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  • 51.
    Linvill, Eric
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation. KTH, School of Chemical Science and Engineering (CHE), Centres, Biofibre Materials Centre, BiMaC.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The Combined Effects of Moisture and Temperature on the Mechanical Response of Paper2014In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 54, no 8, p. 1329-1341Article in journal (Refereed)
    Abstract [en]

    To model advanced 3-D forming strategies for paper materials, the effects of environmental conditions on the mechanical behavior must be quantitatively and qualitatively understood. A tensile test method has been created, verified, and implemented to test paper at various moisture content and temperature levels. Testing results for one type of paper for moisture contents from 6.9 to 13.8 percent and temperatures from 23 to 168 degrees Celsius are presented and discussed. Coupled moisture and temperature effects have been discovered for maximum stress. Uncoupled effects have been discovered for elastic modulus, tangent modulus, hardening modulus, strain at break, tensile energy absorption (TEA), and approximate plastic strain. A hyperbolic tangent function is also utilized which captures the entire one-dimensional stress-strain response of paper. The effects of moisture and temperature on the three coefficients in the hyperbolic tangent function may be assumed to be uncoupled, which may simplify the development of moisture- and temperature-dependent constitutive models. All parameters were affected by both moisture and temperature with the exception of TEA, which was found to only be significantly dependent on temperature.

  • 52. Lobosco, V.
    et al.
    Norman, B.
    Östlund, Sören
    Modelling of forming and densification of fibre mats in twin-wire formers2005In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 20, no 1, p. 16-23Article in journal (Refereed)
    Abstract [en]

    In the forming section of modern paper machines, water is removed from the suspension at low loads applied through the tension in curved wires and by vacuum application. This paper presents a physically based model of the forming and densification of fibre mats in twin-wire formers. The model can calculate the effect of complex pressure profiles that are generated through the forming section. It was developed from mass and momentum balances of the fibre and liquid phases, the fibre mat stress-porosity relation and an expression for the permeability as a function of the porosity. The fibre mat stress porosity relation used is rate- independent and accounts for the generally observed hysteresis. Simulations have been conducted to study the effects of blade pulses, wire tension and beating. The effect of sequential blade pressure pulses after the forming roll on the dewatering and the concentration gradients could be characterised. The simulations also exhibited rewetting by expansion when the fibre mats left the forming roll. Increasing wire tension resulted in increased dewatering, but the rate of increase diminished rapidly with increasing tension. The simulation results also indicated that beating has large influence on dewatering.

  • 53. Lundblad, Joakim
    et al.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Integrating CDIO experiences into a new program environment2007In: Proceedings of 3rd International CDIO Conference, 2007Conference paper (Other academic)
    Abstract [en]

    This paper provides a creative approach on how to implement CDIO-techniques into a Engineering Physics program. A comparison is made between the Vehicle Engineering program and the Engineering Physics program at the Royal Institute of Technology (KTH), in order to establish what the grounds and potentials for implementing CDIO into a new program environment are. The Engineering Physics program is examined not only though the comparison but also through an indicatory survey performed with students, instructors and representatives from the industry. This is done in order to map the perceived nature of the program. Furthermore, these potentials are explored from the view of the Engineering Physics program at KTH, and implementation strategies are suggested and elaborated in accordance with derived results and indicators, using modelling and simulation as a ground for an abstract product orientation.

  • 54.
    Magnusson, Mikael S.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Fischer, Wolfgang J.
    Graz University of Technology.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Hirn, Ulrich
    Graz University of Technology.
    Interfibre joint strength under peeling, shearing and tearing types of loading2013In: Advances in Pulp and Paper Research, Cambridge 2013: Transactions of the 15th Fundamental ResearchSymposium / [ed] S.J. I'Anson, 2013, p. 103-124Conference paper (Refereed)
    Abstract [en]

    The mechanical properties of interfibre joints are essential for the load carrying capacity of fibre network materials such as paper and board. Mechanical measurements of fibre–fibre crosses can been used to characterize the strength of these interfibre joints in order to obtain knowledge on how these hierarchical network materials behave at the micrometer scale. The general method for these interfibre joint strength experiments has been to pull one of the fibres of a fibre–fibre cross and attribute the force at rupture to the shear strength of the interfibre joint. However, without taking the geometry of the fibres and the resulting mixed mode of loading at the interfibre joints into account, limited information on the strength properties can be obtained using this technique. In this study, isolated fibre–fibre crosses have been tested mechanically using four distinctly different load cases; peeling, shearing, tearing and a biaxial type of loading, in order to gain more information on how interfibre joints behave in different modes of loading. The centerline geometry of the fibres, microfibril angles, initial twists as well as the wall thickness of each individual test piece was used to model each experiment using the finite element method, and from the simulation results, the local state of loading in the interfibre joints at failure was obtained for each specific experiment. The force–displacement curve for the experiments as well as the estimated local state of loading was also used to compare the different load cases and to evaluate the information that can be obtained on the strength properties of interfibre joints using these load cases.

  • 55.
    Magnusson, Mikael S.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Zhang, Xiaobo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Experimental Evaluation of the Interfibre Joint Strength of Papermaking Fibres in Terms of Manufacturing Parameters and in Two Different Loading Directions2013In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 53, no 9, p. 1621-1634Article in journal (Refereed)
    Abstract [en]

    The strength properties of interfibre bonds play a vital role in the response of fibrous materials, such as paper and paperboard, under mechanical loading. To help tailor the properties of such materials by chemical or mechanical treatments of the fibre wall and fibre surfaces, improved understanding of the microscopic damage and failure mechanisms of interfibre joints is desirable. In this paper, a method for manufacturing and testing of interfibre joint specimens in two principally different modes of loading is presented. The method was applied to investigate the strength of Kraft pulp interfibre joints with different geometries and in two different modes of loading: the conventional shearing mode and also a peeling mode of loading. The method was also used to investigate the influence of drying pressure, defined as the nominal pressure between two Teflon surfaces or between a rubber surface and a Teflon one as well as a simple comparison of a pulp with two different degrees of refining. The results are presented in terms of rupture force and using different methods of normalization such as nominal overlap area, length, and width of the joint region, measured using a microscope. It was shown in this study that normalising the force at rupture by either geometric parameter reduced the scatter of the strength measurements slightly, but, neither were unambiguously more successful than the other. The results of tests done with the peeling type of loading were about 20 % of those done with the conventional shearing type of loading.

