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Mechanics of Nanocellulose Foams: Experimental and Numerical Studies
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Nanofibrillär cellulosa (NFC) skum är en intressant klass av cellulära material med möjliga applikationer som sträcker sig från fordonsindustrin till biomedicin då det har unika och önskvärda mekaniska egenskaper. I ljuset av de senaste framstegen inom framställning av skum förutspås det tillämpas inom en rad olika områden, inklusive områden där dess mekaniska egenskaper är viktiga. Den makroskopiska responsen är oskiljbart kopplad till mikrostrukturen hos materialet. Det är därför nödvändigt att ha numeriska modeller som inte bara kan förutsäga makroskopisk respons utan också ge insikt vid anpassning av mikrostrukturen så att förbättrade makroskopiska egenskaper kan uppnås. I detta syfte studerar vi 2- och 3-dimensionella slumpmässiga cellulära modeller och karakteriserar genom experiment/simuleringar de  makroskopiska och cellväggens materialegenskaper. 

I Artikel A utforskar vi  lämpligheten av 2-dimensionella slumpmässiga strukturer för att representera skums makroskopiska respons i tryck. Även om den 2-dimensionella modellen inte kan beskriva det exakta beteendet, endast en storleksordning överensstämmelse uppnås, kartlägger vi effekten av inre kontakt på den makroskopiska responsen och studerar effekten av linjär storlek, väggtjocklek och cellväggens kurvatur. Slutsatsen som dras är att 2-dimensionella modeller är otillräckliga och att förbindelserna ut ur planet är icke-triviala. 

I Artikel B framställs NFC skum genom frystorkning och karakteriseras experimentellt vid enaxlig och bi-axiell belastning för att utvärdera materialets strukturella anisotropi. Skummet visas vara isotropiskt i planet. Vidare uppkommer stora icke-reversibla deformationer vid avlastning. En hyperelastisk kontinuum-modell anpassas till experimentell data. 

I Artikel C används tomografibaserade tvärsnittsbilder för att bestämma cellväggens materialegenskaper. Vi rekonstruerar en 3-dimensionell struktur baserad på tomografibilder och använder den i finita element-simuleringar för att bestämma elasticitetsmodulen och sträckgränsen för cellväggens material. Resultaten visar att den beräknade elasticitetsmodulen är jämförbar med den övre gränsen för NFC papper, medan sträckgränsen är jämförbar med uppskattningar från indirekta metoder. Simuleringarna bekräftar även skademekansimen att formering av plastiska gångjärn följs av kollaps,  vilket också observerats i experimentella studier. 

I Artikel D använder vi de materialegenskaper som beräknats i det tomografibaserade arbetet i simuleringar av slumpmässigt genererade 3-dimensionella strukturer. Vi validerar de 3-dimensionella strukturerna mot strukturena som fångats med tomografi. Vi studerar därefter om de slumpmässiga strukturerna kan användas för att representera den makroskopiska responsen tillsammans med studierna av linjärstorlek och effekt av de delvis öppna/slutna cellerna. Vi beräknar även påverkan av cellytans kurvatur på elasticitetsmodulen och på platåspänningen. Vi visar att 3-dimensionella modeller är relativt representativa upp till medelhög töjningsgrad men att förtätningen inte fångas  upp av med den representativa storlek som används.

Abstract [en]

Nanofibrillar cellulose (NFC) foams are an interesting class of cellular materials that are being explored for a variety of applications, ranging from the automotive to the biomedical industries. The cellulose nanofibrils itself has unique and desirable mechanical properties. With recent advances in the preparation of these foams, it is anticipated that these foams will find applications in diverse areas, including those where the mechanical response is important. This macroscopic response is inextricably linked to the microstructure of the material. Thus, it is imperative to have numerical models that can not only predict the macroscopic response but can also provide insights towards tailoring the microstructure such that improved macroscopic properties can be sought. Towards this end, we study 2- and 3-D random cellular models along with characterising through experiments/simulations the macroscopic and cell wall material properties. 

