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  • 1.
    Edvardsson, Sverker
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    On dissipative effects of paper web adhesion strength2011In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 48, no 1, p. 24-30Article in journal (Refereed)
    Abstract [en]

    This work is concerned with the adhesion strength between a paper web and a metal roll surface, which is a common situation in paper machines world-wide. It is shown that the classic expression relating the work of adhesion to the peeling angle and web tension is, in general, insufficient. An improved model is suggested to take into account the energy dissipation due to elastic-plastic deformation behavior of wet paper materials. To judge the model, an industrially relevant example of wet newsprint and a mild steel surface is studied. It is found that the agreement between theory and experimental observations is excellent. A key result is that elastic-plastic material behavior must always be included for wet paper materials in peeling processes.

  • 2.
    Edvardsson, Sverker
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Gulliksson, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    A Note on Wet Paper Web Adhesion Strength2012In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 40, no 4, p. 682-686Article in journal (Refereed)
    Abstract [en]

    This work is concerned with the determination of the adhesion strength between a paper web and an adhesive surface. Edvardsson et al. [Edvardsson, S., Gradin, P., and Isaksson, P., "On Dissipative Effects of Paper Web Adhesion Strength," Int. J. Solids Struct., Vol. 48(1), 2011, pp. 24-30] suggested recently a model that takes into account the energy dissipation caused by elastic plastic deformation in the bent structure of a paper specimen. This model is further developed and investigated in the present work. A linear relation in plastic dissipation is discovered facilitating a novel analysis of the peeling tension and a more convenient determination of the proper adhesion strength. Industrial relevant examples are made with wet newsprint and kraft stock. A straightforward experimental procedure for determining the consistent adhesion strength is suggested. It is found that the agreement between the model and the experimental observations is good.

  • 3.
    Gradin, Per A
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Nyström, Staffan K
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Graham, D
    Kao, D
    Knight, B
    Acoustic Emission applied to mechanically loaded Paper2004In: EWGAE (European Working Group on Acoustic Emission) 2004 proceedings, 2004, p. 423-431Conference paper (Refereed)
  • 4.
    Gradin, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Gulliksson, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    A Note on the Co-linearity of forces and Displacements in an Elastic Structure2011In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 78, no 6, p. Art. no. 4003912-Article in journal (Refereed)
    Abstract [en]

    Theconditions under which force vectors and corresponding displacement vectors becomeco-linear are investigated under the assumption of a linear elasticstructure and for an arbitrary number of loading points. Itis shown that there exist an infinite number of directionsalong which the load and displacement vectors in each loadingpoint coincide. Moreover, the problem of co-linearity is analogous tothe problem of finding the extreme values of the workperformed on an elastic structure under the constraint that eachforce has a given magnitude. The result for a finitenumber of loading points is extended to a continuous loaddistribution on the boundary of an elastic structure, i.e., itis possible to find an infinite number of load distributionssuch that the displacement in a point on the boundaryis co-linear with the boundary stress vector in that samepoint.

  • 5.
    Hellström, Lisbeth M
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Gradin, Per A
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Eriksson, Kjell
    Luleå University of Technology.
    An Analytical and Numerical Study of some aspects of the Wood Chipping Process2009In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 24, no 2, p. 225-230Article in journal (Refereed)
    Abstract [en]

    SUMMARY: In order to model the wood chipping process, the primary process parameters have been identified and their first order interaction studied. The model is analytical and incorporates, in particular, the influence of sliding friction between the wood chipping tool and the log. To estimate the accuracy of the analytical model, a Finite Element (FE) analysis of the problem considered was also performed. The analytical model and the FE analysis are both restricted to small deformations and linear elastic orthotropic material behaviour. The most severe limitation with both the analytical and the FE model is the assumption of linearly elastic material. On the other hand, it is felt that existing models of anisotropic plasticity in metals are lacking too much of physical relevance, if applied to wood. The analytical model predicts the normal and shear strain distribution in the crack-plane prior to crack initiation. The analytical distributions are in reasonable agreement with the corresponding distribution of the FE analysis. Based on experimental findings, it is suggested that the stress field over the entire crack-plane, in conjunction with the stress field close to the tip of the chipping tool, are critical for chip creation, rather than just the latter.

  • 6. Henriksson, Marielle
    et al.
    Berglund, Lars A.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Lindström, Tom
    Nishino, Takashi
    Cellulose nanopaper structures of high toughness2008In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 9, no 6, p. 1579-1585Article in journal (Refereed)
  • 7.
    Hummelgård, Magnus
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Olin, Håkan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Mihailovic, D
    Bending Modulus of Mo6S3I6 Nanowires Studied by Electromechanical ResonancesManuscript (Other academic)
  • 8. Hägglund, R.
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Analysis of localized failure in low-basis weight paper2006In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 43, no 18-19, p. 5581-5592Article in journal (Refereed)
    Abstract [en]

