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Isaksson, Per
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Publications (10 of 52) Show all publications
Åslund, P., Hägglund, R., Carlsson, L. & Isaksson, P. (2015). An analysis of strain localization and formation of face wrinkles in edge-wise loaded corrugated sandwich panels using a continuum damage model. International Journal of Solids and Structures, 56-57, 248-257
Open this publication in new window or tab >>An analysis of strain localization and formation of face wrinkles in edge-wise loaded corrugated sandwich panels using a continuum damage model
2015 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 56-57, p. 248-257Article in journal (Refereed) Published
Abstract [en]

This paper examines the compressive failure mechanism in edge-to-edge loaded corrugated sandwich panels. The formation of face wrinkles is specifically considered. A detailed finite element model of face sheets and web core of a sandwich panel was developed to provide insight on the failure mechanism. A gradient enhanced continuum damage theory was implemented to capture length effects caused by the material microstructure including formation of damage in the face sheets and core. Distributions of strains in the face sheets determined from finite element analysis (FEA) are compared to experimentally measured strains. The predicted location and orientation of the face wrinkle, as indicated by high values of the second principal strain, agrees well with experimental observations.Load vs. out-of-plane deflection curves obtained from FEA with the gradient enhanced damage material model are compared to those obtained from a linear-elastic material model and experimentally determined curves. The gradient enhanced solution gives qualitatively better agreement with experimental results, although the magnitudes of strains are less than those determined experimentally.

Keywords
Buckling, Compression strength, Corrugated board, Gradient enhanced damage modeling
National Category
Applied Mechanics
Identifiers
urn:nbn:se:miun:diva-21908 (URN)10.1016/j.ijsolstr.2014.10.029 (DOI)000349502600020 ()2-s2.0-84921433231 (Scopus ID)
Note

Published online 27th March 2014.

Available from: 2014-05-06 Created: 2014-05-06 Last updated: 2017-08-15Bibliographically approved
Persson, J. & Per, I. (2015). Modeling rapidly growing cracks in planar materials with a view to micro structural effects. International Journal of Fracture, 192(2), 191-201
Open this publication in new window or tab >>Modeling rapidly growing cracks in planar materials with a view to micro structural effects
2015 (English)In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 192, no 2, p. 191-201Article in journal (Refereed) Published
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.

Keywords
Crack growth velocity, Dynamic fracture, Heterogeneous material, Particle method
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-24958 (URN)10.1007/s10704-015-0002-9 (DOI)000352711800005 ()2-s2.0-84939958015 (Scopus ID)
Funder
Swedish Research Council, 2010- 4348
Available from: 2015-05-23 Created: 2015-05-23 Last updated: 2017-12-04Bibliographically approved
Persson, J. & Isaksson, P. (2013). A particle-based method for mechanical analyses of planar fiber-based materials. International Journal for Numerical Methods in Engineering, 93(11), 1216-1234
Open this publication in new window or tab >>A particle-based method for mechanical analyses of planar fiber-based materials
2013 (English)In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 93, no 11, p. 1216-1234Article in journal (Refereed) Published
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.

Keywords
dynamic crack mechanics, fiber network, particle model
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:miun:diva-18645 (URN)10.1002/nme.4428 (DOI)000315399900005 ()2-s2.0-84874193461 (Scopus ID)
Available from: 2013-03-28 Created: 2013-03-27 Last updated: 2017-12-06Bibliographically approved
Edvardsson, S., Gradin, P., Isaksson, P. & Gulliksson, M. (2012). A Note on Wet Paper Web Adhesion Strength. Journal of Testing and Evaluation, 40(4), 682-686
Open this publication in new window or tab >>A Note on Wet Paper Web Adhesion Strength
2012 (English)In: Journal of Testing and Evaluation, ISSN 0090-3973, E-ISSN 1945-7553, Vol. 40, no 4, p. 682-686Article in journal (Refereed) Published
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.

Keywords
adhesion strength, paper web, peeling test
National Category
Physical Sciences
Identifiers
urn:nbn:se:miun:diva-17064 (URN)10.1520/JTE103851 (DOI)000307424100020 ()2-s2.0-84864119770 (Scopus ID)
Available from: 2012-10-01 Created: 2012-09-26 Last updated: 2017-12-07Bibliographically approved
Hägglund, R., Åslund, P., Carlsson, L. A. & Isaksson, P. (2012). Measuring thickness changes of edge-wise compression loaded corrugated board panels using digital image correlation. Journal of Sandwich Structures and Materials, 14(1), 75-94
Open this publication in new window or tab >>Measuring thickness changes of edge-wise compression loaded corrugated board panels using digital image correlation
2012 (English)In: Journal of Sandwich Structures and Materials, ISSN 1099-6362, E-ISSN 1530-7972, Vol. 14, no 1, p. 75-94Article in journal (Refereed) Published
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.

