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  • 1.
    Abed, Kason
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    FEM-analys av torkapparat2011Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 2. Adolfsson, E
    et al.
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Matrix crack closure effects on the thermoelastic properties of [(0/90/+q/-q)s]M composite laminates1994Conference paper (Refereed)
  • 3.
    Agde Tjernlund, Jessica
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Length-scale effects in yielding and damage development in polymer materials2005Licentiate thesis, comprehensive summary (Other scientific)
  • 4.
    Agde Tjernlund, Jessica
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Gamstedt, Kristofer
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Length-scale effects on damage development in tensile loading of glass-sphere filled epoxy2006In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 43, no 24, p. 7337-7357Article in journal (Refereed)
    Abstract [en]

    Particle-reinforced polymers are widely used in load-carrying applications. The effect of particle size on damage development in the polymer is still relatively unexplored. In this study, the effect of glass-sphere size on the damage development in tensile loaded epoxy has been investigated. The diameter of the glass spheres ranged from approximately 0.5-50 mu m. The first type of damage observed was debonding at the sphere poles, which subsequently grew along the interface between the glass spheres and epoxy matrix. These cracks were observed to kink out into the matrix in the radial direction perpendicular to the applied load. The debonding stresses increased with decreasing sphere diameter, whereas the length to diameter ratio of the resulting matrix cracks increased with increasing sphere diameter. These effects could not be explained by elastic stress analysis and linear-elastic fracture mechanics. Possible explanations are that a thin interphase shell may form in the epoxy close to the glass spheres, and that there is a length-scale effect in the yield process which depends on the strain gradients. Cohesive fracture processes can contribute to the influence of sphere size on matrix-crack length. Better knowledge on these underlying size-dependent mechanisms that control damage development in polymers and polymer composites is useful in development of stronger materials. From a methodology point of view, the glass-sphere composite test can be used as an alternative technique (although still in a qualitative way) to hardness vs. indentation depth to quantify length-scale effects in inelastic deformation of polymers.

  • 5.
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Fretting fatigue of a shrink-fit pin subjected to rotating bending: Experiments and simulations2009In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 31, no 10, p. 1559-1570Article in journal (Refereed)
    Abstract [en]

    Fretting fatigue initiation was studied for a shrink-fit pin at rotating bending. Eight assemblies with four different grips were manufactured from soft normalized steel and tested at loads well below bending endurance. All pins displayed rust-red fretting oxides deep into the contact and black oxidised fretting scars with fretting fatigue cracks at the rim. The slip evolution was simulated in a three-dimensional FE model including assembly, bending and sufficiently many rotations to reach a steady-state. The extension of cyclic slip agreed with the black oxidised scar. Deeper into the contact a monotonic slip developed to the positions with rust-red oxides. Asymmetric slip and traction on the interface sides together with a slight twist of the pin in the hub and the slip development process, illustrated that a three-dimensional analysis was required for the interface. Both the stress amplitude and the Findley multi-axial criterion predicted fretting fatigue of the pin although the rotating bend stress was well below the endurance limit.

  • 6.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Arregui, I. Linares
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Low temperature creep in a high strength roller bearing steel2016In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 100, p. 109-125Article in journal (Refereed)
    Abstract [en]

    Noticeable low temperature creep was established for a bainitic and a martensitic microstructure of the 100CrMnMo8 high strength roller bearing steel. The response revealed primary creep that differed between the microstructures, following a power law for martensite and the logarithmic description for bainite. The detected creep was pressure sensitive, higher in tension than in compression for the same stress level, following the strength differential effect (SDE) at material yielding. Two models were proposed where the stress variable for the pressure effect was based on the Drucker-Prager yield function and deviatoric creep strains were derived from a non-associated von Mises potential. Model parameters were determined from experimental series on the respective microstructure. When the models were evaluated against the experiments the accuracy was of the same order as the effects of different heat treatment batches and different load application rates. The importance of different material parameters in the descriptions was discussed.

  • 7.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Dahlberg, Johan
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The role of a single surface asperity in rolling contact fatigue2008In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 264, no 9-10, p. 757-762Article in journal (Refereed)
    Abstract [en]

    The effect of contact loading of single surface irregularities, i.e. asperities, as an underlying mechanism for surface initiated rolling contact fatigue was investigated numerically using FEM. Spalls in the teeth flanks of driving gear wheels were investigated for typical spalling crack initiation properties. The spalling entry angle was documented and some spalls with a convex entry tip were found. The residual surface stresses of the used teeth, with spalls, were measured with the hole drilling technique. The gear contact close to the rolling circle was modelled as two rolling cylinders. A single asperity was introduced into the contact surface of one of them. Due to the presence of the asperity a three dimensional contact model was required. The material description included J(2)-plasticity with isotropic and linear hardening. The simulation included residual stresses from material heat treatment. The first roll cycle introduced plastic deformation which altered the residual stresses. Thus, the stress results were captured during a second roll cycle. The most important result was that asperities will serve as local stress raisers in the contact surfaces. The computed stress cycle at the asperity was compared to stress cycles that gave ring/cone cracks at point loaded experiments. The principal stress trajectory into the material was compared to the cross-section profiles of the spalling entry and ring/cone crack. The surface stress profile at the asperity was compared to the convex surface profiles of the spalling tip and ring/cone crack. The asperity deformation and change in residual surface stresses from moderate plastic deformation during rolling were estimated.

  • 8.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Linares Arregui, Irene
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical analysis of plasticity effects on fatigue growth of a short crack in a bainitic high strength bearing steel2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, p. 36-51Article in journal (Refereed)
    Abstract [en]

    Plasticity effects on fatigue growth were simulated for a physically short crack. The material description comprised the Drucker-Prager yield surface, non-associated flow rule and non-linear combined hardening. The simulated development of the growth limiting parameter agreed with the experimental crack behaviour with early rapid propagation followed by a transition to slow R-controlled growth. The crack was open to the tip without any crack face closure throughout all load cycles. Instead compressive residual stresses developed at the unloaded tip which supplied an explanation to the slow rate of the propagated short crack in this bainitic high strength bearing steel. The material's strength differential effect was the key difference explaining why compressive residual stresses instead of crack face closure was responsible for the short crack effect in this material.