  • 56.
    Magnusson, Mikael S.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical evaluation of interfibre joint strength measurements in terms of three–dimensional resultant forces and moments2012Report (Other academic)
    Abstract [en]

    The interfibre joint is one of the key elements in creating the strength of self–binding fibrous materials such as paper and board. In order to tailor the properties of such materials by chemical and/or mechanical treatments of the fibres, and to learn how such modifications influence the properties at the microscopic level, a greater understanding of how to evaluate the mechanical properties of interfibre joints is desirable. The methods reported in the literature for determining of the interfibre joint strength do not in general distinguish between the contributions of normal and shear stresses in the bonded region. This paper presents a numerical analysis procedure, based on the finite element method, for evaluating interfibre joint strength measurements in terms of the normal, shear, and moment loading components during testing. The method is applied to investigate the strength of Kraft pulp interfibre joints under two principally different modes of loading. The results show that for a typical interfibre joint test of an isolated fibre–fibre cross with long free fibre segments, modes of loading other than pure shear cannot, in general, be neglected, and are strongly dependent on the structural geometry of the fibre–fibre crosses. In addition, the resultant forces and moments were scaled in terms of the interface area and the twisting and bending resistance of the interface approximated as an ellipse to account for differences in interface area between the measurements. These scaled resultants were used to quantify how the mode of loading influences the relation between the amount of normal stress and the amount of shear stress that develop in the interfibre joint.

  • 57.
    Magnusson, Mikael S.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Numerical evaluation of interfibre joint strength measurements in terms of three-dimensional resultant forces and moments2013In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 20, no 4, p. 1691-1710Article in journal (Refereed)
    Abstract [en]

    The interfibre joint is one of the key elements in creating the strength of self-binding fibrous materials such as paper and board. In order evaluate the strength properties of interfibre joints using direct measurements, a greater understanding on how the mode of loading influences the results is desirable. The methods reported in the literature do not in general distinguish between the contributions of normal and shear stresses in the bonded region. This paper presents a numerical analysis procedure, based on the finite element method, for evaluating interfibre joint strength measurements in terms of the normal, shear, and moment loading components during testing. The target is to estimate the resultant forces and moments, that acts in the interfibre joint region at rupture, of Kraft pulp interfibre joints tested under two principally different modes of loading. The results show that for a typical interfibre joint test, modes of loading other than pure shear cannot, in general, be neglected, and are strongly dependent on the structural geometry of the fibre-fibre crosses. In addition, the resultant forces and moments were scaled in terms of the interface area and the twisting and bending resistance of the interface approximated as an ellipse to account for differences in interface area between the measurements. These scaled resultants were used to quantify how the mode of loading influences the relation between the amount of normal stress and the amount of shear stress that develop in the interfibre joint.

  • 58.
    Magnusson, Mikael S.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Zhang, Xiaobo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Experimental evaluation of the interfibre joint strength ofpapermaking fibres in terms of manufacturing parameters and intwo different loading directions2012Report (Other academic)
    Abstract [en]

    The strength properties of interfibre bonds play a vital role in the response of fibrous materials,such as paper and paperboard, under mechanical loading. To help tailor the properties of suchmaterials by chemical or mechanical treatments of the fibre wall and fibre surfaces, improvedunderstanding of the microscopic damage and failure mechanisms of interfibre joints is desirable.In this paper, a method for manufacturing and testing of interfibre joint specimens in twoprincipally different modes of loading is presented.The method was applied to investigate the strength of Kraft pulp interfibre joints with differentgeometries and in two different modes of loading: the conventional shearing mode and also apeeling mode of loading. The method was also used to investigate the influence of dryingpressure, defined as the nominal pressure between two Teflon surfaces or between a rubbersurface and a Teflon one as well as a simple comparison of a pulp with two different degrees ofrefining.The results are presented in terms of rupture force and using different methods of normalizationsuch as nominal overlap area, length, and width of the joint region, measured using a microscope.It was shown in this study that neither of the methods of normalization unambiguously reducedthe large scatter of the strength measurements. The results of tests done with the peeling type ofloading were about 20 % of those done with the conventional shearing type of loading.

  • 59. Makela, P.
    et al.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Orthotropic elastic-plastic material model for paper materials2003In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 40, no 21, p. 5599-5620Article in journal (Refereed)
    Abstract [en]

    Paper and paperboard generally exhibit anisotropic and non-linear mechanical material behaviour. In this work, the development of an orthotropic elastic plastic constitutive model, suitable for modelling of the material behaviour of paper is presented. The anisotropic material behaviour is introduced into the model by orthotropic elasticity and an isotropic plasticity equivalent transformation tensor. A parabolic stress-strain relation is adopted to describe the hardening of the material. The experimental and numerical procedures for evaluation of the required material parameters for the model are described. Uniaxial tensile testing in three different inplane material directions provides the calibration of the material parameters under plane stress conditions. The numerical implementation of the material model is presented and the model is shown to perform well in agreement with experimentally observed mechanical behaviour of paper.

  • 60. Makela, Petri
    et al.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Cohesive crack modelling of thin sheet material exhibiting anisotropy, plasticity and large-scale damage evolution2012In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 79, p. 50-60Article in journal (Refereed)
    Abstract [en]

    The crack tip region in notched structures generally exhibit damage evolution before ultimate failure occurs. In some materials, the damaged regions may reach considerable sizes prior to structural collapse. In this work, a cohesive crack model suitable for static fracture mechanics analysis of thin sheet materials exhibiting anisotropy, plasticity, and large-scale damage evolution was developed. The material parameters of the model were calibrated solely by tensile testing of unnotched test specimens. The predictive capability of the model was verified by comparisons with experiments on notched test specimens with different crack sizes. The predictions of failure were shown to be in excellent agreement with the experiments.

  • 61.
    Malmqvist, Johan
    et al.
    Chalmers University of Technology.
    Edström, Kristina
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    Gunnarsson, Svante
    Linköping University.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Use of CDIO Standards in Swedish National Evaluation of Engineering Education Programs2005In: Proceedings of the 1st International CDIO Conference, 2005, p. 134-137Conference paper (Refereed)
    Abstract [en]

    In this paper, we report on a large-scale application of the CDIO standards, involving approximately 100 educational programs. The context is the Swedish national evaluation of its “civilingenjör” engineering degree programs made by the Swedish National Agency for Higher Education (Högskoleverket, HSV).

    In the paper, we first briefly describe the CDIO standards focusing on the role as a support for continuous program development. We then present the self-evaluation materials used in the HSV evaluation and account for HSV’s motives for including the CDIO standards evaluation in the self-evaluation package and for the modifications made compared to the original CDIO standards.

