| 1. |
- Hägglund, Rickard
(författare)
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Fracture mechanical modelling of embossed paper
- 2001
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of two scientific papers on strength and fracture of embossed tissue paper. In Paper A an experimental study of embossed tissue paper is presented. The mechanical properties of most soft tissue paper are to a large extent governed by the creping and subsequent embossing process. The objective is to study the in-plane fracture mechanical properties of different creped tissue paper products having different embossing patterns. In order to reproduce the embossing process patterned metal plates were used to imprint patterns in paper sheets. Fracture toughness was obtained by tensile testing of paper samples containing pre-fabricated cracks. The fracture process occurring in a sample having a periodic embossing pattern was studied by the aid of a computer controlled CCD-camera system. Inspection of the damage evolution of samples containing a pre-fabricated cracks indicates that damage occur in indentation rows parallel to the crack plane, near the tip of the main crack. This suggests that the stresses at the tip of the crack is shielded by neighbouring 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. Furthermore, below a certain crack lengths, the pre-fabricated crack failed to localise damage. Additionally, it was observed that the stress-strain response of creped tissue paper exhibits a pronounced non-linear behaviour in cross-direction. However, tensile tests of an embossed 2-ply tissue paper made from the same base paper exhibits surprisingly an almost linear behaviour up to failure in cross-direction. In Paper B a model is presented which aims at describing the crack tip shielding effect occurring in an embossed tissue paper material having a periodic indentation pattern. The material is analysed as consisting of representative unit cells. One major objective of homogenising, is the possibility to circumvent the complexity associated with detailed modelling of the periodic geometry of the embossed paper sheet. The effective damage behaviour of a unit cell is determined. This is accomplished by homogenising the cohesive behaviour of the unit cell, based on which an effective cohesive law is established. The result is used to describe the damage behaviour of the body. Additionally, the effective elastic properties of a unit cell are determined. The crack tip of an embossed tissue paper undergoing damage may be visualised to consist of three regions: (i) a fracture process zone ahead of the main crack developing in the direction of the crack (ii); the region immediately outside the crack plane where damage localises to the edges of the round embossing imprints(iii);an entirely elastic undamaged region outside the damage region where no damage and consequently no energy dissipation takes place. The shielding effect may be quantified by means of the shielding ratio WS,/ WMain where WMain is the energy dissipated in the fracture process zone in the plane of the main crack and WS is the energy dissipated outside the plane of the main crack. Both WMain and WS were estimated by the aid of the effective cohesive law determined for the unit cell. The shielding effect is computed and observed to be governed by the density of the indentations and relative size of the indentation.
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| 2. |
- Hägglund, Rickard, et al.
(författare)
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Measuring thickness changes of edge-wise compression loaded corrugated board panels using digital image correlation
- 2012
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Ingår i: Journal of Sandwich Structures and Materials. - : SAGE Publications. - 1099-6362 .- 1530-7972. ; 14:1, s. 75-94
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Hägglund, Rickard
(författare)
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Modelling Damage Evolution and Fracture of Paper Materials
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis contains six papers dealing with different aspects of damage and fracture in paper. The work addresses the problem of material length scales in the context of deformation and fracture of paper materials. In Paper 1, localised failure in low-basis weight paper is studied and a fracture model based on continuum damage mechanics is presented. A gradient enhanced theory is used that incorporate a characteristic length that prevents localisation of strain into an unrealistically small volume. Damage parameters are calibrated using data from acoustic emission (AE) measurements. It is concluded that the model can be used to evaluate the influence of defect size on fracture load. From AE measurements it is concluded that an exponential damage evolution law describes the progress of damage in low-basis weight paper. In Paper 2, an optical non-contact displacement measuring system has been used in mode I fracture testing of low-density paper to determine the strain field in the crack-tip region. Immediately before final fracture, the measured normal strain perpendicular to the crack plane in the near-tip region is approximately sixty percent higher than the computed strain using elastic–plastic theory at corresponding load levels while the strain computed using a non-local damage theory is of the same order of magnitude as the experimental. Hence, it seems physically motivated to include a non-local damage theory in order to obtain agreement in strains in the fracture process zone. In Paper 3, a model describing the fracture behaviour of embossed low-basis-weight paper is presented. It is found that the model captures the development of damage along rows of embossing imprints parallel to the main crack which has been observed in experiments. The model suggests that an embossing pattern could have a toughening effect on the sheet for certain pattern dimensions and embossing pressures. In Paper 4, the deformations near a semi-infinite crack in a linear elastic random fibre network (RFN) under mode I loading is studied using a numerical network model. A square root singular deformation field (K-field) is applied on the periphery of the model domain. An important conclusion of the investigation is that the square root field breaks down in the vicinity of the tip of a main crack due to structural effects caused by the network structure. This type of distortions can not be captured by conventional local continuum mechanics. It is shown that a more realistic strain energy field may be accomplished through the use of non-local field theory. A simple relation between non-local characteristic length and structural parameters of the network is presented. In Paper 5, a closed form relation for the strain energy density in the vicinity of a macroscopic mode I crack in a random fibre network is derived using non-local continuum field theory. The model explains why open network structures seldom localise failure to small macroscopic cracks. It is found that there is a one-to-one relation between the characteristic length controlling non-local actions and the size of the smallest crack that can initiate macroscopic failure. Fibre breakage is a damage process which is active if a paper material is dense or the bonds between fibres are strong. Due to the poor statistics of single fibre measurements, the so-called zero-span strength of paper is often taken as a measure of fibre strength. In Paper 6, some analytical and numerical results concerning the zero-span testing method is presented. Of particular interest is the relationship between an apparent modulus obtained from the zero-span testing method and the elastic properties of the fibres. The apparent elasticity modulus is estimated using two energy theorems in elasto-statics in which the role of span length is explored. Analytical results, derived under the assumption that slippage between specimen and clamps does not occur, clearly show that the apparent modulus strongly depends on the span length. This is verified by the numerical results obtained using the finite element method. Tensile tests at nominal zero span were conducted and it was found that there is qualitative agreement between the experiments and the result of the analysis.
