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  • 1.
    Agrawal, Vishal
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Scapin, Nicolo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway.
    An efficient isogeometric/finite-difference immersed boundary method for the fluid–structure interactions of slender flexible structures2024In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 418, article id 116495Article in journal (Refereed)
    Abstract [en]

    In this contribution, we present a robust and efficient computational framework capable of accurately capturing the dynamic motion and large deformation/deflection responses of highly-flexible rods interacting with an incompressible viscous flow. Within the partitioned approach, we adopt separate field solvers to compute the dynamics of the immersed structures and the evolution of the flow field over time, considering finite Reynolds numbers. We employ a geometrically exact, nonlinear Cosserat rod formulation in the context of the isogeometric analysis (IGA) technique to model the elastic responses of each rod in three dimensions (3D). The Navier–Stokes equations are resolved using a pressure projection method on a standard staggered Cartesian grid. The direct-forcing immersed boundary method is utilized for coupling the IGA-based structural solver with the finite-difference fluid solver. In order to fully exploit the accuracy of the IGA technique for FSI simulations, the proposed framework introduces a new procedure that decouples the resolution of the structural domain from the fluid grid. Uniformly distributed Lagrangian markers with density relative to the Eulerian grid are generated to communicate between Lagrangian and Eulerian grids consistently with IGA. We successfully validate the proposed computational framework against two- and three-dimensional FSI benchmarks involving flexible filaments undergoing large deflections/motions in an incompressible flow. We show that six times coarser structural mesh than the flow Eulerian grid delivers accurate results for classic benchmarks, leading to a major gain in computational efficiency. The simultaneous spatial and temporal convergence studies demonstrate the consistent performance of the proposed framework, showing that it conserves the order of the convergence, which is the same as that of the fluid solver.

  • 2.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Lai, J.
    SKF RTD, Meidoornkade 14, NL-3992 AE Houten, Netherlands..
    Loading rate and temperature effects on the fracture toughness of a high strength bearing steel2021In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 245, article id 107600Article in journal (Refereed)
    Abstract [en]

    Fracture of martensitic AISI 52100 steel with 12% retained austenite was experimentally studied at temperatures below the tempering temperature by K-Ic tests and at extremely low loading rates. Depending on temperature, K-Ic and J

  • 3.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Wiman, Jan
    Planning in Sweden2017In: Reshaping Regional Planning: A Northern Perspective, Taylor & Francis Group, 2017, p. 15-21Chapter in book (Other academic)
    Abstract [en]

    In principle, there is a municipal planning monopoly in Sweden, and the planning system is therefore basically designed for the municipalities. All municipalities must have a comprehensive plan that covers their entire area of responsibility. Although the plan is not binding, it must be kept up to date. The detailed development plan is a legally binding, executive planning instrument - a legal agreement between the municipality, the public and landowners - that makes it possible for the intentions of the comprehensive plan to be implemented. Special area regulations are also binding, and this form of planning is used within limited areas to guarantee compliance with certain comprehensive plan goals. A property regulation plan may be used to facilitate implementation of the detailed development plan. For the planning of matters that are of mutual interest to several municipalities, the national government may appoint a regional planning body with the task of monitoring regional questions and providing basic planning data for municipalities and Government authorities.

  • 4.
    Ali, Asad
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Michael, Danay
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    In-plane bi-axial testing of thin paper2022Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    During its lifetime, a beverage package is subjected to a complex loading history where the loading, in general, is multiaxial. Hence, to optimize the performance of the package, knowledge of the material’s anisotropic multi-axial deformation and subsequent failure is necessary. This report presents a way to investigate the anisotropic loading and fracture of thin paper using bi-axial testing and to analyse the bi-axial test using the Finite Element Method.

    Getting thin paper to fracture in the central region, where the deformation and loading are bi-axial, proved difficult. To overcome this challenge, a new bi-axial specimen, with a specific sample preparation technique, is proposed and implemented in this specific project. This cruciform-shaped thin paper specimen was reinforced with laminating plastic everywhere but in the central region. This was done to avoid material failure in the notched radius area or the clamps, rather to facilitate a material failure in the central region of the material specimen. In order to simulate the bi-axial test, a Hill elastic-plastic material model was calibrated, and the material parameters were obtained from uniaxial tensile tests.

    When subjected to bi-axial loading, the proposed cruciform-shaped specimen fractured systematically and repetitively for the different load cases studied. Moreover, the Hill material model captured the force vs. displacement curve from the experimental results well but overestimated its values. The overestimation was mainly due to sliding during the experimental tests.

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  • 5.
    Alloisio, Marta
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Chatziefraimidou, Marina
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Roy, Joy
    KI Karolinska Inst, Dept Mol Med & Surg, Solna, Sweden..
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Fracture of porcine aorta-Part 1: symconCT fracture testing and DIC2023In: Acta Biomaterialia, ISSN 1742-7061, E-ISSN 1878-7568, Vol. 167, p. 147-157Article in journal (Refereed)
    Abstract [en]

    Tissue failure and damage are inherent parts of vascular diseases and tightly linked to clinical events. Additionally, experimental set-ups designed to study classical engineering materials are suboptimal in the exploration of vessel wall fracture properties. The classical Compact Tension (CT) test was augmented to enable stable fracture propagation, resulting in the symmetry-constraint Compact Tension (symconCT) test, a suitable set-up for fracture testing of vascular tissue. The test was combined with Digital Image Correlation (DIC) to study tissue fracture in 45 porcine aorta specimens. Test specimens were loaded in axial and circumferential directions in a physiological solution at 37 & DEG;C. Loading the aortic vessel wall in the axial direction resulted in mode I tissue failure and a fracture path aligned with the circumferential vessel direction. Circumferential loading resulted in mode I-dominated failure with multiple deflections of the fracture path. The aorta ruptured at a principal Green-Lagrange strain of approximately 0.7, and strain rate peaks that develop ahead of the crack tip reached nearly 400 times the strain rate on average over the test specimen. It required approximately 70% more external work to fracture the aorta by circumferential than axial load; normalised with the fracture surface, similar energy levels are, however, observed. The symconCT test resulted in a stable fracture propagation, which, combined with DIC, provided a set-up for the in-depth analysis of vascular tissue failure. The high strain rates ahead of the crack tip indicate the significance of rate effects in the constitutive description of vascular tissue fracture.

  • 6.
    Alloisio, Marta
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Fracture of porcine aorta. Part 2: FEM modelling and inverse parameter identification2023In: Acta Biomaterialia, ISSN 1742-7061, E-ISSN 1878-7568, Vol. 167, p. 158-170Article in journal (Refereed)
    Abstract [en]

    The mechanics of vascular tissue, particularly its fracture properties, are crucial in the onset and progression of vascular diseases. Vascular tissue properties are complex, and the identification of fracture mechanical properties relies on robust and efficient numerical tools. In this study, we propose a parameter identification pipeline to extract tissue properties from force-displacement and digital image correlation (DIC) data. The data has been acquired by symconCT testing porcine aorta wall specimens. Vascular tissue is modelled as a non-linear viscoelastic isotropic solid, and an isotropic cohesive zone model describes tissue fracture. The model closely replicated the experimental observations and identified the fracture energies of 1.57±0.82 kJ m−2 and 0.96±0.34 kJ m−2 for rupturing the porcine aortic media along the circumferential and axial directions, respectively. The identified strength was always below 350 kPa, a value significantly lower than identified through classical protocols, such as simple tension, and sheds new light on the resilience of the aorta. Further refinements to the model, such as considering rate effects in the fracture process zone and tissue anisotropy, could have improved the simulation results. Statement of significance: This paper identified porcine aorta's biomechanical properties using data acquired through a previously developed experimental protocol, the symmetry-constraint compact tension test. An implicit finite element method model mimicked the test, and a two-step approach identified the material's elastic and fracture properties directly from force-displacement curves and digital image correlation-based strain measurements. Our findings show a lower strength of the abdominal aorta as compared to the literature, which may have significant implications for the clinical evaluation of the risk of aortic rupture.

