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  • 3451.
    Öhman, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    3D localization in digital holography from scattered light from micrometer-sized particles2018Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    When a particle is illuminated by a beam of light it will scatter and redistribute the light in all directions. How it scatters depends on the size, shape and refractive index of the particle. Additionally, it depends on the wavelength and polarization of the illuminating beam. The direction and distance to the observer relative the particle also needs to be considered.  A digital holographic imaging system is used to collect parts of the scattered light from micrometer-sized particles. By utilizing digital holography a three-dimensional reconstruction of the imaged scene is possible. Traditionally, particles are localized based on the intensity in the holographic reconstructions. In this licentiate thesis, the phase response of the scattered light is investigated and utilized. An alternative method for locating spherical particles is presented. The method locate particles based on a simple feature of a propagating wave, namely the fact that the wavefront curvature changes from converging to diverging at the axial location of the particle. The wavefront curvature is estimated using two different methods. The first method estimates the lateral phase-gradients using a finite-difference method. The second method uses a three-dimensional parametric model based on a Chebyshev polynomial expansion. The methods are demonstrated using both simulations and experimental measurements. The simulations are based on the Lorenz-Mie scattering theory for spherical particles and are combined with an imaging system model. Experiments are performed using an off-axis polarization sensitive digital holographic system with a coherent Nd:YAG laser. Measurements of stationary particles are made to validate and evaluate the proposed method. It is found that these methods estimate the true axial position and does not have the offset that is associated with intensity-based methods. Additionally, it is possible to exclude noise that shows up as false particles since noise does not have the same phase response as a real particle. The second method, that uses a parametric model, also improves the standard deviation in the positioning.

  • 3452.
    Öhman, Johan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Axial Particle Positioning by Wavefront Parameterization using Chebyshev Polynomials and Off-axis Digital Holography2017Inngår i: Digital Holography and Three-Dimensional Imaging, Washington: The Optical Society , 2017, artikkel-id M4A.3Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A particle can be axially positioned where its scattered light has a plane wavefront. The phase anomaly compared to a plane wave is fitted to 3D Chebyshev polynomial, where coefficients correspond to the axial position.

  • 3453.
    Öhman, Johan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Improved particle position accuracy from off-axis holograms using a Chebyshev model2018Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 57, nr 1, s. A157-A163Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Side scattered light from micrometer-sized particles is recorded using an off-axis digital holographic setup. From holograms, a volume is reconstructed with information about both intensity and phase. Finding particle positions is non-trivial, since poor axial resolution elongates particles in the reconstruction. To overcome this problem, the reconstructed wavefront around a particle is used to find the axial position. The method is based on the change in the sign of the curvature around the true particle position plane. The wavefront curvature is directly linked to the phase response in the reconstruction. In this paper we propose a new method of estimating the curvature based on a parametric model. The model is based on Chebyshev polynomials and is fit to the phase anomaly and compared to a plane wave in the reconstructed volume. From the model coefficients, it is possible to find particle locations. Simulated results show increased performance in the presence of noise, compared to the use of finite difference methods. The standard deviation is decreased from 3–39 μm to 6–10 μm for varying noise levels. Experimental results show a corresponding improvement where the standard deviation is decreased from 18 μm to 13 μm.

  • 3454.
    Öhman, Johan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Off-axis digital holographic particle positioning based on polarization-sensitive wavefront curvature estimation2016Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, nr 27, s. 7503-7510Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Poor axial resolution in holographic particle imaging applications makes particle positioning in 3D space morecomplex since the positions are not directly obtained. In this paper we estimate the axial position of micrometerparticles by finding the location where the wavefront curvature from the scattered light becomes zero. By record-ing scattered light at 90°using off-axis holography, the complex amplitude of the light is obtained. Byreconstruction of the imaged scene, a complex valued volume is produced. From this volume, phase gradientsare calculated for each particle and used to estimate the wavefront curvature. From simulations it is found that thewavefront curvature became zero at the true axial position of the particle. We applied this metric to track an axialtranslation experimentally using a telecentric off-axis holographic imaging system with a lateral magnification ofM1.33. A silicon cube with molded particles inside was used as sample. Holographic recordings are performedboth before and after a 100μm axial translation. From the estimated positions, it was found that the mean dis-placement of particles between recordings was 105.0μm with a standard deviation of 25.3μm.