    We then discuss the results from a survey and an interview study directed to the program managers that have applied the CDIO standards in the HSV evaluation. The questions in the survey aim to investigate the respondents’ view of the relevance, benefits, limitations and ease of use of the CDIO standards. The questions are aimed both at the overall level – the body of standards – as well as at the level of single standards.

  • 62.
    Malmqvist, Johan
    et al.
    Chalmers University of Technology.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Edström, Kristina
    KTH, School of Education and Communication in Engineering Science (ECE), Lärande.
    Integrated program descriptions: A tool for communicating goals and design of CDIO programs2006In: Proceedings of the 2nd International CDIO Conference, Linköping University Electronic Press, 2006Conference paper (Refereed)
    Abstract [en]

    The CDIO syllabus provides a generic platform for writing program goal statements. Specifically, intended learning outcomes for personal and professional skills and attitudes such as communication, teamwork and ethics can be stated by combining a topic from the CDIO syllabus with an appropriate cognitive verb that reflects the desired proficiency. However, a complete program goal statement must also include goals for mathematical, scientific and technical knowledge. Moreover, while a “pure” goal statement may be suitable for and support discussions with external stakeholders such as industry leaders who are not involved in the program design as such, deliberations with internal stakeholders such as faculty and students often need to address both the goals for the program and they way in which they are realized – the program design.

    In response to these needs, the paper presents a framework which brings together the goals and the design of the program. This achieved by combining the CDIO syllabus and the CDIO curriculum design tools, in a framework that also includes the statement of program-specific goals for disciplinary knowledge. We call this framework integrated program descriptions. In the paper, the contents of these components and the process of implementing them at Chalmers and KTH are discussed. The KTH case involves the CDIO-based Vehicle Engineering program. The Chalmers application spans about 70 engineering programs, both CDIO-based and non-CDIO-based. Benefits and challenges are discussed.

  • 63.
    Malmqvist, Johan
    et al.
    Chalmers University of Technology.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Edström, Kristina
    KTH, School of Education and Communication in Engineering Science (ECE), Learning.
    Using Integrated Programme Descriptions to Support a CDIO Programme Design Process2006In: World Transactions on Engineering and Technology Education, ISSN 1446-2257, Vol. 5, no 2, p. 259-262Article in journal (Refereed)
  • 64.
    Malmqvist, Johan
    et al.
    Chalmers University of Technology.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Gunnarsson, Svante
    Linköping University.
    Edström, Kristina
    KTH, School of Education and Communication in Engineering Science (ECE), Learning.
    The Application of CDIO Standards in the Evaluation of Swedish Engineering Degree Programmes2006In: World Transactions on Engineering and Technology Education, ISSN 1446-2257, Vol. 5, no 2, p. 361-364Article in journal (Refereed)
  • 65.
    Marin, Gustav
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics. RISE Innventia.
    Nygårds, Mikael
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Stiffness and strength properties of five paperboards and their moisture dependency2019In: Proceedings of the 2019 International Paper Physics Conference / [ed] Doug Coffin, Tappi , 2019Conference paper (Refereed)
    Abstract [en]

    Five folding box boards made on the same paperboard machine have been analyzed. The paperboards were from the same product series but had different grammage (235, 255, 270, 315, 340 g/m2) and different bending stiffness. The paperboards are normally used to make packages, and since the bending stiffness and grammage varies the packages performance will be different. Finite element simulations can be used to predict these differences. However, the stiffness and strength properties then need to be known. For efficient determination of the three-dimensional properties in MD, CD and ZD, it is proposed that the whole paperboard should be characterized with the following tests: in-plane tension, ZD tension, shear strength profiles and two-point bending. The stiffness and strength properties have with the proposed setups been determined at different relative humidity (20, 50, 70 and 90 % RH), and the mechanical properties have been evaluated as function of moisture ratio.

    The results showed a linear relation between mechanical properties and moisture ratio for each paperboard. The data was then normalized with data for the standard climate (50 % RH) and investigated as a function of moisture ratio. The results indicated that the normalized mechanical properties for all paperboards coincided along one single line and could therefore be expressed as a linear function of moisture ratio and two constants.

    Consequently, the study indicates that it is possible to obtain the mechanical properties of a paperboard, by knowing the structural properties for the preferred level of RH and the mechanical property for the standard climate (50 % RH and 23 °C).

  • 66. Meng, Guanqun
    et al.
    Trost, Thomas
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Stacking misalignment of corrugated boxes - a preliminary study2007In: Proceedings of the 23rd IAPRI Symposium on Packaging, 2007Conference paper (Other academic)
    Abstract [en]

    Misalignment of corrugated boxes stacked on top of each other, on for example a pallet, considerably reduces the strength and life time of the boxes. The development of analytical tools for prediction of the influence of misalignments on the stacking strength would therefore be of large practical importance. In order to develop such tools, it is important to know the pressure distribution between misaligned boxes.

     

    This paper reports on a preliminary study using a pressure sensitive film to determine the pressure distribution between boxes. A parametric study using filled (90 %) and unfilled corrugated boxes has been performed and the pressure distributions will be presented as functions of the stacking load and parameters describing the misalignment.

  • 67.
    Mäkelä, Petri
    et al.
    STFI Packforsk AB.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Cohesive crack modelling of paper materials2007In: Proceedings of the 2007 International Paper Physics Conference, 2007, p. 357-364Conference paper (Other academic)
    Abstract [en]

    When a notched structure is subjected to sufficiently large tensile loading, it generally exhibits damage evolution in the crack tip region. In paper materials, this damage evolution may reach considerable sizes before ultimate collapse of the structure occurs. Gradual crack growth, characterized by large-scale damage evolution ahead of the crack tip prior to ultimate failure, makes the determination of the initiation of crack growth ill-conditioned and causes the loading in the structure to increase non-proportionally. The applicability of fracture mechanics models that neglect damage evolution, such as classical non-linear fracture mechanics models, therefore becomes questionable for paper materials. The aim of this work was to develop an anisotropic elasticplastic cohesive crack model, i.e. a fracture mechanics model that is capable of accurately describing the anisotropic non-linear constitutive behaviour and the damage behaviour of paper materials. Furthermore, the aim was to use the developed model for studying whether the crack tip state can be characterised by one parameter, as in classical non-linear fracture mechanics models, although large-scale damage evolves. An orthotropic incremental elastic-plastic softening cohesive crack model was formulated. The material parameters of the cohesive crack model were determined for one grade of commercial paper, solely by tensile testing. Short tensile test pieces, allowing for stable post-peak stress behaviour during tensile testing, were used to determine the damage material parameters. The developed cohesive crack model is selfcontained in fracture mechanics applications, i.e. no testing on notched structures was required for calibrating the fracture criterion. The accuracy of the cohesive crack model was investigated by comparing model predictions with experimental data from ordinary tensile tests in different material directions and from tensile testing of notched test pieces with different crack lengths. The calibrated cohesive crack model was shown to predict constitutive behaviour and ultimate failure in excellent agreement with the experiments. The cohesive crack model was further used for examining the crack tip region in more detail. It was demonstrated that the J-integral may accurately characterise the stress and deformation state in the vicinity of the crack tip, although large-scale damage evolves in the crack tip region.