  • 7.
    Alzweighi, Mossab
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Modelling Fiber Network Materials:Micromechanics, Constitutive Behaviour and AI2023Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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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  • 8.
    Alzweighi, Mossab
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Mansour, Rami
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Lahti, Jussi
    Graz Univ Technol, Inst Bioprod & Paper Technol, Inffeldgasse 23, A-8010 Graz, Austria.;CD Lab Fiber Swelling & Paper Performance, A-8010 Graz, Austria..
    Hirn, Ulrich
    Graz Univ Technol, Inst Bioprod & Paper Technol, Inffeldgasse 23, A-8010 Graz, Austria.;CD Lab Fiber Swelling & Paper Performance, A-8010 Graz, Austria..
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Lab Fiber Swelling & Paper Performance, A-8010 Graz, Austria..
    The influence of structural variations on the constitutive response and strain variations in thin fibrous materials2021In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 203, article id 116460Article in journal (Refereed)
    Abstract [en]

    The stochastic variations in the structural properties of thin fiber networks govern to a great extent their mechanical performance. To assess the influence of local structural variability on the local strain and mechanical response of the network, we propose a multiscale approach combining modeling, numerical simulation and experimental measurements. Based on micro-mechanical fiber network simulations, a continuum model describing the response at the mesoscale level is first developed. Experimentally measured spatial fields of thickness, density, fiber orientation and anisotropy are thereafter used as input to a macroscale finite-element model. The latter is used to simulate the impact of spatial variability of each of the studied structural properties. In addition, this work brings novelty by including the influence of the drying condition during the production process on the fiber properties. The proposed approach is experimentally validated by comparison to measured strain fields and uniaxial responses. The results suggest that the spatial variability in density presents the highest impact on the local strain field followed by thickness and fiber orientation. Meanwhile, for the mechanical response, the fiber orientation angle with respect to the drying restraints is the key influencer and its contribution to the anisotropy of the mechanical properties is greater than the contribution of the fiber anisotropy developed during the fiber sheet-making.

  • 9.
    Alzweighi, Mossab
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Mansour, Rami
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. Department of Mechanical and Production Engineering, Aarhus University, 8200 Aarhus N, Denmark.
    Maass, Alexander
    Institute of Bioproducts and Paper Technology, Graz University of Technology, Inffeldgasse 23, 8010 Graz, Austria, Inffeldgasse 23; CD Laboratory for Fiber Swelling and Paper Performance, 8010 Graz, Austria.
    Hirn, Ulrich
    Institute of Bioproducts and Paper Technology, Graz University of Technology, Inffeldgasse 23, 8010 Graz, Austria, Inffeldgasse 23; CD Laboratory for Fiber Swelling and Paper Performance, 8010 Graz, Austria.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Laboratory for Fiber Swelling and Paper Performance, 8010 Graz, Austria.
    Predicting moisture penetration dynamics in paper with machine learning approach2024In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 288, p. 112602-, article id 112602Article in journal (Refereed)
    Abstract [en]

    In this work, we predicted the gradient of the deformational moisture dynamics in a sized commercial paper by observing the curl deformation in response to the one-sided water application. The deformational moisture is a part of the applied liquid which ends up in the fibers causing swelling and subsequent mechanical response of the entire fiber network structure. The adapted approach combines traditional experimental procedures, advanced machine learning techniques and continuum modeling to provide insights into the complex phenomenon relevant to ink-jet digital printing in which the sized and coated paper is often used, meaning that not all the applied moisture will reach the fibers. Key material properties including elasticity, plastic parameters, viscoelasticity, creep, moisture dependent behavior, along with hygroexpansion coefficients are identified through extensive testing, providing vital data for subsequent simulation using a continuum model. Two machine learning models, a Feedforward Neural Network (FNN) and a Recurrent Neural Network (RNN), are probed in this study. Both models are trained using exclusively numerically generated moisture profile histories, showcasing the value of such data in contexts where experimental data acquisition is challenging. These two models are subsequently utilized to predict moisture profile history based on curl experimental measurements, with the RNN demonstrating superior accuracy due to its ability to account for temporal dependencies. The predicted moisture profiles are used as inputs for the continuum model to simulate the associated curl response comparing it to the experiment representing “never seen” data. The result of comparison shows highly predictive capability of the RNN. This study melds traditional experimental methods and innovative machine learning techniques, providing a robust technique for predicting moisture gradient dynamics that can be used for both optimizing the ink solution and paper structure to achieve desirable printing quality with lowest curl propensities during printing.

  • 10.
    Alzweighi, Mossab
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Mansour, Rami
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Tryding, Johan
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Evaluation of Hoffman and Xia plasticity models against bi-axial tension experiments of planar fiber network materials2022In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 238, article id 111358Article in journal (Refereed)
    Abstract [en]

    The anisotropic properties and pressure sensitivity are intrinsic features of the constitutive response of fiber network materials. Although advanced models have been developed to simulate the complex response of fibrous materials, the lack of comparative studies may lead to a dubiety regarding the selection of a suitable method. In this study, the pressure-sensitive Hoffman yield criterion and the Xia model are implemented for the plane stress case to simulate the mechanical response under a bi-axial loading state. The performance of both models is experimentally assessed by comparison to bi-axial tests on cruciform-shaped specimens loaded in different directions with respect to the material principal directions. The comparison with the experimentally measured forces shows the ability of the Hoffman model as well as the Xia model with shape parameter k≤2 to adequately predict the material response. However, this study demonstrates that the Xia model consistently presents a stiffer bi-axial response when k≥3 compared to the Hoffman model. This result highlights the importance of calibrating the shape parameter k for the Xia model using a bi-axial test, which can be a cumbersome task. Also, for the same tension-compression response, the Hill criterion as a special case of the Hoffman model presents a good ability to simulate the mechanical response of the material for bi-axial conditions. Furthermore, in terms of stability criteria, the Xia model is unconditionally convex while the convexity of the Hoffman model is a function of the orthotropic plastic matrix. This study not only assesses the prediction capabilities of the two models, but also gives an insight into the selection of an appropriate constitutive model for material characterization and simulation of fibrous materials. The UMAT implementations of both models which are not available in commercial software and the calibration tool of the Xia model are shared with open-source along with this work. 

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  • 11.
    Alzweighi, Mossab
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Tryding, Johan
    Division of Solid Mechanics, Lund University, Ole Römers väg 1, 223 63 Lund, Sweden;Tetra Pak, Ruben Rausings gata, 221 86 Lund, Sweden.
    Mansour, Rami
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. Department of Mechanical and Production Engineering, Aarhus University, 8200 Aarhus N, Denmark.
    Borgqvist, Eric
    Tetra Pak, Ruben Rausings gata, 221 86 Lund, Sweden.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Anisotropic damage behavior in fiber-based materials: Modeling and experimental validation2023In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 181, article id 105430Article in journal (Refereed)
    Abstract [en]

    This study presents a thermodynamically consistent continuum damage model for fiber-based materials that combines elastoplasticity and damage mechanisms to simulate the nonlinear mechanical behavior under in-plane loading. The anisotropic plastic response is characterized by a non-quadratic yield surface composed of six sub-surfaces, providing flexibility in defining plastic properties and accuracy in reproducing material response. The damage response is modeled based on detailed uniaxial monotonic and cyclic tension-loaded experiments conducted on specimens extracted from a paper sheet in various directions. To account for anisotropic damage, we propose a criterion consisting of three sub-surfaces representing tension damage in the in-plane material principal directions and shear direction, where the damage onset is determined through cyclic loading tests. The damage evolution employs a normalized fracture energy concept based on experimental observation, which accommodates an arbitrary uniaxial loading direction. To obtain a mesh-independent numerical solution, the model is regularized using the implicit gradient enhancement by utilizing the linear heat equation solver available in commercial finite-element software. The study provides insights into the damage behavior of fiber-based materials, which can exhibit a range of failure modes from brittle-like to ductile, and establishes relationships between different length measurements.

  • 12.
    Andreolli, Raphael
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    The influence of post-buckling damage on the tensile properties of single wood pulp fibers2021Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The rapid growth of plastic waste from food packaging around the world demands renewable substitutes, such as natural fibers and biocomposites. Wood fibers are natural fibers extracted from trees and are commonly used in packaging. In order for renewable alternatives to compete against plastics and other non-renewable materials, a better understanding of the mechanical properties of single fibers at the micro-scale are necessary. A great deal of previous research into the mechanical properties of single wood fibers has focused on their tensile behavior, however, little work has been published about their compressive behavior. It is difficult to measure the compressive strength of single fibers directly due to fiber buckling.

    The purpose of this study is to investigate how post-buckling of single wood pulp fibers affects the mechanical properties of fibers in tension. Two alternative hypotheses were tested through experiments in The Odqvist Laboratory for Experimental Mechanics at KTH. The major part of the thesis process has been invested in developing components called grippers, and testing methods for the Single Fiber Testing System, in order to be able to perform the experiments. The existing grippers were tested and alternative grippers were developed, as well as an alternative testing method without grippers, called the Paper frame method (PFM). PFM was used in the final experimental work to test the hypotheses.

    The main finding from this study is that there is not enough evidence to suggest that the tensile strength or tensile stiffness of single wood fibers are significantly reduced by post-buckling damage. This finding is mostly relevant in the research and development of fibrous material with larger distances between individual fibers, such as low-density fiber network materials. The main findings from the single fiber testing methods development were that the existing grippers cannot prevent fiber slippage. Furthermore, the alternative gripper 22A with its arc design generates higher grip force than previous grippers but lacks surface friction in the contact region in order to prevent fiber slippage. PFM has an experimental success rate of over 80 % for trained users and easy usage for the operator. The testing equipment Single Fiber Testing System displays several systematic errors occurring in the post-processing process of tests with cyclic loads.