  • 9.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Nordin, E.
    An Elastic-Plastic Model for Single Shot-Peening Impacts2013In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 52, no 2, p. 231-251Article in journal (Refereed)
    Abstract [en]

    A model was developed for impacts of elastic perfectly plastic spherical particles with impact velocities up to 250 m/s. The model is based on the two master curves, for normalized pressure and projected contact area c (2), which both are functions of the representative strain I > at maximum impact. The model and its parameters were fitted to finite element results for elastic perfectly plastic and strain rate-independent materials. It was applied to a wide range of materials with different ratio between yield stress and elastic properties, different ball sizes and impact velocities. The impact model predicted the results from finite element method for contact radius, maximum impact depth in both target and ball as well as remaining impact depth in target and ball. The remaining impact depth was determined from elastic spring back with Hertzian and quadratic pressure at maximum impact. The rebound velocity was also estimated by following the load-deformation path during spring back. If the strain rate-compensated yield stress was used for the master curve parameters, then the model predicted the impact results also for the investigated strain rate-dependent materials.

  • 10.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Multi-axial fatigue initiation at inclusions and subsequent crack growth in a bainitic high strength roller bearing steel at uniaxial experiments2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 41, p. 130-139Article in journal (Refereed)
    Abstract [en]

    The behaviour of inclusion initiated fatigue was studied for a high strength bearing steel with a bainite micro-structure. The analysis included experiments and numerical simulations. It was realized that the stress-state was multi-axial in the matrix material that met the inclusion also for a uniaxial far field stress. Fatigue initiation risk at the interface between the inclusion and matrix material was therefore predicted with the Findley multi-axial critical plane criterion. The fatigue parameters were determined from independent experiments on smooth specimens with tensile surface stress gradients. Crack growth from the inclusion to final rupture was modelled as a penny shaped crack with closure compensated effective material parameters. The growth simulations suggested that the majority of the fatigue life was consumed as fatigue crack initiation at the non-metallic inclusion.

  • 11.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). STandUP Wind.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. STandUP Wind.
    Wind farms in complex terrains: an introduction2017In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, no 2091Article in journal (Refereed)
    Abstract [en]

    Wind energy is one of the fastest growing sources of sustainable energy production. As more wind turbines are coming into operation, the best locations are already becoming occupied by turbines, and wind-farm developers have to look for new and still available areas-locations that may not be ideal such as complex terrain landscapes. In these locations, turbulence and wind shear are higher, and in general wind conditions are harder to predict. Also, the modelling of the wakes behind the turbines is more complicated, which makes energy-yield estimates more uncertain than under ideal conditions. This theme issue includes 10 research papers devoted to various fluid-mechanics aspects of using wind energy in complex terrains and illustrates recent progress and future developments in this important field. This article is part of the themed issue 'Wind energy in complex terrains'.

  • 12.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Wåtz, Veronica
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fatigue crack initiation and growth at holes in a high strength bainitic roller bearing steel when loaded with non-proportional shear and compressive cycles2011In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 33, no 9, p. 1244-1256Article in journal (Refereed)
    Abstract [en]

    Fatigue initiation from an artificial defect was investigated for a bainitic high strength roller bearing steel. Thin walled pipe specimens with small holes were subjected to multi-axial and non-proportional load cycles. The experimental fatigue crack positions around the hole were predicted with the Findley critical plane criterion. The criterion also ranked the severeness of three load sequences with respect to fatigue risk. Crack growth simulations and crack life measurements with strain gauges confirmed the ranking between the load sequences. Three uni-axial fatigue series with stress gradients were used to determine surface endurance data for the Findley criterion.

  • 13.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Propagation of physically short cracks in a bainitic high strength bearing steel due to fatigue load2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 90, p. 166-180Article in journal (Refereed)
    Abstract [en]

    Physically short cracks in a bainitic high strength bearing steel were fatigue loaded. The rapid propagation rate of early open short cracks agreed with that of long closure free cracks. After some rapid growth, the short cracks entered a transition period to the rate of growth limited long cracks. Potential drop showed that the short cracks were open to the tip throughout the growth sequence, which excluded crack face closure in the wake as the growth limiting mechanism in this material. Instead the short crack effect was related to residual stresses and other mechanisms at the crack tip. Crack manufacturing procedures were determined for straight long and short start cracks in the present material. LEFM with effective material parameters and limit compensation predicted the short crack lives.

  • 14.
    Alfthan, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Linear constitutive model for mechano-sorptive creep in paper2005In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 42, no 24-25, p. 6261-6276Article in journal (Refereed)
    Abstract [en]

    The creep of paper is accelerated by moisture cycling. This effect is known as mechano-sorptive creep. It is assumed that this is an effect of transient stresses produced during moisture content changes in combination with non-linear creep behaviour of the fibres. The stresses produced by the moisture content changes are often much larger than the applied mechanical loads. If this is the case, the mechanical loads are only a perturbation to the internal stress state, and it will appear as if the mechano-sorptive creep is linear in stress. It is possible to take advantage of this feature. In the present report the pure moisture problem is first solved. The mechanical load is then treated as a perturbation of the solution to the moisture problem. Using this strategy, it is possible to linearize a non-linear network model for mechano-sorptive creep and to formulate a continuum model. As a result, the number of variables in the model is reduced. This is a significant improvement as it will be possible to use the linearized model to describe the material in a finite element program and solve problems with complicated geometries.

  • 15.
    Alizadeh, Amir
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fracture Simulation of Electrofusion Joining.2015Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This Master thesis investigates mechanical failures of electrofusion joints. This type of joints are used for weld high density polyethylene pipe systems where tting and pipe will be welded together by the heat caused by the resistance of the copper cables to electric current. There have been some failures where a brittle crack has grown through the fusion zone. Polyethylene as a material has a ductile character but brittle behavior occurs due to an unsuccessful welding by a poor fusion interface. In this study, we have chosen to investigate the plausible load cases that can cause the failure and studied the e ect of the geometry of the tting on the fracture toughness of the welded structure. We used the nite element method numerical analysis. We have approached unsuccessful welding (brittle) with a linear model and non-linear (CZM) model and successful welding (ductile) with a non-linear XFEM model. The material parameters needed for these models are gathered by series of experiments. The results shows that the inside pressure is the critical load case. The linear model and CZM model are consistent in terms of predicted responses to the geometrical parameters for the unsuccessful welding. Decreasing the inner cold zone length, increasing the fusion length and the thickness of the tting will improve the fracture toughness of the welded structure.