  • 68. Nygards, Mikael
    et al.
    Fellers, C.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Measuring out-of-plane shear properties of paperboard2007In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 33, no 2, p. 105-109Article in journal (Refereed)
    Abstract [en]

    Two experimental procedures were used to test the shear properties of three paperboards. First, a douhle-notch shear specimen was used. In this test, an in-plane tensile test specimen was utilized. To generate a shear test from the specimen, two notches were cut through the specimen, one from each side of the specimen. This test procedure is simple and easily can be used for production control. Second, a rigid-block shear test was used. In this test, the paper boards were glued with photo-mounting tissue between two steel blocks. The specimen was tested with displacement control in an MTS system. From both shear tests, maximum shear stresses were calculated and compared. It was shown that reproducible results were achieved by both methods.

  • 69.
    Nygårds, Mikael
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Fellers, Christer
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    DEVELOPMENT OF THE NOTCHED SHEAR TEST2009In: ADVANCES IN PULP AND PAPER RESEARCH, OXFORD 2009, VOLS 1-3 / [ed] IAnson SJ, BURY: PULP & PAPER FUNDAMENTAL RESEARCH SOCIETY , 2009, p. 877-897Conference paper (Refereed)
    Abstract [en]

    The notched shear test (NST) will be proposed to measure shear strength of paperboard by utilizing standard in-plane tensile testing equipment. The test is a further development of the double notch shear test specimen; where plastic lamination has been utilized. With the new test setup it becomes possible to measure shear strength profile in the thickness direction of paperboard. As a spin-off of the NST also the strip shear test (SST) was suggested. The SST test can be used as a quick measure of shear strength. It is shown that the SST measurement correlates well with the traditional rigid shear test (RST). In order to verify that the NST specimen failed due to shear stresses, finite element simulations were performed. The simulations showed that the shear zone had a uniform shear stress field at the time of failure. Moreover, with the finite element simulations it was also possible to predict the force-displacement curve.

  • 70.
    Nygårds, Mikael
    et al.
    Innventia AB.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Mathematical modelling and analysis of converting and end-use2009In: Pulp and Paper Chemistry and Technology: Volume 4: Paper Products Physics and Technology / [ed] Monica Ek, Göran Gellerstedt, Gunnar Henriksson, Berlin: Walter de Gruyter GmbH & Co. KG , 2009, 1, p. 315-334Chapter in book (Refereed)
  • 71.
    Stehr, M.
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    An investigation of the crack tendency on wood surfaces after different machining operations2000In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 54, no 4, p. 427-436Article in journal (Refereed)
    Abstract [en]

    A hypothesis describing tip-crack generation during different machining operations on sapwood of pine surfaces and the differences between the pith side and bark side were investigated. The goal was to determine whether initial tip-cracks are generated already during the machining operation, before weathering. The evaluation of the results showed that about 10 Ic of the annual rings on the pith side and about 1 % of the annual rings on the bark side have cracks after the machining operation. A theoretical analysis of an idealised model using the finite element method (FEM) has also been carried out to investigate the tendency for growth of short initial tip-cracks during a simulation of the compression of a wood piece in a conventional planing operation. The analysis seems to confirm that there is a greater risk for cracks on the pith side than on the bark side.

  • 72. Stenberg, N.
    et al.
    Fellers, C.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Measuring the stress-strain properties of paperboard in the thickness direction2001In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 27, no 6, p. 213-221Article in journal (Refereed)
    Abstract [en]

    In this paper, a device for measuring stress-strain properties in the thickness direction of paperboard is presented. The device is used for out-of-plane tensile, compression and shear loading. In order to measure stress and strain accurately: the deformation of the test piece is restricted to two directions in the desired plane of deformation by means of an attached fixture. The paperboard is first glued io metal blocks with a high-viscosity adhesive and these blocks are then attached ro the device using a fast-curing epoxy adhesive. To find the true strain in the material, knowledge of the penetration nf the adhesive into the surface of the paper structure is important. A method for determining the penetration of the adhesive, based on a comparison of stress-deformation curves for glued and non-glued rest pieces, is presented, Finally: true stress-strain curves in tension, compression, and shear are presented together with an analysis of the accuracy of the method.

  • 73. Stenberg, N.
    et al.
    Fellers, C.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Plasticity in the thickness direction of paperboard under combined shear and normal loading2001In: Journal of engineering materials and technology, ISSN 0094-4289, E-ISSN 1528-8889, Vol. 123, no 2, p. 184-190Article in journal (Refereed)
    Abstract [en]

    Creasing and offset printing are both examples of paperboard converting operations where the stress state is multiaxial. and where elastic-plastic deformation occurs in the thickness direction. Optimization of paperboard for such operations requires both advanced modeling and a better understanding of the mechanical behavior of the material. Today, our understanding and modeling of the our-of-plane properties are nor as well established as our knowledge of the in-plane behavior. In order to bridge this gap, a modification of the Arcan device, which is well known in other fields, was developed for the experimental characterization of the out-of-plane mechanical behavior of paperboard. A fixture attached to the Arcan device was used to control the deformation in the test piece during loading. The test piece was glued to the device with a high viscosity adhesive and left stress-free during curing to achieve an initial state free of stresses, The apparatus proved to work well and to produce reliable results. Measurements of the mechanical behavior in combined normal and shear loading generated data points for the determination of the yield surface in the stress space. The elastic-plastic behavior in the thickness direction of paperboard was modeled assuming small-strain orthotropic linear elasticity and a quadratic yield function. Simulations using this yield function and an associative flow law showed good agreement with the rest results.