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  • 13.
    Arasu, Karthickeyan
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Fatigue life of butt welds - a numerical study on the influence of real geometryIndependent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Welds play a crucial role in the product portfolio of GKN Aerospace. For ease of manufacturing and repairability, butt welds are preferred in the aerospace industry. Weld regions undergo local distortions during manufacturing due to the external heat-input, and the resulting local geometries are complex and stochastic in nature. Under operational loads, the distorted geometry affects the local stress field around the weld region, and this has a significant impact on the fatigue life.

    Traditional design calculations of welds resort to idealization of the local weld geometry. In this thesis, the influence of real weld geometry on the computed fatigue life is investigated. Linear elastic fracture mechanics principles are utilized to calculate the fatigue life of a weld starting from a pre-defined initial crack. The influence of important weld geometric parameters, namely, – edge offset and weld toe radii, on the fatigue life is investigated in detail. A statistical analysis approach, using transfer functions and Monte Carlo simulation, is devised to study the effect of variation in the different weld geometric parameters. Different edge offset measures from a real geometry are identified and these measures are investigated as potential candidates to obtain conservative life estimates using the idealized geometry.

    Investigations in this thesis show that the real geometry has a significant effect on the weld fatigue life. In all cases of local weld geometry, the edge offset has the largest influence on life. For a real weld geometry, the root toe radius has a significant influence on life. In an idealized geometry, for the same normalized edge offset, an increase in the plate thickness leads to a decrease in life. A new edge offset measure is proposed that leads to conservative life estimates when used in conjunction with the idealized geometry, thus enabling computationally efficient design calculations.

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  • 14.
    Asbjörnsson, Sverrir
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    The Sacrificial Casing Project: Optimization and Design of a Fastening Mechanism2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Power companies in the geothermal industry face well-known challenges when exploiting high temperature geothermal reservoirs. To extract the geothermal energy from the reservoirs, geothermal wells are drilled to retrieve the geothermal steam to produce both electricity and hot water for district heating. The geothermal steam can be both highly corrosive and cause high thermal expansions in the steel casings which can result in high repair costs. The Icelandic R&D company Gerosion is developing a protective casing for high temperature wells, called the Sacrificial Casing. The product is a multi-layered design and can protect the other casing strings from corrosion and high thermal expansions. This thesis project aims to design and optimize a new fastening mechanism for the Sacrificial Casing, a so called slip mechanism. The material selection and geometrical dimensions of the mechanism are explored based on computational results. The most feasible design for installation of the mechanism is then used to construct both 2D and 3D FE-models where the mechanism is subjected to high temperatures and pressure and the load-carrying capabilities of the design analyzed. The results indicate that thermal expansions cause extremely high stresses in the mechanism and as a result the S960 high strength steel grade does not offer sufficient strength to withstand the combined loading the mechanism is subjected to.

  • 15.
    Asta, Nadia
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology.
    Kaplan, Magdalena
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Wågberg, Lars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology.
    Influence of density and chemical additives on paper mechanical propertiesManuscript (preprint) (Other academic)
  • 16.
    Atoufi, Zhaleh
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology.
    Gordeyeva, Korneliya
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fiberprocesser.
    Cortes Ruiz, Maria F.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology.
    Hall, Stephen A
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Larsson, Per A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology.
    Wågberg, Lars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology.
    Wet-resilient foams based on heat-treated β-lactoglobulin and cellulose nanofibrilsManuscript (preprint) (Other academic)
  • 17.
    Babbepalli, Venkata Venu Sai Phani Ram
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Implementation of moisturedependent constitutive model for paperboard2020Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    There has been a considerable increase in the usage of paper products due to its sustainability in the product cycle. Many environmental and process variables can affect the mechanical behavior of paper from its making to finished products. Of these variables, moisture is of particular importance and strongly influences both papermaking, converting, and end-use of the paper products.

    Experimental investigations at different humidity levels reveals that normalized in-plane constitutive parameters, such as elastic parameters and the linear hardening modulus, in both MD and CD1) follow a linear relationship with normalized moisture ratio. This relation is found to be acceptable for a wide range of commercial paperboards. To capture this observation, a novel material model with orthotropic elasticity and anisotropic hardening2 is proposed. An associative flow rule for the evolution of plastic strain is proposed. The proposed flow rule is such that all stresses contribute to plastic flow rather than an effective stress. A simple version using anisotropic linear hardening is implemented. The mechanical properties, such as elastic parameters and hardening moduli are considered functions of the moisture ratio. An implicit variant of the material model is implemented in LS-DYNA®. The simulations with the proposed material model at different humidity levels follow the experimental results well for uniaxial loading, but discrepancies are obtained for simulation of biaxial loading tests.

    The moisture is assumed constant in the proposed model since the experiments are done in a moisture-controlled environment.

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  • 18.
    Bayat, Mariam
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Pongpairote, Nichakarn
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.
    Arm Injury Prediction with THUMS SAFER: Improvements of the THUMS SAFER upper extremity2020Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Globally, approximately 1.2 million people die each year due to traffic accidents. Upper extremity injuries account for 18% to 25% of all car accident injuries. In order to be able to analyze these crash-related injuries, Human body models(HBMs) are used as a complement to FE simulations. An example of a HBM is the THUMS SAFER that is based on a 50 percentile American male. The aim of this study was to improve the upper extremity of the THUMS SAFER with respect to Autoliv's requirements to better predict fractures. In addition, this was validated against the Forman experiment(Forman, et al., The journal of trauma and acute care surgery, vol. 77, 2014) where human cadavers of the upper extremity were axially impacted to replicate a car collision. This was done by generating the upper extremity geometry with segmentation of medical images of a right human hand in combination with the complete STL-geometry of the forearm from the Piper project. The STL-geometry of the segmented human hand and Piper forearm was integrated and a complete STL-geometry of the upper extremity was obtained. Based on the complete STL-geometry, the FE-arm HEX 4.0 was built with modelling of bones, ligaments, soft tissue and skin with corresponding material choice in accordance with Autoliv's requirements.

    The model HEX 4.0 was improved considering an increased mesh density from an average of 94% to 98%. HEX 4.0 was also validated against the data from the Forman experiment for experiments 5, 6 and 15. It showed a good correlation with the acceleration curves between the simulated and experimental values for the three experiments. The reaction force in the elbow was compared for experiment 15, where the simulated value 5.7 kN divided by a factor of 1.4 from 4 kN for the experiment. Furthermore, the fi rst principal strains that occurred in HEX 4.0 were analysed by 17 ms were the highest acceleration was achieved for experiments 5 and 6. Both experiments were shown to be close to the failure threshold of bones. However, the highest value e5=9.8E-03 occurred in the radius for experiment 5, while e6=9.3E-03 in a ligament for experiment 6. In addition, the failure threshold for experiment 15 exceeded 5 ms in lunate, schapoid and triquetrum. This indication of fractures is in good agreement with the experimental results where the corresponding bones resulted in fractures in experiment 15. HEX 4.0 was an improved upper extremity of the THUMS SAFER considering an increased mesh density. It is also capable of indicating fractures and corresponding positions in the form of analyzes of occurring stresses and strains. Nevertheless, improvements and further validation of HEX 4.0 has been proposed in the future work section.

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  • 19.
    Berggren, Filip
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Fretting Fatigue Life Analysis of Additively Manufactured Titanium for Compressor Blades2021Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Additive manufacturing (AM) is rising in popularity as a manufacturing technique of industrial components. Compressor blades attached using dove tail joints are a common source of fretting damage and in order to evaluate AM as a manufacturing technique, its fretting fatigue behaviour must be studied. This report treats the fretting fatigue behaviour between AM electron-beam melted (EBM) titanium alloy (Ti-6Al-4V) and forged steel alloy (22NiCrMoV12-7). AM Ti-6Al-4V is compared with forged Ti-6Al-4V to analyse AMs feasibility as an optional manufacturing technique of compressor blades. Further, as AM causes the material to be anisotropic, different print orientations as well as heat-treatments are compared. Performing fretting fatigue and crack propagation experiments alongside computations, through a finite element model and the crack propagation software NASGRO, show that AM Ti-6Al-4V performs similarly to that of forged Ti-6Al-4V. The study concludes that different print orientations from manufacturing show no significant impact on fretting fatigue life. However, the non-heat-treated specimens in the experiments perform slightly better than those that were heat-treated.

  • 20.
    Birgersson, F.
    et al.
    Scania Tech Ctr, SE-15148 Södertälje, Sweden..
    Elmen, P. Mikaelsson
    Scania Tech Ctr, SE-15148 Södertälje, Sweden..
    Andersson, T.
    Scania Tech Ctr, SE-15148 Södertälje, Sweden..
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Measurements and simulations of sliding wear, leakage and acoustic isolation of engine rubber gaskets2020In: International Journal of Vehicle Design. Heavy Vehicle Design, ISSN 1744-232X, E-ISSN 1741-5152, Vol. 27, no 4, p. 549-564Article in journal (Refereed)
    Abstract [en]

    Design and verification of gasket elements between engine mounted components requires computation and physical tests with respect to wear. Wear is a common problem in engines today and mainly comes from engine vibrations and thermal loading. The vibrations are due to inertial loads as well as reaction forces to gas pressure in the cylinders. Here, a new method to correctly simulate the measured wear rate of an oil pan rubber gasket is described. The engine motion is derived directly from measurements and the resulting simulation process has a high efficacy. Much work exists on investigations of gasket sealings between cylinder head and block, where the thermal loading becomes very important. The method described herein, focuses instead on other types of gaskets on the engine, where the main failure mode is due to sliding wear caused by the engine block and component vibrations.

