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  • 1.
    Adolfsson, Erik
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Matrix crack induced stiffness reductions in [(0(m)/90(n)/+theta(p)/-theta(q))(s)](m) composite laminates1995In: Composites Engineering, ISSN 0961-9526, Vol. 5, no 1, 107-123 p.Article in journal (Refereed)
    Abstract [en]

    Two- and three-dimensional linearly elastic glass/epoxy and carbon/epoxy laminates of the type [(0m/90n/ + theta(p)/ - theta(q))s]M containing periodically distributed matrix cracks have been analysed by aid of the finite element method. The presented finite element model enables modelling of several important thick and thin ply stacking sequences like cross-plies, angle plies and quasi-isotropic laminates. Due to periodicity it suffices to model a representative volume element of the laminate. The boundaries of this unit cell represent prospective crack surfaces. In this way varying crack configurations and crack densities could be simulated. By application of periodic boundary conditions the stiffness tensors for laminates containing different crack configurations were calculated. The results are presented in the form of reduced engineering stiffness parameters as functions of matrix crack densities for a thick quasi-isotropic [(0-degree/90-degrees/ + 45-degrees/ -45-degrees)s]M glass/epoxy laminate, a thick [(0-degree/90-degrees + 55-degrees/ - 55-degrees)s]M carbon/epoxy laminate and a thin (0-degree/ +45-degrees/ -45-degrees)s glass/epoxy laminate. Comparisons are made to an approximate analytic model developed previously. An excellent agreement between the analytic predictions and the finite element results was found for all cases under consideration.

  • 2.
    Adolfsson, Erik
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Matrix crack initiation and progression in composite laminates subjected to bending and extension1999In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 36, no 21, 3131-3169 p.Article in journal (Refereed)
    Abstract [en]

    An experimental investigation of matrix crack initiation and progression in glass/epoxy laminates of different stacking sequences is presented. The laminates have been loaded in extension and bending, and the degree of damage as function of the load has been recorded. The changes in certain elastic properties caused by the damage were also measured, and are compared to results from a previously developed approximate analytic model. An energy release rate resistance curve is adopted in an attempt to describe the initiation and progression of matrix cracks in the laminates. The amount of cracking is also viewed in relation to the strain transverse to the fibres in the ply under consideration, and the ply stresses at the onset of cracking are calculated. The different damage evolution criteria are compared to the experimental results, and their validity and reliability are discussed. By use of the ply strain transverse to the fibres as a critical parameter for damage evolution, the load-deformation curves of the tested laminates are simulated taking damage progression into account.

  • 3.
    Adolfsson, Erik
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Thermoelastic properties in combined bending and extension of thin composite laminates with transverse matrix cracks1997In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 34, no 16, 2035-2060 p.Article in journal (Refereed)
    Abstract [en]

    Approximate analytic expressions for the thermoelastic properties in combined bending and extension of composite laminates containing transverse matrix cracks are derived. The model covers two-dimensional laminates of arbitrary lay-up sequences. The derived expressions for the compliances and thermal expansion coefficients merely contain ply property data and crack distributions. In order to check the accuracy and reliability of the presented analytic method, some sample cracked geometries were examined by use of the finite element method. A good agreement was found between the numerically and analytically obtained results for all cases under consideration.

  • 4.
    Agde Tjernlund, Jessica
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gamstedt, Kristofer
    KTH, Superseded Departments, Solid Mechanics.
    Xu, Zhi-Hui
    KTH, Superseded Departments, Materials Science and Engineering.
    Influence of molecular weight on strain-gradient yielding in polystyrene2004In: Polymer Engineering and Science, ISSN 0032-3888, E-ISSN 1548-2634, Vol. 44, no 10, 1987-1997 p.Article in journal (Refereed)
    Abstract [en]

    Experimental observations have indicated that the presence of strain gradients has an influence on the inelastic behavior of polymers as well as in other materials such as ceramics and metals. The present study has experimentally quantified length-scale effects in inelastic deformations of the polymer material polystyrene (PS) with respect to the molecular length. The experimental technique that has been used is nano-indentation to various depths with a Berkovich indenter. The hardness has been calculated with the method by Oliver and Pharr, and also by direct measurements of the area from atomic force microscopy. The experiments showed that the length-scale effects in inelastic deformations exist in polystyrene at ambient conditions. The direct method gave a smaller hardness than the Oliver-Pharr method. It was also shown that the length-scale parameter according to Nix and Gao increases with increasing molecular weight. For high molecular weights above a critical value of entanglement, there was no pertinent increase in the length-scale parameter. The length-scale parameter for strain-gradient plasticity has a size of around 0.1 μm for polystyrene.

  • 5.
    Alfredsson, Bo
    KTH, Superseded Departments, Solid Mechanics.
    A study on contact fatigue mechanisms2000Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Surfaces subjected to rolling and sliding contacts maysuffer from contact fatigue. This thesisdeals with solidmechanic aspects of contact fatigue including the descriptionand verificationof explaining mechanisms. The new mechanism forsurface initiated contact fatigue is basedon tensile surfacestresses from local asperity contacts. It is also realised thatsub-surfaceinitiated contact fatigue is the result of tensileresidual stresses that emanate from plasticdeformation belowthe surface. These mechanisms clearly show that contact fatiguecracksfollow the same rules as ordinary fatigue cracks inhardened steel.

