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  • 151.
    Brandberg, August
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Insights in paper and paperboard performance by fiber network micromechanics2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fiber networks are ubiquitous due to their low cost and high ratio of mechanical performance to weight. Fiber networks made of cellulose fibers from trees are used as information carriers (paper) and as packaging (board). Often the ideal product is both mechanically sturdy and possible to print on. This thesis investigates the underlying reasons for the mechanical performance of paper and board through the discretization and direct simulation of every fiber in the network.

    In Paper A the effect of fiber-fiber bond geometry on sheet stiffness is investigated. Many packaging products seek to maximize the bending stiffness by employing stiff outer layers and a bulkier layer in the middle. In bulky sheets, the fibers are frequently uncollapsed resulting in a more compliant bonded segment. Because all the loads in the network are transferred via the bonds, such compliance can cause unexpectedly large decreases in mechanical performance. Although many models have been presented which aim to predict the tensile stiffness of a sheet, these predictions tend to overestimate the resulting stiffness. One reason is that the bonds are generally considered rigid. By finite element simulations, we demonstrated the effect of the lumina configuration on the stiffness of the bonded segment on the scale of single fiber-to-fiber bonds, and that the average state of the fiber lumen has a marked effect on the macroscopic response of fiber networks when the network is bulky, has few bonds, or has a low grammage.

    Compression strength is central in many industrial applications. In paper B we recreated the short span compression test in a simulation setting. The networks considered are fully three-dimensional and have a grammage of 80 to 400 gsm, which is the industrially relevant range. By modeling compression strength at the level of individual fibers and bonds, we showed that fiber level buckling or bifurcation phenomena are unlikely to appear at the loads at which the macroscopic sheet fails.

    In paper C, we developed a micromechanical model to study the creation of curl in paper sheets subjected to a moisture gradient through the sheet. A moisture gradient is always created during the printing process, which may lead to out-of-plane dimensional instability. We showed that the swelling anisotropy of individual fibers bonded at non-parallel angles causes an additional contribution to the curl observed on the sheet level.

  • 152.
    Brandberg, August
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The Effect of Geometry on the Mechanical Properties of Paper Fiber Bonds2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 153.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Compression failure in dense non-woven fiber networks2019Report (Other academic)
  • 154.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The effect of geometry changes on the mechanical stiffness of fibre-fibre bonds2017In: Advances in Pulp and Paper Research, Manchester, 2017, p. 683-719Conference paper (Refereed)
  • 155.
    Brandberg, August
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Motamedian, Hamid Reza
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hirn, Ulrich
    Technische Universität Graz.
    The Role of the Fiber and the Bond in the Hygroexpansion and Curl of Thin Freely Dried Paper Sheets2019Report (Other academic)
    Abstract [en]

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

  • 156.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Cossu, C
    Henningson, Dan S
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Chomaz, J M
    Huerre, P
    Numerical studies of streak instability in boundary layers2006In: Sixth IUTAM Symposium on Laminar-Turbulent Transition / [ed] Govindarajan, R, DORDRECHT: SPRINGER , 2006, Vol. 78, p. 121-126Conference paper (Refereed)
    Abstract [en]

    Numerical results on the stability of boundary layers in the presence of streaks, assumed steady and spanwise periodic, are presented. The instability features are retrieved both from stability analysis and from the numerical simulation of the flow impulse response. It is found that the presence of streaks of moderate amplitudes is able to quench the viscous Tollmien-Schlichting waves. However, a threshold exists beyond which secondary inflectional instabilities occur. Streaky basic flows unstable to both sinuous and varicose perturbations are considered. To gain physical understanding of the instability mechanisms the equation for the perturbation kinetic energy is analysed. To investigate the sinuous instability modes an analytical model streak is also proposed.

  • 157.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Direct numerical simulations of streak breakdown in boundary layers2004In: Direct and Large-Eddy Simulation V, Proceedings / [ed] Friedrich, R; Geurts, BJ; Metais, O, 2004, Vol. 9, p. 175-196Conference paper (Refereed)
    Abstract [en]

    Numerical simulations of bypass transition in Blasius boundary layers are presented. The breakdown of streamwise streaks is first considered in the case of the steady, spanwise periodic basic flows arising from the nonlinear saturation of optimal perturbations and then in the case of transition in boundary layers subject to free-stream turbulence. Similarity and differences with previous work are discussed.

  • 158.
    Brauer, Jesper
    KTH, Superseded Departments, Machine Design.
    A general finite element model of involute gears2004In: Finite elements in analysis and design (Print), ISSN 0168-874X, E-ISSN 1872-6925, Vol. 40, no 13-14, p. 1857-1872Article in journal (Refereed)
    Abstract [en]

    Involute gears comprise primarily spur gears, helical gears, straight conical involute gears and conical involute gears. Robust and effective parameterised finite element models of involute gears should be based on analytically derived mathematical representations of their shape. In this paper we derive a mathematical description of conical involute gears that is also capable of representing three other types of involute gear. The equations and the intervals for the surface parameters are then used to create a general finite element model of involute gears.