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| 4. |
- Isaksson, Per, et al.
(författare)
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Strain energy distribution in a crack-tip region in random fiber networks
- 2009
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Ingår i: International Journal of Fracture. - : Springer Science and Business Media LLC. - 0376-9429 .- 1573-2673. ; 156:1, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Isaksson, Per, et al.
(författare)
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Structural effects on deformation and fracture of random fiber networks and consequences on continuum models
- 2009
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 46:11-12, s. 2320-2329
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Kao-Walter, Sharon, et al.
(författare)
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Fracture toughness of a laminated composite
- 2003
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Ingår i: Fracture of Polymers, Composites and Adhesives II. - : Elsevier. - 0080441955 ; , s. 355-364
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The fracture toughness of a polymer-metal laminate composite is obtained by mechanical testing of a specimen containing a pre-crack. The result is compared with a calculated fracture toughness based on the measured fracture toughness of the individual layers. The laminate is a material used for packaging. It consists of a thin aluminium foil and a polymer coating. A centred crack panel test geometry is used. Each of the layers forming the laminate is also tested separately. It is observed that the load carrying capacity increases dramatically. At the strain when peak load is reached for the laminate only aluminium is expected to carry any substantial load because of the low stiffness of the LDPE. However, the strength of the laminate is almost twice the strength of the aluminium foil. The reason seems to be that the aluminium forces the polymer to absorb large quantities of energy at small deformation. The result is compared with the accumulated toughness of all involved layers. A more elaborate model is proposed in the light of non-linear material behaviour and development of a fracture process zone at the crack tip. Possible fracture of the interface between the layers is discussed.
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| 7. |
- Simonson, Oscar E., et al.
(författare)
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In Vivo Effects of Mesenchymal Stromal Cells in Two Patients With Severe Acute Respiratory Distress Syndrome
- 2015
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Ingår i: Stem Cells Translational Medicine. - : Oxford University Press (OUP). - 2157-6564 .- 2157-6580. ; 4:10, s. 1199-1213
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Tidskriftsartikel (refereegranskat)abstract
- Mesenchymal stromal cells (MSCs) have been investigated as a treatment for various inflammatory diseases because of their immunomodulatory and reparative properties. However, many basic questions concerning their mechanisms of action after systemic infusion remain unanswered. We performed a detailed analysis of the immunomodulatory properties and proteomic profile of MSCs systemically administered to two patients with severe refractory acute respiratory distress syndrome (ARDS) on a compassionate use basis and attempted to correlate these with in vivo anti-inflammatory actions. Both patients received 2 x 10(6) cells per kilogram, and each subsequently improved with resolution of respiratory, hemodynamic, and multiorgan failure. In parallel, a decrease was seen in multiple pulmonary and systemic markers of inflammation, including epithelial apoptosis, alveolar-capillary fluid leakage, and proinflammatory cytokines, microRNAs, and chemokines. In vitro studies of the MSCs demonstrated a broad anti-inflammatory capacity, including suppression of T-cell responses and induction of regulatory phenotypes in T cells, monocytes, and neutrophils. Some of these in vitro potency assessments correlated with, and were relevant to, the observed in vivo actions. These experiences highlight both the mechanistic information that can be gained from clinical experience and the value of correlating in vitro potency assessments with clinical effects. The findings also suggest, but do not prove, a beneficial effect of lung protective strategies using adoptively transferred MSCs in ARDS. Appropriate randomized clinical trials are required to further assess any potential clinical efficacy and investigate the effects on in vivo inflammation. STEM CELLS TRANSLATIONAL MEDICINE 2015;4:1199-1213
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| 8. |
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| 9. |
- Åslund, Pär, et al.
(författare)
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An analysis of strain localization and formation of face wrinkles in edge-wise loaded corrugated sandwich panels using a continuum damage model
- 2015
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 56-57, s. 248-257
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
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