    The thesis contains four papers that treat a new testprocedure named Standing ContactFatigue (SCF). The results ofthe test procedure have played an important role inthedevelopment and verification of the mechanisms for surfaceand sub-surface contact fatigue.

    The first part of the research work was experimental. Inthis part the SCF test properties wasdecided, crack resultsconfirmed and crack detection methods developed. Herecomparativestudies were performed using some differentmaterials and mechanical properties. It wasverified that SCFcould detect differences in contact fatigue resistance.

    Next a finite element model of the SCF test was evaluatedthrough the general-purposeprogram MARC. The model includedgraded material properties that originate from heattreatment.The residual surface deformation and surface compliance wereverified againstexperimental results. Crack initiation wasinvestigated in two ways. Firstly, the principalstresses atcritical locations were computed and plotted in a Haighdiagram. The diagramshowed that the cracks initiate in adirection perpendicular to the principal stress with thelargeststress range provided that the principal stress is tensilesometime during the load cycle.

    Secondly, some high cycle multiaxial fatigue criteria,including the Haigh principal stresscriterion, was evaluatedagainst the SCF crack initiation results. The surface cracklocation waspredicted by including statistical effects using aweakest-link criterion and a three-parameterWeibulldistribution.

    The SCF crack propagation was investigated by numericalevaluation ofJ1 andJ2integrals. The crack initiation and propagationphases were separated with a threshold criterionand a directioncriterion. It was found that during crack propagation bothsurface andsub-surface contact fatigue cracks follow thedirection with minimum mode II loading.

    Key words: contact fatigue mechanism; spall; spalling;surface crack; sub-surface crack;elasto-plastic indentation;contact compliance measurement; mixed-mode fatigue;fatiguecrack growth;J-integral; multiaxial fatigue; weakest-link.

  • 6.
    Alfredsson, Bo
    KTH, Superseded Departments, Solid Mechanics.
    An experimental and numerical study on contact fatigue initiation1998Licentiate thesis, comprehensive summary (Other scientific)
  • 7.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Cadario, Alessandro
    KTH, Superseded Departments, Solid Mechanics.
    A study on fretting friction evolution and fretting fatigue crack initiation for a spherical contact2004In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 26, no 10, 1037-1052 p.Article in journal (Refereed)
    Abstract [en]

    A new design for fretting experiments is presented. The normal and tangential contact loads as well as the specimen bulk stress are separately controlled. The separate control of load systems enables more accurate simulations of the fretting situations in component interfaces. Also, the influence of the salient parameters can be investigated individually. The initial test series comprised a spherical indenter and constant normal load and bulk stress. The evolution of the slip zone coefficient of friction at a spherical fretting contact was evaluated in four different ways. For two of these methods new equations were derived. The importance of a correct coefficient of friction and the advantages of each evaluation method are discussed. The experimental results were evaluated with respect to fretting fatigue crack initiation. Five multi-axial fatigue criteria were evaluated and ranked with respect to their ability to predict fretting fatigue initiation properties. The endurance limits of all criteria were too high as compared to the experimental fretting fatigue endurance level. A qualitative explanation for the discrepancy was found in the surface profile of the slip zone.

  • 8.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Applying multiaxial fatigue criteria to standing contact fatigue2001In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 23, no 6, 533-548 p.Article in journal (Refereed)
    Abstract [en]

    The ability of some multiaxial fatigue criteria to predict initiation of standing contact fatigue cracks is investigated. In the standing contact fatigue test an indenter subjects a case-hardened rest specimen to a stationary but pulsating contact load. The initiation of two axi-symmetric crack types appearing in the test specimen is investigated. The surface initiated ring/cone cracks circumscribe the contact area, whereas the lateral cracks are horizontal sub-surface cracks. The effect of the stress-state history at possible crack Locations is evaluated through the Sines, Haigh principal stress, Findley, Me Diarmid and Dang Van multiaxial fatigue criteria. The material fatigue parameters of each criterion are determined from independent bending and torsion fatigue testing. Finally, the mean and spread in radial position of the ring/cone crack are evaluated by considering the statistical effects of a weakest link assumption using a three parameter Weibull distribution. The investigation shows that it is hard to distinguish a single criterion that well describes all aspects of the experimental results. For the current contact situation with highly compressive mean stresses in combination with tensile maximum values the Findley criterion shows the best overall performance followed by the Haigh principal stress criterion

  • 9.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Inclined standing contact fatigue2003In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 26, no 7, 589-602 p.Article in journal (Refereed)
    Abstract [en]

    An experimental method is presented, in which a sphere is repeatedly pressed against a surface with an inclined contact load. The method is a development of the normally loaded standing contact fatigue test. Experiments are performed for three inclination angles below the angle of friction and the results are compared to those of the normally loaded standing contact fatigue test. The influence of tangential load on endurance limit load, number of cycles to crack initiation, contact mark appearance and crack behaviour in the surface as well as in cut views are evaluated. The surface crack behaviour outside the contact mark is analysed based on the cyclic contact stresses in the test specimen. The trajectories of the largest principal stresses are followed in both the surface view and in the cut view on the symmetry plane. These stress trajectories are compared to the experimental crack results. The connection between the inclined standing contact fatigue cracks and surface distress micro-cracks is also discussed.