  • 159.
    Brethouwer, Gert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Matsuo, Y.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    DNS of rotating homogeneous shear flow and scalar mixing2006In: Direct and Large-Eddy Simulation VI / [ed] Lamballais, E; Friedrich, R; Geurts, BJ; Metais, O, DORDRECHT: SPRINGER , 2006, Vol. 10, p. 225-232Conference paper (Refereed)
  • 160. Bru, T.
    et al.
    Waldenström, Paul
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Gutkin, R.
    Olsson, R.
    Vyas, G. M.
    Development of a test method for evaluating the crushing behaviour of unidirectional laminates2017In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 51, no 29, p. 4041-4051Article in journal (Refereed)
    Abstract [en]

    More fundamental test methods are needed to assist the development of physically based and truly predictive simulation tools for composite materials under crash conditions. In this paper, a unidirectional flat specimen that can be used to validate the predicted behaviour from a simulation to the physical behaviour in the experiment is developed. A systematic experimental investigation is conducted to evaluate the influence of the trigger geometry on the crushing response by selecting two trigger types and different trigger angles. For longitudinal crushing, the traditional bevel trigger leads to out-of-plane failure by splaying with a limited amount of in-plane fracture, while the proposed trigger achieves a high amount of compressive fragmentation failure. For transverse crushing, the symmetry of the proposed new trigger improves the specimen stability during the crushing process. It is also observed that the weft threads of the unidirectional fabric reinforcement used for the tests have a strong influence on the longitudinal crushing response. The boundary conditions of the test and the information on the specimen failure gleaned from video recordings and microscopic inspections are discussed in order to facilitate a future correlation with modelling results.

  • 161. Bustamante, R.
    et al.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Methods to compute 3D residual stress distributions in hyperelastic tubes with application to arterial walls2010In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 48, no 11, p. 1066-1082Article in journal (Refereed)
    Abstract [en]

    In this paper the problem of modeling three-dimensional residual stress distributions in hyperelastic tubes is addressed. First, the problem of a radially opened straight and bent tube, where the opening angle depends on the axial position, is explored with the semi-inverse method. As a result a rather complicated system of nonlinear partial differential equations is achieved which is difficult to solve. Second, a different approximate method considers the tube as a composition of two, three, four or more rings in the axial direction. Also here the opening angle of the tube depends on the axial position. Some numerical solutions for the stress components in the radial, circumferential and axial directions are analyzed in more detail. Third, the tube wall is divided into a number of radial layers, with different mechanical properties, and an approximate method to treat that problems is presented. It is emphasized that the proposed approach can also be used to compute 3D residual stress distributions in arterial walls. A final conclusion points to possible future research directions.

  • 162.
    Bäckström, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Model of dynamic behavior for frame mounted truck components2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In the truck industry, it is crustal to test components against fatigue to make sure that the trucks stand up to the high demands on durability. Today’s testing methods have some disadvantages; it is quite a time-consuming process, but more important, similar tested components cannot easily be compared due to the load spread the components are subjected to. It is therefore desirable to test the components in a standardized way. One way to do this is to use a synthetic signal which is a large number of unique truck measurements combined. The synthetic signal only contains information of the frame’s vibration and not any components. The purpose of this project was to create a model that uses the synthetic signal to describe the motion of components.

     

    Two approaches were used, the first was to base the model on previous measurements, the second one was to base the model on analytical equations. These models were experimentally tested in a 4 channel shake rig, and a silencer was the component chosen to be tested. For the model based on measurements, the load was shown to have a large spread which was hard to control due to the spread in the measurements. The second model was easier to control where the damping factor can be chosen and varied. A promising model was the analytical model using 10% damping applied to the synthetic signal, it covers most measurements without overestimate the load of the component. However, the model was only developed for the silencer acceleration in the z-direction, and it is recommended to develop it for the x-direction as well. The method used in this project could also be used to develop models for other components.

  • 163. Bölcs, A
    et al.
    Fransson, Torsten
    KTH, School of Industrial Engineering and Management (ITM).
    Schläfli, D
    AGARD Conference Proceedings No. 468 on "Unsteady Aerodynamic Phenomena in Turbomachines", Luxemburg, August, 1989.1989Conference paper (Refereed)
  • 164. Bölcs, A
    et al.
    Fransson, Torsten
    KTH, School of Industrial Engineering and Management (ITM).
    Schläfli, D
    Time-Dependent Measurements on Vibrating Annular Turbine Cascades Under Various Steady-State Operating Conditions1989Conference paper (Refereed)
  • 165.
    Bölke, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Swahn, Lars
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Att simulera en uthållighetsidrottare2015Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The two most important physiological properties to determine an individuals endurance are maximumoxygen uptake and the anaerobic threshold. These properties can be obtained with a threshold test anda max test, which literature and an interview with an expert in the field showed.

    The properties would then be linked to a Matlab model developed to simulate exhaustion for anindividual on a given track. To achieve this, a threshold test and a max test were performed on twoindividuals to measure the maximum oxygen uptake and the anaerobic threshold. The tests wereperformed in Solna by Aktivitus. Afterwards the max tests were simulated in the Matlab model for thetwo individuals.

    It turned out that the anaerobic threshold is directly linked to the Matlab model which means that youcan put measured values directly into the model while the maximum oxygen uptake need furtherinvestigation.

    The experiments have been performed as part of a project lead by Professor Anders Eriksson fromKTH and his colleagues from Karolinska Institutet and Mittuniversitetet. Technique which is animportant factor in endurance has been overlooked. To obtain better and more accurate results moretests should be performed.