  • 16.
    Almgren, Karin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Stress-transfer mechanisms in wood-fibre composites2007Licentiate thesis, comprehensive summary (Other scientific)
  • 17.
    Andersson, D. C.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lindskog, P.
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Inverse modeling applied for material characterization of powder materials2015In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 43, no 5, p. 1005-1019Article in journal (Refereed)
    Abstract [en]

    An investigation is performed concerning the applicability of inverse procedures, using optimization and simple experiments, for characterization of WC/Co powder materials. The numerical procedure is combined with uniaxial die-compaction experiments using an instrumented die, which allows direct measurement of the distribution of radial stress during the experiments. Finite-element (FE) methods and an advanced constitutive description of powder materials are relied upon to model the compaction experiment. Optimization using a surrogate model is used to determine some of the parameters in the constitutive description. These parameters in the material model are said to be found (with some accuracy) if the output from the FE simulation is similar to the experimental data. It is found that even though a complete constitutive description of the powder materials investigated cannot be achieved using this approach, many important material parameters can be determined with good accuracy.

  • 18.
    Andersson, Daniel C.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Material Characterization of Powder Materials using Inverse Modeling2014Doctoral thesis, comprehensive summary (Other academic)
  • 19.
    Andersson, Daniel C.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Cadario, Alessandro
    Lindskog, Per
    On the influence from punch geometry on the stress distribution at powder compaction2010In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 202, no 1-3, p. 78-88Article in journal (Refereed)
    Abstract [en]

    A numerical analysis, using the finite element method, of the mechanical behavior at powder compaction at higher densities was performed. In this investigation the material behavior is modeled using an advanced macroscopic constitutive description initially presented by Brandt and Nilsson [1]. This material model, like many other models describing powder compaction at higher densities, includes a large number of constitutive parameters and as a result, a complete material characterization is a difficult task to perform or at least requires a large number of different experiments. A remedy to this problem is to apply inverse modeling, i.e. optimization, for determination of relevant material properties from comparatively simple experiments. It is then of course important, in order to achieve high accuracy results from the optimization procedure, that the stress fields produced during the experiments involves high gradients of stress. Adhering to simple uniaxial die compaction experiments the main parameter that can be used in order to achieve such a feature is the geometry of the punch used for load application. In the present investigation a number of punch profiles are studied and it is found that a skewed punch geometry is the most appropriate one to be used for experimental die compaction aiming at a constitutive description of the powder material based on inverse modeling. The main efforts are devoted towards an analysis based on the previously mentioned material model by Brandt and Nilsson [1] but also relevant results for another type of constitutive model will be presented. (C) 2010 Elsevier B.V. All rights reserved.

  • 20.
    Andersson, Daniel C.
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Lindskog, Per
    Staf, Hjalmar
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A Numerical Study of Material Parameter Sensitivity in the Production of Hard Metal Components Using Powder Compaction2014In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 23, no 6, p. 2199-2208Article in journal (Refereed)
    Abstract [en]

    Modeling of hard metal powder inserts is analyzed based on a continuum mechanics approach. In particular, one commonly used cutting insert geometry is studied. For a given advanced constitutive description of the powder material, the material parameter space required to accurately model the mechanical behavior is determined. These findings are then compared with the corresponding parameter space that can possibly be determined from a combined numerical/experimental analysis of uniaxial die powder compaction utilizing inverse modeling. The analysis is pertinent to a particular WC/Co powder and the finite element method is used in the numerical investigations of the mechanical behavior of the cutting insert.

  • 21.
    Andersson, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Determination of yield surfaces in advanced constitutive powder models using inverse modeling2011In: Proceedings of the Euro PM 2011 Congress and Exhibition, European Powder Metallurgy Association , 2011, Vol. 3, p. 247-252Conference paper (Refereed)
    Abstract [en]

    In order to decrease the extensive experimental work in product development of WC/Co powder tooling equipment, an investigation of the applicability of inverse procedures to industrially relevant material models is performed. The powder die compaction process of WC/Co powder is modeled using the finite element method and an advanced plasticity-based constitutive model. An angled indenter is used to increase the sensitivity of constitutive parameters with respect to the measurable data from experiment, which is done using an instrumented die. Here, the inverse modeling (optimization) is performed (using iterative reduction of variable space and a combination of genetic (GA) and gradient search algorithms) in order to determine the yield surface of the investigated material. The results indicate that inverse modeling can be a useful tool in order to reduce the experimental efforts at material characterization of powder materials described by advanced material models.

  • 22.
    Andersson, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Cadario, Alessandro
    Sandvik Tooling AB, Sweden.
    Lindskog, Per
    Sandvik Tooling AB.
    Inverse modeling for parameter determination when using advanced material models to describe powder compaction2010In: Proceedings of the World Powder Metallurgy Congress and Exhibition, World PM 2010, European Powder Metallurgy Association , 2010, Vol. 5, p. 77-84Conference paper (Refereed)
    Abstract [en]

    In order to decrease the amount of experimental work at material characterization of powder mixtures, optimization procedures are often used. In the present study, characterization of WC/Co powders described by an advanced plasticity model is at issue. In particular, uniaxial die compaction is analyzed numerically in order to determine the correlation between material parameters, scalar and functional, and measurable information from experiments. Such information include radial (contact) pressure between powder and die walls, press force as function of indentation depth and frictional effects between powder and die walls. The commercial finite element solver LS-Dyna and the optimization module LS-OPT are used in the present investigation.

  • 23.
    Andersson, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Cadario, Alessandro
    Sandvik Tooling AB, Sweden .
    Lindskog, Per
    Sandvik Tooling AB.
    Parameter Sensitivity at Advanced Constitutive Modeling of Powder Materials2009In: Proceedings of the Euro International Powder Metallurgy Congress and Exhibition, Euro PM 2009, European Powder Metallurgy Association (EPMA) , 2009, Vol. 3, p. 293-298Conference paper (Refereed)
    Abstract [en]

    Dry pressing powder compaction is analyzed using the finite element method (FEM) and in particular then the explicit commercial FEM program LS-DYNA. The aim is then to determine the sensitivity of mechanical local and global parameters with respect to different constitutive as well as geometrical variables. A very advanced constitutive description (not available as standard in commercial FEM packages but implemented into LS-DYNA for the present investigation) is used in the numerical analysis, Brandt and Nilsson [1], and the results are particularly helpful for a forthcoming inverse analysis where it is crucial, for reasons of efficiency, to reduce the number of variables in the optimization procedure aiming at a accurate description of the mechanical behavior of the powder material. It should be clearly stated that the geometrical parameters to be investigated include different possible press geometries to be used for experimental verification of the results.

