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Influence of inhomogeneities on the tensile and compressive mechanical properties of paperboard
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0003-0848-4305
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The in-plane properties of paperboard have always been of interest to paper scientists. Tensile properties are crucial when the board is fed through converting machines at high speeds. Compressive properties are essential in the later use. Inhomogeneities affect both the compressive and tensile properties. For the tensile properties, it is the inherent heterogeneity of the paperboard that might cause problems for the board-maker. Varying material properties, through the thickness of the paperboard, are on the other hand used to achieve high bending stiffness with low fiber usage. It is of interest to know how this practice affects the local compressive properties. Papers A and B aims to address this, while CD and E focus on in-plane heterogeneities. Paper A investigates the mechanism that causes failure in the short span compression test (SCT). It was concluded that the main mechanism for failure in SCT is delamination due to shear damage. In paper B the effect of the through-thickness profiles on the local compression strength was examined. It was concluded that the local compression is governed by in-plane stiffness and through thickness delamination. The latter was in turn dependent on the local shear strength and in-plane stiffness gradients. In paper C the tensile test is investigated with focus on sample size and strain distributions. The strain behavior was dependent on the length to width ratio of the sample and was caused by activation of local zones with high strainability. Paper D focuses on the strain zones seen in C. The thermal response in paper was studied. It was observed that an inhomogeneous deformation pattern arose in the paper samples during tensile testing. It was concluded that the heat patterns observed coincided with the deformation patterns. It could be shown that the formation was the cause of the inhomogeneous deformation. In final paper, E, the virtual field method was applied on data from C.

Abstract [sv]

Egenskaperna hos ett kartongark kan grovt delas upp i två kategorier: i-planet egenskaper och ut-ur-planet egenskaper. I-planet egenskaperna har länge varit ett område som pappersmekanister och andra pappersforskare visat intresse för. Anledningen till detta är att de är avgörande för hur väl det går att konvertera kartongen till färdiga förpackningar, samt hur väl de förpackningarna klarar sin uppgift. Dragegenskaperna prövas när kartongen dras genom tryck- och konverteringsmaskiner i hög hastighet. Tryckegenskaperna spelar stor roll för hur väl en förpackning klarar att staplas och hålla sitt innehåll intakt. Inhomogeniteter påverkar både drag och tryckegenskaper. Papprets naturliga variation påverkar dragegenskaperna hos kartongen och kan orsaka problem för kartongmakarna. Särskilt när utvecklingen går mot mer avancerade kartong utseenden. Å andra sidan så använder sig kartongmakare flitigt av egenskapsvariationer genom tjockleken på kartongen, när dom vill åstadkomma böjstyva kartonger utan att slösa med fibrer. I detta fall är det intressant att veta hur de lokala kompressionsegenskaperna påverkas av kartongens ut-ur-planet profil. Det första två uppsatserna i denna avhandling, A och B, handlar om just detta. Uppsatserna C, D och E avhandlar hur i-planet variationer påverkar kartongens egenskaper.

I Artikel A undersöks vilka skademekanismer som aktiveras under ett kortspannskompressionstest (SCT). Tre flerskiktskartonger undersöktes. De hade valts så att de hade distinkt olika skjuvstyrkeprofiler. Kartongerna karakteriserades och datan användes som materialdata i en finit element modell av SCT-testet. Modellen bestod av skikt, betraktade som kontinuum, mellan vilka det fanns kohesiva ytor. Huvudmekanismen i SCT var att kartongen delaminerade på grund av skjuvskador.

Den andra uppsatsen, Artikel B, var en fortsättning på den första. Denna gång undersöktes fem flerskiktskartonger framtagna så att de hade olika skjuvstyrka beroende på positionen i tjockleksled. Det konstaterades att kompressionsegenskaperna lokalt styrs av skjuvstyrkeprofilen och styvhetsgradienter. Vidare konstaterades det att mekanismerna innan kartongen delaminerar är, i huvudsak, elastiska.