  • 166.
    Cameron, Christopher
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Lind, Eleonora
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Wennhage, Per
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Balancing structural and acoustic performance of sandwich panels for vehicle applications with topology, property, and size optimization2010In: 7th Asian-Australasian Conference on Composite Materials 2010, ACCM 2010, ACCM-7 Organizing Committee , 2010, Vol. 2, p. 835-838Conference paper (Refereed)
    Abstract [en]

    Within this paper, a process for the design of a multifunctional sandwich body panel for vehicle applications is proposed. The method, presented with a case study, attempts to achieve a balance between structural and acoustic performance using numerical tools for topology optimization and combined size and property optimization. The goal of the work is to achieve an optimal distribution of traditional sandwich foam material and light weight acoustic foam within the core of the panel. The significance of the coupling between the panels inner face sheet and the acoustic foam is examined and proves to be a critical parameter in the design. An adaptation to existing topology optimization schemes is proposed to deal with the presence or absence of such a coupling. The results show promise in simplifying construction, reducing weight, and streamlining the assembly process.

  • 167.
    Cameron, Christopher
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Wennhage, Per
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics.
    Multi-Scale Structural-Acoustic Optimization of a Multi-Functional Vehicle Body Panel2009Conference paper (Other academic)
  • 168.
    Canton, Jacopo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Auteri, F.
    Carini, M.
    Linear global stability of two incompressible coaxial jets2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 824, p. 886-911Article in journal (Refereed)
    Abstract [en]

    The linear stability of two incompressible coaxial jets, separated by a thick duct wall, is investigated by means of both a modal and a non-modal approach within a global framework. The attention is focused on the range of unitary velocity ratios for which an alternate vortex shedding from the duct wall is known to dominate the flow. In spite of the inherent convective nature of jet flow instabilities, such behaviour is shown to originate from an unstable global mode of the dynamics linearised around the axisymmetric base flow. The corresponding wavemaker is located in the recirculating-flow region formed behind the duct wall. At the same time, the transient-growth analysis reveals that huge amplifications (up to 20 orders of magnitude) of small flow perturbations at the nozzle exit can occur in the subcritical regime, especially for high ratios between the outer and the inner velocities.

  • 169.
    Carlsson, Susanna
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    On Microindentation of Viscoelastic Polymers1998In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 17, no 1, p. 49-75Article in journal (Refereed)
  • 170.
    Cederholm, Alex
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A study of the velocity and temperature boundary layers over a heated rotating disk1998Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 171.
    Cha, Yingying
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Hedberg, Yolanda
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. Karolinska Institutet, Sweden.
    Mei, Nanxuan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Olofsson, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Airborne Wear Particles Generated from Conductor Rail and Collector Shoe Contact: Influence of Sliding Velocity and Particle Size2016In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 64, no 3, article id 40Article in journal (Refereed)
    Abstract [en]

    The mechanical wear of train components is one of the main sources of airborne particles in subway air. A certain contribution is suspected to derive from third-rail systems due to the sliding of two metallic surfaces between conductor rail and collector shoe during operation. In this study, a pin-on-disc apparatus was used to simulate the friction between such two sliding partners (shoe-to-rail). Airborne particles generated from the sliding contact were measured by particle counters (a fast mobility particle sizer spectrometer and an optical particle sizer) and were collected by an electrical low-pressure impactor for physical and chemical analysis. Interface temperature for each test was measured by a thermocouple. The influence of sliding velocity and temperature on particulate number concentration, size distribution, and chemical composition was investigated. Atomic absorption spectroscopy, cyclic voltammetry, and energy-dispersive spectroscopy measurements were carried out to determine the chemical compositions. Results show that increasing sliding velocity results in a higher temperature at the frictional interface and a higher concentration of ultrafine particles. The ratio of manganese to iron surface oxides increased strongly with smaller particle size. A copper compound was observed in some particle samples, probably gerhardite (Cu2NO3(OH)(3)) formed due to high temperature.

  • 172.
    Charton, Aurelien
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Simulation for chiseling process in concrete based on the Discrete Element Method2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The development of chisels is currently mainly based on experiments and empirical researcheswithin the company. Recently it was decided to develop a simulation tool aiming at predictingthe performances and the capabilities of its chisels. The simulation of concrete during thechiselling process is based on a Cohesive Particle Model for concrete developed in partnershipwith external universities and implemented in the open-source software YADE using the DiscreteElement Method.The goal of this thesis is first to continue the development of the simulation tool by improvingthe calibration method of the material parameters in order to better describe the concretebehaviour under static loading. Afterward a validation phase, aiming at evaluating the real capabilitiesof the simulation tool to predict the demolition process, is performed by comparingsimulations and experiments results. The last objective is to define a simulation method in orderto evaluate the clamping phenomenon during chiselling in an acceptable amount of time.The new calibration algorithm has produced significant improvements in the determinationof the material parameters. Moreover, it was discovered that the ratio between the ultimatetensile and compressive strengths as well as the concrete brittleness are key parameters forthe material calibration accuracy. On the other hand, the validation phase was performed byevaluating the influence of seven parameters, such as the impact energy, the chisel length, onthe demolition process for both experiments and simulations. The procedure and the key resultsare presented in the thesis report. Concerning the clamping phenomenon, it was discovered thatthere is a relation between the pull-out force and the contact force during chiselling. This resultoffers a new and quick possibility for the evaluation of chisels sticking behaviour.