  • 10.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Initiation and Growth of Standing Contact Fatigue Cracks2000In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 65, no 1, 89-106 p.Article in journal (Refereed)
    Abstract [en]

    In the standing contact fatigue test an indenter subjects a case hardened test specimen to a stationary but pulsating contact load. Two crack types appearing in the test specimen are investigated. The ring/cone cracks are surface cracks that circumvent the contact area, whereas the lateral cracks are horizontal sub-surface cracks. The initial crack lengths are determined for both crack types. Actual crack paths from experiments are evaluated numerically. For each crack tip position, stress intensity factors are determined from J(1) and J(2) integrals. The stress intensity ranges are compared to criteria for fatigue crack propagation rate and direction. As the cracks propagate, they orient in the direction with mode II loading close to zero.

  • 11.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Standing Contact Fatigue1999In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 22, no 3, 225-237 p.Article in journal (Refereed)
    Abstract [en]

    A novel experimental method for testing the resistance of a material to contact fatigue, called standing contact fatigue (SCF), is presented. It comprises a spherical indenter, repeatedly pressed onto a plane specimen in pure normal contact without lubrication, friction or wear. The SCF method is here applied to three case-hardened steels, and results in ring/cone cracks initiated at the surface. The connection between SCF and spalling is discussed.

    The experimental results are presented in the form of P-N curves, where P is the normal contact load and N the number of cycles required for fatigue crack initiation. The experimental results are supported by numerical simulations of the tests. The elasto-plastic properties of case-hardened materials are graded, i.e. functions of the depth from the carburized surface. The gradation is estimated from independent experiments and is included in the analysis.

  • 12.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Standing Contact Fatigue Testing of a Ductile Material: Surface and Sub-Surface Cracks2000In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 23, no 3, 229-240 p.Article in journal (Refereed)
    Abstract [en]

    During standing contact fatigue testing of case hardened steel plates, four different fatigue crack types are found: ring/cone; lateral; radial; and median cracks. Fatigue results are presented as load versus cycle number, with endurance limits and initiation laws for the ring/cone and lateral cracks. The behaviour of the radial surface strain outside the contact is altered by the presence of cracks. In particular this makes in situ crack detection possible for the lateral crack.

    The ductility of the tested material is found to be important for fatigue crack initiation. Numerical elastoplastic computations are used to derive the stress cycles responsible for each crack type. Stress cycles at different locations and in different directions are compared in order to explain why a particular crack type initiates. It is noted that cracks are produced normal to principal stresses of sufficient range, which are tensile sometime during the load cycle. Implications for spalling are discussed.

  • 13.
    Alfthan, Johan
    KTH, Superseded Departments, Solid Mechanics.
    A simplified network model for mechano-sorptive creep in paper2003In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 29, no 7, 228-234 p.Article in journal (Refereed)
    Abstract [en]

    A simplified network model for mechanosorptive creep is presented. The model resembles Cox's model for fibrous materials, but creep and the influence of bonds are included in addition to the elastic behaviour of the fibres. Three different creep laws describing the creep of individual fibres are applied in the simulations of creep of the network. Results from simulations using the model are presented. The influence of the amplitude of moisture content changes is discussed. It is shown that the model may produce macroscopic strains that are linear in stress, even though the creep of the fibres is nonlinear This may explain why both regular creep and mechanosorptive creep at small loads appear to be linear in stress.

  • 14.
    Alfthan, Johan
    KTH, Superseded Departments, Solid Mechanics.
    Micro-mechanical modelling of mechano-sorptive creep in paper2002Licentiate thesis, comprehensive summary (Other scientific)
  • 15.
    Alfthan, Johan
    KTH, Superseded Departments, Solid Mechanics.
    Micro-mechanically based modeling of mechano-sorptive creep in paper2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The creep of paper is accelerated by moisture content changes. This acceleration is known as mechano-sorptive creep, which is also found in wood and some other materials. Mechano-sorptive creep has been known for several decades but it is still not well understood, and there is no generally accepted model explaining the effect.

    In this thesis, it is assumed that mechano-sorptive creep is the result of transient redistributions of stresses during moisture content changes in combination with non-linear creep behaviour of the material. The stress redistributions are caused by the anisotropic hygroexpansion of the fibres, which will give a mismatch of hygroexpansive strains at the bonds and hence large stresses each time the moisture content changes. This redistribution will lead to an uneven stress state. If the creep of the material depends non-linearly on stresses this will give an increase in creep rate where the stresses are high, that is larger than the decrease of creep rate where stresses are low, so in average there will be an increase in creep rate. The stress distribution evens out as the stresses relax during creep, and the moisture content has to change again to create a new uneven stress state and maintain the accelerated creep.