  • 24.
    Andersson, Niklas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Modellering och svängningsstudie av asfaltsbelagd träbro2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 25.
    Andersson, Oscar
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering. KTH, Centres, XPRES, Excellence in production research.
    Budak, Nesrin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Melander, Arne
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering. KTH, Centres, XPRES, Excellence in production research.
    Palmquist, Niclas
    Experimental measurements and numerical simulations of distortions of overlap laser-welded thin sheet steel beam structures2017In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 61, no 5, p. 927-934Article in journal (Refereed)
    Abstract [en]

    Distortions of mild steel structures caused by laser welding were analyzed. One thousand-millimeter U-beam structures were welded as overlap joints with different process parameters and thickness configurations. Final vertical and transverse distortions after cooling were measured along the U-beam. Significant factors, which affect distortions, were identified. Heat input per unit length, weld length, and sheet thickness showed a significant effect on welding distortions. Furthermore, the welding distortions were modeled using FE simulations. A simplified and computationally efficient simulation method was used. It describes the effect of shrinkage of the weld zone during cooling. The simulations show reasonable computation times and good agreement with experiments.

  • 26.
    Andersson, Stefan
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Drivning av skidlift - friktionsmodeller och utveckling av det elektriska linspänningssystemet2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 27.
    Appelsved, Peter
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Investigation of Mechanical Properties of Thermoplastics with Implementations of LS-DYNA Material Models.2012Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The increased use of thermoplastics in load carrying components, especially in the automotive industry, drives the needs for a better understanding of its complex mechanical properties. In this thesis work for a master degree in solid mechanics, the mechanical properties of a PA 6/66 resin with and without reinforcement of glass fibers experimentally been investigated. Topics of interest have been the dependency of fiber orientation, residual strains at unloading and compression relative tension properties. The experimental investigation was followed by simulations implementing existing and available constitutive models in the commercial finite element code LS-DYNA.

    The experimental findings showed that the orientation of the fibers significantly affects the mechanical properties. The ultimate tensile strength differed approximately

    50% between along and cross flow direction and the cross-flow properties are closer to the ones of the unfilled resin, i.e. the matrix material. An elastic-plastic model with Hill’s yield criterion was used to capture the anisotropy in a simulation of the tensile test. Residual strains were measured during strain recovery from different load levels and the experimental findings were implemented in an elastic-plastic damage model to predict the permanent strains after unloading. Compression tests showed that a stiffer response is obtained for strains above 3% in comparison to tension. The increased stiffness in compression is although too small to significantly influence a simulation of a 3 point bend test using a material model dependent of the hydrostatic stress.

  • 28.
    Arbegard, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Jakobsson, Håkan
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Evaluation of Boundary Conditions in Chassis Test Rig.2014Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Trucks are constantly developed in order to achieve new goals in terms of demands from the customers and regulation. In order to develop the truck chassis to withstand the load, different tests are conducted and among those life time testing. At Scania, service life of frames and suspensions is tested using a purpose built test rig known as Ramriggen. The test rig subjects the frame to a quasi-static load created of measurements from a test track.

    The current rig for frame testing has boundary conditions that distribute the loads unevenly within the chassis, especially for trucks with added frame twisting. In the current rig, the front axle load is absorbed solely by the engine and the frame twisting is inserted by a cradle. With increased axle load the error in vertical loading is also increased and with the current configuration for frame twisting, unexpected cracks in the frame has been observed. In order to improve the emulation of a real truck the boundary conditions requires development. The purpose of this study is to compare different boundary conditions at the front of the truck.

    An investigation of how new boundary conditions at the front of truck can redistribute the loads has been conducted. This investigation utilized FEM as a tool and method to reach the end results. In order to compare different boundary conditions two synthetic load cases were defined from measurements conducted on a truck at a test track. The two load cases consisted of a vertical load and two cases of frame twisting. This investigation resulted in new boundary conditions for a concept test rig. The new boundary conditions consist of a cab mock-up that is able to absorb vertical load and induce frame twisting.

    The result of the investigation of the two load cases shows that the engine beam is overloaded at most 40% and the base mount with 150% with the current boundary conditions compared with the new suggested concept.

  • 29. Auer, M.
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Reconstruction and Finite Element Mesh Generation of Abdominal Aortic Aneurysms From Computerized Tomography Angiography Data With Minimal User Interactions2010In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 29, no 4, p. 1022-1028Article in journal (Refereed)
    Abstract [en]

    Evaluating rupture risk of abdominal aortic aneurysms is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. According to the current clinical practice aneurysm rupture risk is (mainly) estimated from its maximum diameter and/or expansion rate; an approach motivated from statistics but known to fail often in individuals. In contrast, recent research demonstrated that patient specific biomechanical simulations can provide more reliable diagnostic parameters, however current structural model development is cumbersome and time consuming. This paper used 2D and 3D deformable models to reconstruct aneurysms from computerized tomography angiography data with minimal user interactions. In particular, formulations of frames and shells, as known from structural mechanics, were used to define deformable modes, which in turn allowed a direct mechanical interpretation of the applied set of reconstruction parameters. Likewise, a parallel finite element implementation of the models allows the segmentation of clinical cases on standard personal computers within a few minutes. The particular topology of the applied 3D deformable models supports a fast and simple hexahedral-dominated meshing of the arising generally polyhedral domain. The variability of the derived segmentations (luminal: 0.50(SD 0.19) mm; exterior 0.89(SD 0.45) mm) with respect to large variations in elastic properties of the deformable models was in the range of the differences between manual segmentations as performed by experts (luminal: 0.57(SD 0.24) mm; exterior: 0.77(SD 0.58) mm), and was particularly independent from the algorithm's initialization. The proposed interaction of deformable models and mesh generation defines finite element meshes suitable to perform accurate and efficient structural analysis of the aneurysm using mixed finite element formulations.

  • 30. Auer, M.
    et al.
    Regitnig, P.
    Stollberger, R.
    Ebner, F.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    A methodology to study the morphologic changes in lesions during in vitro angioplasty using MRI and image processing2008In: Medical Image Analysis, ISSN 1361-8415, E-ISSN 1361-8423, Vol. 12, no 2, p. 163-173Article in journal (Refereed)
    Abstract [en]

    The assessment of morphologic changes in atherosclerotic lesions during interventional procedures such as transluminal balloon angioplasty is an issue of highest clinical importance. We propose a methodology that allows realistic 3D morphomechanical modeling of the vessel, the plaque and the lumen at different stages of in vitro angioplasty. We elaborate on a novel device designed to guide angioplasty under controlled experimental conditions. The device allows to reproduce in vivo conditions as good as possible, i.e. axial in situ pre-stretch, 100 mmHg intraluminal pressure, 37 degrees C Tyrode solution, balloon inflation without external constraints using a high-pressure syringe and contrast medium. With a standard 1.5 T MR-system we accomplish multi-spectral images at different stages of the angioplasty experiment. After MR image acquisition the specimen is used for histopathological analysis and biomechanical tests. A segmentation process is used to generate NURBS-based 3D geometric models of the individual vessel and plaque components at different balloon pressures. Tissue components are segmented automatically using generalized gradient vector flow active contours. We investigated 10 human femoral arteries. The effects of balloon compression on the individual artery components is particularly described for two obstructed arteries with an intact collagenous cap, a pronounced lipid pool and with calcification. In both arteries we observe a significant increase in lumen area after angioplasty. Dissection between intima and media and reduction of the lipid pool are primary mechanisms of dilatation. This methodology provides a basis for studying plaque biomechanics under supra-physiological loading conditions. It has the potential to improve and validate finite element models of atherosclerotic plaques which may allow a better prediction of angioplasty procedures.