    Continuum damage mechanics (CDM) is used to describe the post-elastic behavior of low-basis-weight paper. The relevance of undertaking studies of the mechanical behavior of low-basis-weight paper is that it enables characterization, optimization and quality control. In accordance with a CDM theory, an internal variable is introduced that represent the degree to which the material has degraded in a continuum sense and details inherent in a damage evolution law contain information about the rupture mechanism. To account for long ranging micro-structural effects, because of the fiber structure in the paper material, a non-local formulation of the constitutive law is considered. Of particular interest is the fracture toughness of the material, i.e. the ability to resist further crack propagation, as it is often a good measure of flaw tolerance and durability in the context of paper. The constitutive model discussed is verified against tensile tests on rectangular paper specimens containing pre-fabricated cracks. Acoustic emission was used to study the damage evolution in paper specimens during tensile loading. An orthotropic material description has been utilized. The model is contrasted with a purely isotropic formulation. It seems that for the type of problem analyzed in this work, an orthotropic material description does not significantly improve the predictive capability as compared to an isotropic formulation. It is concluded that the model can be used to evaluate the influence of arbitrary defect geometries, defect size and loading conditions and can easily be incorporated into a finite element code.

  • 9. Hägglund, R.
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Influence of damage in the vicinity of a crack-tip in embossed low-basis-weight paper2007In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 74, no 11, p. 1758-1769Article in journal (Refereed)
    Abstract [en]

    It is experimentally observed that the fracture process developed in an embossed paper sample having a periodic matrix organized pattern is distributed over several indentation rows in the region near a macroscopic crack. This observation suggests that the stresses at the tip of the crack is shielded by damage in neighboring rows of indentation: energy dissipation may occur not only at the tip of the main crack, but in several indentation rows parallel to the main crack. In this investigation, a model describing the in-plane fracture behavior of embossed low-basis-weight paper is developed. It is found that the model is capable of capturing the development of damage in rows parallel to the main crack and compares well with experimental results.

  • 10. Hägglund, R.
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Mechanical analysis of folding induced failure in corrugated board: A Theoretical and Experimental Comparison2008In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 42, no 9, p. 889-908Article in journal (Refereed)
    Abstract [en]

    The problem of failure in the facings of a corrugated board is investigated by means of a mechanical model utilizing the principles of continuum damage mechanics. To evaluate the predictive capability of the model, experiments have been conducted on in-plane isotropic single- and two-ply paper sheets manufactured in the laboratory. It is found that there is a qualitative agreement between model and experiments. The model can predict if delamination will occur before the top-ply loses its in-plane load carrying capacity due to material degradation. To circumvent delamination problems, a material with low stiffness should be used in the top-ply (as compared to the bottom-ply) and a very thin top-ply should be avoided. It is exemplified that it is possible to steer the failure mode by optimizing the degree of beating or the ply thickness. It is demonstrated that plasticity may be omitted in the analysis if one is only interested in characterizing the expected type of failure mode. An important key result is that, if the circumstances are that severe damage growth takes place in the crack-tip region, then delamination most likely occur which may be in contradiction to what is anticipated from classical theories of fracture mechanics.

  • 11. Hägglund, R
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    On the coupling between macroscopic material degradation and interfiber bond fracture in an idealized fiber network2008In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 45, no 3-4, p. 868-878Article in journal (Refereed)
    Abstract [en]

    The numerical analysis performed here, using a finite element network model, provides a number of important results regarding the evolution of micro fractures in planar random fiber networks where the only active microscopic fracture mechanism is bond fracture. The fibers are randomly distributed in the network meaning that the network is considered having in-plane isotropic properties on the macroscopic scale. The network is loaded so that, in an average sense, homogenous macroscopic stress and strain fields are present. Several conclusions are drawn. It is found that the development of macroscopic material degradation follows an exponential two-parameter law, consisting of an onset parameter and a fracture rate parameter, justifying a previous theory derived by the authors. The fracture rate parameter is linearly related to the inverse of the bond density above a certain density limit (percolation) and increases with increasing slenderness ratio of the fibers when keeping the bond density at a constant level. The strain energies stored in interfiber bonds are exponentially distributed over the whole network. The numerical analysis reveals that there is a linear relation between the ratio of fractured and initial number of loaded bonds, and the network's macroscopic material stiffness normalized with its pristine stiffness, confirming earlier findings based on experimental observations. At localization the analyzed theory looses its validity because the fracture process is no longer randomly distributed over the whole network. Localization coincides with location of peak load in force-displacement tensile tests.

  • 12. Hägglund, R
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    The influence of embossing pattern on in-plane fracture behavior of paper2007In: 7th World Congress on Computational Mechanics WCCM VII, Los Angeles, California, July 16 - 22, 2006 (CD-ROM), 2007Conference paper (Refereed)
  • 13.
    Hägglund, Rickard
    et al.
    SCA R&D Centre, Sundsvall, Sweden.
    Åslund, Pär
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Carlsson, Leif A.
    Florida Atlantic University, Department of Engineering Physics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
    Measuring thickness changes of edge-wise compression loaded corrugated board panels using digital image correlation2012In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972, Vol. 14, no 1, p. 75-94Article in journal (Refereed)
    Abstract [en]

    This study examines thickness changes in web-core sandwich panels under edgewise compressive loading. Both undamaged and damaged panels were examined. Three-dimensional full-field digital image correlation systems were used to determine deflections on both sides of loaded panels. The change in thickness at any given point in the panel was obtained as the difference between the two displacement fields. It was observed that the thickness was reduced in the post-buckling regime. Damage introduced into the corrugated core by lateral compression proved to significantly reduce the load-carrying capability panels and elevate the thickness reduction of the panels.