  • 3455.
    Örtegren, Viola
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013).
    Water jet steering concept: - evaluation of an environmental design, Part 12014Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    The current hydraulic system that function as the power source for operating the water jet steering device, need to be located inside of the hull to avoid possible environmental damage. This will cause a height difference from where the power supply will be located and where the output is needed. The research of literature and the limitations given by Rolls-Royce laid the basis for the simulation work. The lever concept is a development of the original layout from Rolls-Royce. The current lever concept was formed by simulation of the individual parts it consists of. The modeling work is a starting point for further design changes and improved solutions depending on what results are achieved when simulation is performed on the parts. The modeling work is not part of this report but can be seen in the other master thesis “Water jet steering concept - evaluation of an environmental design, Part 2” which is not yet done but it will be soon. All simulations made are simplified and they are a solid starting point for further work with dimensioning, material selection and calculations. The results of the simulation show that further development is required before a theoretically functioning concept is achieved.

  • 3456.
    Östberg, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Numerisk akustik.
    Göransson, Peter
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Numerisk akustik.
    On cylindrical waves in anisotropic Cartesian materials and its implications on the harmonic mode separability assumption2011Konferansepaper (Annet vitenskapelig)
  • 3457.
    Östberg, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Göransson, Peter
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Hörlin, Nils
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Kari, Leif
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Weak forms for modelling of rotationally symmetric, multilayered structures, including anisotropic poro-elastic media2012Inngår i: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 90, nr 8, s. 1035-1052Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A weak form of the anisotropic Biot's equation represented in a cylindrical coordinate system using a spatial Fourier expansion in the circumferential direction is presented. The original three dimensional Cartesian anisotropic weak formulation is rewritten in an arbitrary orthogonal curvilinear basis. Introducing a cylindrical coordinate system and expanding the circumferential wave propagation in terms of orthogonal harmonic functions, the original, geometrically rotationally symmetric three dimensional boundary value problem, is decomposed into independent two-dimensional problems, one for each harmonic function. Using a minimum number of dependent variables, pore pressure and frame displacement, a computationally efficient procedure for vibro-acoustic finite element modelling of rotationally symmetric three-dimensional multilayered structures including anisotropic porous elastic materials is thus obtained. By numerical simulations, this method is compared with, and the correctness is verified against, a full three-dimensional Cartesian coordinate system finite element model.

  • 3458.
    Östberg, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Kari, Leif
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Dynamic stiffness of hollowed cylindrical rubber vibrationisolators: The wave-guide solutionArtikkel i tidsskrift (Annet vitenskapelig)
  • 3459.
    Östberg, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Kari, Leif
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Transverse, tilting and cross-coupling stiffness of cylindrical rubber isolators in the audible frequency range-The wave-guide solution2011Inngår i: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 330, nr 13, s. 3222-3244Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Audio-frequency wave-guide models for antisymmetric dynamic stiffness of arbitrary long elastomer cylinders are presented. The locally non-mixed boundary conditions at the lateral and radial surfaces are simultaneously satisfied by using the modes corresponding to the dispersion relation for axial waves in cylinders satisfying the stress free boundary conditions at the curved radial boundaries, while the displacement conditions on the flat cylinder ends are satisfied by mode matching. The elastomer is modelled as nearly incompressible with deviatoric visco-elasticity based on a fractional derivative, standard linear solid embodying a Mittag-Leffler relaxation kernel, the main advantage being the minimum parameter number required to successfully model the material properties over a broad frequency band. The stiffness is found to depend strongly on frequency: displaying resonances and anti-resonances. The method is compared with and verified against finite element models. In addition, comparison to thin beam theories, i.e. Euler and Timoschenko theory and a simple shear model, is presented, illustrating the limitations of these models.

  • 3460.
    Österlind, Tomas
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Kari, Leif
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Nicolescu, Cornel-Mihai
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Analysis of stationary displacement patterns in rotating machinery subject to local harmonic excitation2017Inngår i: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 389, s. 224-235Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Rotor vibration and stationary displacement patterns observed in rotating machineries subject to local harmonic excitation are analysed for improved understanding and dynamic characterization. The analysis stresses the importance of coordinate transformation between rotating and stationary frame of reference for accurate results and estimation of dynamic properties. A generic method which can be used for various rotor applications such as machine tool spindle and turbo machinery vibration is presented. The phenomenon shares similarities with stationary waves in rotating disks though focuses on vibration in shafts. The paper further proposes a graphical tool, the displacement map, which can be used for selection of stable rotational speed for rotating machinery. The results are validated through simulation of dynamic response of a milling cutter, which is a typical example of a variable speed rotor operating under different load conditions.

