| 1. |
- Alfthan, Johan, et al.
(författare)
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A micromechanical model for mechanosorptive creep in paper
- 2002
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Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 28:3, s. 98-104
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Alfthan, Johan
(författare)
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A simplified network model for mechano-sorptive creep in paper
- 2003
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Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 29:7, s. 228-234
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Alfthan, Johan
(författare)
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Experimental study of non-linear stress relaxation and creep of paper materials and the relation between the two types of experiments
- 2010
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Ingår i: Nordic Pulp & Paper Research Journal. - 0283-2631 .- 2000-0669. ; 25:3, s. 351-357
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Tidskriftsartikel (refereegranskat)abstract
- The non-linear time-dependent mechanical behaviour of three different commercial paper grades was investigated. Stress relaxation and creep tests were carried out in MD and CD using different load levels. The strain in each test was split in a linear part and a non-linear part. From the stress and the linear part of the strain in the test, a stress relaxation modulus or a creep compliance was calculated. The stress relaxation moduli and creep compliances determined in this way were observed to be independent of the load level. The stress relaxation moduli and creep compliances for each paper were further independent of the loading direction (MD/CD) when scaled by the elastic modulus. It was also shown that the stress relaxation modulus was approximately equal to the inverse of the creep compliance, which is what would have been expected if linear viscoelastic theory had been applicable.
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| 4. |
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| 5. |
- Alfthan, Johan, et al.
(författare)
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Linear constitutive model for mechano-sorptive creep in paper
- 2005
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 42:24-25, s. 6261-6276
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Tidskriftsartikel (refereegranskat)abstract
- The creep of paper is accelerated by moisture cycling. This effect is known as mechano-sorptive creep. It is assumed that this is an effect of transient stresses produced during moisture content changes in combination with non-linear creep behaviour of the fibres. The stresses produced by the moisture content changes are often much larger than the applied mechanical loads. If this is the case, the mechanical loads are only a perturbation to the internal stress state, and it will appear as if the mechano-sorptive creep is linear in stress. It is possible to take advantage of this feature. In the present report the pure moisture problem is first solved. The mechanical load is then treated as a perturbation of the solution to the moisture problem. Using this strategy, it is possible to linearize a non-linear network model for mechano-sorptive creep and to formulate a continuum model. As a result, the number of variables in the model is reduced. This is a significant improvement as it will be possible to use the linearized model to describe the material in a finite element program and solve problems with complicated geometries.
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| 6. |
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| 7. |
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| 8. |
- Alfthan, Johan, 1974-
(författare)
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Micro-mechanically based modeling of mechano-sorptive creep in paper
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
- Alfthan, Johan
(författare)
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The effect of humidity cycle amplitude on accelerated tensile creep of paper
- 2004
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Ingår i: Mechanics of time-dependant materials. - : Springer Science and Business Media LLC. - 1385-2000 .- 1573-2738. ; 8:4, s. 289-302
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Helmersson, Johanna, et al.
(författare)
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Serum selenium predicts levels of F2-isoprostanes and prostaglandin F2alpha in a 27 year follow-up study of Swedish men
- 2005
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Ingår i: Free radical research. - : Informa UK Limited. - 1071-5762 .- 1029-2470. ; 39:7, s. 763-770
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Tidskriftsartikel (refereegranskat)abstract
- Low concentrations of selenium (Se) predict mortality and cardiovascular diseases in some populations. The effect of Se on in vivo indicators of oxidative stress and inflammation, two important features of atherosclerosis, in human populations is largely unexplored. This study investigated the longitudinal association between serum selenium (s-Se) and a golden standard indicator of oxidative stress in vivo (8-iso-prostaglandin F2, a major F2-isoprostane), an indicator of cyclooxygenase (COX)-mediated inflammation (prostaglandin F2), high sensitive C-reactive protein (hsCRP), interleukin-6 (IL-6) and serum amyloid A protein (SAA) in a follow-up study of 27 years. The s-Se was measured in 615 Swedish men at 50 years of age in a health investigation. The status of oxidative stress and inflammation was evaluated in a re-investigation 27 years later by quantification of urinary 8-iso-PGF2 and 15-keto-dihydro- PGF2 (a major metabolite of PGF2) and serum hsCRP, SAA and IL-6. Men in the highest quartile of s-Se at age 50 had decreased levels of 8-iso-PGF2 compared to all lower quartiles and decreased levels of PGF2 compared to all lower quartiles at follow-up. These associations were independent of BMI, diabetes, hyperlipidemia, hypertension, smoking, -tocopherol and β-carotene at baseline. The s-Se was not associated with hsCRP, SAA or IL-6 at follow-up. In conclusion, high concentrations of s-Se predict reduced levels of oxidative stress and subclinical COX-mediated (but not cytokine-mediated) inflammation in a male population. The associations between Se, oxidative stress and inflammation, respectively, might be related to the proposed cardiovascular protective property of Se.
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