  • 21.
    Blomqvist, Fredrik
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    FEM (Finite Element Method) Optimization of Load Bearing Nodular Cast Iron Component in Truck Chassis: Topology Optimization with Reaction Force Constraints on Simplified Bolt Models2020Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In the automotive industry, there is great competition as the quality of vehicles constantly improves while at the same time fulfilling the emission and sustainability requirements. In the truck industry, Scania is a market leader who provides durability, drivability, comfort and low fuel consumption. For some trucks used in South Latin America, they failed because of fracture at one side of the truck’s frames. This was due to excessive stresses that arose within the frame because of extreme loads in the vicinity of the air spring bracket. The investigation of this thesis is whether redesign of this component would decrease the stress levels within the frame. The approach to redesign the component was solely by the use of Finite Element Analysis (FEA) and topology optimization. The optimization was formulated as minimize the compliance subject to constrained volume but also constrained reaction force at bolts that experienced higher loads. Additionally to the formulation, manufacturing constraints were applied such as casting, minimum member size and symmetry. Further, the approach was to setup a simplified Finite Element (FE)-model of the chassis including the air spring bracket to which topology optimization was employed. Based on the results of the optimization, realisation of the topologies were produced and evaluated to what extent they affected the stresses within the frame. From these results a relative change of a maximum stress-measure was obtained between -7.1 % to 2.7 % based on constraining either two, four or five bolts. Regarding the mass, a relative change in the range of -11.3 % to -5.7 % corresponding to a reduction of 3.2 kg and 1.6 kg respectively, was obtained. Conclusively, performing topology optimization such as minimize compliance with bolts’ reaction forces constrained may result in reduced stress levels in adjacent components. It would be interesting to see if a further decrease in stress levels could be obtained using other topology optimization setups or other optimization methods. Further if the interpreted designs candidates to be implemented in production, some validation of these first needs to be considered such as whether the manufacturing costs are compensated by the reduced mass and stress levels. Additionally some validation of the strength should be performed for the realised geometries.

  • 22.
    Blomström, Johan
    et al.
    Dept. of Maintenance, Analytical Services, Ringhals AB, SE-43285 Väröbacka, Sweden.
    Roudén, Jenny
    Dept. of Maintenance, Analytical Services, Ringhals AB, SE-43285 Väröbacka, Sweden.
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. Dept. of Maintenance, Analytical Services, Ringhals AB, SE-43285 Väröbacka, Sweden.
    Experience with Embrittlement Trend Curves in Swedish PWRs2023In: Radiation Embrittlement Trend Curves and Equations and Their Use for RPV Integrity Evaluations, ASTM International , 2023, Vol. STP 1647, p. 382-397Conference paper (Refereed)
    Abstract [en]

    There are currently two operating pressurized water reactors in Sweden, currently planning for 60 years of operation until 2041 and 2043. The acceptance of operation time is continuously evaluated at least every 10 years in a comprehensive mandatory periodic safety review that requires the utilities to continuously update and implement the developments in science and technology. The RPV welds have been shown by the applied surveillance program to be the limiting material for operation. The welds are manufactured according to the same specifications with a chemical composition with high nickel and manganese content. The welds show a large increase in transition temperature shift with an almost linear relationship to neutron fluence that is underestimated by most of the established embrittlement trend curves (ETCs). The current regulations from the Swedish Radiation Safety Authority are in general not detailed and prescriptive and hence permit plant-specific ETCs if they are sufficiently justified and based on proper material and plant conditions. This paper describes the bases for the ETCs and an ongoing work to revise the ETCs to enable the use of a material-specific master curve for crack initiation, KIC, with compliance with the reactor vessel integrity analyses.

  • 23.
    Boestad, Albin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Investigation of the Applicability of Fracture Mechanics for Tissue Paper2023Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Tissue paper is a common type of paper material and is used in a variety of products. For tissue paper, several properties are of interest, such as absorbency, softness, bulk and mechanical properties. Embossing is an operation used to apply a pattern on tissue paper. It is used to improve several properties, but is known to reduce mechanical properties. Currently, no models can predict the loss of strength due to embossing. In this report base tissue paper is embossed with two different embossing patterns and tensile tests are conducted with and without edge notches. The edge notch length was varied between 0 mm to 12 mm. From the experiment, a modified Linear Elastic Fracture Mechanics model was applied on both base tissue paper and embossed tissue paper tensile test results. The experimental procedure is described. In total, four different paper qualities were tested. Two that are designed for toilet paper and two that are designed for kitchen paper. The tissue sheets were embossed using 3D-printed plates and conducted in a laboratory environment. Tensile tests with edge-notch specimens were performed. The notch lengths tested were between 0 mm and 12 mm long. It was investigated if any trends of the parameters in the model could be noticed due to embossing. The model worked well for all base tissue qualities. The embossing reduces the material's tensile strength compared to the base material. With increasing embossing load, longer notches are needed to drop the tensile strength of the specimen. Some general trends were noted. However, the impact of the embossing was different for different paper qualities and the embossing pattern used. The most significant difference between plates was noted in specimens with high embossing load. With increasing embossing load, the edge-notch must also be longer to reduce tensile strength. The model parameters changed more for machine direction (MD) specimens than crossmachine direction (CD) specimens.

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  • 24.
    Boivie, Tove
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Modelling of a Glued Bolt Joint in Finite Element2023Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This report presents a master thesis project at KTH at the school of Engineering Science. The project is done in collaboration with ABB Robotics, a company who develops industrial robots.

    A robot consists of several parts and the last structural part is the wrist. During the project, a glued bolt joint, commonly named hybrid joint, in the wrist was investigated. The glued bolt joint mounts a protecting cover to the wrist and is a vital component to prevent leakage and provide structural strength to the wrist. The project's purpose was to create a time efficient and accurate methodology how to model the glue bolt joint in Finite element.

    Four different modelling methods of the adhesive layer were developed. One method includes friction contact, one method includes glue spring contact, one method includes solid element, and the last method includes cohesive elements. The developed methods were based on the pre-study and a previous master thesis project collaborated with the company. The modelling methods were verified against lab measurement where stresses in the wrist and bolt stresses were measured. Furthermore, to include the bolt preload in the models, a modelling and post-processing method was developed.

    The results showed similar simulated and measured von Mises stress in the structure of the wrist. However, the simulated bolt stress was significantly higher compared to the measured bolt stress for all methods. The main conclusion with the project is that none of the methods is accurate enough and further verification of the methods needs to be done before an accurate method can be determined.

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  • 25.
    Boåsen, Magnus
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Modeling of structural integrity of aged low alloy steels using non-local mechanics2020Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ageing of low alloy steels affects the structural integrity assessment as it most commonly causes embrittlement and a hardening of the material. This is due to theevolution of the microstructure during operation in the specific application. In nuclear applications, the most common causes of ageing of low alloy steels areirradiation and thermal ageing. Embrittlement in this type of materials is generally divided into hardening and non-hardening embrittlement. The formation of clusters or precipitates of solute atoms typically cause the former, and the weakening of grain boundaries generally cause the latter. This thesis is devoted to thedevelopment of models that can be used to describe the material properties of aged low alloy steels in terms of plastic properties and fracture toughness, and to thestudy of the effects of thermal ageing on the mechanical properties of a low alloy steel.

    In Paper I, a strain gradient plasticity framework is applied in order to capture length scale effects. The constitutive length scale is assumed to be related to the dislocation mean free path and the changes it undergoes during plastic deformation. Several evolution laws for the length scale were developed and implemented in a FEM-code. This was used to solve a test problem in order to probe the effects of the length scale evolution. All length scale evolution laws considered in this study results in a decreasing length scale, which causes an overall softening in cases where the strain gradient dominates the solution. The results are in tentative agreement with phenomena of strain localization that occurs in highly irradiated materials.

    In Paper II, a scalar stress measure for cleavage fracture is developed and generalized, here called the effective normal stress measure. This is used in a nonlocal weakest link model which is applied to two datasets from literature in order to study the effects of the effective normal stress measure, as well as to experiments considering four-point bending of specimens containing a semi-elliptical surface crack. The model is shown to reproduce the failure probability of all considered datasets, i.e. well capable of transferring toughness information between different geometries.