    Two different network models based on this mechanism are developed in this thesis. Numerical simulations show that the models produce results similar to the mechano-sorptive creep found in paper. In the first model it is assumed that creep takes place in the fibre-fibre interfaces at the bonds, in the second the creep of the fibres themselves is accelerated. The second model is further developed. Experiments verify model predictions of the dependence of the amplitude of moisture changes.

    The second model shows a linear relationship between mechanical load and deformation, although creep of the fibres depends non-linearly on stresses. This linear behaviour is also found in applications. Further analysis shows that the mechanical load can be treated as a small perturbation of the internal stress state caused by moisture content changes. This can be used to develop a linearized model, from which a continuum model can be derived. This leads to a reduction of the necessary number of variables, and a significant increase in speed of calculations. Hence, this linearized continuum model can be used as a constitutive law of paper in problems with complicated geometries, for example a corrugated board box in varying humidity.

  • 16.
    Alfthan, Johan
    KTH, Superseded Departments, Solid Mechanics.
    The effect of humidity cycle amplitude on accelerated tensile creep of paper2004In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 8, no 4, 289-302 p.Article in journal (Refereed)
    Abstract [en]

    The creep of paper is accelerated by moisture cycling, an effect known as mechanosorptive creep. In this paper, the effect of different amplitudes of the moisture content is investigated experimentally and numerically. Tensile creep tests were made in a climate chamber. Low basis weight isotropic sheets were used in the tests. The moisture content history was measured during each creep test using a balance placed in the climate chamber. The experimental results are compared to predictions using a theoretical network model. A brief description of the model is given. In the model it is assumed that the anisotropic hygroexpansion of the fibres produces large stresses at the fibre-fibre bonds when moisture changes. The resulting stress state will accelerate creep if the material obeys creep laws that are nonlinear in stress. A quite good fit between the theoretical model and the experimental creep curves is obtained.

  • 17.
    Alfthan, Johan
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Linear constitutive model for mechano-sorptive creep in paper2004Report (Other academic)
  • 18.
    Alfthan, Johan
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    A micromechanical model for mechanosorptive creep in paper2002In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 28, no 3, 98-104 p.Article in journal (Refereed)
    Abstract [en]

    The creep of paper is accelerated by moisture cycling, a phenomenon known as mechanosorptive creep or accelerated creep. In this paper stress created at bonds due to anisotropic swelling during absorption and desorption of moisture, in combination with nonlinear creep, are proposed to be the cause for mechanosorptive creep. Two simplifled models are first discussed in order to demonstrate the suggested mechanism. A three-dimensional fibre network model composed of elastic fibres and inelastic bonds is then studied by finite element calculations. The relative sliding in the bonds is described by a nonlinear creep model which, in combination with anisotropic hygroexpansion of the fibres results in accelerated creep of the network.

  • 19.
    Alpman, Johan
    KTH, Superseded Departments, Solid Mechanics.
    Numerical methods for analysis of damage in aircraft structures1999Licentiate thesis, comprehensive summary (Other scientific)
  • 20.
    Asp, Leif E.
    et al.
    Polymer Engineering, Luleå University of Technology.
    Berglund, Lars A.
    Polymer Engineering, Luleå University of Technology.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Effects of a composite-like stress state on the fracture of epoxies1995In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, no 1, 27-37 p.Article in journal (Refereed)
    Abstract [en]

    The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure.

  • 21.
    Bankel, Johan
    et al.
    Chalmers University of Technology.
    Berggren, Karl-Fredrik
    Linköping University.
    Blom, Karin
    KTH.
    Crawley, Edward F.
    Massachussetts Institute of Technology.
    Wiklund, Ingela
    Linköping University.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    The CDIO syllabus: a comparative study of expected student proficiency2003In: European Journal of Engineering Education, ISSN 0304-3797, E-ISSN 1469-5898, Vol. 28, no 3, 297-315 p.Article in journal (Refereed)
    Abstract [en]

    Engineering students must graduate with command of a vast body of technical knowledge. They must possess personal, interpersonal and system-building skills to function in teams, and be prepared to produce products and systems. Their education must have been structured under a curriculum blending ability to combine technical expertise with ethical, innovative, philosophical and humanistic acumen. This paper describes a unique international collaboration among four universities to reform engineering education. The collaborators agreed to a statement of goals, which includes descriptions of knowledge, skills and attitudes vital to an effective education, and codifies proficiency levels expected of graduates. We developed and utilized unique stakeholder surveys both to validate our prototype and to determine desired proficiency levels. This collaboration resulted in The CDIO Syllabus, A Statement of Goals for Undergraduate Engineering Education. The syllabus is both a template and a process that can be used to customize the syllabus to others' programmes. It can define new educational initiatives/and be employed as the basis for rigorous assessment. This paper details how, with the input of industry, academia and others, we employed an engineering problem-solving paradigm to effect our redesign. It outlines the syllabus and the unique process employed to create it.