  • 14.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    A new etching method for silicon micro machining (master thesis)1995Other (Other academic)
  • 15.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    An implicit stress gradient plasticity model for describing mechanical behavior of planar fiber networks on a macroscopic scale2010In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 77, no 8, p. 1240-1252Article in journal (Refereed)
    Abstract [en]

    The plasticity behavior of fiber networks is governed by complex mechanisms. This study examines the effect of microstructure on the macroscopic plastic behavior of two-dimensional random fiber networks such as strong-bonded paper. Remote load is a pure macroscopic mode I opening field, applied via a boundary layer assuming small scale yielding on the macroscopic scale. It is shown that using a macroscopic classical homogeneous continuum approach to describe plasticity effects due to (macroscopic) singular-dominated strain fields in planar fiber networks leads to erroneous results. The classical continuum description is too simple to capture the essential mechanical behavior of a network material since a structural effect, that alters the macroscopic stress field, becomes pronounced and introduces long-ranging microstructural effects that have to be accounted for. Because of this, it is necessary to include a nonlocal theory that bridges the gap between microscopic and macroscopic scales to describe the material response in homogeneous continuum models. An implicit stress gradient small deformation plasticity model, which is based on a strong nonlocal continuum formulation, is presented here that has the potential to describe the plasticity behavior of fiber networks on a macroscopic scale. The theory is derived by including nonlocal stress terms in the classical associated J2-theory of plasticity. The nonlocal stress tensor is found by scaling the local Cauchy stress tensor by the ratio of nonlocal and local von Mises equivalent stresses. The model is relatively easy to implement in ordinary finite element algorithms for small deformation theory. Fairly good agreements are obtained between discrete micromechanical network models and the derived homogeneous nonlocal continuum model.

  • 16.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Contact fracture: a review2001Report (Other academic)
  • 17.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Crack growth under high pressure governed by a fully plastic crack path criterion2000In: Proceedings of Plasticity 2000. The eight International Symposium on Plasticity and Its current Applications. Whistler, British Columbia, Canada, 17–21 July, 2000., 2000Conference paper (Other academic)
  • 18.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    FE calculations of the stress distribution and crack growth in the top of a ABB Atom BWR control blade CR-82M due to swelling of boron carbide pellets1996Report (Other academic)
  • 19.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Mechanical behaviour of, and crack paths in, a compressed neutron absorber1998Licentiate thesis, monograph (Other academic)
  • 20.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    On crack growth under compressive stresses2001Doctoral thesis, monograph (Other academic)
    Abstract [en]

    This thesis concerns fractures subjected to compressive stresses. In the four papers appended fracture behavior in brittle as well as ductile materials is studied. In the first paper, an expression for the mode II stress intensity factor at a straight extended kink has been calculated under the condition that crack opening is suppressed during crack growth. The expression has been found as a function of the mode II stress intensity factor K2 at the parent crack, the direction and length of the kink, and the difference between the remote compressive normal stresses perpendicular to, and parallel with, the plane of the parent crack. Crack growth directions have been suggested based on the result. At a sufficiently high non-isotropic compressive normal stress, so that the crack remains closed, the crack will propagate along a curved path maximizing the mode II stress intensity factor. Only at an isotropic compressive normal stress will the crack continue straight ahead in its original plane without directional change. By analyzing experimental crack growth patterns in paper two, the conclusion is that crack paths experimentally observed indicate that mode II crack growth under compression in some brittle materials follow a propagation path described by a function y=gx^b. In fact, the agreement between the experiments and the propagation path prescribed by the model, in which b=3/2, is astonishingly good since b was found in the interval [1.43-1.58] in all the experiments studied. Further, the investigation of the curvature parameter g has revealed that g also agree with the simplified model, even though not as good as the exponent b. However, the experimentally observed g differs in general less than 15% from the theoretical value predicted by the analytical model discussed in paper I. In paper three, a directional crack growth criterion in a compressed elastic perfectly-plastic material is considered. A slip-line solution is derived for evaluation of the stresses at the crack tip, which considers hydrostatic pressure and friction between the crack surfaces. Based upon the slip-line solution a projection stress based model is discussed for prediction of the direction of initiated crack growth. The opening displacement of an extended kink has been examined in paper four, using a finite element procedure. The conclusion is that an over-critical pressure in the plastic zone surrounding the crack tip suppresses crack opening regardless the direction of crack growth. The only possibility seems to be shear mode crack growth, which occur straight ahead in the crack plane if the crack is assumed to follow the plane of maximum shear stress. At a sub-critical hydrostatic pressure, or lower friction between the crack surfaces, the crack can extend via a kink subjected to local opening mode. An expression for the critical value determining fracture mode has been found as a function of hydrostatic pressure and friction between the crack surfaces assuming the fracture process to be predominantly controlled by local tensile stresses at the crack tip. The crack growth directions predicted by the projection stress based criterion in paper three are comparable with the directions maximizing the opening displacement of an extended kink computed in paper four.