In Paper A, we explore the suitability of two-dimensional random structures in representing the macroscopic compressive response of foams. Though the two-dimensional model fails to capture the exact response, only an order of magnitude agreement is found, we map the effect of internal contact on the macroscopic response and study the effect of linear size, wall thickness and non-straightness of the cell walls. It is concluded that 2-D models are inadequate and that the out of plane connectivity is non-trivial. 

In Paper B, NFC foams prepared from freeze-drying are experimentally characterised under uniaxial and biaxial loading conditions, with a view towards testing for structural anisotropy. It is found that the prepared foam is isotropic in the plane. The experiments also reveal that there are large irreversible deformations, when unloaded. A continuum hyperelastic model is fitted to the experimental data. 

In Paper C, tomography based scans of the NFC foams are used to arrive at the material properties of the cell walls. We reconstruct the three-dimensional structure from the tomography scans and use it in finite element simulations to determine the elastic modulus and yield strength of the cell wall material. It is seen that the estimated elastic modulus is comparable to the upper limit for NFC paper, while the yield strength is comparable to estimates from indirect methods. The simulations also corroborate the damage mechanism, i.e. by plastic hinge formations followed by the collapse of the inner structure, as observed by experimental studies. 

In Paper D, we utilise the material properties derived from the tomography-based work in simulating three-dimensional random structures. We validate the three-dimensional reconstruction method against the foam structures derived in microtomography. We then study the applicability of these random structures in representing the macroscopic response, together with studies on linear size and effect of partially open/closed cells. We also estimate the influence of cell face curvature on the elastic modulus and plateaus stress. It is concluded that 3-D models provide a reasonable representation of the response up to intermediate strain levels, but the densification regime is not captured by the considered representative size.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 29
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0610
National Category
Paper, Pulp and Fiber Technology
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-202590ISBN: 978-91-7729-290-6 (print)OAI: oai:DiVA.org:kth-202590DiVA, id: diva2:1077761
Public defence
2017-03-16, F3, Lindstedstvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170301

Available from: 2017-03-01 Created: 2017-03-01 Last updated: 2017-03-01Bibliographically approved
List of papers
1. Analysis of the compressive response of Nano Fibrillar Cellulose foams
Open this publication in new window or tab >>Analysis of the compressive response of Nano Fibrillar Cellulose foams
2015 (English)In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 80, no Part A, p. 13-26Article in journal (Refereed) Published
Abstract [en]

Nano Fibrillar Cellulose (NFC) is fast emerging as a biomaterial with promising applications, one of which is cellular foam. The inner structure of the foam can take various shapes and hierarchical micro-structures depending on the manufacturing parameters. The compressive response of foams developed from these materials is currently a primary criterion for the material development. In this work, we focus on the connection between the non-linear part of the response and the inner structure of the material. We study the effect of internal contact and its contribution to gradual stiffening in the energy absorbing region and accelerated stiffening in the densification region of the large strain compressive response. We use the finite element method in this study and discuss the applicability and efficiency of different modelling techniques by considering well defined geometries and available experimental data. The relative contribution of internal contact is singled out and mapped onto the overall compressive response of the material. The effect of initial non-straightness of the cell walls is studied through superposing differing percentages of the buckling modes on the initial geometry. The initial non-straightness is seen to have a significant effect for only strains up to 1%. The secant modulus measured at slightly higher strains of 4%, demonstrates lesser effect from the non-straightness of cell walls. The simulations capture the compressive response well into the densification regime and there is an order of magnitude agreement in between simulations and experiments. We observed that internal contact is crucial for capturing the trend of compressive response.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
NFC foams, Internal contact, Voronoi structures, Foam densification, Effect of foam porosity
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-159620 (URN)10.1016/j.mechmat.2014.09.006 (DOI)000347493100002 ()2-s2.0-84908214965 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20150209

Available from: 2015-02-09 Created: 2015-02-05 Last updated: 2017-12-04Bibliographically approved
2. Experimental characterisation of nanofibrillated cellulose foams
Open this publication in new window or tab >>Experimental characterisation of nanofibrillated cellulose foams
2015 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882XArticle in journal (Refereed) Published
Abstract [en]