    In Paper III, a thermally aged weld from the Ringhals nuclear power plant is studied experimentally and compared to a reference material using fracture toughness testing. The main objective of the study was to investigate the effect of thermal ageing on the cleavage or brittle fracture toughness, with a specific focus on the effect of crack tip constraint. The testing showed that thermal ageing had enabled brittle fracture initiation from grain boundaries, resulting in a bimodal toughness distribution due to multiple mechanisms for brittle fracture initiation.

    In Paper IV, the non-local weakest link model in Paper II is further developed to account for multiple mechanism brittle fracture. The model is developed for brittle fracture initiation from grain boundaries and second phase particles. The grain boundary mechanism is inferred from simulations of polycrystalline aggregates using crystal plasticity. When applied to the experimental results of Paper III, the model is able to describe the fracture toughness distribution with a remarkable accuracy.

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    Boåsen [2020] Modeling of structrual integrity of aged low alloy steels using non-local mechanics
  • 26.
    Boåsen, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Dahlberg, Carl F. O.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    A weakest link model for multiple mechanism brittlefracture - Model development and application2020Report (Other academic)
    Abstract [en]

    A multiple mechanism weakest link model for intergranular and transgranularbrittle fracture is developed on the basis of experimental observations in a thermallyaged low alloy steel. The model development is carried out in tandemwith micro mechanical analysis of grain boundary cracking using crystal plasticitymodeling of polycrystalline aggregates with the purpose to inform theweakest link model. The fracture modeling presented in this paper is carriedout by using a non-local porous plastic Gurson model where the void volumefraction evolution is regularized over two separate length scales. The ductilecrack growth preceding the nal brittle fracture is well predicted using this typeof modeling. When applied to the brittle fracture tests, the weakest link modelpredicts the fracture toughness distribution remarkably well, both in terms ofthe constraint and the size eect. Included in the study is also the analysis of areference material.

  • 27.
    Boåsen, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Lindgren, Kristina
    Chalmers, Microstructure Physics, Department of Physics .
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Thuvander, Mattias
    Chalmers, Microstructure Physics, Department of Physics .
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Analysis of thermal embrittlement of a low alloy steel weldment using fracture toughness and microstructural investigations2020Report (Other academic)
    Abstract [en]

    A thermally aged low alloy steel is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of thermal embrittlement on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal as a result. The consequence is that the constraint effect is significantly reduced when grain boundary initiation dominates the toughness distribution, as compared to the reference material where the constraint effect is significant. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Cu-rich clusters are found primarily situated on dislocation lines.

  • 28.
    Boåsen, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Lindgren, Kristina
    Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden..
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Thuvander, Mattias
    Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden..
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Analysis of thermal embrittlement of a low alloy steel weldment using fracture toughness and microstructural investigations2022In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 262, article id 108248Article in journal (Refereed)
    Abstract [en]

    A thermally aged low alloy steel weld metal is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of embrittlement due to thermal ageing on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. The comparison of the investigated materials has been made at temperatures that give the same median fracture toughness of the high constraint specimens, ensuring comparability of the low constraint specimens. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal. Consequently, this appears to reduce the facture toughness of the low constraint specimens of the aged material as compared to the reference material. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Ni-Mn-rich clusters, precipitated during ageing, are found primarily situated on dislocation lines.

  • 29.
    Brandberg, August
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Micro-mechanical characterization and modeling of paper and paperboard2021Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fiber networks made of cellulose fibers from trees are used as information carriers (paper) and as packaging (paperboard). This thesis investigates the mechanical performance of paper and paperboard via micro-mechanical modeling and presents new methods for the mechanical characterization of the micro scale, necessary in such models.        In Paper A the effect of the fiber-fiber bond geometry on the sheet stiffness is investigated. In thick, low density sheets, the fiber lumen remains open resulting in a more compliant bonded segment. By finite element simulations, we demonstrate the effect of the lumen configuration on the stiffness of the bonded segment. Most important for the stiffness of the segment is the average state of the fiber lumen which has a marked effect on the macroscopic response of fiber networks when the network is sparse.        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 three-dimensional and have a grammage of 80--400 gm^-2. By modeling compression strength at the level of individual fibers and bonds, we show that widespread fiber level buckling is unlikely to appear at the loads at which the macroscopic sheet fails.        

    In Paper C we develop a micro-mechanical model to study the creation of curl in paper sheets subjected to a moisture gradient through the thickness of a sheet. A moisture gradient is created during the printing process if the ink is water based, which may lead to  out-of-plane deformations (curl). The effect of transverse fiber shrinkage is captured using a multiscale model where the fiber-fiber bond is modeled with volume elements. We show how the swelling anisotropy of individual fibers contributes to the curl of the sheet in such settings. 

    In Paper D we present how to uniquely and compactly describe the distribution of fiber shapes (length, width, wall thickness, curl) used in network simulations. Using a canonical vine structure, fiber shapes measured using an optical image analyzer are used to construct a multivariate distribution function. New fiber geometries can then be generated by sampling from this distribution. Having access to such a complete description with both the distribution of fiber properties and the dependence between properties is shown to be superior to previously presented methods using micro-mechanical simulations of thermo-mechanical (TMP) long fiber sheets.        In Paper E we compare sheet testing, micro-mechanical tensile testing, and nanoindentation as methods to extract the elastic material properties of individual pulp fibers. Nanoindentations are performed parallel to and orthogonal to the axis of the fiber after it has gone through all steps of papermaking, and indentation moduli are extracted. By relating the indentation modulus to the components of the anisotropic stiffness tensor, the longitudinal and transverse elastic modulus can be determined via an iterative error minimization scheme. We show that nanoindentation is an alternative to traditional methods with the advantage of yielding the transverse modulus and enabling measurement of the fiber properties after papermaking.

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    Kappa
  • 30.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Compression failure in dense non-woven fiber networks2020In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Vol. 27, no 10, p. 6065-6082Article in journal (Refereed)
    Abstract [en]

    Investigating the compression properties of randomly ordered fiber networks experimentally is difficult which has resulted in ongoing disputes as to the mechanisms controlling the compression strength in such materials. In this work, we investigated compression properties of randomly oriented fiber networks with a special emphasis on cellulose products such as paperboard. We numerically reconstructed the conditions of the short span compression test widely used to quantify the compression strength of paperboard. We found that the phenomenological failure mode of such networks is elasto-plastic buckling. The x-shaped failure mode observed in physical experiments appears when test specimen restraints are included in the model. The most significant improvements to sheet strength can be obtained by improving the elastic properties while the strain to failure is increased most by an improvement of the plastic yield and hardening properties of individual fibers. Bond breaks were confirmed to have a smaller influence on the overall response. Fiber level microscopic buckling was investigated in depth, providing quantitative estimates of the fraction of mass likely to buckle at the microscopic level. The analysis indicated that only a low to moderate number of load carrying fibers can be expected to buckle. The inherent strength reserve in non-ordered fiber networks was investigated by introducing hinge mechanisms throughout the network, and the effect was shown to be small for a small to moderate number of hinges.

  • 31.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    New insights into compressive strength of paper & board2019In: Paper Conference and Trade Show, PaperCon 2019, TAPPI Press , 2019, p. 1308-1313Conference paper (Refereed)
    Abstract [en]

    Compressive properties of fiber-based materials are linked to their performance as packaging materials. This is not only due to use under compressive loads such as stacking but also since compressive strength is lower than tensile strength, causing compressive properties to be the limit in all applications involving bending. We examine the effect of changing the network structure on short span compression strength using a numerical model. In this way, we overcome one of the major challenges of working on non-woven randomly oriented composited: performing controlled parametric studies. We show that the effect of changes made to fiber in- and out-of-plane orientation as well as non-uniform through-thickness density on compressive strength is small, but that stiffness and strain-tofailure may be altered using comparatively small structural modifications.

  • 32.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Motamedian, Hamid Reza
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Hirn, U.
    The role of the fiber and bond in hygroexpansion and curl of paper2019In: Paper Conference and Trade Show, PaperCon 2019, TAPPI Press , 2019, p. 1256-1271Conference paper (Refereed)
    Abstract [en]

    The underlying mechanism of hygroexpansion of paper products' deformation resides on the microscale and is a product of the coupling between hydrophilic fibers and micromechanical details of the fiber network, including the geometry and alignment of fibers and bonds. We present a micromechanical framework suitable for studying hygroexpansion from the fiber- and bond-level. Using the developed model, we show that due to the threedimensionality of the bond, the transverse deformations of the fibers are not transferred effectively to the in-plane deformation of the sheet. At the same time, the longitudinal deformation of the fiber accounts for a large portion of the hygroexpansion even in highly oriented sheets. On the other hand, the out-plane deformation of paper is predominantly controlled by the strain gradient in the bonds which stems from transverse shrinkage or expansion of the fibers in the bond region. Therefore, considering the bonds as three-dimensional entities is vital for the analysis since a two-dimensional approximation overestimates the impact of the transverse deformation of the bonds to the in-plane properties and underestimates moisture-induced out-of-plane deformations of the fiber network.