  • 22.
    Barbier, Christophe
    KTH, Superseded Departments, Solid Mechanics.
    An experimental and numerical investigation of folding of coated papers2002Licentiate thesis, comprehensive summary (Other scientific)
  • 23.
    Barbier, Christophe
    KTH, Superseded Departments, Solid Mechanics.
    On folding of coated papers2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The mechanical behaviour of coated papers during folding has been investigated. This problem has been studied with experimental techniques and numerical analyses in order to give a better understanding of the folding properties of coated papers pertinent to the mechanical behaviour in general, and particularly cracking along the fold.

    A microscopy investigation has been performed. The surface of the folded paper has been carefully examined to study the event of fracture and related issues. The influence of the grammage on the cracking event has been studied and it was shown that the coating material would not fail if the paper sample was sufficiently thin. It was found that a stress or strain based criterion is sufficient to describe the cracking of the coating layers and that the anisotropy of paper should be taken into account when studying the folding process.

    The finite element method has been used for the numerical analyses remembering that the geometry of the problem is rather complicated, excluding a solution in analytical form. Using different constitutive models for the base stock, it has been shown that the deformation of the coated paper during folding is much governed by the paper substrate. The numerical results also suggested that particular forms of plastic anisotropy can substantially reduce the maximum strain levels in the coating. Furthermore, it has also been shown that delamination buckling, in the present circumstances, has a very small influence on the strain levels in the coating layer subjected to high tensile loading.

    Dynamic effects have also been studied and it has been shown that a quasi-static analysis of the problem is sufficient in order to describe many of the important features related to cracking. An attempt to model strong anisotropy of paper has been presented and the results indicate that the large anisotropy in the thickness direction of coated papers needs to be taken into account in order to fully understand the mechanics of folding.

    Finally, an experimental investigation has been presented in order to study if important mechanical properties of the coating material could be determined by microindentation techniques. The results presented indicate that microindentation can be a powerful tool for characterization of these materials, but only if careful efforts are made in order to account for the influence from plasticity as well as from boundary effects.

    KEYWORDS: folding, coated papers, finite element method, cracking, indentation, anisotropy, plasticity.

  • 24.
    Barbier, Christophe
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Experimental investigation of damage at folding of coated papers2002In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 17, no 1, 34-38 p.Article in journal (Refereed)
    Abstract [en]

    An experimental investigation of damage occuring at folding of coated paper has been performed. For this purpose an experimental device was constructed in such a way that close resemblance with an industrial situation was achieved. During the experiments the influence on the damage levels in the coating from such features as delamination, humidity and paper thickness have been studied using an optical microscope. The behaviour of two different paper materials has been investigated. A stress (or strain) based fracture criterion is relevant for the present problem but biaxiality of stresses as well as in-plane anisotropy must be taken into account. It was observed that cracking of the coating would not lead to subsequent cracking of the paper substrate and that delamination occurred during folding, in the base stock and not at the paper/coating interface, but its quantitative influence as regards cracking could not be determined. The influence from sheet grammage was investigated and it was found that the only case when (visible) cracks did not appear was at low values. High humidity did not affect the cracking.

  • 25.
    Barbier, Christophe
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    On the effect of high anisotropy at folding of coated papers2004Report (Refereed)
  • 26.
    Berggren, Karl-Fredrik
    et al.
    Linköping University.
    Brodeur, Doris
    Massachusetts Institute of Technology.
    Crawley, Ed F.
    Massachusetts Institute of Technology.
    Ingemarsson, Ingemar
    Linköping University.
    Litant, William T. G.
    Massachusetts Institute of Technology.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    An International Initiative for Reforming Engineering Education2003In: World Transactions on Engineering and Technology Education, Vol. 2, no 1, 49-52 p.Article in journal (Refereed)
  • 27. Blanco, N.
    et al.
    Gamstedt, E. Kristofer
    KTH, Superseded Departments, Solid Mechanics.
    Asp, L. E.
    Costa, J.
    Mixed-mode delamination growth in carbon-fibre composite laminates under cyclic loading2004In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 15, 4219-4235 p.Article in journal (Refereed)
    Abstract [en]

    Delamination growth under fatigue loads in real composite components generally develops in a non-constant propagation mode. The aim of the investigation described in this article was to develop a model capable of predicting the fatigue delamination growth in a general case, under varying mode mix conditions. The crack growth development in essentially unidirectional laminates of carbon-fibre reinforced epoxy was analysed in terms of the Paris law for different constant propagation modes: mode I (double-cantilever beam test), mode II (end-notched flexure test) and different mixed-modes I/II (mixed-mode bending test). The dependence of the Paris law parameters oil mode mix is compared with the existing models in the literature. It is shown that these models do not reproduce the non-monotonic dependence on mode mix which has been observed in experimental data. Therefore, an improved phenomenological model is introduced and compared with the experimental data obtained by other researchers. To check the ability of the model to predict variable mixed-mode fatigue delamination, the mixed-mode end-loaded split test was employed and the experimental results were compared to the predictions of the model. The underlying mechanisms responsible for the dependency of the crack propagation rates on the degree of mode mix are also discussed on the basis of fractographic analysis.

