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The Role of the Fiber and the Bond in the Hygroexpansion and Curl of Thin Freely Dried Paper Sheets
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).ORCID iD: 0000-0001-5580-5023
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).ORCID iD: 0000-0002-5112-1289
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).ORCID iD: 0000-0003-3611-2250
Technische Universität Graz.ORCID iD: 0000-0002-1376-9076
2019 (English)Report (Other academic)
Abstract [en]

A computationally efficient method to study the in-plane and out-of-plane dimensional instability of thin paper sheets under the influence of moisture changes is presented. The method explicitly resolves the bonded and the free segments of fibers in the network, capturing the effect of anisotropic hygroexpansion at the fiber level. The method is verified against a volumetric model. The importance of longitudinal fiber hygroexpansion is demonstrated in spite of the absolute value of longitudinal hygroexpansion being an order of magnitude lower than the transverse hygroexpansion component. Finally, the method is used to demonstrate the formation of macroscopic sheet curl due to a moisture gradient in structurally uniform sheets in the absence of viscoelastic or plastic constitutive behavior and through-thickness residual stress profiles.

Place, publisher, year, edition, pages
2019. , p. 28
Series
TRITA-SCI-RAP ; 2019:005
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-260371OAI: oai:DiVA.org:kth-260371DiVA, id: diva2:1355440
Funder
Swedish Research Council, 2015-05282Swedish National Infrastructure for Computing (SNIC), 2017-1-175Available from: 2019-09-28 Created: 2019-09-28 Last updated: 2019-09-28
In thesis
1. Insights in paper and paperboard performance by fiber network micromechanics
Open this publication in new window or tab >>Insights in paper and paperboard performance by fiber network micromechanics
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Fiber networks are ubiquitous due to their low cost and high ratio of mechanical performance to weight. Fiber networks made of cellulose fibers from trees are used as information carriers (paper) and as packaging (board). Often the ideal product is both mechanically sturdy and possible to print on. This thesis investigates the underlying reasons for the mechanical performance of paper and board through the discretization and direct simulation of every fiber in the network.

In Paper A the effect of fiber-fiber bond geometry on sheet stiffness is investigated. Many packaging products seek to maximize the bending stiffness by employing stiff outer layers and a bulkier layer in the middle. In bulky sheets, the fibers are frequently uncollapsed resulting in a more compliant bonded segment. Because all the loads in the network are transferred via the bonds, such compliance can cause unexpectedly large decreases in mechanical performance. Although many models have been presented which aim to predict the tensile stiffness of a sheet, these predictions tend to overestimate the resulting stiffness. One reason is that the bonds are generally considered rigid. By finite element simulations, we demonstrated the effect of the lumina configuration on the stiffness of the bonded segment on the scale of single fiber-to-fiber bonds, and that the average state of the fiber lumen has a marked effect on the macroscopic response of fiber networks when the network is bulky, has few bonds, or has a low grammage.

Compression strength is central in many industrial applications. In paper B we recreated the short span compression test in a simulation setting. The networks considered are fully three-dimensional and have a grammage of 80 to 400 gsm, which is the industrially relevant range. By modeling compression strength at the level of individual fibers and bonds, we showed that fiber level buckling or bifurcation phenomena are unlikely to appear at the loads at which the macroscopic sheet fails.

In paper C, we developed a micromechanical model to study the creation of curl in paper sheets subjected to a moisture gradient through the sheet. A moisture gradient is always created during the printing process, which may lead to out-of-plane dimensional instability. We showed that the swelling anisotropy of individual fibers bonded at non-parallel angles causes an additional contribution to the curl observed on the sheet level.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. p. 38
Series
TRITA-SCI-FOU ; 2019:44
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-260374 (URN)978-91-7873-317-0 (ISBN)
Presentation
2019-10-22, Seminar room, Dept. of Solid Mechanics, Teknikringen 8D, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

Examinator: Professor Mårten Olsson, KTH, Stockholm

QC 20190930

Available from: 2019-09-30 Created: 2019-09-28 Last updated: 2019-09-30Bibliographically approved

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