (SCT). It was concluded that the main mechanism for failure in SCT is delamination due to shear damage. In paper B the effect of the through-thickness profiles on the local compression strength was examined. It was concluded that the local compression is governed by in-plane stiffness and through thickness delamination. The latter was in turn dependent on the local shear strength and in-plane stiffness gradients. In paper C the tensile test is investigated with focus on sample size and strain distributions. The strain behavior was dependent on the length to width ratio of the sample and was caused by activation of local zones with high strainability. Paper D focuses on the strain zones seen in C. The thermal response in paper was studied. It was observed that an inhomogeneous deformation pattern arose in the paper samples during tensile testing. It was concluded that the heat patterns observed coincided with the deformation patterns. It could be shown that the formation was the cause of the inhomogeneous deformation. In final paper, E, the virtual field method was applied on data from C.
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| 15. |
- Marin, Gustav
(författare)
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On the relation between paperboard properties and packaging performance
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper-based materials, such as paperboard, are commonly used as packaging materials. Inaddition to the advantage that wood as a raw material is renewable, there are also many otherbenefits of paperboard. From a mechanical point of view, paperboard has a high bendingstiffness compared to its relatively low weight and has a high foldability, which both areproperties of significance in the design of packages. However, a distinct drawback withpaperboard is its significant sensitivity to moisture. The moisture reduces the mechanicalproperties of the paperboard and consequently reduces the performance of the package. Thisthesis is starting with an investigation of the relation between moisture and differentmechanical properties on a continuum material level, and then these relations are applied onthe packaging design level through experimental testing and simulations.In Paper A, a material characterization was performed on a series of five paperboards withdifferent grammages from the same producer. Five types of mechanical tests to characterizethe paperboards’ material properties were performed:• In-plane tensile test,• Out-of-plane tensile test,• Short-span Compression Test (SCT),• Bending stiffness test,• Double-notch shear test.All tests were performed at several levels of relative humidity (RH). Linear relations betweenthe mechanical properties normalized with their respective value at 50 % RH and moistureratio were found.Paper B examined whether the linear relationships discovered in Paper A are true also forother paperboard series as well. Therefore, 15 paperboards from four producers wereinvestigated in this study, at the same levels of RH as before. Here, the in-plane stiffnessesand strengths and SCT-values were evaluated as a function of moisture. When also themoisture ratios in the investigated paperboards were normalized, it turned out that allpaperboards followed the same linear relationship between normalized mechanical propertyand normalized moisture ratio. Additionally, a bilinear elastic-plastic in-plane model wasdeveloped, that can predict the stress-strain relation of an arbitrary paperboard at an arbitrarymoisture level, and without requiring any mechanical testing except at standard condition(50% RH, 23 °C).In Paper C, this relation was used to estimate input material parameters for simulating a BoxCompression Test (BCT) at different moisture levels. The result showed that it was possibleto accurately predict the load-compression curve of a BCT when moisture was accountedfor.
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| 16. |
- Mattsson, Amanda
(författare)
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Characterisation of Time-dependent Statistical Failure of Fibre Networks : Applications for Light-weight Structural Composites
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The future of a sustainable society requires that materials not only be renewable, but also leave as small a carbon foot-printin the environment as possible. One such product is light-weight composite material for transportation packages. Cellulose fibres have been and will continue to be ideal for this purpose.The strength design of light-weight composites is becoming increasingly important. The challenge is to neither over- nor under-design, but instead to target the right strength underrealistic loading conditions. The question then is: What is right strength? Under realistic loading conditions (e.g., fatigue, random loading, and creep), materials fail differently from what one expects from tests of static strength: materials often fail at much lower stresses than are measured in these tests, the failure is time-dependent, and time to failure is highly variable. Therefore, to answer the above question, we have set up the following objectives: (1) theoretically formulate time-dependent statistical failure (TSF), and examine the validity of the model; (2) define material parameters describing the multi-faceted strength characteristics based on this formulation; (3) develop an experimental method to determine the material parameters; (4) investigate the impacts of fibre properties and network structures; and finally (5) characterise containerboard (the fibre material used in corrugated boxes) samples in terms of the new material parameters. The results for these five objectives are presented below, one by one.(1) A general formulation of TSF, originally proposed byColeman [1] for fibre failures, has been used. We have found that this model is indeed valid, even at the fibre network level, with only two restrictions: the existence of a lower bound onweakest-link scaling and an approximate nature of the Weibull distribution.(2) Accordingly, we have defined three material parameters that characterise different aspects of material strength: shortterm strength, durability/brittleness, and reliability. We call these parameters the new strength metrics.(3) Although the newly defined material parameters are most comprehensive, it takes up to several months to determine them by using creep tests. We have developed a new method, using constant loading rate (CLR) tests, that not only gives values comparable to those from creep tests, but also requires only about one day, allowing a drastic reduction in the testing time.(4) Monte-Carlo simulations of lattice networks have been performed to determine the basic relationships between fibre properties and network failures. The brittleness of an individual fibre (or a breaking element) influenced both brittleness and reliability of the fibre network, the higher the brittleness, the lower the reliability. Reliability, on the other hand, exhibited more intricate relationships with fibre properties and network structures. Several important analytical relationships have been derived.(5) Finally, using the CLR tests, we have characterised commercial containerboards in terms of the new strength metrics. Containerboard, as a cellulose fibre network, is quite comparable to typical stiff polymer-based fibre composites (e.g., glassfibres and aramid fibres). However, the reliability and durability/brittlenessof containerboard varied considerably within the operating windows, suggesting ample opportunities to fine-tune these properties even using current papermaking practices.The fact that the multi-faceted nature of strength can be expressed by three parameters is remarkable, and the implications are profound for how materials are designed and new materials developed. It is the author’s hope that this thesis will be of some use when it comes to redefining materials for a sustainable society, particularly the renewable alternative –cellulose fibres.
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