Keywords
Corrugated board; compression strength; digital image correlation; thickness
National Category
Applied Mechanics
Identifiers
urn:nbn:se:miun:diva-15742 (URN)10.1177/1099636211419133 (DOI)000299347100004 ()2-s2.0-84856193907 (Scopus ID)
Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2017-12-08Bibliographically approved
Gradin, P., Isaksson, P. & Gulliksson, M. (2011). A Note on the Co-linearity of forces and Displacements in an Elastic Structure. Journal of Applied Fluid Mechanics, 78(6), Art. no. 4003912
Open this publication in new window or tab >>A Note on the Co-linearity of forces and Displacements in an Elastic Structure
2011 (English)In: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 78, no 6, p. Art. no. 4003912-Article in journal (Refereed) Published
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.

Keywords
Linear elasticity co-linearity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:miun:diva-15086 (URN)10.1115/1.4003912 (DOI)000295615500011 ()2-s2.0-80052433090 (Scopus ID)
Available from: 2011-12-08 Created: 2011-12-08 Last updated: 2017-12-08Bibliographically approved
Åslund, P. & Isaksson, P. (2011). A note on the nonlinear mechanical behavior of planar random network structures subjected to in-plane compression. Journal of composite materials, 45(25), 2697-2703
Open this publication in new window or tab >>A note on the nonlinear mechanical behavior of planar random network structures subjected to in-plane compression
2011 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 45, no 25, p. 2697-2703Article in journal (Refereed) Published
Abstract [en]

The microstructural effect on the mechanical behavior of idealized two-dimensional random fiber networks subjected to in-plane compression is studied. A finite element model utilizing nonlinear beam elements assuming a linearly elastic material is developed. On a macroscopic level, random fiber networks often display an asymmetric material behavior when loaded in tension and compression. In mechanical models, this nonlinearity is traditionally described using continuum elastic-inelastic and/or damage models even though using a continuum approach risks overlooking microstructural effects. It is found that even though a linear elastic material model is used for the individual fibers, the network gives a nonlinear response in compression. The nonlinearity is found to be caused by buckling of individual fibers. This reversible nonlinear mechanism is limited in tensile loading and hence offers an alternative explanation to the global asymmetry of random fiber networks.

Keywords
random fiber network; compression; mechanical behavior; nonlinearity
National Category
Applied Mechanics
Identifiers
urn:nbn:se:miun:diva-15741 (URN)10.1177/0021998311422749 (DOI)000297519800009 ()2-s2.0-82955222951 (Scopus ID)
Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2017-12-08Bibliographically approved
Edvardsson, S., Gradin, P. & Isaksson, P. (2011). On dissipative effects of paper web adhesion strength. International Journal of Solids and Structures, 48(1), 24-30
Open this publication in new window or tab >>On dissipative effects of paper web adhesion strength
2011 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 48, no 1, p. 24-30Article in journal (Refereed) Published
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.

Keywords
Adhesion strength; Paper web; Peeling test
National Category
Other Physics Topics
Identifiers
urn:nbn:se:miun:diva-9805 (URN)10.1016/j.ijsolstr.2010.09.006 (DOI)000284671800003 ()2-s2.0-78049462102 (Scopus ID)
Available from: 2009-09-22 Created: 2009-09-22 Last updated: 2017-12-13Bibliographically approved
Isaksson, P., Gradin, P. & Östlund, S. (2010). A simplified treatise of the Scott bond testing method. Experimental mechanics, 50(6), 745-751
Open this publication in new window or tab >>A simplified treatise of the Scott bond testing method
2010 (English)In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 50, no 6, p. 745-751Article in journal (Refereed) Published
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.

Keywords
Scott bond testing - Internal bond strength - Delamination - Cohesive zone model
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-10096 (URN)10.1007/s11340-009-9269-y (DOI)000278900500008 ()2-s2.0-77953872865 (Scopus ID)
Available from: 2009-10-20 Created: 2009-10-20 Last updated: 2017-12-12Bibliographically approved
Isaksson, P. (2010). An implicit stress gradient plasticity model for describing mechanical behavior of planar fiber networks on a macroscopic scale. Engineering Fracture Mechanics, 77(8), 1240-1252
Open this publication in new window or tab >>An implicit stress gradient plasticity model for describing mechanical behavior of planar fiber networks on a macroscopic scale
2010 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 77, no 8, p. 1240-1252Article in journal (Refereed) Published
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.

Keywords
Crack mechanics; Gradient plasticity; Network material; Nonlocal theory
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:miun:diva-11639 (URN)10.1016/j.engfracmech.2010.03.023 (DOI)000279026500003 ()2-s2.0-77952891436 (Scopus ID)
Available from: 2010-06-10 Created: 2010-06-10 Last updated: 2017-12-12Bibliographically approved
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