  • 173.
    Chen, Feng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Balieu, Romain
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Kringos, Nicole
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Thermodynamics-based finite strain viscoelastic-viscoplastic model coupled with damage for asphalt material2017In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 129, p. 61-73Article in journal (Refereed)
    Abstract [en]

    A thermodynamics based thermo-viscoelastic-viscoplastic model coupled with damage using the finite strain framework suitable for asphalt material is proposed in this paper. A detailed procedure for model calibration and validation is presented, utilizing a set of experimental measurements such as creep recovery, constant creep, and repeated creep-recovery tests under different loading conditions. The calibrated constitutive model is able to predict the sophisticated time- and temperature-dependent responses of asphalt material, both in tension and in compression. Moreover, a scenario case study on permanent deformation (rutting) prediction of a practical asphalt pavement structure is presented in this work. This paper presents the main features of this new constitutive model for asphalt: (1) A thermodynamics-based framework developed in the large strain context to derive the specific viscoelastic, viscoplastic and damage constitutive equations; (2) A viscoelastic dissipation potential involving deviatoric and volumetric parts, in which Prony series representations of the Lame constants are used; (3) A modified Perzyna's type viscoplastic formulation with non-associated flow rule adopted to simulate the inelastic deformation, using a Drucker-Prager type plastic dissipation potential; (4) A specific damage model developed for capturing the evolution disparity between tension and compression. As such, the developed model presents a robust, fully coupled and validated constitutive framework that includes the major behavioral components of asphalt materials, enabling thus an optimized simulation of predicted performance under various conditions. Further development improvements to the model in continued research efforts can be to include further environmental and physico-chemical material behavior such as ageing, healing or moisture induced damage.

  • 174.
    Chernysheva, Olga V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Fransson, Torsten H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Kielb, R. E.
    Barter, J.
    Influence of a vibration amplitude distribution on the aerodynamic stability of a low-pressure turbine sectored vane2006In: Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines / [ed] Hall, KC; Kielb, RE; Thomas, JP, 2006, p. 17-29Conference paper (Refereed)
    Abstract [en]

    A parametrical analysis summarizing the effect of the reduced frequency and sector mode shape is carried out for a low-pressure sectored vane cascade for different vibration amplitude distributions between the airfoils in sector as well as the numbers of the airfoils in sector. Critical reduced frequency maps are provided for torsion- and bending-dominated sector mode shapes. Despite the different absolute values of the average aerodynamic work between four-, five- and six-airfoil sectors a high risk for instability still exists in the neighborhood of realistic reduced frequencies of modern low-pressure turbine. Based on the cases studied it is observed that a sectored vane mode shape with the edge airfoils in the sector dominant provides the most unstable critical reduced frequency map.

  • 175.
    Cloots, Rudy J.H.
    et al.
    Eindhoven University of Technology, Department of Mechanical Engineering.
    van Dommelen, J.A.W.
    Eindhoven University of Technology, Department of Mechanical Engineering.
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Neuronic Engineering.
    Geers, Marc
    Eindhoven University of Technology, Department of Mechanical Engineering.
    Multi-scale mechanics of traumatic brain injury: predicting axonal strains from head loads2013In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 12, no 1, p. 137-150Article in journal (Refereed)
    Abstract [en]

    The length scales involved in the development of diffuse axonal injury typically range from the head level (i.e., mechanical loading) to the cellular level. The parts of the brain that are vulnerable to this type of injury are mainly the brainstem and the corpus callosum, which are regions with highly anisotropically oriented axons. Within these parts, discrete axonal injuries occur mainly where the axons have to deviate from their main course due to the presence of an inclusion. The aim of this study is to predict axonal strains as a result of a mechanical load at the macroscopic head level. For this, a multi-scale finite element approach is adopted, in which a macro-level head model and a micro-level critical volume element are coupled. The results show that the axonal strains cannot be trivially correlated to the tissue strain without taking into account the axonal orientations, which indicates that the heterogeneities at the cellular level play an important role in brain injury and reliable predictions thereof. In addition to the multi-scale approach, it is shown that a novel anisotropic equivalent strain measure can be used to assess these micro-scale effects from head-level simulations only.

  • 176.
    Coja, Michael
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    A new computational method based on waveguides for modeling of preload nonlinear effects on cylindrical vibration isolators2004In: Nordic Seminar on Computational Mechanics, 2004Conference paper (Other academic)
  • 177.
    Coja, Michael
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Kari, Leif
    KTH, Superseded Departments, Vehicle Engineering.
    A self similarity audio-frequency stiffness model of pre-compressed vibration isolators2004In: Nordic Vibration Research, 2004Conference paper (Other academic)
  • 178.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A simple engineering audible-frequency stiffness model for a preloaded conical rubber isolatorManuscript (preprint) (Other academic)
  • 179.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    An analytical, closed-form stiffness model for preloaded conical rubber mounting in the audible frequency range2005In: CONSTITUTIVE MODELS FOR RUBBER IV / [ed] Per-Erik Austrell, Leif Kari, London: Taylor & Francis, 2005Conference paper (Refereed)
  • 180.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    An effective waveguide model for pre-compressed vibration isolatorsIn: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959Article in journal (Refereed)
  • 181.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Axial audio-frequency stiffness of a bush mounting: the waveguide solution2007In: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 31, no 1, p. 38-53Article in journal (Refereed)
    Abstract [en]