  • 21.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Using acoustic emission monitoring to evaluate the through the thickness and in-plane strength properties of TMP rejects fractionsManuscript (Other academic)
  • 22.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Shear buckling in the core of a corrugated board structure2009In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 88, no 4, p. 610-614Article in journal (Refereed)
    Abstract [en]

    In some situations, a corrugated board package can experience loading of such a kind so that the corrugated core, or as it is most often referred to, the fluting, is loaded in transverse shear. This might cause the fluting to collapse and through this, cause a decrease in the bending stiffness. In this paper, an exact solution for the elastic instability of an infinite linear elastic strip with an initial curvature and loaded in pure in-plane shear is given. The solution is a modification of an existing solution for an isotropic strip. To experimentally investigate the accuracy of the model, a special device has been developed in which paper strips are loaded in pure in-plane shear to observe the buckling behavior. According to the experiments performed, the model seems to quantitatively well capture the main buckling behavior and lends some confidence to the theory. With use of the model it is easy to judge which deformation process most likely will occur, shear buckling or plastic deformation, and thereby allows one to structurally optimize the geometric ratio, i.e. the ratio between thickness and height of the fluting, when designing the corrugated core structure. Finally, it is illustrated that going below a certain critical thickness of the fluting may cause the structural strength of the board panel to decrease drastically.

  • 23.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Gradin, Per A
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    A bending specimen for constant energy release rate under controlled displacement conditions2005In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 72, no 11, p. 1770-1775Article in journal (Refereed)
    Abstract [en]

    A bending specimen to give a constant energy release rate under given displacement conditions is developed. The specimen is shown by experiments to give a, in essence, constant energy release rate.

  • 24.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Gradin, Per A
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Kulachenko, Artem
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Acoustic Emission applied to mechanically loaded Paper ??Manuscript (Other academic)
  • 25.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Östlund, Sören
    Solid Mechanics, KTH.
    A simplified treatise of the Scott bond testing method2007In: Proceedings of the 2007 International Paper Physics Conference, May 6-11, 2007, Gold Coast, Qld, Australia, 1-6, 2007, 2007Conference paper (Other academic)
    Abstract [en]

    The Scott bond test method has been used extensively in the paper industry over the years as a mean to assess the bond strength of paper. The method has been a subject of some controversy lately since it does not always correlate to the sensitivity of the material to fracture by delamination.

     

    To gain some further insight into which parameters govern the fracture process in a Scott bond test a simplified approach has been chosen in order to formulate an analytical mathematical/mechanical model of the test. The model is dynamic in the sense that inertia effects are included. The material model utilised is a simple cohesive theory that assumes a linear behaviour between stress and crack opening when the material has started to degrade. This choice of material model makes the mathematical model very nonlinear. In fact, a system of three coupled nonlinear second order partial differential equations have to be solved and adjusted to the correct initial conditions.

     

    The material parameters needed for the model are the elastic modulus in the thickness direction, the transverse shear (elastic) modulus, the tensile strength (in the thickness direction) and the fracture work (per unit area) for a delamination crack.

     

    To investigate the ability of the model, a Scott bond testing apparatus have been equipped with a piezoelectric load sensor. The load cell was mounted on the apparatus' pendulum so that the load acting on the sample holder could be recorded during the whole impact stage. This was done for a number of different initial velocities of the pendulum and it is found that the model gives a fair prediction of the contact load.

  • 26.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Östlund, Sören
    KTH, Hållfasthetslära.
    A simplified treatise of the Scott bond testing method2010In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 50, no 6, p. 745-751Article in journal (Refereed)
    Abstract [en]

    The Scott bond test method has been used extensively in the paper industry over the years as a means to assess the bond strength of paper. The method has been a subject of some controversy lately since it does not always correlate to the sensitivity of the material to fracture by delamination. To gain some further insight into which parameters govern the fracture process in a Scott bond test a simplified approach has been chosen in order to formulate an analytical mathematical/mechanical model of the test. The model is dynamic in the sense that inertia effects are included. The material model utilised is a simple cohesive theory that assumes a linear behaviour between stress and crack opening when the material has started to degrade. This choice of material model makes the mathematical model very nonlinear. In fact, a system of three coupled nonlinear second order partial differential equations have to be solved and adjusted to the correct initial conditions. The material parameters needed for the model are the elastic modulus in the thickness direction, the transverse shear (elastic) modulus, the tensile strength (in the thickness direction) and the fracture work (per unit area) for a delamination crack. To investigate the ability of the model, a Scott bond testing apparatus have been equipped with a piezoelectric load sensor. The load cell was mounted on the apparatus’ pendulum so that the load acting on the sample holder could be recorded during the whole impact stage. This was done for a number of different initial velocities of the pendulum and it is found that the model gives a fair prediction of the contact load.