  • 31. Auer, M.
    et al.
    Stollberger, R.
    Regitnig, P.
    Ebner, F.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    3-D reconstruction of tissue components for atherosclerotic human arteries using ex vivo high-resolution MRI2006In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 25, no 3, p. 345-357Article in journal (Refereed)
    Abstract [en]

    Automatic computer-based methods are well suited for the image analysis of the different components in atherosclerotic plaques. Although several groups work on such analysis some of the methods used are oversimplified and require improvements when used within a computational framework for predicting meaningful stress and strain distributions in the heterogeneous arterial wall under various loading conditions. Based on high-resolution magnetic resonance imaging of excised atherosclerotic human arteries and a series of two-dimensional (2-D) contours we present a segmentation tool that permits a three-dimensional (3-D) reconstruction of the most important tissue components of atherosclerotic arteries. The underlying principle of the proposed approach is a model-based snake algorithm for identifying 2-D contours, which uses information about the plaque composition and geometric data of the tissue layers. Validation of the computer-generated tissue boundaries is performed with 100 MR images, which are compared with the results of a manual segmentation performed by four experts. Based on the Hausdorff distance and the average distance for computer-to-expert differences and the interexpert differences for the outer boundary of the adventitia, the adventitia-media, media-intima, intima-lumen and calcification boundaries are less than 1 pixel (0.234 mm). The percentage statistic shows similar results to the modified Williams index in terms of accuracy. Except for the identification of lipid-rich regions the proposed algorithm is automatic. The nonuniform rational B-spline-based computer-generated 3-D models of the individual tissue components provide a basis for clinical and computational analysis.

  • 32. Auer, M.
    et al.
    Stollberger, R.
    Regitnig, P.
    Ebner, F.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    In Vitro Angioplasty of Atherosclerotic Human Femoral Arteries: Analysis of the Geometrical Changes in the Individual Tissues Using MRI and Image Processing2010In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 38, no 4, p. 1276-1287Article in journal (Refereed)
    Abstract [en]

    Existing atherosclerotic plaque imaging techniques such as intravascular ultrasound, multidetector computed tomography, optical coherence tomography, and high-resolution magnetic resonance imaging (hrMRI) require computerized methods to separate and analyze the plaque morphology. In this work, we perform in vitro balloon angioplasty experiments with 10 human femoral arteries using hrMRI and image processing. The vessel segments contain low-grade to high-grade lesions with very different plaque compositions. The experiments are designed to mimic the in vivo situation. We use a semi-automatic image processing tool to extract the three-dimensional (3D) geometries of the tissue components at four characteristic stages of the angioplasty procedure. The obtained geometries are then used to determine geometrical and mechanical indices in order to characterize, classify, and analyze the atherosclerotic plaques by their specific geometrical changes. During inflation, three vessels ruptured via helical crack propagation. The adventitia, media, and intima did not preserve their area/volume during inflation; the area changes of the lipid pool during inflation were significant. The characterization of changes in individual 3D tissue geometries, together with tissue-specific mechanical properties, may serve as a basis for refined finite element (FE) modeling, which is key to better understand stress evolution in various atherosclerotic plaque configurations.

  • 33.
    Auer, Martin
    et al.
    Develoment, Vascops GmbH, Graz, Austria.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Portugaller, R.
    Automatic Displacement and Strain measuring in the Aorta from dynamic electrocardiographically-gated Computed Tomographic Angiography2010Conference paper (Refereed)
    Abstract [en]

    Introduction

    Image modalities like Duplex Ultrasound, Transesophageal Echocardiography, Intravascular Ultrasound, Computed Tomography and Magnetic Resonance provide vascular interventionists and surgeons with useful diagnostic information for treatment planning. Recent developments in cross-sectional imaging, including multi-modality image fusion and new contrast agents have resulted in improved spatial resolution. Specifically, dynamic Electrocardiographically-Gated Computed Tomographic Angiography (ECG-gated CTA) provides valuable information regarding motion and deformation of the normal and diseased aorta during the cardiac cycle. Extracting and presenting (visualization) of accurate quantitative information from the recorded image data, however remains a challenging task of image post processing.

    Method

    The algorithm proposed within this paper processes ECG-gated CTA data (here goes the scanner model and manufacturer) in DICOM (digital imaging and communication in medicine) format, within which the user manually defines an Eulerian Region of Interest (ROI). 2D deformable (active) contour models are used to pre-segment the luminal surfaces of the selected vessels at an arbitrary time point during the cardiac cycle. A tessellation algorithm is used to define the initial configuration of a 3D deformable (active) contour model, which in turn is used for the final segmentation of the luminal surfaces continuously during the cardiac cycle. Specifically, Finite Element (FE) formulations [1] for frames and shells, as known from structural mechanics, are used to define the deformable contour modes. This allows a direct mechanical interpretation of the applied set of reconstruction parameters and leads to an efficient FE implementation of the models [2]; parallel processor architecture is used to solve the global set of non-linear FE equations. Finally displacement and strain measures are derived from the dynamic segmentations and color coded plots are used to visualize them.

    Results and Conclusions

    The clinical relevance of dynamic imaging has not been fully exploited and accurate and fast image processing tools are critical to extract valuable information from ECG-gated CTA data. Such information is not only of direct clinical relevance but also critical to process our current understanding regarding normal and pathological aortic motions and deformations. The image processing concept proposed in this paper leads to efficient and clinically applicable software that facilitates an analysis of the entire aorta on a standard Personal Computer within a few minutes. Deformable (active) contour models are known to be more accurate compared to threshold based segmentation concepts [3] and the accuracy of the present approach is in the range of the in-plane image resolution. Apart from direct diagnostic information the extracted geometrical data could also be used (once enriched by accurate pressure measurements) for none invasive (minimal invasive) estimation of biomechanical aortic tissue properties.