    An axial, dynamic stiffness model of an arbitrary wide and long rubber bush mounting is developed within the audible-frequency range, where influences of audible frequencies, material properties, bush mounting length and radius, are investigated. The problems of simultaneously satisfying the locally non-mixed boundary conditions at the radial and end surfaces are solved by adopting a waveguide approach, using the dispersion relation for axially symmetric waves in thick-walled infinite plates, while satisfying the radial boundary conditions by mode matching. The rubber is assumed nearly incompressible, displaying dilatation elasticity and deviatoric viscoelasticity 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 wide-frequency band material properties. The stiffness is found to depend strongly on frequency, displaying acoustical resonance phenomena; such as stiffness peaks and troughs. The presented model agrees fully with a simplified, long-bush model while diverging from it for increased diameter-to-length ratios. To a great extent, the increased influences of higher order modes and dispersion explain the discrepancies reported for the approximate approach.

  • 182.
    Coja, Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Rubber versus steel vibration isolators - The audible frequency contest2005In: KGK Kautschuk Gummi Kunststoffe, ISSN 0948-3276, Vol. 58, no 11, p. 564-569Article in journal (Refereed)
    Abstract [en]

    The audible frequency axial dynamic stiffness of three vibration isolators plausibly used in the design of a ship main engine suspension system are examined and compared for different static preloads. An indirect measurement method is used to investigate the blocked dynamic transfer stiffness using a specially designed test rig displaying a strong frequency dependence where resonance and antiresonance phenomena appear in the form of troughs and peaks respectively. The significant influence of the preload effects is also assessed for each isolator. Clearly, the rubber isolator presents superior performances in comparison with the two others, steel and combined steel-rubber isolators, over the studied frequency range 200 to 1000 Hz for each preload 30, 40 and 50 kN.

  • 183. Connor, M.
    et al.
    Toll, Staffan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Manson, J. -AE.
    Gibson, A. G.
    Model for the consolidation of aligned thermoplastic powder impregnated composites1995In: Journal of Thermoplastic Composite Materials, ISSN 08927057 (ISSN), Vol. 8, no 2, p. 138-162Article in journal (Refereed)
    Abstract [en]

    This paper describes the role of surface energy effects, externally applied pressure, resin flow and fiber bed elasticity on the consolidation of thermoplastic-matrix composites manufactured by the powder impregnation route. Surface energy effects in the spreading of polymer droplets on fiber surfaces are discussed; then a model for the consolidation process is developed, relating the variables mentioned above. Consolidation experiments on powder-impregnated composites of the FIT type (Fibres Impregnees de Thermoplastique) were carried out using a mold attached to a servo-hydraulic testing machine. The model accurately predicts variations in void content during consolidation of carbon fiber/PEEK (CF/PEEK) and carbon fiber/PEI (CF/PEI) laminates. It was found that, at the pressures needed to achieve rapid consolidation, surface energy has a negligible influence on impregnation rate, but its effects on the void topology can be considerable. It was also shown that, when laminates of low void content are required, a minimum pressure is needed to overcome the effect of fiber bed elasticity.

  • 184. Connor, M.
    et al.
    Toll, Staffan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Månson, J. -AE.
    On surface energy effects in composite impregnation and consolidation1995In: Composites Manufacturing, ISSN 09567143 (ISSN), Vol. 6, no 3-4, p. 289-295Article in journal (Refereed)
    Abstract [en]

    Macroscopic capillary pressure and microscopic interparticle forces due to surface tension are examined. A general equation for the capillary pressure during impregnation is derived and subsequently specialized to particular processes. For fibre composites, the capillary pressure can be of the order of ±104 Pa, the sign depending on the contact angle between solid and liquid. Next, the attractive and repulsive forces between particles connected by liquid droplets are analysed by two different model geometries. At contact angles between π/2 and π, an equilibrium particle separation distance is obtained in the absence of applied force. At lower contact angles, spontaneous impregnation can be achieved. The effect of capillary action on impregnation rate may be significant if applied pressures are small (e.g. filament winding) but negligible at applied pressures greater than ∌100 kPa (e.g. compression moulding). The topology and concentration of voids may, however, be greatly influenced by surface energies. © 1995.

  • 185. Correa, E.
    et al.
    Gamstedt, E. Kristofer
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Paris, F.
    Mantic, V.
    Effects of the presence of compression in transverse cyclic loading on fibre-matrix debonding in unidirectional composite plies2007In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 38, no 11, p. 2260-2269Article in journal (Refereed)
    Abstract [en]

    Fatigue of composite materials is of great concern in load-carrying structures. The first type of damage to appear is generally transverse cracks in off-axis plies. These cracks form when fibre-matrix debonds coalesce. The underlying mechanism is hence fatigue growth of debonds at the fibre-matrix interfaces. In the present study, debond growth has been characterized under tensile and compressive cyclic loading of single glass fibres embedded in polymer matrix. The debond length was deter-mined by in situ microscopy with transmitted polarized light showing the more damaging effect of tension-compression cyclic loading than tension-tension cyclic loading. A boundary element model has been developed and interfacial fracture mechanics concepts applied over the numerical results aiming to give an explanation of this experimental fact. These results may be used to formulate a fatigue growth law at a local microscopic level, at a stage prior to the formation of any visible damage, i.e. transverse cracks. Ideas of how to develop this methodology further are also discussed.