  • 74.
    Svensson, Anna
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
    Lindström, Tom
    Ankerfors, Mikael
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    3D-shapeable thermoplastic paper materials2013In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 28, no 4, p. 602-610Article in journal (Refereed)
    Abstract [en]

    The purpose of this work was to investigate to what extent it is possible to improve the thermoplastic properties of paper materials so that 3D-shapeable paper products can be manufactured. For that purpose, the addition of various chemical adjuvants, known to improve both tensile strength index and strain at break, was investigated. Adding polylactide latex was found to significantly improve both the tensile strength properties and strain at break of paper materials. To enhance their strainability, the paper sheets were cured at an elevated temperature of 150 degrees C. The improved strainability after curing is hypothesized to relate to the spreading of the polylactide latex (minimum film-forming temperature of 90 degrees C) on the fibre surfaces, improving the relative bonded area. Both the tensile strength index and strain at break improved significantly with no densification of the paper sheets. A second aim was to make double-curved board structures in a hydroforming equipment, using the sheets treated with polylactide latex under various conditions. Double-curved sheets with a nominal strain at break of over 20% could be formed by adding 20% polylactide latex. Hydroforming had to be done at temperatures exceeding the minimum film-forming temperature of the polylactide latex to significantly improve the strain at break during the forming operation.

  • 75.
    Tjahjanto, Denny D.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). ABB AB, Sweden.
    Girlanda, O.
    Östlund, Stefan
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Anisotropic viscoelastic-viscoplastic continuum model for high-density cellulose-based materials2015In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 84, p. 1-20Article in journal (Refereed)
    Abstract [en]

    A continuum material model is developed for simulating the mechanical response of high-density cellulose-based materials subjected to stationary and transient loading. The model is formulated in an infinitesimal strain framework, where the total strain is decomposed into elastic and plastic parts. The model adopts a standard linear viscoelastic solid model expressed in terms of Boltzmann hereditary integral form, which is coupled to a rate-dependent viscoplastic formulation to describe the irreversible plastic part of the overall strain. An anisotropic hardening law with a kinematic effect is particularly adopted in order to capture the complex stress-strain hysteresis typically observed in polymeric materials. In addition, the present model accounts for the effects of material densification associated with through-thickness compression, which are captured using an exponential law typically applied in the continuum description of elasticity in porous media. Material parameters used in the present model are calibrated to the experimental data for high-density (press)boards. The experimental characterization procedures as well as the calibration of the parameters are highlighted. The results of the model simulations are systematically analyzed and validated against the corresponding experimental data. The comparisons show that the predictions of the present model are in very good agreement with the experimental observations for both stationary and transient load cases.

  • 76.
    Tjahjanto, Denny
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Girlanda, Orlando
    ABB Corporate Research.
    Ask, Anna
    ABB Corporate Research.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Constitutive model for high-density cellulose-based materials2014In: Proceedings of 27th Nordic Seminar on Computational Mechanics / [ed] A. Eriksson, A. Kulachenko, M. Mihaescu and G. Tibert, KTH, 2014Conference paper (Other academic)
  • 77.
    Tojaga, Vedad
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH Royal Inst Technol, Dept Solid Mech, SE-10044 Stockholm, Sweden..
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation2019In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 182, article id 107758Article in journal (Refereed)
    Abstract [en]

    Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber matrix level and predicting the compressive strength of laminates.

  • 78.
    Tojaga, Vedad
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation2019In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 182, article id 107758Article in journal (Refereed)
    Abstract [en]

    Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber-matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber-matrix level and predicting the compressive strength of laminates.

  • 79.
    Trädegård, Annika
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Nilsson, Fred
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    FEM-remeshing technique applied to crack growth problems1998In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 160, no 1-2, p. 115-131Article in journal (Refereed)
    Abstract [en]

    The remeshing option of the commercial FE-code ABAQUS is used to study crack propagation in elastic-plastic materials. Propagation is accomplished with a combination of remeshing and nodal relaxation. The results are compared with analyses using nodal release only. Substantial crack growth is achieved with less computer time and higher accuracy than in conventional FE analysis.

  • 80.
    Trädegård, Annika
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Nilsson, Fred
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    J-Q characterization of propagating cracks1998In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 94, no 4, p. 357-370Article in journal (Refereed)
    Abstract [en]

    An investigation is performed to determine to what extent the state at a growing crack tip vicinity can be characterised by J and Q calculated from FE analyses of successively stationary crack tip positions. FE models in two-dimensionals of single edge notch bend and double edge cracked panel specimens with several different crack lengths are used to cover a range of load and constraint levels. The stress and strain fields are compared between different specimens keeping J- and Q-values equal. A remeshing technique in the commercial FE-code ABAQUS is used to enhance the efficiency of the analysis. The results show that the J-Q-theory provides reasonably accurate crack tip characterization also for growing cracks. This leads to the conclusion that FE analyses of successive stationary cracks rather than full FE propagation analyses are sufficient. The limit of validity for propagation is similar to the validation limit for the stationary case, although somewhat more restrictive.

  • 81.
    Trädegård, Annika
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Nilsson, Fred
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    J-Q characterization of propagating cracks by FEM-remeshing1997In: Computational Mechanics, ISSN 0178-7675, E-ISSN 1432-0924, Vol. 20, no 1-2, p. 181-185Article in journal (Refereed)
    Abstract [en]

    An investigation is performed as to the extent J and Q calculated for successively stationary crack positions can be used to characterize the state at a growing crack in a corresponding geometry. FE models of single-edge notch bend and double-edge cracked panel specimens are used to cover a variation of constraint levels. The stress and strain fields are compared between different specimens at equal J-values and Q-values, respectively. A remeshing technique is used to enhance the efficiency of the analysis. The commercial FE-code ABAQUS is used and substantial crack growth is achieved with less computer time and better accuracy than in conventional FE analysis.

  • 82. Wallmeier, Malte
    et al.
    Linvill, Eric
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation. KTH, School of Chemical Science and Engineering (CHE), Centres, Biofibre Materials Centre, BiMaC.
    Hauptmann, Marek
    Majschak, Jens-Peter
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Chemical Science and Engineering (CHE), Centres, Biofibre Materials Centre, BiMaC. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Explicit FEM analysis of the deep drawing of paperboard2015In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 89, p. 202-215, article id 2441Article in journal (Refereed)
    Abstract [en]

    An explicit finite element model of the deep-drawing of paperboard has been developed utilizing a custom yet simple material model which describes the anisotropy and plasticity of paperboard. The model was verified with a variety of tests and was then utilized to compare the punch force that was measured during the deep-drawing experiments to the punch force that was calculated during the deep-drawing simulations. All material parameters were calibrated based on individual experiments; thus, no parameter fitting was utilized to match the experimental deep-drawing results. The model was found to predict the experimental results with reasonable accuracy up to the point when wrinkling began to dominate the material response. Since most failures during paperboard deep-drawing occur before wrinkling begins to play a major role, this model can probably be utilized to study and predict the failure of deep-drawn paperboard cups. The overall trends and the effects of major process parameters are predicted by the model. The process parameters that were varied and compared for both experiments and simulations were: blankholder force, die temperature, and thickness. The model was utilized to discover that friction of the blankholder and die have significant effects on the punch force and thus the stress, implying that low-friction dies and blankholders can considerably reduce the failure probability and thus also improve the quality of deep-drawn paperboard cups.