  • 27.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Hägglund, R.
    A mechanical model of damage and delamination in corrugated board during folding2005In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 72, no 15, p. 2299-2315Article in journal (Refereed)
  • 28.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Hägglund, R.
    Analysis of the strain field in the vicinity of a crack tip in an in-plane isotropic paper material2007In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 2, p. 659-671Article in journal (Refereed)
    Abstract [en]

    Strains, computed by the finite element method, are evaluated and compared to an experimentally determined strain field. The analyzed low-density paper has been designed to ensure bond-breakage as the dominating damage mechanism and the paper material is approximately in-plane isotropic. An optical non-contact displacement measuring system has been used in fracture tests to determine the strain field in the crack-tip region of a pre-fabricated crack. Additionally, acoustic emission monitored tensile tests have been conducted to determine onset and evolution of damage processes and thereby enabling calibration of required constitutive parameters. The results suggest that the investigated paper material can tolerate significantly higher strains than what is predicted by a classic elastic-plastic J2-flow theory. Immediately before onset of the final fracture (i.e., localization), the experimental measured normal strain in the near-tip region is around 60% higher than the computed strain when using exclusively an elastic-plastic theory for the corresponding load while the strain computed utilizing a non-local damage theory is of the same order of magnitude as the experimentally measured strain. Hence, it seems essential to include a non-local continuum theory to describe strains in the near-tip region quantitatively correct for paper materials. It is demonstrated that path independence of the well-known J-integral does not prevail for this class of material models. Only for the special situation of a homogenous damage field in the crack-tip region may the stress and strain fields be described by the well-known HRR-solutions.

  • 29.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Hägglund, R.
    Evolution of bond fractures in a randomly distributed fiber network2007In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 18-19, p. 6135-6147Article in journal (Refereed)
    Abstract [en]

    Fracture in a planar randomly ordered fiber network subjected to approximately homogenous macroscopic stress and strain field is considered. A theory describing material degradation on a macroscopic scale is derived via Griffith's energy balance for an internal fractured area in the network assuming the active fracture process on the microscopic level is fiber-fiber bond breakage. Attention is confined to a purely mechanical theory assuming isothermal processes and the theory relies on equations commonly used in theories of statistical physics. In the theory, a bond breaking driving force is stated to be equal to the elastic strain energy density of a non-fractured network. A debond fraction can be coupled to a linearly decrease of the network's macroscopic stiffness. The rate of the fracture processes is determined by the network's inherent properties (bond and fiber density, bond strength, etc.). During the loading process, until onset of localization, the bond breaks occur at randomly distributed locations spread over the fiber network and the theory estimate material degradation on a macroscopic level. When localization takes place, the fracture process changes from a two-dimensional randomly distributed process to a one-dimensional process and other theories have to be included to describe post-localization behavior. An approximately in-plane isotropic low-density paper is used in tensile experiments while monitoring acoustic emission activity to evaluate the theory. The experimentally obtained results support the theory surprisingly well.

  • 30.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Hägglund, R
    On the nonlocal mechanical behavior in idealized fiber networksManuscript (Other academic)
  • 31.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Hägglund, R
    Gradin, P
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    A continuum damage elastic-plastic model for packaging paper2003In: 2003 International Paper Physics Conference. Proceedings, 2003, p. 207-210Conference paper (Refereed)
  • 32.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Hägglund, R
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Continuum damage mechanics applied to paper2004In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 16-17, p. 4731-4755Article in journal (Refereed)
    Abstract [en]

    The mechanical behavior of two packaging paper materials subjected to tensile loading up to complete breakage has been investigated. A model for isotropic strain hardening elastic anisotropic plasticity, coupled to anisotropic damage, is discussed. The constitutive relations, including a gradient enhanced damage model, are developed within a thermodynamical framework. The Helmholtz free energy in the continuum is assumed to depend not only on the strain and stress components but also on the damage in the material.The model has been analyzed in a non-linear finite element procedure. The capability of the model to properly capture and simulate the failure of a paper material subjected to tensile loading is demonstrated by means of several numerical examples that are compared to, and verified with, experiments on packaging paper specimens of varying geometry.

  • 33.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Hägglund, Rickard
    Strain energy distribution in a crack-tip region in random fiber networks2009In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 156, no 1, p. 1-9Article in journal (Refereed)
    Abstract [en]

    A closed form relation for the strain energy density in the vicinity of a macroscopic mode I crack in a random fiber network is derived using an implicit gradient nonlocal continuum field theory. An expression for the characteristic length, used in the nonlocal formulations, in terms of microstructural properties is derived and it is found that the characteristic length is proportional to the average fiber segment length to the power of two. It is illustrated that the crack-tip singularity vanishes for a characteristic length greater than zero. An open fiber structure exhibits a distributed strain energy field in the crack tip vicinity. As the network becomes relatively denser, the characteristic length decreases and the networks mechanical behavior approaches the behavior of a classic elastic continuum. Only for an infinitely dense network is the r −1-singularity in strain energy field achieved. The theory explains why open network structures have difficulties in localizing failure to macroscopic cracks. It is found that there is a one-to-one relation between characteristic length and size of the smallest crack that can initiate macroscopic failure.