    References

    [1] O. C. Zienkiewicz and R. L. Taylor, vol.1,2, 5th ed. Oxford: Butterworth Heinemann, 2000.

    [2] M. Auer and T. C. Gasser,

    IEEE T. Med. Imaging, 2010 (in press).

    [3] M. Sonka and J. M. Fitzpatrick, editors.,

    Bellingham: Spie press, 2000

  • 34.
    AZİZOĞLU, YAĞIZ
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Micromechanical Numeric Investigation of Fiber Bonds in 3D Network Structures.2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In manufacturing of paper and paperboard, optimized fiber usage has crucial importance for process efficiency and profitability. Dry strength of paper is one of the important quality criteria, which can be improved by adding dry strength additive that affect fiber to fiber bonding. This study is using the micromechanical simulations which assist interpretation of the experimental results concerning the effect of strength additives.

    A finite element model for 3D dry fiber network was constructed to study the effect of bond strength, bond area and the number of bonds numerically on the strength of paper products. In the network, fibers’ geometrical properties such as wall thickness, diameter, length and curl were assigned according to fiber characterization of the pulp and SEM analyses of dry paper cross-section. The numerical network was created by depositing the fibers onto a flat surface which should mimic the handsheet-making procedure. In the FE model, each fiber was represented with a number of quadratic Timoshenko beam elements where fiber to fiber bonds were modelled by beam-to-beam contact. The contact model is represented by cohesive zone model, which needs bond strength and bond stiffness in normal and shear directions. To get a reasonable estimate of the bond stiffness, a detailed finite element model of a fiber bond was used. Additionally, the effect of different fiber and bond geometries on bond stiffness were examined by this model since the previous work [13] indicated that the bond stiffness can have a considerable effect on dry strength of paper.

    The network simulation results show that the effect of the strength additive comes through improving the bond strength primarily. Furthermore, with the considered sheet structure, both the fiber bond compliance and the number of bonds affect the stiffness of paper. Finally, the results of the analyses indicated that the AFM measurements of the fiber adhesion could not be used directly to relate the corresponding changes in the bond strength.

    The fiber bond simulation concluded that fiber wall thickness has the most significant effect on the fiber bond compliance. It was also affected by micro-fibril orientation angle, bond orientation and the degree of pressing.

  • 35.
    Ballesteros, Antonio
    et al.
    JRC Petten, Holland.
    Hein, Hieronymus
    AREVA Gmbh Germany.
    May, Johannes
    AREVA Gmbh Germany.
    Planman, Tapio
    VTT Finland.
    Todeshini, Patrick
    EdF France.
    Brumowski, Milan
    UJV Czech Republic.
    Roudén, Jenny
    Gillemot, Ferenc
    MTA Hungary.
    Chaouadi, Rachid
    SCK-CEN, Belgium.
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Altstadt, Eberhard
    Forschung Center Rossendorff, Germany.
    Reactor Pressure vessel surveillance2014In: Nuclear Engineering International, ISSN 0029-5507, Vol. 59, no 12, p. 19-20Article in journal (Refereed)
    Abstract [en]

    This publication summarizes techniques suitable for surveillance program for the objective of  long term operation (LTO) on European NPPs and provides recommendations on reactor pressure vessel (RPV) irradiation surveillance based on the work preformed in the work package 7 "Surveillance guidelines" of the LONGLIFE international project. The LONGLIFE project "treatment of long term irradiation embrittlement effects in RPV safety assessment" was 50% funded by the Euratom 7th framework programme of the European commision. The project coordinated by the Helmholtz-centrum Dresden Rossendorf successfully finalized in 2014.

  • 36. Balzani, D.
    et al.
    Holzapfel, Gerhard
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics. Graz University of Technology, Institute of Biomechanics, Center of Biomedical Engineering.
    Brinkhues, S.
    Modeling of damage in soft biological tissues and application to arterial walls2011In: Computational Plasticity XI - Fundamentals and Applications, 2011, p. 764-775Conference paper (Refereed)
    Abstract [en]

    A new material model is proposed for the description of stress-softening observed in cyclic tension tests performed on soft biological tissues. The modeling framework is based on the concept of internal variables introducing a scalar-valued variable for the representation of fiber damage. Remanent strains in fiber direction can be represented as a result of microscopic damage of the fiber crosslinks. Particular internal variables are defined able to capture the nature of soft biological tissues that no damage occurs in the physiological loading domain. A specific model is adjusted to experimental data taking into account the supra-physiological loading regime. For the description of the physiological domain polyconvex functions are used which also take into account fiber dispersion in a phenomenological approach. The applicability of the model in numerical simulations is shown by a representative example where the damage distribution in an arterial cross-section is analyzed.

  • 37. Balzani, Daniel
    et al.
    Brinkhues, Sarah
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Constitutive framework for the modeling of damage in collagenous soft tissues with application to arterial walls2012In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 213, p. 139-151Article in journal (Refereed)
    Abstract [en]

    In this paper a new material model is proposed for the description of stress-softening observed in cyclic tension tests of collagenous soft tissues such as arterial walls, for applied loads beyond the physiological level. The modeling framework makes use of terms known from continuum damage mechanics and the concept of internal variables introducing a scalar-valued variable for the representation of fiber damage. A principle is given for the construction of damage models able to reflect remanent strains as a result of microscopic damage in the reinforcing collagen fiber families. Particular internal variables are defined able to capture the nature of arterial tissues that no damage occurs in the physiological loading domain. By application of this principle, specific models are derived and fitted to experimental data. Finally, their applicability in numerical simulations is shown by some representative examples where the damage distribution in arterial cross-sections is analyzed.

  • 38. Bankel, J.
    et al.
    Berggren, K. F.
    Engstrom, M.
    Wiklund, I.
    Crawley, E. F.
    Söderholm, D.
    El Gaidi, K.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Benchmarking engineering curricula with the CDIO syllabus2005In: International journal of engineering education, ISSN 0949-149X, Vol. 21, no 1, p. 121-133Article in journal (Refereed)
    Abstract [en]

    Four internationally-renowned universities-Chalmers University of Technology, Linkoping University, Royal Institute of Technology (Sweden), and the Massachusetts Institute of Technology (USA)-developed a benchmark survey that may be used by any engineering school to benchmark curricula for teaching of personal, interpersonal and system building skills. These skills are enumerated in the CDIO Syllabus. Teaching activities were categorized as Introduce, Teach or Utilize, based on intent, time spent, and linkage to learning objectives, assignments and assessment criteria. Interviews were used to collect the data from instructors of the schools' engineering programs. The data was then reduced and analyzed to illuminate patterns of teaching. The results indicate that much effort is expended in covering these topics, but often in an inefficient, uncoordinated and unplanned manner. For example, there are often frequent repetitions of introducing a topic, without ever teaching it. In other instances, students are expected to utilize knowledge without having been taught it. The results of the benchmark survey indicate that a consistent and deliberately designed curriculum in this area could demand no additional resources, yet provide a much more effective education. The survey gives useful indications of how to begin such a curriculum redesign process.