  • 83.
    Östlund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Borodulina, Svetlana
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Influence of Paperboard Structure and Processing Conditions on Forming of Complex Paperboard Structures2011In: Packaging technology & science, ISSN 0894-3214, E-ISSN 1099-1522, Vol. 24, no 6, p. 331-341Article in journal (Refereed)
    Abstract [en]

    Experiments were made with a laboratory apparatus for forming paper sheets into double-curved structures. The purpose was to learn more about the design criteria for forming of paper materials into complex shapes. The influence of forming parameters such as moisture and temperature was studied, and the performance of some common pulps as raw materials for the paper sheets was evaluated. Papers made from short hardwood fibres performed worse than papers from other pulps, and the method of moisture application was more important than the amount of moisture that was applied.

  • 84.
    Östlund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Mäkelä, Petri
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    The influence of through-thickness variation on the mechanics of paper drying2005In: ADVANCES IN PAPER SCIENCE AND TECHNOLOGY: TRANSACTIONS OF THE 13TH FUNDAMENTAL RESEARCH SYMPOSIUM, VOLS 1-3 / [ed] IAnson SJ, 2005, p. 213-240Conference paper (Refereed)
    Abstract [en]

    A material model for drying paper is presented. Moisture-dependent material parameters, hygroscopic shrinkage, the elastic and the time-dependent responses of the material to load, and the effect of unloading at a higher stiffness than the load was applied at are modelled. The model is used to determine the effects of a varying moisture ratio through the paper during drying on free shrinkage development and stiffness development at free drying. Simulation results for the stress development during drying and the state of residual stress immediately after drying are also presented. The model predicts a variation of in-plane elastic moduli through the paper, a prediction that is studied by experiments.

  • 85.
    Östlund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Carlsson, Leif
    Fellers, Christer
    Experimental determination of residual stresses in paperboard2005In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 45, no 6, p. 493-497Article in journal (Refereed)
    Abstract [en]

    A method for the experimental determination of the through-thickness residual stress distribution in paper-board is presented.The successive removal of thin layers from strips of board through surface grinding changes the stress-state and the bending stiffness resulting in a changed curvature, which is measurable. From tests of strips in both in-plane directions, stress distributions can then be evaluated using the Treuting-Read method. Geometrically nonlinear effects at the large deformations taking place are avoided through a proper choice of strip dimensions. Typical results are presented and factors influencing the accuracy of the determination are thoroughly discussed.

  • 86.
    Östlund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Carlsson, Leif
    Fellers, Christer
    Residual stresses in paperboard through the manufacturing process2005In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 31, no 4, p. 197-201Article in journal (Refereed)
    Abstract [en]

    The residual stresses in a machine-made multi-ply board with chemical pulp in all plies were monitored as the board progressed after the initial drying through calendering, moisture treatments and coating. After drying, the stresses were compressive near the surfaces. In the coated board at the end of the machine, the stress state was reversed to a large extent with tensile stresses in the outermost layers. The various moisture applications on the board in the post-processing steps are regarded to have caused the change in the residual stress state. In another machine-made board, with mechanical pulp in the middle plies, the stresses after drying were limited to the surface plies made from chemical pulp, but were again compressive at the surfaces. This stress state was largely unaffected by post-processing. The possibility for stress reversal and the implications on in-plane and out-of-plane dimensional stability of the boards are discussed.

  • 87.
    Östlund, Magnus
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Carlsson, Leif
    Fellers, Christer
    The influence of drying conditions on residual stress build-up in paperboard2004In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 30, no 11, p. 312-316Article in journal (Refereed)
    Abstract [en]

    The through-thickness distributions of in-plane residual stress have been determined for some laboratory-made paperboards. The effects of drying variables such as temperature, moisture flow resistance at the surfaces and thickness of the boards on residual stresses were studied. The drying rate was shown to influence residual stresses only for relatively slow drying. Residual stresses correlate well with the moisture gradient during drying for various drying methods. Some effects of the drying conditions on paper tensile stiffness are discussed. One-sided heating for curl control is also discussed.

  • 88.
    Östlund, Magnus
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Carlsson, Leif
    Fellers, Christer
    The influence of drying restraints and beating degree on residual stress build-up in paperboard2004In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 30, no 11, p. 289-293Article in journal (Refereed)
    Abstract [en]

    The through-thickness distribution of in-plane residual stress has been determined for laboratory-made paperboards, to study the causes of these stresses. The degree of restraint during drying is shown not to influence the residual stresses in a significant way. However, a higher level of beating of the pulp leads to higher residual stresses. The drying gradient through the sheet combined with the shrinkage of paper as moisture content decreases is suggested as the main reason for residual stress build-up. No effect an free shrinkage was seen when attempting to limit the drying gradients by decreasing the grammage.

  • 89.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Fracture modelling of brittle-matrix composites with spatially dependent crack bridging1995In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 18, no 10, p. 1213-1230Article in journal (Refereed)
    Abstract [en]

    In brittle-matrix composites cracking of the matrix is often accompanied by bridging of the crack surfaces. The bridging will reduce the net stress intensity factor at the crack tip and consequently increase the toughness of the composite material. The bridging mechanism is due to for example unbroken whiskers, fibres, ductile particles or interlocking grains.

    Analysis of the bridging mechanism in cracked structures is conveniently carried out using the concept of cohesive zone modelling. In this case the action of the bridging elements is replaced by a distribution of forces, so called cohesive forces trying to close the crack. The commonly used approach in such modelling has been to replace the action from individual bridging elements by a continuous spatially independent distribution of closing tractions whose magnitude is a function of the crack opening displacement only.

    In this paper the influence of the spatial distribution of bridging elements is considered for plane crack problems. The cross section of the bridging elements is assumed to be circular and the distance between the different bridging elements is determined by the volume fraction, the radius and the geometrical distribution of the bridging elements.

    Damage resistance curves have been calculated for typical whiskers-reinforced ceramic composites, and the results from the present spatially dependent models are compared with results from calculations with spatially independent models. The influence of the radius of the bridging element, the volume fraction of whiskers and the material properties are illustrated and the use of spatially independent models is discussed.