  • 34.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Hägglund, Rickard
    SCA R&D Centre.
    Structural effects on deformation and fracture of random fiber networks and consequences on continuum models2009In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 46, no 11-12, p. 2320-2329Article in journal (Refereed)
    Abstract [en]

    The mechanical behavior of fibrous networks is governed by complex multiple mechanisms. This study examines the effect of microstructure on the macroscopic deformation and fracture of two-dimensional random fiber networks and its practical implications for understanding the material failure in paper materials by using finite element models. Remote load is a pure mode I opening field, applied via a boundary layer. Characteristic networks, consisting of the union of solutions of several unique networks, are interpolated on a rectangular grid covering the whole problem domain. The interpolated solutions are interpreted as network-equivalent continuums representing the mechanical behavior, on average, for a specific set of structural properties. A regularization routine is included in a variational procedure in order to minimize potential energy in the body and produce continuous strains at cell borders in the grid. It is shown that using a classical continuum linear elastic fracture mechanics (LEFM) approach to describe macroscopic singular-dominated fields in fiber networks, can lead to erroneous results especially in networks having a low degree of bonds per fiber. The classical continuum description is too simple to capture the essential mechanical behavior for this class of material since a structural effect, that alters the displacement field, becomes pronounced. It is necessary to include a nonlocal theory to describe the mechanical behavior at a continuum level. By using an appropriate characteristic length in a nonlocal continuum formulation, strain energies, in the neighborhood of a dominant macroscopic singularity, are calculated that agree well with characteristic network models and hence produce fairly good agreements between the networks and the nonlocal continuum models. A key conclusion found is that, only for networks with a high degree of bonding, can the mechanical behavior around a macroscopic singularity be captured by the classical local continuum theory. In networks with a low degree of bonds per fiber, there are regions far away from the macroscopic singularity that have relatively higher magnitudes of strain energy than predicted by the classical theory. A relation between an internal length scale parameter, used in the nonlocal continuum model, and the structural properties of the network is approximated by a simple function.

  • 35.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Krusper, Aleksandra
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Gradin, Per A
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Shear correction factors for corrugated core structures2007In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 80, no 1, p. 123-130Article in journal (Refereed)
    Abstract [en]

    Because of the high computational cost involved when modeling a corrugated panel (in e.g., finite element analyses) the corrugated core sandwich is in this investigation homogenized and the panel is transformed to an equivalent continuous homogenous layer with effective equal properties. Mathematically, the corrugated board panel is divided into an arbitrary number of thin virtual layers. For each virtual layer, unique effective elastic modules are calculated. Then, the elastic properties in all layers are assembled together in order to be able to analyze a corrugated board as a continuous structure having equivalent mechanical properties to a real structure. It is shown that by using shear correction factors derived from an equilibrium stress field, improvements in the calculated stiffness and deflections can be achieved in circumstances when a corrugated board panel is subjected to bending. Following the algorithm outlined, the shear correction factors are easy to calculate and become a valuable tool when performing mechanical analyses of corrugated boards. The capability of the model to properly capture and simulate the mechanical behavior of corrugated boards subjected to plate bending as well as three-point-bending has been demonstrated by means of several numerical examples, which are compared to experiments on corrugated board panels of varying geometry.

  • 36.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Kulachenko, Artem
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Gradin, Per A
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    The onset and progression of damage in isotropic paper sheets2006In: International Journal of Solids and Structures, ISSN 0020-7683, Vol. 43, no 3-4, p. 713-726Article in journal (Refereed)
    Abstract [en]

    The validity of existing criteria and relations for the onset and evolution of damage in paper is investigated and it is found that a non - local theory has to be utilized.

  • 37.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Stahle, P
    Prediction of shear crack growth direction under compressive loading2002In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 113, no 2, p. 175-194Article in journal (Refereed)
    Abstract [en]

    A directional crack growth prediction in a compressed homogenous elastic isotropic material under plane strain conditions is considered. The conditions at the parent crack tip are evaluated for a straight stationary crack. Remote load is a combined biaxial compressive normal stress and pure shear. Crack surfaces are assumed to be frictionless and to remain closed during the kink formation wherefore the mode I stress intensity factor KI is vanishing. Hence the mode II stress intensity factor KII remains as the single stress intensity variable for the kinked crack. An expression for the local mode II stress intensity factor k2 at the tip of a straight kink has been calculated numerically with an integral equation using the solution scheme proposed by Lo (1978) and refined by He and Hutchinson (1989). The confidence of the solution is strengthened by verifications with a boundary element method and by particular analytical solutions. The expression has been found as a function of the mode II stress intensity factor KII of the parent crack, the direction and length of the kink, and the difference between the remote compressive normal stresses perpendicular to, and parallel with, the plane of the parent crack. Based on the expression, initial crack growth directions have been suggested. At a sufficiently high non-isotropic compressive normal stress, so that the crack remains closed, the crack is predicted to extend along a curved path that maximizes the mode II stress intensity factor k2. Only at an isotropic remote compressive normal stress the crack will continue straight ahead without change of the direction. Further, an analysis of the shape of the crack path has revealed that the propagation path is, according the model, required to be described by a function y = cxγ , where the exponent γ is equal to 3/2. In that case, when γ = 3/2, predicts the analytical model a propagation path that is selfsimilar (i.e. the curvature c is independent of any length of a crack extension), and which can be described by a function of only the mode II stress intensity factor KII at the parent crack tip and the difference between the remote compressive normal stress perpendicular to, and parallel with, the parent crack plane. Comparisons with curved shear cracks in brittle materials reported in literature provide limited support for the model discussed.