  • 39.
    Baradaran, Mohammad Ali
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.). KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hydrogen Embirttlement in Weldox 1300 and Hardox 5002014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Abstract

    Upon trying to reach higher strengths, when designing steels, inevitably susceptibility to one

    type of cracking known as hydrogen cracking increases. In present work, this complexity with

    regard to high strength structural steel of Weldox 1300 and wear plate of Hardox 500 was

    studied.

    Effect of low temperature tempering (200 ) and hard leveling on hydrogen embrittlement was

    qualitatively explored by fracture toughness testing. Tests were performed on SENB specimens of two types of Weldox 1300 in air and 3.5% NaCl solution. In-situ testing of as-quenched Weldox caused K value for crack growth initiation to drop to almost 20% of that for reference specimen tested in air. However, Weldox 1300 in tempered and leveled condition exhibited considerably improved resistance against hydrogen cracking by almost 50% compared to asquenched condition.

    It is believed that formation of transit carbides acting as strong traps due to tempering, and alteration in dislocations’ structure and level of tensile residual stresses thanks to combined effects of tempering and leveling have considerable impact on crack growth kinetics which results in improved resistance. The influence of tempering and leveling was not investigated separately.

    Additionally, by using four-point-bending test it was attempted to screen a method suitable for study of hydrogen embrittlement. Test variables were tried to be adjusted to meet the failure criteria. Precharged samples were subjected to bending stresses and left in outdoor atmosphere. Hydrogen measurement after passing 41 days on one of the samples containing a stress concentrator showed that hydrogen had been trapped and still present into the sample. Although hydrogen measurement showed the effect of stress fields on hydrogen trapping, test results along with FEM simulation indicated that such a test method might not be practicable for this special combination of materials and expectations.

  • 40.
    Barbier, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical investigation of folding of coated papers2005In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 67, no 4, p. 383-394Article in journal (Refereed)
    Abstract [en]

    Folding of coated paper is examined numerically using the finite element method. Particular emphasis is put on the behaviour of field variables relevant for cracking of the coating layers. In the numerical analysis, the basepaper is modelled as an anisotropic elastic-plastic material (both elastic and plastic anisotropy is accounted for) while the constitutive behaviour of the coating layers are approximated by classical (Mises) elastoplasticity. The numerical results suggest, among other things, that particular forms of plastic anisotropy can substantially reduce the maximum strain levels in the coating. It is also shown that delamination buckling, in the present circumstances, will have a very small influence on the strain levels in the coating layer subjected to high tensile loading.

  • 41.
    Barbier, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    On dynamic effects at folding of coated papers2005In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 67, no 4, p. 395-402Article in journal (Refereed)
    Abstract [en]

    Folding of coated papers is examined numerically using the finite element method. The analysis is focused on the influence from dynamic effects on the folding process. In particular, the behaviour of field variables relevant for cracking of the coating layers are studied in some detail. The results presented indicate that dynamic effects are of little importance as regards maximum strain levels in the coating but will influence the stress and strain distributions. Accordingly, a quasi-static analysis of the problem will be sufficient in order to describe many of the important features related to cracking.

  • 42.
    Barbier, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    On the effect of high anisotropy at folding of coated papers2006In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 72, no 3, p. 330-338Article in journal (Refereed)
    Abstract [en]

    A finite element procedure, developed in order to account for the effect of high anisotropy at folding of coated papers, is presented. The anisotropic behaviour (with very low stiffness in the thickness direction) is modelled using stiff structural elements (trusses and beams). The numerical results indicate that high elastic anisotropy leads to lower strain levels at folding than reported in previous analyses where this effect was not accounted for. High plastic anisotropy, on the other hand, will contradict this result.

  • 43.
    Barbier, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Eklund, Johan
    DPC, MidSweden University.
    Folding of Printed Papers: Experiments and Numerical Analysis2003In: Preprints of the 2003 International Paper Physics Conference, 2003, p. 193-196Conference paper (Refereed)
  • 44.
    Barbier, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Larsson, Per-Lennart
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hallbäck, N.
    Karathanasis, M.
    On material characterization of paper coating materials by microindentation testing2005In: Journal of Coatings Technology Research, ISSN 1547-0091, Vol. 2, no 6, p. 463-471Article in journal (Refereed)
    Abstract [en]

    Microindentation as a method for determining important material properties of paper coating materials is studied experimentally and numerically The bulk of the investigation is concentrated upon the short-lived elastic part of a spherical indentation test, but determination of the failure stress of the coating is also discussed. The results indicate that microindentation can be a powerful tool for material characterization of these materials, but only if careful efforts arc, made to account for the influence from plasticity as well as from boundary effects.

  • 45.
    Barrskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Implementation of methods for computation of Tooth Interior Fatigue Fracture2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    For a company like Scania CV AB, a vast number of laws and regulations has to be considered when developing a truck. In the constant struggle to keep the generated noise below the allowed levels, the gears are made more slender and flexible. The slenderness in combination with case hardening has brought a new type of gear fracture into the light.The Tooth Interior Fatigue Fracture, TIFF. A 2D-method, and a tool for engineers, was developed in the early 2000’s. However, this tool did not provide sufficient accuracy andcompatibility with the current design process to be adopted by the engineers at Scania.This thesis expands on the current 2D-model and attempts to improve the accuracy by bringing the analysis to 3D. Furthermore, the computational tool is developed in Pythonto allow for a more streamlined interface with the current workflow.The proposed method approximates the tooth as a cantilever-beam, and is only evaluated for this case. However, the stresses are computed with good accuracy. The onlydiscrepancy is one of the stress components, where the error is about 50%. This error isderived from the decision to, in torsion, model the cross-section of the gear tooth as an ellipse. The method has potential to be incorporated into the current design process, but the accuracy of the stresses due to torsion has to be improved, and some of the equations has to be adapted before real gear geometries can be considered.