  • 186. Cossu, C.
    et al.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    On Tollmien-Schlichting-like waves in streaky boundary layers2004In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 23, no 6, p. 815-833Article in journal (Refereed)
    Abstract [en]

    The linear stability of the boundary layer developing on a flat plate in the presence of finite-amplitude, steady and spanwise periodic streamwise streaks is investigated. The streak amplitudes considered here are below the threshold for onset of the inviscid inflectional instability of sinuous perturbations. It is found that, as the amplitude of the streaks is increased, the most unstable viscous waves evolve from two-dimensional Tollmien-Schlichting waves into three-dimensional varicose fundamental modes which compare well with early experimental findings. The analysis of the growth rates of these modes confirms the stabilising effect of the streaks on the viscous instability and that this stabilising effect increases with the streak amplitude. Varicose subharmonic modes are also found to be unstable but they have growth rates which typically are an order of magnitude lower than those of fundamental modes. The perturbation kinetic energy production associated with the spanwise shear of the streaky flow is found to play an essential role in the observed stabilisation. The possible relevance of the streak stabilising role for applications in boundary layer transition delay is discussed.

  • 187. Cossu, C.
    et al.
    Chevalier, M.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Secondary optimal growth and subcritical transition in the plane Poiseuille flow2010In: 7th IUTAM Symposium on Laminar-Turbulent Transition, Springer Netherlands, 2010, p. 129-134Conference paper (Refereed)
    Abstract [en]

    Nonlinear optimal perturbations leading to subcritical transition with minimum threshold energy are searched in the plane Poiseuille flow at Re = 1500. To this end we proceed in two steps. First a family of optimally growing primary streaks U issued by the optimal vortices of the Poiseuille laminar solution is computed by direct numerical simulation for a set of finite amplitudes AI of the primary vortices. An adjoint technique is then used to compute the maximum growth and the finite time Lyapunov exponents of secondary perturbations growing on top of these primary base flows. The secondary optimals take into full account the non-normality and the local instabilities of the tangent operator all along the temporal evolution of the primary flows. The most amplified optimal perturbations are sinuous and realized in correspondence of streaks that are locally unstable. The combinations of primary and secondary perturbations optimal for transition are then explored using direct numerical simulations. It is shown that the minimum initial energy is realized by a large set of these combinations, revealing new paths to transition. Surprisingly we find that transition can be efficiently obtained even using secondary perturbations alone, in the absence of primary optimal vortices.

  • 188.
    Crespo, Juan
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Stiffness Design of Paperboard Packages using the Finite Element Method.2012Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis focuses on FEM analysis of paperboard global stiffness. Simulations in Abaqus and experiments were carried out where the deformation was measured. The experimental results were compared with simulation results in order to verify the FEM simulations. Different types of boxes were used to carry out the empirical experiments.

    The analyses are based on one model that simulates the mechanic behavior of the used boxes. The influence of the creasing stiffness in the global stiffness is specially analyzed. Different gluing zones, materials and structural geometries were used in boxes. The model predicted the experimental results well except from the gluing zones that in some experiments had a higher impact concluding in worse results. Moreover, the results of the work indicate that the deformation of the boxes mostly depend on the bending stiffness of the paperboard while the influence of the creasing stiffness is low.

  • 189.
    Cuenca, Jacques
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics.
    Inverse estimation of the elastic and anelastic properties of the porous frame of anisotropic open-cell foams2011Conference paper (Other academic)
  • 190.
    Cuenca, Jacques
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Inverse estimation of the elastic and anelastic properties of the porous frame of anisotropic open-cell foams2012In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 132, no 2, p. 621-629Article in journal (Refereed)
    Abstract [en]

    This paper presents a method for simultaneously identifying both the elastic and anelastic properties of the porous frame of anisotropic open-cell foams. The approach is based on an inverse estimation procedure of the complex stiffness matrix of the frame by performing a model fit of a set of transfer functions of a sample of material subjected to compression excitation in vacuo. The material elastic properties are assumed to have orthotropic symmetry and the anelastic properties are described using a fractional-derivative model within the framework of an augmented Hooke's law. The inverse estimation problem is formulated as a numerical optimization procedure and solved using the globally convergent method of moving asymptotes. To show the feasibility of the approach a numerically generated target material is used here as a benchmark. It is shown that the method provides the full frequency-dependent orthotropic complex stiffness matrix within a reasonable degree of accuracy.

  • 191.
    Cuenca, Jacques
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics. Siemens Industrial Software, Leuven Belgium.
    Van der Kelen, Christophe
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics. Centre for ECO2 Vehicle Design.
    Inverse estimation of the elastic and anelastic properties of anisotropic foams: study of the static/dynamic separation.2015In: EURONOISE 2015, 2015Conference paper (Refereed)
    Abstract [en]

    This paper investigates the modelling and characterisation of the porous frame of anisotropic open-cell foams. The main objective is to nd a suitable model for describing the elastic and anelasticproperties of the material by making as few assumptions as possible. The proposed model is basedon a fractional di erential equation, taking into account the deformation memory of the materialin a versatile and compact manner. In the frequency domain, this results in an augmented Hooke'slaw, where the sti ness matrix of the porous frame consists of a superposition of a fully-relaxed,frequency-independent elastic part, and a dynamic, frequency-dependent anelastic part. In order toestimate the properties of the material and to determine if the elastic and anelastic parts sharethe same material symmetry, two separate experiments are performed. A static photometry setup isdesigned, where a cubic sample of material is compressed along each of the three directions of spacewhile the deformation is recorded on the four exposed faces. Furthermore, a dynamic measurementof a set of transfer functions between each pair of opposed faces of the sample is performed. Thecharacterisation methodology consists of an inverse estimation of the parameters of the model. Thisis acheved by replicating each experiment as a nite element simulation and tting the model byusing an optimisation algorithm. The static and dynamic observations serve as a basis for discussingthe independence of the elastic and anelastic properties of the material.