  • 3461.
    Östlund, Rickard
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Microstructure based modelling of ductile fracture in quench-hardenable boron steel2015Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Reduction of fuel consumption and emissions by vehicle weight minimization constitute a major driving force for the development of new materials and manufacturing processes in the automotive industry. Simultaneously formed and quenched boron steel components have higher strength to weight ratio than conventional mild steel components. Additionally, hot formed components can be tailored to have regions with lower strength and higher ductility, improving their crash performance. This is often realized via dierential in-die cooling rates, thus yielding a variable microstructure compositiongiving rise to distributed mechanical properties. Predicting the performance envelopes of these types of components poses some challenges in terms of constitutive modelling, due to the dierential material composition and mechanical properties. Moreover, fractureinitiation is often a limiting design factor. This thesis aims to contribute to the constitutive and ductile fracture modelling of quench-hardenable boron steels, with reference to microstructure composition and hence process history. Modelling techniques which in an approximate manner can estimate the eective material properties based on the properties of the constituents in combination with ductile fracture models are presented.Computational issues concerning numerical nite element modelling of material instabilities are also addressed, essentially via two dierent methods. Introducing a discretization dependent parameter in the constitutive description, or by kinematic enhancements with respect to the localization problem. Both aim to reduce mesh sensitivity and provide improved predictions of post-instability response with industrially relevant mesh sizes.Additionally, an experimental investigation on the ow and fracture properties of boron steel, with a comprehensive range of dierent microstructure compositions, is presented. A full-eld measurement technique enabled the direct evaluation of mechanical properties and fracture relevant data from tensile tests. These results have supported the establishment of models and enabled their calibration, and they provide further insight to the inuence of microstructure and processing conditions on the ductile fracture properties. Comparisons between simulations and experiments indicate that useful predictions of the overall hardening behaviour and fracture elongations can be obtained by the suggested microstructure based modelling approach.

  • 3462.
    Östlund, Rickard
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling and characterisation of fracture properties of advanced high strength steels2011Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Growing demands for passenger safety, vehicle performance and fueleconomy is a continuous driving force for the increase in use of advancedhigh strength steels (AHSS) in the automotive industry. Thesesteels area characterised by improved formability and crash worthinesscompared to conventional steel grades. An important prerequisite of theapplication of new material grades is the characterization of its mechanicalproperties. Post-localization and fracture predictive technologiesgreatly facilitate the design of components which make optimal use ofthese steel grades. In this thesis, press hardened boron alloyed steelsubjected to differential thermo-mechanical processing is characterized.Fracture properties in relation to the different microstructures obtainedis studied. Furthermore a dual phase (DP) cold forming steel is chosenfor evaluation of ductility limit in shear loading. throughout thiswork a strategy for modelling post-localization response and predictingductility limit using shell elements larger then the typical width of thelocalized neck is used. The studied material is assumed to be in a stateof plane stress. Mesh dependency is alleviated by the introduction of aelement size dependent parameter into the constitutive description. Thisparameter acts as a hardening parameter, controlling the evolution ofthe yield surface depending on loading, strain history and shell elementsize. Model calibration relies on a full field measurement technique, DigitalSpeckle Photography (DSP), to record the plane deformation field oftensile specimens. Quantitative measurements of the severely localizeddeformation preceding crack initiation are feasible. With the proposedstrategy, mesh sensitivity in terms of post localization load responseand fracture elongation predictions is reduced significantly compared toresults obtained without the element size dependent parameter. It wasfound that high strain hardening favours strain localization of shear band type, and accelerates the formation of a localized neck. The hardeningcharacteristics is determinant to which deformation mode dissipates theminimum energy. For the DP steel, the Tresca yield surface more accuratelydescribes the yielding point compared to the von Mises planestress elipse. Furthermore, the exponential ductility function dependenton the stress triaxiality parameter agrees well with experimental fracturedata in the ductile loading regime for both DP and boron steel.In shear loading, the maximum shear (MS) stress criterion successfullydescribes the ductility limit. Due to the significantly different ductilityof the various microstructures obtainable by the thermo-mechanicalprocessing of boron alloyed steel, a modelling strategy is needed. It wasfound that in ductile loading, local equivalent fracture strain can be relatedto the hardness of that material point. An exponential decrease inductility with increased hardness describes experimental data collectedfor five different microstructures.