  • 38.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Ståhle, P
    Crack kinking under high pressure in an elastic-plastic material2001In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 108, no 4, p. 351-366Article in journal (Refereed)
    Abstract [en]

    Directional crack growth criteria in compressed elastic–plastic materials are considered. The conditions at the crack tip are evaluated for a straight stationary crack. Remote load is a combined hydrostatic stress and pure shear, applied via a boundary layer assuming small scale yielding. Strains and deformations are assumed to be small. Different candidates for crack path criteria are examined. Maximum non-negative hoop stress to judge the risk of mode I and maximum shear stress for mode II extension of the crack are examined in some detail. Crack surfaces in contact are assumed to develop Coulumb friction from the very beginning. Hence, a condition of slip occurs throughout the crack faces. The plane in which the crack extends is calculated using a finite element method. Slip-line solutions are derived for comparison with the numerically computed asymptotic field. An excellent agreement between numerical and analytical solutions is found. The agreement is good in the region from the crack tip to around halfway to the elastic–plastic boundary. The relation between friction stress and yield stress is varied. The crack is found to extend in a direction straight ahead in shear mode for sufficiently high compressive pressure. At a limit pressure a kink is formed at a finite angle to the crack plane. For lower pressures the crack extends via a kink forming an angle to the parent crack plane that increases with decreasing pressure.

  • 39.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Ståhle, P
    Mode II crack paths under compression in brittle solids: A theory and experimental comparison2002In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 39, no 8, p. 2281-2297Article in journal (Refereed)
    Abstract [en]

    A study of crack propagation paths in the situation where the crack is suppressed to open during growth due to high compressive forces has been performed. This problem was analyzed theoretically very recently by the authors and is here extended to involve a limited number of illustrative experimental results reported elsewhere in the literature. By analyzing the experimental crack growth patterns, the conclusion is that the model cannot describe the more realistic microscopic failure in detail. Since shear crack growth on the microscale strongly depend on inhomogenities in the material, like cavities, grains or inclusions; the closed crack growth patterns observed are not smooth or free of kinks. Nevertheless, the model show good agreement with the reported experimental observations of the paths of closed macroscopic mode II cracks on samples in brittle materials, induced under overall compression. Failure patterns experimentally observed supports the theory that the growth of mode II cracks under compression in brittle materials follow a propagation path described by a function y ¼ kxb, where b ¼ 3=2. This is strongly supported by the measured values obtained from various experiments. In all the studied experiments, the exponent b was found in the interval [1.43–1.58]. Further, an investigation of the curvature parameter k has been performed and the conclusion is that k does also agree with the simplified model, even though not as good as the exponent b. However, k differs in general <15% from the theoretical value predicted by the model. The process of crack growth is in the simplified model assumed to be controlled by the mode II stress intensity factor KII of the main crack and the difference between the compressive remote normal stress parallel with the crack plane (r111) and the compressive remote normal stress perpendicular to the crack plane (r122).

  • 40.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Ståhle, P
    Massih, A
    Modelling the consolidation and swelling of boron carbide powder in nuclear reactors1997In: Svenska mekanikdagar -97, Luleå 16-18 mars, Luleå tekniska universitet :: [sammanfattning av föredrag], Stockholm: Svenska nationalkommittén för mekanik , 1997, , p. 150Conference paper (Other academic)
  • 41.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Ståhle, Per
    Directional crack path criterion for crack growth in ductile materials subjected to shear and compressive loading under plane strain conditions2003In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 40, no 13-14, p. 3523-3536Article in journal (Refereed)
    Abstract [en]

    A directional crack growth criterion in a compressed elastic perfectly plastic material is considered. The conditions at the crack-tip are evaluated for a straight stationary crack with a small incipient kink. Remote load is a combined hydrostatic pressure and pure shear applied via a boundary layer. Crack surfaces in contact are assumed to develop homogenous Coulomb friction. The crack opening displacement of an extended kink is examined in a finite element analysis to judge the risk of opening mode failure. It has been found that the direction that maximizes the crack opening displacement of an extended kink tip coincides very well with a prediction of the crack growth direction obtained by using a criterion for continued crack growth direction discussed by the authors elsewhere [Int. J. Fract. 108 (2001) 351]. Moreover, the by the model predicted incipient crack growth directions are qualitatively comparable with reported crack paths obtained in ductile materials in a limited number of experiments performed under a combined load of inplane shear and compression.

  • 42.
    Isaksson, Per
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Ståhle, Per
    Massih, Ali
    Mechanics of control element during irradiation1997In: Structural mechanics in reactor technology : transactions of the 14th International Conference on Structural Mechanics in Reactor Technology, Lyon, France, August 17-22, 1997: International Conference on Structural Mechanics in Reactor Technology (14 : Lyon : 1997), [Gif-sur-Yvette]: SMiRT 97 , 1997Conference paper (Refereed)
  • 43.
    Krusper, Aleksandra
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Gradin, Per A
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Modelling of out-of-plane compression loading of corrugated paper board structures2007In: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 133, no 11, p. 1171-1177Article in journal (Refereed)
    Abstract [en]

    A simple analytical model for calculating the nonlinear deformations of a corrugated core (fluting) during out-of-plane compression is developed. The results from the analytical model are compared to a more comprehensive finite-element model and to experiments. A connection between the boundary conditions and the damage state of the corrugated board is discussed. Including a modified set of boundary conditions gives an almost perfect match of the initial stiffness to the experimental results, indicating that the core might be significantly damaged as a result of the manufacturing process. Possibly, the strength of corrugated boards might be increased about 20% if one could avoid damaging the fluting during the manufacturing process.