  • 28.
    Bonnaud, Etienne
    KTH, Superseded Departments, Solid Mechanics.
    On effects of eleastic and strength in composite structures2001Licentiate thesis, comprehensive summary (Other scientific)
  • 29.
    Bonnaud, Etienne L.
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Neumeister, Jonas M.
    KTH, Superseded Departments, Solid Mechanics.
    Inertia and damping effects on the damage-dependent burst size distribution of fiber bundles2001In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 290, 261-269 p.Article in journal (Refereed)
    Abstract [en]

    Distributions of simultaneous fiber failures—bursts—in loaded fiber bundles are studied considering inertia and damping. Resulting burst size distributions have universal properties: all approach the power law DΔΔ−2.5 for larger burst sizes Δ. Momentary burst size distributions evolve with increasing damage and do not follow power laws but are still universal. Finally, it is briefly outlined how to use distribution progression to assess damage state.

  • 30.
    Bonnaud, Etienne L.
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Neumeister, Jonas M.
    KTH, Superseded Departments, Solid Mechanics.
    Proper Boundary Conditions for Infinitely Layered Orthotropic Media2000In: Journal of applied mechanics, ISSN 0021-8936, E-ISSN 1528-9036, Vol. 67, 629-632 p.Article in journal (Refereed)
    Abstract [en]

    A stress analysis of a plane infinitely layered medium subjected to surface loadings is performed using Airy stress functions, integral transforms, and a revised transfer matrix approach. Proper boundary conditions at infinity are for the first time established, which reduces the problem size by one half. Methods and approximations are also presented to enable numerical treatment and to overcome difficulties inherent to such formulations.

  • 31.
    Cadario, Alessandro
    KTH, Superseded Departments, Solid Mechanics.
    On fretting fatigue with spherical indenter2004Licentiate thesis, comprehensive summary (Other scientific)
  • 32. Carlsson, S.
    et al.
    Biwa, S.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    On frictional effects at inelastic contact between spherical bodies2000In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 42, no 1, 107-128 p.Article in journal (Refereed)
    Abstract [en]

    Normal inelastic contact between spherical bodies is examined theoretically and numerically. The analysis is focused on viscoplastic material behaviour. In particular the effect of Coulomb friction is analysed in some detail, both regarding global and field variables. It is shown that the solution to the problem of contact between two deformable spherical bodies is provided by the solution of the fundamental problem of indentation of a viscoplastic half-space by a rigid sphere. The indentation analysis is based on self-similarity and cumulative superposition of intermediate flat die solutions as outlined in detail in a previous study by Storakers et al. (International Journal of Solids and Structures 1997;34:3061-83). The results show that frictional effects, when global properties such as the contact area and the mean contact pressure are at issue, will only be of importance at close to perfectly plastic material behaviour. Even in such circumstances the difference between values given by the solutions for frictionless and for full adhesive contact is no more than approximately 10%. Accordingly, it can be concluded that frictional effects are essentially negligible, when, for example, material characterization of viscoplastic solids by Brinell indentation is of interest. The situation is, however, quite different when field variables are at issue. In this case, stress and strain fields can be substantially influenced by friction with possible implications for features such as crack initiation and crack growth,

  • 33. Carlsson, S.
    et al.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    On the determination of residual stress and strain fields by sharp indentation testing. Part I: Theoretical and numerical analysis2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 12, 2179-2191 p.Article in journal (Refereed)
    Abstract [en]

    Sharp indentation tests, presently represented by cone and Vickers indenters, are analysed theoretically and numerically in order to explore how equi-biaxial residual stress and strain fields can be determined from the global properties, i.e. the size of the contact area between indenter and material and the hardness. given by such tests. It is shown that the residual strain fields can be accurately correlated with the hardness value while residual stresses are related to the size of the contact area. The latter feature is explained by the Fact that the size of the: contact area is sensitive to elastic effects. The results are summarized in simple closed form relations. well-suited to be used in an experimental situation, and the range of validity for the resulting formulae is discussed. The predictions are compared with corresponding results taken from the literature and good agreement is found. An experimental scheme for determination of residual fields by indentation is also suggested.

  • 34. Carlsson, S.
    et al.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    On the determination of residual stress and strain fields by sharp indentation testing. Part II: Experimental investigation2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 12, 2193-2203 p.Article in journal (Refereed)
    Abstract [en]

    An experimental investigation has been carried out in order to study how residual stress and strain fields can be determined by sharp indentation testing. Aiming at a thorough understanding of the influence from general residual surface stress: and strain fields on global indentation parameters, i.e. hardness and contact area, Vickers indentation tests have been performed on specimens first exposed to standard four-point bend (4PB) or single edge notch bend (SEN(B)) loading. The experimental results have been evaluated based on the findings in a parallel theoretical/numerical investigation and are compared with finite element simulations as well as with corresponding results taken from the literature. Good agreement between experiments and numerical results has been found, particularly in a situation with no or little plastic deformation due to preloading.