  • 3463.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, Daniel
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Failure analysis of a hat profile with tailored properties subjected to axial compression2013Inngår i: 4th International Conference Hot Sheet Metal Forming of High-performance Steel CHS: June 9-12, Luleå, Sweden : Proceedings / [ed] Mats Oldenburg; Braham Prakash; Kurt Steinhoff, Auerbach: Verlag Wissenschaftliche Scripten , 2013, s. 23-30Konferansepaper (Fagfellevurdert)
  • 3464.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Golling, Stefan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Microstructure based modeling of ductile fracture initiation in press-hardened sheet metal structures2016Inngår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 302, s. 90-108Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The manufacturing of sheet metal components with spatially varying microstucture composition and mechanical properties using press-hardening technology is now an established practice in the automotive industry. To estimate the performance envelopes of such components, a multi-scale approach to ductile fracture prediction based on mean-field homogenization is proposed. Two non-interacting fracture criteria are formulated in terms of the local average stress field, referring to inter-phase and intra-phase fracture mechanisms. The overall ductility is governed by the weakest constituent or interface present in the multiphase material. Moreover, instabilities related to the strain localization problem at the macroscale are treated by embedding discontinuities in the element formulation. These are triggered by a localization criterion derived via bifurcation analysis of the homogenized material. Issues concerning numerical implementation include a forward Euler scheme for integrating the mean-field equations, suitable for explicit finite element analysis of heterogeneous materials. Tensile specimens with ten distinctly different microstructure compositions are evaluated, for which useful predictions of the overall force-displacement response and fracture elongations are demonstrated.

  • 3465. Östlund, Rickard
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Microstructure-Based Modelling of Ductile Failure2015Inngår i: Hot Sheet Metal Forming of High-Performance Steel 5th International Conference: May 31-June 3, Toronto, Canada : Proceedings / [ed] Kurt Steinhoff; Mats Oldenburg; Braham Prakash, Auerbach: Verlag Wissenschaftliche Scripten , 2015, s. 149-156Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This study is concerned with plasticity and ductile fracture modelling aspects of press hardened steels, where multiple phases of different yield strength, work hardening and ductility are present. Macroscopic constitutive properties are determined using a semi-analytical approach termed Mean Field Homogenization (MFH), based on the properties of individual phases and micro-topology. This is combined with a phenomenological ductile fracture initiation criterion formulated in stress space at the microscopic scale. Localization enhanced kinematics are introduced at the macro-scale when instability is signaled by a local bifurcation analysis of the homogenized material. Discontinuities in the strain field are introduced into a quadrilateral shell based on the weak discontinuity approach. Fracture properties and flow curves up to large strains are determined from single phase tensile experiments using digital image correlation. The predictions of the integrated MFH and fracture model is compared with experimental results on steel sheets with different thermal histories, in terms of fracture prediction and and overall mechanical properties

  • 3466.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, D.
    Gestamp R&D.
    Evaluation of localization and failure of boron alloyed steels with different microstructure compositions2014Inngår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 214, nr 3, s. 592-598Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Within the press hardening technology, where hot sheet blanks are simultaneously formed fixed and quenched, new methods with differential thermal treatment come to light. With controlled tool temperature variation, components with tailored properties can be produced. Automotive components combining high energy absorption and intrusion protection in a crash situation are feasible. In the present work the mechanical properties of three different material qualities, beginning with the same base sheet metal subjected to different thermal histories, are investigated. A strategy for modelling post-necking response and crack initiation using shell elements larger then the typical bandwidth of the localized neck is used. The model relies on a sequence of full field measurements throughout a tensile test; i.e. Digital Speckle Photography(DSP). The full field experimental method allows for evaluation of mechanical and failure properties at different analysis lengths, providing parameters for a model which accounts for shell element size. Additionally the model contains a strain based failure criteria as a function of stress triaxiality. Good correlations between a simulated tensile test and experimental results were found. A detailed metallographic study of the three grades was performed and is presented.

  • 3467.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, Daniel
    Gestamp Hardtech AB, Luleå, Sweden.
    Failure model evaluation for varying microstructure based on material hardness2011Inngår i: Proceedings: 3rd International Conference Hot Sheet Metal Forming of High Performance Steel : June 13 - 17, 2011, Kassel, Germany / [ed] Mats Oldenburg, Auerbach: Verlag Wissenschaftliche Scripten , 2011Konferansepaper (Fagfellevurdert)
  • 3468.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, Daniel
    Gestamp R&D.
    Numerical failure analysis of steel sheets using a localization enhanced element and a stress based fracture criterion2015Inngår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 56, s. 1-10Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Strain localization and fracture initiation of elasto-plastic thin steel sheets is analyzed. A shell element enhanced by embedded discontinuities is developed to improve coarse-mesh accuracy in terms of fracture initiation prediction and to regularize the post-instability response. Discontinuities in the strain field are introduced when instability is signaled by a local bifurcation analysis. The enhancements are implemented for the Belytschko-Lin-Tsay shell element. This is combined with a stress based fracture criterion which relates the magnitude of the stress vector and the first invariant of the stress tensor. A robust experimental procedure based on full-field measurements enable direct calibration in stress space, and provides a flow curve up to large strains. Numerical examples involving tensile samples with different localization behavior are presented to demonstrate significant reduction of spurious mesh dependence. Moreover, the engineering feasibility of the direct use of a stress based fracture criterion in combination with the enhanced element is evaluated by comparison of measured and calculated fracture elongations.

