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Micromechanics of Fiber Networks
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The current trends in papermaking involve, but are not limited to, maintaining the dry strength of paper material at a reduced cost. Since any small changes in the process affect several factors at once, it is difficult to relate the exact impact of these changes promptly. Hence, the detailed models of the network level of a dry sheet have to be studied extensively in order to attain the infinitesimal changes in the final product.

In Paper A, we have investigated a relation between micromechanical processes and the stress–strain curve of a dry fiber network during tensile loading. The impact of “non-traditional” bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds, is discussed. In Paper B, we studied the impact of the chemical composition of the fiber cell wall, as well as its geometrical properties, on the fiber mechanical properties using the three-dimensional model of a fiber with helical orientation of microfibrils at a range of different microfibril angles (MFA). In order to accurately characterize the fiber and bond properties inside the network, via statistical distributions, microtomography studies on the handsheets have been carried out. This work is divided into two parts: Paper C, which describes the methods of data acquisition and Paper D, where we discuss the extracted data. Here, all measurements were performed at a fiber level, providing data on the fiber width distribution, width-to-height ratio of isotropically oriented fibers and contact density. In the last paper, we utilize data thus obtained in conjunction with fiber morphology data from Papers C and D to update the network generation algorithm in order to produce more realistic fiber networks. We also successfully verified the models with the help of experimental results from dry sheets tested under uniaxial tensile tests. We carry out numerical simulations on these networks to ascertain the influence of fiber and bond parameters on the network strength properties.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 32 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 97
Keyword [en]
Network simulation, Mechanical properties, Fibers, Fiber-to-fiber bonds, Free fiber length, Number of contacts, Contact density, Paper properties, X-ray microtomography
National Category
Paper, Pulp and Fiber Technology
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-188481ISBN: 978-91-7595-994-8 (print)OAI: oai:DiVA.org:kth-188481DiVA: diva2:935146
Public defence
2016-09-02, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160613

Available from: 2016-06-13 Created: 2016-06-10 Last updated: 2016-06-13Bibliographically approved
List of papers
1. Stress-strain curve of paper revisited
Open this publication in new window or tab >>Stress-strain curve of paper revisited
2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 2, 318-328 p.Article in journal (Refereed) Published
Abstract [en]

We have investigated a relation between micromechanical processes and the stress-strain curve of a dry fiber network during tensile loading. By using a detailed particle-level simulation tool we investigate, among other things, the impact of "non-traditional" bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds. This is probably the first three-dimensional model which is capable of simulating the fracture process of paper accounting for nonlinearities at the fiber level and bond failures. The failure behavior of the network considered in the study could be changed significantly by relatively small changes in bond strength, as compared to the scatter in bonding data found in the literature. We have identified that compliance of the bonding regions has a significant impact on network strength. By comparing networks with weak and strong bonds, we concluded that large local strains are the precursors of bond failures and not the other way around.

Keyword
Network simulation, Mechanical properties, Fibers, Bonds, Paper properties, Damage
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-104884 (URN)10.3183/NPPRJ-2012-27-02-p318-328 (DOI)000315696000021 ()2-s2.0-84865252133 (Scopus ID)
Note

QC 20121114

Available from: 2012-11-14 Created: 2012-11-14 Last updated: 2017-12-07Bibliographically approved
2. Constitutive modeling of a paper fiber in cyclic loading applications
Open this publication in new window or tab >>Constitutive modeling of a paper fiber in cyclic loading applications
2015 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 110, 227-240 p.Article in journal (Refereed) Published
Abstract [en]

The tensile response of dense fiber-based materials like paper or paperboard is mainly dependent of the properties of the fibers, which store most of the elastic energy. In this paper, we investigate the influence of geometrical and material parameters on the mechanical response of the pulp fibers used in paper manufacturing. We developed a three-dimensional finite element model of the fiber, which accounts for microfibril orientation of cellulose fibril, and the presence of lignin in the secondary cell wall. The results showed that the change in the microfibril orientation upon axial straining is mainly a geometrical effect, and is independent of the material properties of the fiber, as long as the deformations are elastic. Plastic strain accelerates the change in microfibril orientation and thus makes it material-dependent. The results also showed that the elastic modulus of the fiber has a non-linear dependency on microfibril angle, with elastic modulus being more sensitive to the change of microfibril angle around small initial values of microfibril angles. Based on numerical results acquired from a 3D fiber model supported by available experimental evidence, we propose an anisotropic-kinematic hardening plasticity model for a fiber within a beam framework. The proposed fiber model is capable of reproducing the main features of the cyclic tensile response of a pulp fiber, such as stiffening due to changing microfibril angle. The constitutive model of the fiber was implemented in a finite-element model of the fiber network. By using the fiber network model, we estimated the level of strain that fiber segments accumulate before the typical failure strain of the entire network is reached.