  • 192.
    Cuenca, Jacques
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics. Siemens Industrial Software Leuven Belgium.
    Van der Kelen, Christophe
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Göransson, Peter
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Numerical acoustics.
    Inverse estimation of the elastic and anelastic properties of anisotropic open-cell foams2014Conference paper (Other academic)
  • 193. Cui, Zhao Ying
    et al.
    Famaey, Nele
    Depreitere, Bart
    Ivens, Jan
    Kleiven, Svein
    KTH, School of Technology and Health (STH), Medical Engineering, Neuronic Engineering.
    Vander Sloten, Jos
    On the assessment of bridging vein rupture associated acute subdural hematoma through finite element analysis2017In: Computer Methods in Biomechanics and Biomedical Engineering, ISSN 1025-5842, E-ISSN 1476-8259, Vol. 20, no 5, p. 530-539Article in journal (Refereed)
    Abstract [en]

    Acute subdural hematoma (ASDH) is a type of intracranial haemorrhage following head impact, with high mortality rates. Bridging vein (BV) rupture is a major cause of ASDH, which is why a biofidelic representation of BVs in finite element (FE) head models is essential for the successful prediction of ASDH. We investigated the mechanical behavior of BVs in the KTH FE head model. First, a sensitivity study quantified the effect of loading conditions and mechanical properties on BV strain. It was found that the peak rotational velocity and acceleration and pulse duration have a pronounced effect on the BV strains. Both Young's modulus and diameter are also negatively correlated with the BV strains. A normalized multiple linear regression model using Young's modulus, outer diameter and peak rotational velocity to predict the BV strain yields an adjusted -value of 0.81. Secondly, cadaver head impact experiments were simulated with varying sets of mechanical properties, upon which the amount of successful BV rupture predictions was evaluated. The success rate fluctuated between 67 and 75%. To further increase the predictive capability of FE head models w.r.t. BV rupture, future work should be directed towards improvement of the BV representation, both geometrically and mechanically.

  • 194.
    Dadfar, Reza
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Active Control and Reduced-Order Modeling of Transition in Shear Flows2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis direct numerical simulation is used to investigate the possibilityto delay the transition from laminar to turbulent in boundary layer flows.Furthermore, modal analysis is used to reveal the coherent structures in highdimensional dynamical systems arising in the flow problems.Among different transition scenarios, the classical transition scenario isanalysed. In this scenario, the laminar-turbulent transition occurs when Tollmien-Schlichting waves are triggered inside the boundary layer and grow exponentiallyas they move downstream in the domain. The aim is to attenuate the amplitudeof these waves using active control strategy based on a row of spatiallylocalised sensors and actuators distributed near the wall inside the boundarylayer. To avoid the high dimensional system arises from discretisation of theNavier Stokes equation, a reduced order model (ROM) based on EigensystemRealisation Algorithm (ERA) is obtained and a linear controller is designed.A plasma actuator is modelled and implemented as an external forcing on theflow. To account for the limitation of the plasma actuators and to further reducethe complexity of the controller several control strategies are examinedand compared. The outcomes reveal successful performance in mitigating theenergy of the disturbances inside the boundary layer.To extract coherent features of the wind turbine wakes, modal decompositiontechnique is employed where a large scale dynamical system is reduced toa fewer number of degrees of freedom. Two decomposition techniques are employed:proper orthogonal decomposition and dynamic mode decomposition.In the former procedure, the flow is decomposed into a set of uncorrelated structureswhich are rank according to their energy. In the latter, the eigenvaluesand eigenvectors of the underlying approximate linear operator is computedwhere each mode is associated with a specific frequency and growth rate. Theresults revealed the structures which are dynamically significant to the onsetof instability in the wind turbine wakes.

  • 195. Daehli, L. E. B.
    et al.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Borvik, T.
    Hopperstad, O. S.
    Unit cell simulations and porous plasticity modelling for recrystallization textures in aluminium alloys2016In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), Elsevier, 2016, p. 2535-2542Conference paper (Refereed)
    Abstract [en]

    The well-known Gurson model has been heuristically extended to incorporate effects of matrix anisotropy on the macroscopic yielding of porous ductile solids. Typical components of recrystallization textures for aluminium alloys were used to calibrate the Barlat Y1d2004-18p yield criterion using a full-constraint Taylor homogenization method. The resulting yield surfaces were further employed in unit cell simulations using the finite element method. Unit cell calculations are invoked to investigate the evolution of the approximated microstructure under pre-defined loading conditions and to calibrate the proposed porous plasticity model. Numerical results obtained from the unit cell analyses demonstrate that anisotropic plastic yielding has great impact on the mechanical response of the approximated microstructure. Despite the simplifying assumptions that underlie the proposed constitutive model, it seems to capture the overall macroscopic response of the unit cell. However, to further enhance the numerical predictions, the model should be supplemented with a void evolution expression that accounts for directional dependency, and a void coalescence criterion in order to capture the last stages of deformation.