  • 46. Barsoum, I.
    et al.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Pingle, S.
    The effect of stress state on ductility in the moderate stress triaxiality regime of medium and high strength steels2012In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 65, no 1, p. 203-212Article in journal (Refereed)
    Abstract [en]

    Experiments on double notched tube specimens subjected to tension and torsion were conducted by Barsoum and Faleskog (2007) [8,9]. In this study a complementary experimental investigation was conducted on tensile round circumferentially notched bar specimens. The results from the current study were compared with the results from the double circumferentially notched tube specimens with stress triaxiality larger than 0.7 in order to asses the influence of the Lode parameter on ductility in the moderate stress triaxiality regime. The effective plastic strain, the stress triaxiality T and the Lode parameter L were determined at the center of the notch up to the point of onset of failure by means of finite element. The influence of the Lode parameter on the failure strain was significant for the high strength and low hardening material, whereas for the medium strength and high hardening material the influence of the Lode parameter was less distinguished. The experimental results were then analyzed with the micromechanical model proposed by Barsoum and Faleskog (2011) [15], which is based on the assumption that ductile failure is a consequence of that plastic deformation localizes into a band. The band consists of a square array of equally sized cells, with a spherical void located in the center of each cell, which allows for studying a single 3D unit cell with fully periodic boundary conditions. The unit cell is subjected to a proportional loading such that it resembles the stress state, in terms of T and L, from the experiments. The micromechanical model captures the experimental trend and the influence of L on ductility very well. It is found that the Lode parameter sensitivity increases by the combination of increase in the yield strength and decrease in strain hardening. The fractographical analysis reveals that this Lode parameter sensitivity is associated with the failure characteristics of the material.

  • 47. Barsoum, I.
    et al.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Pingle, S.
    The influence of the lode parameter on ductile failure strain in steel2011In: ICM11, 2011, p. 69-75Conference paper (Refereed)
    Abstract [en]

    In this study an experimental investigation was conducted on tensile round circumferentially notched bar specimens. The results were compared to the experimental result on double notched tube specimens subjected to tension and torsion conducted in [5]. The comparison was done for moderate stress triaxiality levels larger than 0.7 with the objective to assess the influence of the Lode parameter on the ductile failure strain. The effective plastic strain, the stress triaxiality T and the Lode parameter L were determined at the center of the notch up to the point of failure by means of finite element based on J2-plasticity. The influence of the Lode parameter on the failure strain was remarkable for the high strength and low hardening material, whereas for the medium strength and high hardening material the influence of the Lode parameter was less prominent. The experimental results were then analyzed with the micromechanical model proposed in [6-7] which is based on the assumption that ductile failure is a consequence of that plastic deformation localizing into a band of imperfections. It is found that the micromechanical model captures the experimental trend and thus the influence of L on the ductility very well. It is found that the Lode parameter sensitivity increases with increase in the yield strength. The fractographical analysis reveals that Lode parameter sensitivity is associated with the failure characteristics of the material.

  • 48.
    Barsoum, Imad
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Ductile failure and rupture mechanisms in combined tension and shear2006Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This licentiate thesis is generally concerned with the ductile failure and rupture mechanisms encountered under combined tension and torsion loading. In the first part entitled Paper A, an experimental investigation of the rupture mechanisms in a mid-strength and a high strength steel was conducted employing a novel test configuration. The specimen used was a double notched tube specimen loaded in combined tension and torsion at a fixed ratio. The effective plastic strain, the stress triaxiality and the Lode parameter was determined in the centre of the notch at failure. Scanning electron microscopy of the fractured surfaces revealed two distinctively different ductile rupture mechanisms depending on the stress state. At high stress triaxiality the fractured surfaces were covered with large and deep dimples, suggesting that growth and internal necking of voids being the governing rupture mechanism. At low triaxiality it was found that the fractured surfaces were covered with elongated small shear dimples, suggesting internal void shearing being the governing rupture mechanism. In the fractured surfaces of the high-strength steel, regions with quasi-cleavage were also observed. The transition from the internal necking mechanism to the internal shearing mechanism was accompanied by a significant drop in ductility.

    In the second part entitled Paper B, a micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The periodic arrangement of the cells allows the study of a single unit cell for which fully periodic boundary conditions are applied. The micromechanics model is applied to analyze failure by ductile rupture in experiments on double notched tube specimens subjected to combined tension and torsion carried out by the present authors. The stress state is characterized in terms of the stress triaxiality and the Lode parameter. Two rupture mechanisms can be identified, void coalescence by internal necking at high triaxiality and void coalescence by internal shearing at low triaxiality. For the internal necking mechanism, failure is assumed to occur when the deformation localizes into a planar band and is closely associated with extensive void growth. For the internal shearing mechanism, a simple criterion based on the attainment of a critical value of shear deformation is utilized. The two failure criteria capture the transition between the two rupture mechanisms successfully and are in good agreement with the experimental result.

  • 49.
    Barsoum, Imad
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The effect of stress state in ductile failure2008Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The industrial application of high strength steels in structural components has increased the demand on understanding the ductile failure behavior of this type of materials. In practical situations the loading experienced on components made out of these materials can be very complex, which may affect the failure behavior.

    The objective of this work is to study the effect of stress state on ductile failure and the mechanisms leading to rupture in high strength steels. The stress state is characterized by the stress triaxiality T and the Lode parameter L, which is a deviatoric stress state parameter that discriminates between axisymmetric or shear dominated stress states. For this purpose experiments on two different specimen configurations are performed; a double notched tube (DNT) specimen tested in combined tension and shear and a round notched bar (RNB) specimen tested in uniaxial tension. The two specimens give rise to different stress states at failure in terms of T and L. The failure loci for the DNT specimen show an abrupt change in ductility, indicating a transition between the rupture mechanisms necking of intervoid ligaments and shearing of intervoid ligaments. A clear difference in ductility between the two specimen configurations is also observed, which is closely associated with the difference in stress state at failure.

    A micromechanical model is developed, which assumes that ductile material failure occurs when the deformation becomes highly non-linear and localizes into a band. The model, which is applied to analyze the experiments, consists of a band with a square array of equally sized cells, with a spherical void located in the center of each cell. The model, extended with a shear criterion, captures the experimental trend rather well. The model also shows that the effect of the deviatoric stress state (L) on void growth, void shape evolution and coalescence is significant, especially at low levels of T and shear dominated stress state.

  • 50.
    Barsoum, Imad
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Influence of the Lode parameter on ductility in the moderate stress triaxiality regime2008Report (Other academic)
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