Keyword
Microfibril angle, Cell wall, Twist, Cyclic loading
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-175896 (URN)10.1016/j.commatsci.2015.08.039 (DOI)000362010800029 ()2-s2.0-84940925038 (Scopus ID)
Note

QC 20151113

Available from: 2015-11-13 Created: 2015-10-26 Last updated: 2017-12-01Bibliographically approved
3. Characterisations of fibre networks in paper using micro computed tomography images
Open this publication in new window or tab >>Characterisations of fibre networks in paper using micro computed tomography images
2014 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 29, no 3, 468-475 p.Article in journal (Refereed) Published
Abstract [en]

Although several methods exist for characterisation of the morphology of wood fibres, the application of these procedures for the analysis of paper microstructure has been limited due to their complexity or shortcomings. Here, a methodology for microstructure characterisation of individual fibres, as well as paper, is presented which is based on three dimensional computed tomography images of paper at micrometer resolution. The first step of the method consists of a graphical user interface (GUI), designed to minimize the amount of manual labour. To manually identify a fibre from a 2x2 mm(2) paper sheet takes about one minute with this GUI. Then several algorithms are available to analyse the image data automatically guided by the user input. With this approach it is possible to measure several characteristic properties without complete segmentation of the individual fibres. The methodology includes a method to calculate the contact areas between fibres even in extreme cases of severely deformed fibres, which are naturally present in paper. Among the measurable properties are also estimators for the free fibre lengths and fibre wall thickness.

Keyword
Complex networks, Computerized tomography, Fibers, Graphical user interfaces, Microstructure, Paper, Tomography Characteristic properties, Computed tomography images, Contact areas, Fibre network, Fibre wall thickness, Graphical user interfaces (GUI), Microcomputed tomography, Microstructure characterisation
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-155816 (URN)10.3183/NPPRJ-2014-29-03-p468-475 (DOI)000342682700012 ()2-s2.0-84907662472 (Scopus ID)
Note

QC 20141113

Available from: 2014-11-13 Created: 2014-11-13 Last updated: 2017-12-05Bibliographically approved
4. Extracting fiber and network connectivity data using microtomography images of paper
Open this publication in new window or tab >>Extracting fiber and network connectivity data using microtomography images of paper
2016 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 31, no 3, 469-478 p., 07315Article in journal (Refereed) Accepted
Abstract [en]

We apply image analysis methods based on micro-computed tomography (μCT) to extract the parameters that characterize the structure and bonding parameters in the fiber network of paper. The scaling and variational properties of μCT images are examined by analyzing paper structural properties of two 1 × 1 mm2 test pieces, which have been cut out from a low-grammage handsheet. We demonstrate the applicability of the methods for extracting the free fiber length, fiber cross-sectional data, the distances between the fibers, and the number of fiber-to-fiber bonds, which are the key properties required for the adequate representation of the network in numerical models. We compare the extracted connectivity data with the early reported analytical estimations and conclude that the number of contacts in three-dimensional networks is controlled by the fiber aspect ratio. In addition, we compare the cross-sectional data with those measured by the fiber morphology characterization tools and estimate the fiber shrinkage from completely wet to dry state to be nearly 20%.

Place, publisher, year, edition, pages
Stockholm: Arbor Publishing AB, 2016
Keyword
Fiber network, X-ray microtomography, Fibers, Fiber-to-fiber bonds, Free fiber length, Number of contacts, Contact density
National Category
Paper, Pulp and Fiber Technology
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-188479 (URN)000387976000007 ()2-s2.0-84982182500 (Scopus ID)
Note

QC 20160613

Available from: 2016-06-10 Created: 2016-06-10 Last updated: 2017-11-30Bibliographically approved
5. Effect of fiber and bond strength variations on the tensile stiffness and strength of fiber networks
Open this publication in new window or tab >>Effect of fiber and bond strength variations on the tensile stiffness and strength of fiber networks
2016 (English)Report (Refereed)
Abstract [en]

As fiber and bond characterization tools become more sophisticated, the information from the fiber scale becomes richer. This information is used for benchmarking of different types of fibers by the paper and packaging industries. In this work, we have addressed a question about the effect of variability in the fiber and fiber bond properties on the average stiffness and strength of fiber networks. We used a fiber-scale numerical model and reconstruction algorithm to address this question. The approach was verified using the experimental sheets having fiber data acquired by a fiber morphology analyzer and corrected by microtomographic analysis of fibers in these sheets. We concluded, among other things, that it is sufficient to account for the average bond strength value with an acceptable number of samples to describe dry network strength, as long as the bond strength distribution remains symmetric. We also found that using the length-weighted average for fiber shape factor and fiber length data neglects the important contribution from the distribution in these properties on the mechanical properties of the sheets.

Publisher
25 p.
Keyword
fibers, bonds, stress-strain curve, paper strength, network simulation
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-188480 (URN)
Note

QC 20160613

Available from: 2016-06-10 Created: 2016-06-10 Last updated: 2016-06-13Bibliographically approved

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