  • 33.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Laboratory for Fiber Swelling and Paper Performance, Graz University of Technology, Inffeldgasse 23, Graz, 8010, Austria.
    Motamedian, Hamid Reza
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Laboratory for Fiber Swelling and Paper Performance, Graz University of Technology, Inffeldgasse 23, Graz, 8010, Austria.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Laboratory for Fiber Swelling and Paper Performance, Graz University of Technology, Inffeldgasse 23, Graz, 8010, Austria.
    Hirn, U.
    The role of the fiber and the bond in the hygroexpansion and curl of thin freely dried paper sheets2020In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 193-194, p. 302-313Article in journal (Refereed)
    Abstract [en]

    A computationally efficient method to study the in-plane and out-of-plane dimensional instability of thin paper sheets under the influence of moisture changes is presented. The method explicitly resolves the bonded and the free segments of fibers in the sheet, capturing the effect of anisotropic hygroexpansion at the fiber level. The method is verified against a volumetric model. The importance of longitudinal fiber hygroexpansion is demonstrated in spite of the absolute value of longitudinal hygroexpansion being an order of magnitude lower than the transverse hygroexpansion component. Finally, the method is used to demonstrate the formation of macroscopic sheet curl due to a moisture gradient in structurally uniform sheets in the absence of viscoelastic or plastic constitutive behavior and through-thickness residual stress profiles.

  • 34.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Laboratory for Fiber Swelling and Paper Performance, Graz University of Technology, Inffeldgasse 23, Graz, 8010, Austria.
    Österling, Sofia Reyier
    Dalarna Univ, Hogskolegatan 2, SE-79188 Falun, Sweden..
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. CD Laboratory for Fiber Swelling and Paper Performance, Graz University of Technology, Inffeldgasse 23, Graz, 8010, Austria.
    Hirn, Ulrich
    Graz Univ Technol, CD Lab Fiber Swelling & Paper Performance, Inffeldgasse 23, A-8010 Graz, Austria.;Graz Univ Technol, Inst Paper Pulp & Fibre Technol, Inffeldgasse 23, A-8010 Graz, Austria..
    Characterization and impact of fiber size variability on the mechanical of fiber networks with an to materials2022In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 239, article id 111438Article in journal (Refereed)
    Abstract [en]

    Cellulose fibers come in a wide range of shapes and sizes. The heterogeneity of the fiber length, width, wall thickness, curl and external fibrillation is detrimental to the mechanical performance of products such as paper and paperboard. Although micro-mechanical models of these materials sometimes incorporate features of this heterogeneity, so far there is no standardized method of fully incorporating this.We examine a large number of industrial mechanical fiber pulps to determine what information such a standardized method would have to have. We find that the method must allow for both non-Gaussian distributions and dependence between the variables. We present a method of characterizing mechanical pulp under these conditions that views the individual fiber as outcome of a sampling process from a multivariate distribution function. The method is generally applicable to any dataset, even a non-Gaussian one with dependencies.Using a micro-mechanical model of a paper sheet the proposed method is compared with previously presented methods to study whether incorporating both a varying fiber size and dependencies is necessary to match the response of a sheet modeled with measured characterization data. The results demonstrate that micro-mechanical models of paper and paperboard should not neglect the influence of the dependence between the characteristic shape features of the fibers if the model is meant to match physical experiments.

  • 35.
    Buckler, Andrew J.
    et al.
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden.;Elucid Bioimaging Inc, Boston, MA USA..
    Karlof, Eva
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden..
    Lengquist, Mariette
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden.;Karolinska Inst, Dept Med Solna, Stockholm, Sweden..
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Maegdefessel, Lars
    Perisic Matic, Ljubica
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden..
    Hedin, Ulf
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden..
    Virtual Transcriptomics Noninvasive Phenotyping of Atherosclerosis by Decoding Plaque Biology From Computed Tomography Angiography Imaging2021In: Arteriosclerosis, Thrombosis and Vascular Biology, ISSN 1079-5642, E-ISSN 1524-4636, Vol. 41, no 5, p. 1738-1750Article in journal (Refereed)
    Abstract [en]

    Objective: Therapeutic advancements in atherosclerotic cardiovascular disease have improved prevention of ischemic stroke and myocardial infarction, but diagnostic methods for atherosclerotic plaque phenotyping to aid individualized therapy are lacking. In this feasibility study, we aimed to elucidate plaque biology by decoding the molecular phenotype of plaques through analysis of computed-tomography angiography images, making a predictive model for plaque biology referred to as virtual transcriptomics. Approach and Results: We employed machine intelligence using paired computed-tomography angiography and transcriptomics from carotid endarterectomies of 40 patients undergoing stroke-preventive surgery for carotid stenosis. Computed tomography angiographies were analyzed with novel software for accurate characterization of plaque morphology and plaque transcriptomes obtained from microarrays, followed by mathematical modeling for prediction of molecular signatures. Four hundred fourteen coding and noncoding RNAs were robustly predicted using supervised models to estimate gene expression based on plaque morphology. Examples of predicted transcripts included ion transporters, cytokine receptors, and a number of microRNAs whereas pathway analyses demonstrated enrichment of several biological processes relevant for the pathophysiology of atherosclerosis and plaque instability. Finally, the ability of the models to predict plaque gene expression was demonstrated using computed tomography angiographies from 4 sequestered patients and comparisons with transcriptomes of corresponding lesions. Conclusions: The results of this pilot study show that atherosclerotic plaque phenotyping by image analysis of conventional computed-tomography angiography can elucidate the molecular signature of atherosclerotic lesions in a multiscale setting. The study holds promise for optimized personalized therapy in the prevention of myocardial infarction and ischemic stroke, which warrants further investigations in larger cohorts.

  • 36.
    Buckler, Andrew J.
    et al.
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden.;Elucid Bioimaging Inc, Boston, MA USA..
    van Wanrooij, Max
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Andersson, Måns
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Karlof, Eva
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden..
    Matic, Ljubica Perisic
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden..
    Hedin, Ulf
    Karolinska Inst, Dept Mol Med & Surg, Stockholm, Sweden..
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. Univ Southern Denmark, Fac Hlth Sci, Odense, Denmark..
    Patient-specific biomechanical analysis of atherosclerotic plaques enabled by histologically validated tissue characterization from computed tomography angiography: A case study2022In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 134, article id 105403Article in journal (Refereed)
    Abstract [en]

    Background: Rupture of unstable atherosclerotic plaques with a large lipid-rich necrotic core and a thin fibrous cap cause myocardial infarction and stroke. Yet it has not been possible to assess this for individual patients. Clinical guidelines still rely on use of luminal narrowing, a poor indicator but one that persists for lack of effective means to do better. We present a case study demonstrating the assessment of biomechanical indices pertaining to plaque rupture risk non-invasively for individual patients enabled by histologically validated tissue characterization. Methods: Routinely acquired clinical images of plaques were analyzed to characterize vascular wall tissues using software validated by histology (ElucidVivo, Elucid Bioimaging Inc.). Based on the tissue distribution, wall stress and strain were then calculated at spatial locations with varied fibrous cap thicknesses at diastolic, mean and systolic blood pressures. Results: The von Mises stress of 152 [131, 172] kPa and the equivalent strain of 0.10 [0.08, 0.12] were calculated where the fibrous cap thickness was smallest (560 mu m) (95% CI in brackets). The stress at this location was at a level predictive of plaque failure. Stress and strain at locations with larger cap thicknesses were calculated to be lower, demonstrating a clinically relevant range of risk levels. Conclusion: Patient specific tissue characterization can identify distributions of stress and strain in a clinically relevant range. This capability may be used to identify high-risk lesions and personalize treatment decisions for individual patients with cardiovascular disease and improve prevention of myocardial infarction and stroke.

  • 37.
    Ceccato, Chiara
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Brandberg, August
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Barbier, Christophe
    BillerudKorsnäs, Grums, Värmland County, Sweden.
    Micro-mechanical modeling of the paper compaction process2021In: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 232, no 9, p. 3701-3722Article in journal (Refereed)
    Abstract [en]

    Double-roll compaction is a process to create extensible paper and paperboard suitable for replacing plastic in 3D forming applications. Understanding the macro- and micro-mechanisms governing the compaction process allows increasing the stretch potential while maintaining sufficient strength and bending stiffness. In this work, we approach the compaction process of paperboard with micro-mechanical methods featuring the unprecedented level of details otherwise inaccessible with currently available experimental tools. The loading scheme is based on experiments and continuum level simulations. The different levels of compaction and their continuous impact on the fibers’ geometry, void closures, and irreversible deformation of the fibers are thoroughly characterized. We find that the structural changes are concentrated in the fibers oriented within 30 degrees of the direction of compaction. The deformation accumulates primarily in the wall of the fibers in the form of irreversible strains. The spring-back effect beyond the compaction is negligible. For the first time, the role of normal and frictional fiber-to-fiber interactions in the compaction process is investigated and quantified. The frictional interaction between the fibers has a surprisingly low impact on the outcome of the compaction process, and the normal interaction between the fibers has a dominant response. The consequence of this finding is potentially limited impact of the surface modifications targeting the friction.