  • 90.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Large scale yielding for dynamic crack growth in a strip geometry1991In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 49, no 3, p. 219-237Article in journal (Refereed)
    Abstract [en]

    Dynamic crack growth in a strip geometry has been studied by a convective formulation of the finite element method. The strip is assumed to be made of a material described by an elastic-viscoplastic constitutive equation and the loading conditions are plane strain mode I. The plastic strain-rates are characterized by a power-law overstress model giving an asymptotic elastic singularity at the crack tip. Two different types of loading on the strip geometry have been investigated. In the first geometry the horizontal boundaries parallel to the crack plane were subjected to prescribed displacements perpendicular to the crack plane and the crack tip was loaded in essentially the same way as for a crack in a plate subjected to tensile loading perpendicular to the crack. The second type of loading consisted of a prescribed rotation of the trailing edge in the moving finite element mesh. This created primarily bending loading of the structure.The crack tip energy flow has been calculated for different levels of yielding around the crack tip. The results are compared to the true small scale yielding solution obtained from a boundary layer analysis.The results reported indicate that for the tensile loading there exists a rather wide range of load levels for which the boundary layer solution gives a good description of the fields around the crack tip. For the bending loading the results indicate that the boundary layer solution is a correct description only in cases of extremely small scale yielding. Comparisons between large scale yielding and the boundary layer solution are also made for the effective stress and the effective plastic strain.

  • 91.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    On numerical modeling and fracture criteria of dynamic elastic-viscoplastic crack growth1990In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 44, no 4, p. 283-299Article in journal (Refereed)
    Abstract [en]

    Dynamic steady-state small-scale yielding crack propagation in an elastic-viscoplastic structure is studied by a convective formulation of the finite element method. The loading condition is mode I plane strain.The plastic strain-rates are characterized by a power-law overstress model based on a general theory by Perzyna. For values of the stress exponent less than 3, the elastic strain-rates are more singular than the plastic strain-rates and consequently the near tip fields will exhibit an asymptotic elastic behaviour. The size of the zone where this elastic singularity dominates is typically of the order 10–3–10–5 of the size of the active plastic zone. These relatively small dimensions severely complicate the finite element modeling. Although the energy flow is calculated with a path-independent integral of J-integral type, its value will be dependent on the size of the near-tip elements unless extremely small finite elements are used. This is an important difference compared with many other situations, for example the elastic case and a stationary crack in a power-law hardening material, where the use of a path-independent integral improves the numerical accuracy even if the crack tip region is modeled with a rather coarse mesh. This size effect is discussed in detail. An application of the results with numerical data for a realistic situation shows that the crack tip energy flow might be vanishingly small compared to the energy dissipation in the plastic region. This indicates that the energy flow to the crack tip is perhaps not an appropriate parameter for the description of small-scale yielding crack growth in the present type of material model.The paper also contains an investigation of the introduction of a variable order singular element at the element positions adjacent to the crack tip. It is shown that this type of element does not improve the numerical behaviour in the present formulation.

  • 92.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Three-Dimensional Deformation and Damage Mechanisms in Forming of Advanced Structures in Paper2017In: ADVANCES INPULP AND PAPERRESEARCH,OXFORD 2017: Transactions of the 16th Fundamental Research Symposium, Pembroke College, Oxford, England, September 2017 / [ed] Warren Batchelor and Daniel Söderberg, 2017, Vol. 2, p. 489-594Conference paper (Refereed)
    Abstract [en]

    There is a large potential for wood-fibre based materials such as paper and board to contribute to lightweight structures in several applications, particularly packaging.  Fibre-based packaging materials have important advantages in comparison to fossil-based plastics regarding biodegradability, recyclability and renewability.  Individualisation has become a crucial criterion for the use of packaging solutions and forming of advanced paperboard structures is a key technology for manufacturing of such packaging shapes.  New sustainable packaging concepts are creating a need for paper materials with considerably enhanced properties.

    Paper and board are in processes for manufacturing of geometrically advances structures in general subjected to complex and often little known multi-axial states of loading and deformation that are not necessarily quantified by conventional measures for paper performance.  Today, commercial paperboard is optimised for folding and printing, and not for applications involving forming of advanced structures. It is likewise important to design the manufacturing processes to meet the particular properties of paperboard.  Manufacturing methods that are suitable for metals and plastics are inevitably not suitable for paper and board since the deformation and damage mechanisms of fibre network materials are different from metals and plastics.

    In this paper recent findings in the literature on 3D forming of paper and paperboard structures are reviewed.  In particular, deformation and damage mechanisms involved in pertinent forming operations and how they are related to paper and board properties in order to enhance the development of new advanced paper materials and structures are analysed.

    In the last decade, there have been major advancements in the development of geometrically advanced 3D paperboard structures including technological advances of various forming processes, enriched understanding of the importance and influence of process parameters, and new paperboard materials with significantly improved forming properties.  However, there is still a lack of knowledge regarding the deformation mechanisms of these complex systems, and particularly regarding the influence of friction.  One remedy would be the enhancement of numerical simulation tools.  Optimisation of existing forming processes and development of new ones as well as tailored paper and board materials with properties customised to the demands of existing and new 3D forming processes will also play important roles.  This development is only in its beginning and major progress is expected in the near future.

  • 93.
    Östlund, Sören
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Blom, Karin
    KTH, School of Engineering Sciences (SCI).
    Hjorth, P.-O.
    SAM Samarbetande konsulter AB.
    Ahlstrand, Jonas
    SAM Samarbetande konsulter AB.
    Continuous improvement of a CDIO program using management by means2007In: Proceedings of the 3rd International CDIO Conference, 2007Conference paper (Refereed)
    Abstract [en]

    The Vehicle Engineering program at KTH was one of the first programs that adopted the CDIO-model for engineering education. The implemented changes were during the first three years subjected to a number of internal and external evaluations, with very positive judgments. Perhaps, the most important task for the program management after such a major overhaul of a program is to motivate faculty for continuous improvement work. Faculty have put a lot of effort into the change process, and they are in general not easily susceptible to further demands from the program management. Thus, there was an appropriate opportunity to try a new management philosophy.

    This paper describes how the management by means concept is introduced in the continuous improvement work of the Vehicle Engineering program. This concept, which is in contrast with traditional management by results concepts, has been used successfully in industrial engineering development work. The main feature of the management by means concept is that the development work is driven not by explicit preconceived targets, but by common values and principles.