There is a growing interest in applications for nanofibrillated cellulose based materials owing to their exceptional mechanical properties. Nanofibrillated cellulose (NFC) foam is one such derivative which has potential applications in a wide array of fields. Here, we characterise the mechanical properties of two particular high porosity NFC foams (98.13 and 98.96 %) prepared by a freeze drying process. We evaluate their behaviour in uni-axial and bi-axial compression with cyclic loading. The secondary loading cycles reveal complete irreversible damage of the microstructure, with the secondary loading path being characterised by near zero plateau stress. In force controlled tests, negligible hysteresis corroborates the idea that there is no energy dissipation owing to near complete microstructural damage. Furthermore, we observe no indications of preferential orientation of the microstructure in these tests. The stress responses in mutually perpendicular directions are seen to be identical, within statistical considerations. We then utilise the “pseudo-elastic” model developed and adopt it to the case of highly compressible Ogden strain energy formulation with a modified neo-Hookean for the unloading, with a view of fitting a continuum hyperelastic model to the experimental data. The material parameters obtained from uni-axial data are seen to be insufficient to describe the more general bi-axial deformation. The parameters obtained from the bi-axial test describe uni-axial deformation up to stretches of 0.5 but overestimate the stress levels beyond that point.

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2015
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-174593 (URN)10.1007/s10570-015-0753-5 (DOI)000364513800019 ()2-s2.0-84946497469 (Scopus ID)
Note

QC 20151207

Available from: 2015-12-07 Created: 2015-10-07 Last updated: 2017-12-01Bibliographically approved
3. Material properties of the cell walls in nanofibrillar cellulose foams from finite element modelling of tomography scans
Open this publication in new window or tab >>Material properties of the cell walls in nanofibrillar cellulose foams from finite element modelling of tomography scans
2017 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, no 24, p. 519-533Article in journal (Refereed) Published
Abstract [en]

The mechanical properties of the nanofibrillar cellulose foam depend on the microstructure of the foam and on the constituent solid properties. The latter are hard to extract experimentally due to difficulties in performing the experiments on the micro-scale. The aim of this work is to provide methodology for doing it indirectly using extracted geometry of the microstructure. X-ray computed tomography scans are used to reconstruct the microstructure of a nanofibrillar cellulose foam sample. By varying the levels of thresholding, structure of differing porosities of the same foam structure are obtained and their macroscopic properties of the uni-axial compression are computed by finite element simulations. A power law relation, equivalent to classical foam scaling laws, are fit to the data obtained from simulation at different relative densities for the same structure. The relation thus obtained, is used to determine the cell wall material properties, viz. elastic modulus and yield strength, by extrapolating it to the experimental porosity and using the measured response at this porosity. The simulations also provide qualitative insights into the nature of irreversible deformations, not only corroborating the experimental results, but also providing possible explanation to the mechanisms responsible for crushable behaviour of the nanofibrillar cellulose foams in compression.

Place, publisher, year, edition, pages
Springer Netherlands, 2017
Keyword
Nanofibrillar cellulose foam, Elastic modulus, Yield strength, X-ray tomography
National Category
Paper, Pulp and Fiber Technology
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-202575 (URN)10.1007/s10570-016-1179-4 (DOI)000394145500006 ()2-s2.0-85007236686 (Scopus ID)
Note

QC 20170228

Available from: 2017-02-28 Created: 2017-02-28 Last updated: 2017-11-29Bibliographically approved
4. Three-dimensional random structure representation for nanofibrillar cellulose foams: Validation and representative volume simulations
Open this publication in new window or tab >>Three-dimensional random structure representation for nanofibrillar cellulose foams: Validation and representative volume simulations
2017 (English)Report (Other academic)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-202576 (URN)
Note

QC 20170228

Available from: 2017-02-28 Created: 2017-02-28 Last updated: 2017-03-01Bibliographically approved

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