  • 38.
    Ceccato, Chiara
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Barbier, C.
    BillerudKorsnäs, Grums, Värmland County, Sweden.
    Investigation of rolling contact between metal and rubber-covered cylinders governing the paper compaction process2019In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 163, article id 105156Article in journal (Refereed)
    Abstract [en]

    With the goal of partial or complete replacement of plastic components with paper-based products, 3D paper structures have seen a growing interest in industrial applications: from packaging to more complex daily life objects. However, the extensibility of paper has become a key issue within this context and is the main factor determining the formability of these products and determining the depth of the achievable shapes. The most effective way to increase paper's stretch potential is by subjecting the moist paper web to a compaction process, which can be achieved through an extensible unit that is located in the drying section of a paper machine, where the network experiences in-plane compression in the machine direction (MD), under out-of-plane lateral constraints. The objective of this work is to clarify the mechanisms governing the compression process and to evaluate the influence of various parameters on the final material properties to optimize the industrial production of extensible paper. The system operation has been simulated realistically and shows that paper experiences a compressive state passing through the nip, with plastic strains in MD direction being of the same order of the applied speed difference, which was expected experimentally in optimal conditions of compaction. Starting from an initial reference case, a sensitivity study has been performed to identify and address the following factors that may affect the compression state: (a) friction coefficients; (b) indentation level; (c) speed difference; and (d) rubber properties. The analysis of the numerical results gives an insight into the mechanisms governing the compaction operations and allows a better comprehension of the features controlling the process outcome.

  • 39.
    Chatterjee, Abhiroop
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Approximating Damping Coefficient of Bolted Joints using the Finite Element Method2023Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Bolted joints are important due to their energy dissipation property in structures,but the damping mechanism is also highly nonlinear and localized. The goal ofthis thesis is to develop an accurate method for modeling bolted joint dampingin large structures using finite element (FE) software. To model bolted jointdamping, the first step is to study the mechanism and define the terms like slip,micro-slip, and macro-slip. An extensive literature review identified the necessarymethods: detailed contact model, thin-layer elements, and connector elements.These methods are compared based on parameters such as computation time,modeling time, etc. The thin-layer method was used for modeling bolted jointdamping in large structures.

    To evaluate parameters for thin-layer element modeling, a local joint model wasbuilt using contact formulation of an engine housing and ladder frame assembly.The computed parameters include normal stiffness, tangential stiffness, and lossfactor. Analysis reveals that the loss factor depends on pre-load and amplitudeload. The micro-slip is the region of interest where the loss factor was computed. Using curve-fitting, a range of amplitude-dependent loss factors was calculated.

    Finally, the thin-layer elements are used in the engine housing and ladder frameassembly to model bolted joint damping. The parameters estimated using the localjoint model are used to define the properties of the thin-layer elements such thatthe elements are a phenomenological representation of bolted joint. A mode-basedsteady-state analysis has been performed to estimate the loss factor on a systemlevel. The frequency response of such an analysis accurately captures the frequencyresponse curves of structures with bolted joints. The two important behaviors thathave been captured are the shifting of the resonance peak to a lower value and thewidening of the frequency response curve as the applied load increases. However,the resonance frequency shifting to a lower frequency (softening) has not beencaptured due to modeling limitations in the FE software. A substructure couplingmodel using the Craig-Bampton formulation of the engine housing and ladderframe assembly has been analyzed using a constant loss factor. The frequencyresponse of such a system appears to give an approximate behavior of a structurewith bolted joint damping.

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  • 40.
    Chevallier, Yohan
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Design of a custom motocompressor for aeronautic cooling applications2021Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis is part of a project that aims to develop a custom motocompressor for aeronautic cooling applications from an existing compressor that will no longer be manufactured. The main goals are to prevent obsolescence issues and better master all the motocompressor characteristics. In this context, various tasks are to be performed, including the design of the entire compressor froman existing model.

    Firstly, the work is to understand how the existing compressor is built and how it can be redesigned and improved. Then, the work focuses on designing the new motocompressor, and particularly the scroll section. The others main tasks are improving the existing design, in particular concerning the sealing and the balancing system, as well as checking the mechanical resistance. Finally, the 2D technical drawings are realised, a functional dimensioning is performed on all the parts drawings and discussed with the manufacturer in order toensure a proper production quality. The main results of the design process will be discussed and the remaining tasks investigated.

  • 41.
    Chlebek, David
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Simulation of ultrasonic time of flight in bolted joints2021Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Ultrasonic measurements of the preload in bolted joints is a very accurate method since it does not depend on the friction and other factors which cause difficulties for common methods. The ultrasonic method works by emitting an ultrasonic pulse into the bolt which is reflected at the end and returned to the transducer, the change in the time of flight (TOF) can be related to the elongation of the bolt and therefore the preload. One must account for the acoustoelastic effect which is the change in sound speed due to an initial stress state. The goal of this thesis project was to implement a Murnaghan hyperelastic material model in order to account for the acoustoelastic effect when conducting a numerical simulation using the finite element method (FEM). An experiment was also performed to validate the numerical simulation. The DeltaTOF as a function of a tensile force was obtained for an M8 and M10 test piece from the experiment. The material model was implemented by creating a user subroutine written in Fortran for the explicit solver Radioss. Hypermesh was used to set-up the numerical simulation. The material model has shown an expected behavior with an increased sound speed with compressive stresses and a decreased speed with tensile stresses. The numerical simulation showed a good correspondence to the experimental results.

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  • 42.
    Croné, Philip
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Continuum modelling of work hardening in precipitation hardened alloys2022Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis deals with prediction of macroscopic work hardening in a precipitation hardened alloy. The focus is on the particle contribution. A hierarchical modelling approach is adopted where work hardening in a representative material volume on the microscale is homogenized and used to represent the macroscopic hardening. The modelling on the smaller scale is carried out within the framework of an isotropic continuum strain gradient plasticity theory where particlesare modelled as elastic zones embedded in a continuous isotropic elastic-plasticmatrix. Effects of plastic deformation in smaller particles are included as well.Moreover, the interface between a particle and its surrounding matrix is modelled as a separate region of zero thickness. The end result is an analytical model that highlights the particle contribution under cyclic deformation assuming small plastic strains, and a small to moderate volume fraction of particles. The model moreover allows effects of plastic relaxation around particles to be included in a straightforward manner, which in turn allows larger plastic strains to be considered. Validation of the model is carried out by comparison with experimental uniaxial tension/compression data on a maragin stainless 15-5 steel containingspherical Cu-precipitates. In the first validation, only monotonic loading is considered and the model is brought to close agreement with the data up to a plasticstrain of 7.5% via the implementation of a plastic relaxation model. In the second validation, the model is compared to cyclic tension/compression experiments with plastic strain amplitudes up to 1%. Generally excellent agreement between model and experimental data is obtained.

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  • 43.
    Croné, Philip
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    A model for reversed plasticity in dispersion hardened metals validated by uniaxial tension/compression experimentsManuscript (preprint) (Other academic)
    Abstract [en]

    Room temperature cyclic tensile/compression tests were carried out on a precipitation hardened martensitic 15-5 PH stainless steel in order to validate a previously developed work hardening model by the authors, based on strain gradient plasticity. Identical tests were also made on a 15-5 steel containing no precipitates to serve as model input for the matrix material. The model was calibrated to the experimental data up to a forward plastic strain of 1% and the rest of the cyclic stress strain curves were predicted by the model with generally very good agreement. We believe that the model's capability to predict the cyclic stress strain behaviour of the composite strengthens the role of continuum modelling within material micro mechanics. Despite low plastic strain amplitudes (0.25%, 0.5% and 1%) no signs of inflection on the reverse flow curves were observed, even when tested at a temperature of -50 °C. Moreover, the results suggest that the strain gradient plasticity related higher order stresses that exist in close vicinity to the particles most likely have a dissipative character in the current alloy. However, this does not constitute a general conclusion as it should depend on parameters such as temperature, alloy stacking fault energy, etc.

  • 44.
    Croné, Philip
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Analytical prediction of yield stress and strain hardening in a strain gradient plasticity material reinforced by small elastic particles2022In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 151, p. 103200-103200, article id 103200Article in journal (Refereed)
    Abstract [en]

    The influence on macroscopic work hardening of small, spherical, elastic particles dispersedwithin a matrix is studied using an isotropic strain gradient plasticity framework. An analyticalsolution for strain hardening, i.e. the flow stress as a function of plastic strain, based ona recently developed model for initial yield strength is proposed. The model accounts forrandom variations in particle size and elastic properties, and is numerically validated againstFE solutions in 2D/3D unit cell models. Excellent agreement is found as long as the typicalparticle radius is much smaller than the material length scale, given that the particle volumefraction is not too large (< 10%) and that the particle/matrix elastic mismatch is within arealistic range. Finally, the model is augmented to account for strengthening contribution from shearable particles using classic line tension models and successfully calibrated againstexperimental tensile data on an 𝐴𝑙 − 2.8𝑤𝑡%𝑀𝑔 − 0.16𝑤𝑡%𝑆𝑐 alloy.