    The paper presents the development of common values and principles for the Vehicle engineering program, and the implementation of them in the practical work, carried out in a change group covering all first year courses. Benefits and difficulties are described and the experiences gained so far are discussed, particularly, the dual loyalties of faculty that arise in the educational system at KTH where a program, in general, is not owned by a department.

  • 94.
    Östlund, Sören
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Asymptotic crack tip fields for dynamic fracture of linear strain-hardening solids1988In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 24, no 11, p. 1141-1158Article in journal (Refereed)
    Abstract [en]

    Asymptotic crack tip fields, for dynamic crack propagation in an elastic-plastic material, have been calculated. The material is characterized by J2flow theory with linear-strain hardening. The possibility of plastic reloading on the crack flank is taken into account. Numerical results for the strength of the crack tip singularity, the angular positions of elastic unloading and possible plastic reloading regions, and the angular variation of the stress and velocity fields, are presented as functions of the crack tip speed and the ratio between tangent modulus and elastic modulus. Calculations have been performed for crack tip speeds below a certain limit velocity which depends on the tangent modulus and the loading conditions. The different loading modes which have been studied are modes I and II (plane strain and plane stress) and mode III (antiplane strain).

  • 95.
    Östlund, Sören
    et al.
    Swedish Institute of Composites.
    Gudmundson, Peter
    Swedish Institute of Composites.
    Numerical-Analysis Of Matrix-Crack-Induced Delaminations In [+/-55-Degrees] Gfrp Laminates.1992In: COMPOSITES ENGINEERING 2, ISSN 0961-9526, Vol. 2, no 3, p. 161-175Article in journal (Refereed)
    Abstract [en]

    In this paper a three-dimensional glass epoxy angle-ply laminate of the type [+/- 55-degrees] containing matrix cracks and two types of matrix-crack-induced interlaminar local delaminations has been numerically analyzed by the finite element method. Due to periodicity and symmetry only a representative volume element in the form of a parallelepiped bounded by neighbouring matrix cracks needs to be modelled. The additional contributions to the reduction of stiffness coefficients due to local delaminations are compared to the stiffness of undamaged laminates and laminates containing only matrix cracks. Furthermore, the growth of delaminations and progressive matrix cracking are investigated by calculations of their respective total energy release rates as functions of the delamination size and matrix crack density. Implications of the influence of matrix-crack-induced delaminations on the mechanical behaviour of the composite laminate are discussed.

  • 96.
    Östlund, Sören
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    The Application Of Moving Finite-Elements For The Study Of Crack-Propagation In Linear Elastic Solids.1987In: Computers & structures, ISSN 0045-7949, E-ISSN 1879-2243, Vol. 25, no 5, p. 765-774Article in journal (Refereed)
    Abstract [en]

    Mode I crack propagation in linear elastic solids is studied by a convecting finite element mesh. The finite element formulation is briefly described. Two different approaches for the determination of the stress intensity factor K1 have been investigated, one using the COD relation to K1 and one using the dynamic G-integral. Different test problems, including stationary cracks, steady-state crack propagation and transient crack propagation, have been thoroughly examined.

  • 97.
    Östlund, Sören
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Karenlampi, P.
    Structural geometry effect on the size-scaling of strength2001In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 109, no 2, p. 141-151Article in journal (Refereed)
    Abstract [en]

    The sensitivity of the empirical exponent of Bazant's size-effect scaling law on structural geometry is clarified through numerical experiments. For large centre- cracked tension panels, made of a linearly softening material, the best-fitting exponent is 0.90, whereas for large edge-cracked panels it is 0.75. For edge-cracked panels, the value of the exponent increases as a function of increasing crack-length-to-width-ratio. The results indicate that with structures of brittleness numbers below unity, reliable predictions of strength require the size-effect scaling law to be fitted for any particular structural geometry.

  • 98.
    Östlund, Sören
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center BiMaC Innovation.
    Linvill, Eric
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Wallmeier, M
    Continuum Modeling of Wrinkles and Explicit FEM Modeling of Paperboard Deep-Drawing2016In: 2016 Progress in Paper Physics Seminars / [ed] Samuel Schabel, 2016Conference paper (Other academic)
    Abstract [en]

    Deep drawing of paperboard requires the creation of wrinkles for successful forming of a paperboard cup. A phenomenological model for the mechanical response of wrinkles is proposed and based on the assumption that the wrinkles have a mechanical response equivalent to that of a plastic hinge (i.e. the internal moment in the hinge is held constant during deformation). This 1-D model has been verified with two experiments.

                Furthermore, this phenomenological model has been expanded into a 3-D continuum constitutive model, which considers the initiation and propagation of wrinkles, large deformations due to wrinkle formation and wrinkle compression, and permanent deformations. This 3-D continuum model has been compared to and verified against experimental deep-drawing results. Experiments and simulations have been compared in terms of springback, wrinkle propagation, and punch force. Additionally, the model provided insight into the deep-drawing process by establishing better understanding of the initiation of wrinkling.

  • 99.
    Östlund, Sören
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Mäkelä, Petri
    Tetra Pak Packaging Solutions AB.
    Fracture properties2011In: Mechanics of Paper Products / [ed] Kaarlo Niskanen, Berlin: Walter de Gruyter GmbH & Co. KG , 2011, 1, p. 67-89Chapter in book (Refereed)
  • 100.
    Östlund, Sören
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Nilsson, Fred
    KTH, Superseded Departments, Solid Mechanics.
    Cohesive modelling of process regions for cracks in linear elastic structures-fundamental aspects1993In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 16, no 2, p. 215-235Article in journal (Refereed)
    Abstract [en]

    The process region at the tip of a crack in a linear elastic structure has been modelled by a cohesive zone. Growth of the front end of the cohesive zone is governed by a critical stress intensity factor criterion, and advance of the original traction free crack is determined by a critical crack opening at the rear end of the cohesive zone.

    Damage resistance curves relating the applied stress intensity factor to the growth of the cohesive zone have been calculated for an idealized structure containing two characteristic dimensions. Instability resulting in failure of the structure is found to occur either by unstable growth of the front end of the cohesive zone, without a fully developed cohesive zone, or by unstable growth of the original flaw, when the crack opening displacement at the rear end of the cohesive zone reaches a critical value.

    The influence of the size of the structure compared to the length of the cohesive zone is investigated, and conditions for the limits of validity of the small scale yielding assumption are discussed. Comparisons are made between the maximum load and the length of the cohesive zone at instability resulting from the present analysis, and the values predicted by linear elastic fracture mechanics.

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