  • 3469.
    Östlund, Sören
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Fracture modelling of brittle-matrix composites with spatially dependent crack bridging1995Inngår i: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 18, nr 10, s. 1213-1230Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 3470.
    Östlund, Sören
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Large scale yielding for dynamic crack growth in a strip geometry1991Inngår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 49, nr 3, s. 219-237Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 3471.
    Östlund, Sören
    KTH, Tidigare Institutioner, Hållfasthetslära.
    On numerical modeling and fracture criteria of dynamic elastic-viscoplastic crack growth1990Inngår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 44, nr 4, s. 283-299Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 3472.
    Östlund, Sören
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, VinnExcellens Centrum BiMaC Innovation.
    Three-Dimensional Deformation and Damage Mechanisms in Forming of Advanced Structures in Paper2017Inngår i: 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, s. 489-594Konferansepaper (Fagfellevurdert)
    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.

  • 3473.
    Östlund, Sören
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, VinnExcellens Centrum BiMaC Innovation.
    Linvill, Eric
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Wallmeier, M
    Continuum Modeling of Wrinkles and Explicit FEM Modeling of Paperboard Deep-Drawing2016Inngår i: 2016 Progress in Paper Physics Seminars / [ed] Samuel Schabel, 2016Konferansepaper (Annet vitenskapelig)
    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.

  • 3474.
    Östlund, Sören
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Mäkelä, Petri
    Tetra Pak Packaging Solutions AB.
    Fracture properties2011Inngår i: Mechanics of Paper Products / [ed] Kaarlo Niskanen, Berlin: Walter de Gruyter GmbH & Co. KG , 2011, 1, s. 67-89Kapittel i bok, del av antologi (Fagfellevurdert)
  • 3475.
    Östlund, Sören
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Nilsson, Fred
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Cohesive modelling of process regions for cracks in linear elastic structures-fundamental aspects1993Inngår i: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 16, nr 2, s. 215-235Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 3476.
    Östlund, Sören
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Nilsson, Fred
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Cohesive zone modelling of damage at the tip of cracks in slender beam structures1993Inngår i: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 16, nr 6, s. 663-676Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The use of simple beam theory for cohesive zone modelling of the damage response at the crack tip in linear elastic isotropic double cantilever beam (DCB) specimens has been investigated. Damage resistance curves (DR-curves) relating the applied stress intensity factor to the growth of the cohesive zones for beam theory modelling has been compared with two-dimensional elasticity calculations for different material parameters and specimen dimensions. A substantial difference is observed between DR-curves for the two types of models. As expected this difference vanishes for decreasing beam heights. For large beam heights the DR-curves calculated by two-dimensional elasticity are approaching small-scale yielding DR-curves, i.e. DR-curves for an edge crack in an infinite plate. The beam height for which beam theory is applicable could be up to 10-3 times the height for which small scale bridging DR-curves are applicable.

  • 3477.
    Östlund, Sören
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Niskanen, Kaarlo
    KCL.
    Kärenlampi, Petri
    University of Helsinki.
    On the prediction of the strength of paper structures with a flaw1999Inngår i: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 25, nr 10, s. 356-360Artikkel i tidsskrift (Fagfellevurdert)
  • 3478.
    Östlund, Sören
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, VinnExcellens Centrum BiMaC Innovation.
    Nygårds, Mikael
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Through-thickness mechanical testing and computational modelling of paper and board for efficient materials design2009Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    The objective of most methods for mechanical testing of paper and paperboard is production control, i.e. to secure that the performance of the produced material fulfils certain minimum end‐use requirements. There is in the industry an ever increasing demand for improved efficiency in paper and board converting and end‐use, and this demand paves the way for the development of new test methods that enables a better understanding of the material design. There is a need for better control of changes in the pulping and papermaking processes at different structural levels, and for efficient characterization of these changes. To achieve such control, novel test methods and computational modelling are required.

    The objective of this paper is to discuss some newly proposed test methods and to illustrate how these in combination with simulations are a valuable tool for efficient materials design. The mechanical properties that are covered in the paper include in‐plane residual stresses and transverse shear properties of paper and paperboard. The methods will be shortly described and some important experimental results will be presented. Finally, some examples of computation modelling related to these properties will be discussed.

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