  • 44. Massih, Ali R
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Ståhle, Per
    Modelling the behaviour of a control-element blade during irradiation1997In: Computers & Structures, 1997, Vol. 64, no 5/6, p. 1113-1127Conference paper (Refereed)
    Abstract [en]

    The control-rod elements in a boiling-water reactor contain natural boron carbide (B4C) powder, used as neutron-absorber material and clad in stainless-steel blades. During in-reactor service, the internal production of helium gas and point defects in neutron-irradiated boron carbide cause swelling which can induce significant contact stresses in the blade causing, eventually, stress-corrosion cracking of the blades. In this work, a finite-element analysis of a control-rod blade consisting of B4C powder and stainless-steel cladding has been performed using ADINA. An algorithm for the finite-element calculation of a porous material such as B4C powder has been developed and which models both swelling and consolidation behaviour of B4C powder. The Drucker-Prager constitutive law has been used to model the consolidation effect. The model has been verified with an analytical solution for a simple geometry. A number of cases with B4C powder in contact with stainless steel and using the actual blade design have been studied for which contact stresses, the displacements and the effective stresses are calculated. Finally, the model has been used to predict the deformation of the blade during irradiation under B4C swelling and irradiation-induced creep of stainless steel.

  • 45.
    Norgren, Sven
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Höglund, Hans
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Acoustic emission monitoring to evaluate the z-direction and in-plane strength properties of TMP reject fractionManuscript (Other academic)
  • 46.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    A particle-based method for mechanical analyses of planar fiber-based materials2013In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 93, no 11, p. 1216-1234Article in journal (Refereed)
    Abstract [en]

    A new discrete element model to deal with rapid deformation and fracture of flat fibrous materials is derived. The method is based on classical mechanical theories and is a combination of traditional particle dynamics and nonlinear engineering beam theory. It is assumed that a fiber can be seen as a beam that is represented by discrete particles, which are moving according to Newton's laws of motion. Damage is dealt with by fracture of fiber-segments and fiberfiber bonds when the potential energy of a segment or bond exceeds the critical fracture energy. This allows fractures to evolve as a result of material properties only. To validate the model, four examples are shown and compared with analytical results found in literature. Copyright (c) 2013 John Wiley & Sons, Ltd.

  • 47.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Gradin, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Dynamic mode I crack growth in a notched foam specimen under quasi static loadingManuscript (preprint) (Other academic)
  • 48.
    Persson, Johan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Per, Isaksson
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Modeling rapidly growing cracks in planar materials with a view to micro structural effects2015In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 192, no 2, p. 191-201Article in journal (Refereed)
    Abstract [en]

    Dynamic fracture behavior in both fairly continuous materials and discontinuous cellular materials is analyzed using a hybrid particle model. It is illustrated that the model remarkably well captures the fracture behavior observed in experiments on fast growing cracks reported elsewhere. The material's microstructure is described through the configuration and connectivity of the particles and the model's sensitivity to a perturbation of the particle configuration is judged. In models describing a fairly homogeneous continuous material, the microstructure is represented by particles ordered in rectangular grids, while for models describing a discontinuous cellular material, the microstructure is represented by particles ordered in honeycomb grids having open cells. It is demonstrated that small random perturbations of the grid representing the microstructure results in scatter in the crack growth velocity. In materials with a continuous microstructure, the scatter in the global crack growth velocity is observable, but limited, and may explain the small scattering phenomenon observed in experiments on high-speed cracks in e.g. metals. A random perturbation of the initially ordered rectangular grid does however not change the average macroscopic crack growth velocity estimated from a set of models having different grid perturbations and imply that the microstructural discretization is of limited importance when predicting the global crack behavior in fairly continuous materials. On the other hand, it is shown that a similar perturbation of honeycomb grids, representing a material with a discontinuous cellular microstructure, result in a considerably larger scatter effect and there is also a clear shift towards higher crack growth velocities as the perturbation of the initially ordered grid become larger. Thus, capturing the discontinuous microstructure well is important when analyzing growing cracks in cellular or porous materials such as solid foams or wood.

  • 49. Yang, S
    et al.
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Engineering, Physics and Mathematics.
    Mechanical modeling of boron carbide. Part III: Contact force and displacement analysis1996Report (Other academic)
  • 50.
    Åslund, Pär
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Hernández-Pérez, Adrián
    Hägglund, Rickard
    Isaksson, Per
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    In situ XCT study of failure mechanisms in corrugated boards subjected to out-ofplane compressionManuscript (preprint) (Other academic)
12 1 - 50 of 52
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