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  • 45.
    Croné, Philip
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Strain gradient plasticity modelling of cyclic loading in dispersion hardened materials2022In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 96, article id 104741Article in journal (Refereed)
    Abstract [en]

    An analytical model, based on an isotropic strain gradient plasticity theory, describing work hardening during cyclic straining in a metal reinforced by a dispersion of non shearable particles is presented. The yield criterion is expressed in terms of isotropic and kinematic hardening contributions and the model is validated against full field finite element (FE) solutions on a 2D axi-symmetric unit cell model. Excellent agreement between analytical and FE results is obtained. The theory presented includes mixed energetic/dissipative contributions from higher order stresses in both bulk and at particle/matrix interfaces. In particular, the influence of a quadratic interface free energy that transitions into a linear form at some threshold value of plastic strain is investigated. It is shown that such an energy is capable of capturing the experimentally observed phenomenon of inflections in the reverse stress-strain curve. It is argued, based on the well known phenomenon where particles are shielded by Orowan dislocation loops during reverse strain, that an energetic interface contribution could be physically relevant for low plastic strains.

  • 46.
    Croné, Philip
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Zhou, Tao
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures.
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Continuum plasticity modelling of work hardening for precipitation-hardened martensitic steel guided by atom probe tomography2022In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 215, article id 110463Article in journal (Refereed)
    Abstract [en]

    An analytical flow stress model, based on isotropic strain gradient plasticity theory, for precipitation hardened materials, is proposed and evaluated against tensile data on a 15 wt% Cr - 5 wt% Ni (15-5) PH stainless steel. The 15-5 PH material was aged at 500 °C for 1 h, 2 h, 5 h and 50 h to obtain a wide range of precipitate sizes. Detailed characterisation of precipitates was obtained using atom probe tomography (APT). A second material, a 15-5 stainless steel without added Cu was heat treated to obtain a similar matrix microstructure as in the 15-5 PH, but without Cu precipitates. Tensile testing revealed that the heat treated 15-5 PH material covered the full range from under- to overaged conditions. The analytical model, which accounts for stress reducing effects of plastic relaxation around particles, manages to capture the experimental data in a very satisfying manner using only a total of three tunable parameters. It is believed that the proposed model can offer an alternative to the much more commonly used work hardening models based on the internal variable approach.

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  • 47.
    Czibula, Caterina
    et al.
    Univ Leoben, Inst Phys, Franz Josef Str 18, A-8700 Leoben, Austria.;Graz Univ Technol, Inst Bioprod & Paper Technol, Inffeldgasse 23, A-8010 Graz, Austria.;Graz Univ Technol, Christian Doppler Lab Fiber Swelling & Paper Perf, Inffeldgasse 23, A-8010 Graz, Austria..
    Brandberg, August
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. Graz Univ Technol, Christian Doppler Lab Fiber Swelling & Paper Perf, Inffeldgasse 23, A-8010 Graz, Austria..
    Cordill, Megan J.
    Austrian Acad Sci, Erich Schmid Inst Mat Sci, Jahnstr 12, A-8700 Leoben, Austria..
    Matkovic, Aleksandar
    Univ Leoben, Inst Phys, Franz Josef Str 18, A-8700 Leoben, Austria..
    Glushko, Oleksandr
    Univ Leoben, Dept Mat Sci, Jahnstr 12, A-8700 Leoben, Austria..
    Czibula, Chiara
    Graz Univ Technol, Inst Bioprod & Paper Technol, Inffeldgasse 23, A-8010 Graz, Austria..
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. Graz Univ Technol, Christian Doppler Lab Fiber Swelling & Paper Perf, Inffeldgasse 23, A-8010 Graz, Austria..
    Teichert, Christian
    Univ Leoben, Inst Phys, Franz Josef Str 18, A-8700 Leoben, Austria.;Graz Univ Technol, Christian Doppler Lab Fiber Swelling & Paper Perf, Inffeldgasse 23, A-8010 Graz, Austria..
    Hirn, Ulrich
    Graz Univ Technol, Inst Bioprod & Paper Technol, Inffeldgasse 23, A-8010 Graz, Austria.;Graz Univ Technol, Christian Doppler Lab Fiber Swelling & Paper Perf, Inffeldgasse 23, A-8010 Graz, Austria..
    The transverse and longitudinal elastic constants of pulp fibers in paper sheets2021In: Scientific Reports, E-ISSN 2045-2322, Vol. 11, no 1, article id 22411Article in journal (Refereed)
    Abstract [en]

    Cellulose fibers are a major industrial input, but due to their irregular shape and anisotropic material response, accurate material characterization is difficult. Single fiber tensile testing is the most popular way to estimate the material properties of individual fibers. However, such tests can only be performed along the axis of the fiber and are associated with problems of enforcing restraints. Alternative indirect approaches, such as micro-mechanical modeling, can help but yield results that are not fully decoupled from the model assumptions. Here, we compare these methods with nanoindentation as a method to extract elastic material constants of the individual fibers. We show that both the longitudinal and the transverse elastic modulus can be determined, additionally enabling the measurement of fiber properties in-situ inside a sheet of paper such that the entire industrial process history is captured. The obtained longitudinal modulus is comparable to traditional methods for larger indents but with a strongly increased scatter as the size of the indentation is decreased further.

  • 48.
    Czibula, Caterina
    et al.
    Graz University of Technology.
    Brandberg, August
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Cordill, Megan J.
    Erich Schmid Institute of Materials Science.
    Matković, Aleksandar
    Montanuniversitaet Leoben.
    Glushko, Oleksandr
    Montanuniversitaet Leoben.
    Czibula, Chiara
    Graz University of Technology.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics.
    Teichert, Christian
    Montanuniversitaet Leoben.
    Hirn, Ulrich
    Graz University of Technology.
    The transverse and longitudinal elastic constants of pulp fibers in paper sheets2021Report (Other academic)
    Abstract [en]

    Cellulose fibers are a major industrial input, but due to their irregular shape and anisotropic material response, accurate material characterization is difficult. Single fiber tensile testing is the most popular way to estimate the material properties of individual fibers. However, such tests only determine the longitudinal modulus of the fiber. Here, we compare sheet testing, micromechanical testing, and nanoindentation as methods to extract the elastic material properties of the individual fibers. We show that nanoindentation can be used to determine both the longitudinal and the transverse elastic modulus using only two indentations, additionally enabling the measurement of fiber properties in-situ inside a sheet of paper where the complete process history is captured. For the longitudinal modulus, the accuracy is comparable for larger indents, but with an increase of scatter of unknown origin as the probe size is decreased using an atomic force microscopy tip.

  • 49.
    Dahlberg, Carl F. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Correlation of Global Quantities at Material Characterization of Pressure-Sensitive Materials Using Sharp Indentation Testing2021In: LUBRICANTS, ISSN 2075-4442, Vol. 9, no 3, article id 29Article in journal (Refereed)
    Abstract [en]

    Correlation of sharp indentation problems is examined theoretically and numerically. The analysis focuses on elastic-plastic pressure-sensitive materials and especially the case when the local plastic zone is so large that elastic effects on the mean contact pressure will be small or negligible as is the case for engineering metals and alloys. The results from the theoretical analysis indicate that the effect from pressure-sensitivity and plastic strain-hardening are separable at correlation of hardness values. This is confirmed using finite element methods and closed-form formulas are presented representing a pressure-sensitive counterpart to the Tabor formula at von Mises plasticity. The situation for the relative contact area is more complicated as also discussed.

  • 50.
    Dahlberg, Carl F. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Ortiz, Michael
    CALTECH, Div Engn & Appl Sci, Pasadena, CA 91125 USA..
    Size Scaling of Plastic Deformation in Simple Shear: Fractional Strain-Gradient Plasticity and Boundary Effects in Conventional Strain-Gradient Plasticity2020In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 87, no 3, article id 031017Article in journal (Refereed)
    Abstract [en]

    A recently developed model based on fractional derivatives of plastic strain is compared with conventional strain-gradient plasticity (SGP) models. Specifically, the experimental data and observed model discrepancies in the study by Mu et al. (2016, "Dependence of Confined Plastic Flow of Polycrystalline Cu Thin Films on Microstructure," MRS Com. Res. Let. 20, pp. 1-6) are considered by solving the constrained simple shear problem. Solutions are presented both for a conventional SGP model and a model extension introducing an energetic interface. The interface allows us to relax the Dirichlet boundary condition usually assumed to prevail when solving this problem with the SGP model. We show that the particular form of a relaxed boundary condition does not change the underlying size scaling of the yield stress and consequently does not resolve the scaling issue. Furthermore, we show that the fractional strain-gradient plasticity model predicts a yield stress with a scaling exponent that is equal to the fractional order of differentiation.

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