  • 35.
    Carlsson, Susanna
    KTH, Superseded Departments, Solid Mechanics.
    Normal contact of inelastic solids1998Licentiate thesis, comprehensive summary (Other scientific)
  • 36.
    Carlsson, Susanna
    KTH, Superseded Departments, Solid Mechanics.
    On indentation methods for characterization of inelastic materials2000Doctoral thesis, comprehensive summary (Other scientific)
  • 37.
    Chandrasekaran, Dilip
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Nygårds, Mikael
    KTH, Superseded Departments, Solid Mechanics.
    Comparison of surface displacement measurements in a ferritic steel using AFM and non-local plasticity2004In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 365, no 1-2, 191-195 p.Article in journal (Refereed)
    Abstract [en]

    An attempt to experimentally study deformation characteristics around grain boundaries and to analyze the presence of strain gradients is presented. The evolution of surface profiles is studied by atomic force microscopy (AFM) at relatively small strains. The results indicate that this method can be used to draw conclusions about the deformation characteristics, e.g. in large grains the surface profile seems to vary within a grain. This latter effect can be seen as an indication of the inhomogeneous deformation occurring within large grains. The results are also compared with FEM calculations using a non-local crystal plasticity theory that incorporates strain gradients in the hardening moduli.

  • 38.
    Dahlberg, Magnus
    KTH, Superseded Departments, Solid Mechanics.
    Fatigue in nodular cast iron1997Licentiate thesis, comprehensive summary (Other scientific)
  • 39.
    Dahlin, Peter
    KTH, Superseded Departments, Solid Mechanics.
    Fatigue crack growth under non-proportional mixed model I+II loading2003Licentiate thesis, comprehensive summary (Other scientific)
  • 40.
    Dahlin, Peter
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Reduction of mode I fatigue crack growth rate due to occasional mode II loading2004In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 26, no 10, 1083-1093 p.Article in journal (Refereed)
    Abstract [en]

    The effect of mode 11 loading on subsequent mode I crack growth is experimentally investigated on steel AISI 01. The results show that the mode I crack growth rate decreases after a single mode 11 load cycle. This effect remains also after the crack has propagated through the mode 11 plastic zone. The crack growth reduction is shown to be caused by crack closure due to tangential displacement of crack-surface irregularities. The durability of this reduction has a decisive influence on the fatigue life when the mode I R-ratio is not as high as to keep the entire load cycle above the closure level.

  • 41.
    Dahlin, Peter
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    The effect of plasticity on incipient mixed-mode fatigue crack growth2003In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 26, no 7, 577-588 p.Article in journal (Refereed)
    Abstract [en]

    Several criteria for the prediction of incipient crack path direction of non-proportional mixed-mode fatigue cracks, immediately after a change of load from steady mode I conditions are investigated. The analysis is based on two-dimensional plane strain FE-simulations in which the actual elasto-plastic stress distribution is used for the MTS, MTSR and MEPSR criteria. The purpose of the analysis is to compare the numerical results with experimental results as well as with previous predictions based on solely elastic stress analyses, taken from the literature. It is shown that the influence from elasto-plastic deformation on crack branching direction is of utmost importance. It is found that the incipient crack growth of metals falls into two categories: high strength metals follow the MTSp criterion whereas more ductile metals follow the MTSRp criterion. The subscript p indicates that the elasto-plastic evaluation of the respective criterion should be used.

  • 42.
    Dovstam, Krister
    KTH, Superseded Departments, Solid Mechanics.
    On material damping modelling and modal analysis in structural dynamics1998Doctoral thesis, comprehensive summary (Other scientific)
  • 43. Eriksson, C. L.
    et al.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    Rowciffe, D. J.
    Strain-hardening and residual stress effects in plastic zones around indentations2003In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 340, no 02-jan, 193-203 p.Article in journal (Refereed)
    Abstract [en]

    Plastic zones were revealed by polishing away Vickers indentations made in soda-lime glass, WC-11% Co, W and 7075 Al. Micro and nanohardness traces were used to explore the local mechanical response. The hardness value within the deformed zone increased up to 21% depending on the material. Soda-lime glass was the only material not to show a hardening effect, in fact it showed a small decrease in hardness. Finite element calculations were used to qualitatively determine the influence from residual stresses at indentation of soda-lime glass. The results are discussed in the context of the influence from work-hardening and residual stresses on indentation quantities.

  • 44.
    Faleskog, Jonas
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Kroon, Martin
    KTH, Superseded Departments, Solid Mechanics.
    Öberg, Hans
    KTH, Superseded Departments, Solid Mechanics.
    A probabilistic model for cleavage fracture with a length scale - parameter estimation and predictions of stationary crack experiments2004In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 71, no 1, 57-79 p.Article in journal (Refereed)
    Abstract [en]

    This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different crack depth over specimen width ratios to capture the strong constraint effect on fracture toughness. Three test temperatures were considered, covering a range of 85 degreesC. All specimens failed by cleavage fracture prior to ductile tearing. A recently proposed probabilistic model for the cumulative failure by cleavage was applied to the comprehensive sets of experimental data. This modified weakest link model incorporates a length scale, which together with a threshold stress reduce the scatter in predicted toughness distributions as well as introduces a fracture toughness threshold value. Model parameters were estimated by a robust procedure, which is crucial in applications of probabilistic models to real structures. The conformity between predicted and experimental toughness distributions, respectively, were notable at all the test temperatures.