  • 196. Daehli, Lars Edvard Bryhni
    et al.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Borvik, Tore
    Hopperstad, Odd Sture
    Unit cell simulations and porous plasticity modelling for strongly anisotropic FCC metals2017In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 65, p. 360-383Article in journal (Refereed)
    Abstract [en]

    The macroscopic behaviour of anisotropic porous solids made from an aggregate of spherical voids embedded in a plastically anisotropic matrix material is investigated by means of unit cell simulations. Plastic yielding of the polycrystalline matrix is governed by the anisotropic yield criterion Yld2004-18p. Generic texture components for face-centred cubic crystals resembling those that typically emerge during rolling and annealing processes are applied in the study. A numerical method for systematic prescription of external stress states is presented and employed in the unit cell calculations. To preclude shear effects in the unit cell model, the material symmetry axes are restricted to coincide with the principal stress directions. This excludes the possibility to properly study the ductile failure mechanism and the current work is thus mainly concerned with the void growth phase. Various stress states ranging from biaxial tension to highly constrained regions in the vicinity of crack tips are employed in the study. The numerical results demonstrate that the matrix anisotropy has a marked effect on the unit cell response, both in terms of void growth and stress-strain curves. Furthermore, the void shape evolves quite differently depending upon the direction of the major principal stress relative to the material axes. A heuristic extension of the Gurson model that incorporates matrix plastic anisotropy is presented and subsequently used to describe the constitutive behaviour of the porous ductile solid. Numerical data from the unit cell analyses are used as target curves in the calibration process of the porous plasticity model. A sequential least-square optimization procedure is invoked to minimize the overall discrepancy between the unit cell calculations and the homogenized response of the plastically anisotropic porous solid for all the imposed stress states. The anisotropic porous plasticity model demonstrates predictive capabilities for the range of stress states covered in this study.

  • 197.
    Dahlberg, Carl
    Department of Solid Mechanics, KTH.
    The Functional Response of Mesenchymal Stem Cells to Electron-Beam Patterend Elastomeric Surfaces Presenting Micrometer to Nanoscale Heterogeneous Rigidity2017In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095Article in journal (Refereed)
  • 198. Dahlberg, Carl F. O.
    Spatial distribution of the net Burgers vector density in a deformed single crystal2016In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 85, p. 110-129Article in journal (Refereed)
    Abstract [en]

    A two-dimensional deformation field on an indented single crystal, where the only nonzero lattice rotation occurs in the plane of deformation and only three effective in-plane slip systems are activated, is investigated both experimentally and numerically. ElectronBackscatter Diffraction (EBSD) is utilized to probe the lattice rotation field on the sample. The lattice rotation field is utilized to calculate the two non-zero components of Nye'sdislocation density tensor, which serves as a link between plastic and elastic deformation states. The enhanced accuracy of EBSD enabled measurements of the net Burgers vector density, and a new quantity β, which monitors the activity of slip systems in the deformed zone. The β-field is compared to the slip system activity obtained by analytical solution and also by crystal plasticity simulations. A qualitative comparison of the three methods confirms that the β-field obtained experimentally agrees with the slip system activity obtained analytically and by numerical methods.

  • 199.
    Dahlberg, Carl F. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Boåsen, Magnus
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Evolution of the length scale in strain gradient plasticity2019In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 112, p. 220-241Article in journal (Refereed)
    Abstract [en]

    An equivalence is assumed between a microstructural length scale related to dislocation density and the constitutive length scale parameter in phenomenological strain gradient plasticity. An evolution law is formed on an incremental basis for the constitutive length scale parameter. Specific evolution equations are established through interpretations of the relation between changes in dislocation densities and increments in plastic strain and strain gradient. The length scale evolution has been implemented in a 2D-plane strain finite element method (FEM) code, which has been used to study a beam in pure bending. The main effect of the length scale evolution on the response of the beam is a decreased strain hardening, which in cases of small beam thicknesses even leads to a strain softening behavior. An intense plastic strain gradient may develop close to the neutral axis and can be interpreted as a pile-up of dislocations. The effects of the length scale evolution on the mechanical fields are compared with respect to the choice of length evolution equation.

  • 200.
    Dahlberg, Carl F. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    An improved strain gradient plasticity formulation with energetic interfaces: theory and a fully implicit finite element formulation2013In: Computational Mechanics, ISSN 0178-7675, E-ISSN 1432-0924, Vol. 51, no 5, p. 641-659Article in journal (Refereed)
    Abstract [en]

    A fully implicit backward-Euler implementation of a higher order strain gradient plasticity theory is presented. A tangent operator consistent with the numerical update procedure is given. The implemented theory is a dissipative bulk formulation with energetic contribution from internal interface to model the behavior of material interfaces at small length scales. The implementation is tested by solving some examples that specifically highlight the numerics and the effect of using the energetic interfaces as higher order boundary conditions. Specifically, it is demonstrated that the energetic interface formulation is able to mimic a wide range of plastic strain conditions at internal boundaries. It is also shown that delayed micro-hard conditions may arise under certain circumstances such that an interface at first offers little constraints on plastic flow, but with increasing plastic deformation will develop and become a barrier to dislocation motion.

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