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Micro-mechanically based modeling of mechano-sorptive creep in paper
KTH, Superseded Departments, Solid Mechanics.
2004 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

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

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

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

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

Place, publisher, year, edition, pages
Stockholm: Hållfasthetslära , 2004.
Series
Trita-HFL, ISSN 1104-6813 ; 0376
Keyword [en]
Materials science, applied mechanics, materials science, packaging
Keyword [sv]
Materialvetenskap
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-41OAI: oai:DiVA.org:kth-41DiVA: diva2:10721
Public defence
2004-11-11, D2, KTH, Lindstedtsvägen 5, Stockholm, 13:15
Opponent
Supervisors
Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2012-03-19
List of papers
1. A micromechanical model for mechanosorptive creep in paper
Open this publication in new window or tab >>A micromechanical model for mechanosorptive creep in paper
2002 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 28, no 3, 98-104 p.Article in journal (Refereed) Published
Abstract [en]

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

Keyword
creep, moisture content, sorption, models, swelling, fiber bonding, fiber networks
National Category
Mechanical Engineering Materials Engineering
Identifiers
urn:nbn:se:kth:diva-6101 (URN)000174392900006 ()
Note
QC 20101022Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2010-10-22Bibliographically approved
2. A simplified network model for mechano-sorptive creep in paper
Open this publication in new window or tab >>A simplified network model for mechano-sorptive creep in paper
2003 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 29, no 7, 228-234 p.Article in journal (Refereed) Published
Abstract [en]

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

Keyword
creep, mathematical models, simulation, moisture content, fiber bonding, fiber networks, elasticity
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-6102 (URN)000185031700004 ()
Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2011-09-20Bibliographically approved
3. The effect of humidity cycle amplitude on accelerated tensile creep of paper
Open this publication in new window or tab >>The effect of humidity cycle amplitude on accelerated tensile creep of paper
2004 (English)In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 8, no 4, 289-302 p.Article in journal (Refereed) Published
Abstract [en]

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

Keyword
creep test, mechano-sorptive creep, moisture accelerated creep, network model, paper
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-6103 (URN)10.1007/s11043-004-0536-0 (DOI)000229288900001 ()2-s2.0-18244386209 (Scopus ID)
Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2011-09-20Bibliographically approved
4. Linear constitutive model for mechano-sorptive creep in paper
Open this publication in new window or tab >>Linear constitutive model for mechano-sorptive creep in paper
2004 (English)Report (Other academic)
Publisher
15 p.
Series
Report / Royal Institute of Technology, KTH Solid Mechanics, ISSN 1654-1472 ; 375
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-6104 (URN)
Note
QC 20101022Available from: 2004-11-30 Created: 2004-11-30 Last updated: 2010-10-22Bibliographically approved

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