  • 45.
    Faleskog, Jonas
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Nilsson, Fred L.
    KTH, Superseded Departments, Solid Mechanics.
    Conditional failure probabilities in weakest link modelling2004In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 125, no 04-mar, 349-351 p.Article in journal (Refereed)
  • 46. Fredriksson, P.
    et al.
    Gudmundson, P.
    KTH, Superseded Departments, Solid Mechanics.
    FE-modelling of interface conditions within a strain gradient plasticity theory2006Conference paper (Refereed)
  • 47. Fredriksson, P.
    et al.
    Gudmundson, P.
    KTH, Superseded Departments, Solid Mechanics.
    Size dependent yield strength and surface energies of thin films2004Conference paper (Other academic)
  • 48.
    Gamstedt, E. Kristofer
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Sjogren, B. A.
    An experimental investigation of the sequence effect in block amplitude loading of cross-ply composite laminates2002In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 24, no 04-feb, 437-446 p.Article in journal (Refereed)
    Abstract [en]

    The Palmgren-Miner rule has been shown to be inexact in many cases for various composite materials. Several empirical models have been conceived to account for this discrepancy, as well as the effect of block sequence. The approach taken here is based on the underlying mechanisms. A cross-ply laminate was used as a model material. In general, composites show both initiatory and progressive mechanisms under fatigue loading. The former is active at high static stresses, whereas the latter predominates at lower stress amplitudes where they are given sufficient time to propagate, Initiatory mechanisms give rise to damage from which the progressive mechanisms can start, and conversely the progressive mechanisms continually alter the local stress state which results in further damage accumulation caused by the initiation controlled mechanisms. In a cross-ply laminate, the initiatory mechanism is the formation of transverse cracks, and the progressive mechanism is mainly delamination growth initiated from the transverse cracks. In an experimental investigation of carbon fiber/epoxy cross-ply laminates, the interaction of these mechanisms has shown why a sequence of high-low amplitude levels results in shorter lifetimes than a low-high order. Such a sequence effect seems to be a common behavior for many other composite materials, and can be mechanistically explained by a similar kind of interaction. Advantages and drawbacks of the mechanistic approach compared with empirical rules are also discussed.

  • 49.
    Gamstedt, E. Kristofer
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Skrifvars, M.
    Jacobsen, T. K.
    Pyrz, R.
    Synthesis of unsaturated polyesters for improved interfacial strength in carbon fibre composites2002In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 33, no 9, 1239-1252 p.Article in journal (Refereed)
    Abstract [en]

    Carbon fibres are gaining use as reinforcement in glass fibre/polyester composites for increased stiffness as a hybrid composite. The mechanics and chemistry of the carbon fibre-polyester interface should be addressed to achieve an improvement also in fatigue performance and off-axis strength. To make better use of the versatility of unsaturated polyesters in a carbon fibre composite, a set of unsaturated polyester resins have been synthesized with different ratios of maleic anhydride, o-phthalic anhydride and 1,2-propylene glycol as precursors. The effective interfacial strength was determined by micro-Raman spectroscopy of a single-fibre composite tested in tension. The interfacial shear strength with untreated carbon fibres increased with increasing degree of unsaturation of the polyester, which is controlled by the relative amount of maleic anhydride. This can be explained by a contribution of chemical bonding of the double bonds in the polymer to the functional groups of the carbon fibre surface.

  • 50. Gamstedt, E. Kristofer
    et al.
    Östlund, Sören
    KTH, Superseded Departments, Solid Mechanics.
    Fatigue propagation of fibre-bridged cracks in unidirectional polymer-matrix composites2001In: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 8, no 6, 385-410 p.Article in journal (Refereed)
    Abstract [en]

    In order to improve the fatigue resistance of polymer-matrix composites by materials design, or to conceive micromechanics based models for life predictions, the underlying micromechanisms must be understood. Experimental investigations have revealed fibre-bridged cracking as a toughening micromechanism that retards further fatigue crack growth in a unidirectional 0 degrees carbon-fibre-reinforced epoxy. The bridging fibres exert a closing traction on the crack surfaces, thereby reducing the driving force for crack growth. In this study, the growth of bridged cracks has been quantified by a surface replication technique. The da/dN-DeltaK curve defined in terms of nominal stress-intensity factors shows a crack retarding behaviour. The crack growth curve can be replotted in terms of the effective stress-intensity factor where the contribution of the cohesive crack surface forces from the bridging fibres are taken into account. This curve falls somewhat closer to that of the neat matrix material, but the difference is still considerable, and it shows a decelerating propagation. Therefore, there must be other active toughening mechanisms besides fibre bridging, that slow the crack propagation down, and account for the fatigue resistant behaviour of the tested material. Ways by microstructural design to promote the fatigue resistant mechanisms are discussed.

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