Den tredje artikeln, Artikel C, fokuserade på hur dragprov på kartong påverkas av provstorleken och töjningsvariationen. Tre olika flerskiktskartonger användes som provmaterial och provbitar med olika storlek analyserades. Förutom dragprov så användes digital image correlation (DIC) för analysen. Det visade sig att den globala töjbarheten varierade med storleken på provet beroende på kvoten mellan längd och bredd. DIC visade att detta i sin tur berodde på att zoner med hög töjbarhet aktiverades i provet. Dessa zoner hade samma storlek oberoende av provstorlek och påverkade därför den totala töjbarheten olika mycket.

Artikel D undersöker töjningszonerna som sågs i Artikel C samt hur de påverkas av kreppning. Vidare undersöktes pappersproverna med hjälp av termografi. Termografin visade att varma zoner uppstod i proven när det töjdes. Zonerna blev synliga när provet töjdes plastiskt. Termografi kördes parallellt med DIC på några prover. Det visade sig att de varma zonerna överenstämde med zoner med hög lokal töjning. Vidare kunde det visas att dessa zoner övenstämde med papperets mikrostruktur, formationen. En finit element analys av hur papper med olika formation töjs gjordes. Delar av provningen gjordes på kreppade papper som har högre töjbarhet. Det visades sig att någon form av skada hade överlagrats på papprets mikrostruktur under kreppningen, och att den deformationen återtogs när pappret töjdes.

I den sista artikeln, Artikel E, behandlas hur VFM (Virtual Field Method) kan användas på DIC-data från kartong. DIC-datan som användes hämtades från Artikel C. Detta gjordes för att visa på hur olika VFM-formuleringar kan användas för att karakterisera styvhetsvariationen hos kartong. Provet delades upp i tre subregioner baserat på den axiella töjningsgraden. VFM-analysen visade att dessa subregioners styvhet och tvärkontraktionstal sjönk monotont, men att skillnaden mellan regionerna ökade med ökande spänning. även om endast ett prov undersöktes, så indikerade resultaten att områden med hög styvhet endast förbättrar de mekaniska egenskaperna marginellt. Analysen visade också att även om subregionerna inte är sammanhängande, så har dom liknande mekaniska egenskaper.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 39 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0596
Keyword [en]
Paperboard, Inhomogeneities, In-plane properties, Tension, Compression, Shear properties, Delamination
National Category
Paper, Pulp and Fiber Technology
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-185917ISBN: 978-91-7595-990-0 (print)OAI: oai:DiVA.org:kth-185917DiVA: diva2:924588
Public defence
2016-05-26, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Note

QC 20160429

Available from: 2016-04-29 Created: 2016-04-28 Last updated: 2016-05-04Bibliographically approved
List of papers
1. Investigation of shear induced failure during SCT loading of paperboards
Open this publication in new window or tab >>Investigation of shear induced failure during SCT loading of paperboards
2013 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 28, no 3, 415-429 p.Article in journal (Refereed) Published
Abstract [en]

In-plane compression has been analyzed experimentally and numerically using three machine made multiply paperboards. The paperboards had different shear strength profiles. Both short span compression (SCT) and long span compression (LCT) were performed. A finite element model of the SCT setup was developed, and the experimental results in MD and CD could be well predicted by the model. Using the model we could identify that the SCT failure was initiated by shearing of the interfaces in combination with the onset of plasticity in the loading direction. The model was used to make a parameter study. It showed that increased SCT values can be achieved by increasing the stiffness of the board or increase the failure displacement. The increase of stiffness was associated with ply properties, while the failure displacement was associated with interface properties.

Keyword
Paperboard, Elastic-plastic properties, In-plane compression, Shear
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-133503 (URN)10.3183/NPPRJ-2013-28-03-p415-429 (DOI)000325145900012 ()
Note

QC 20131106

Available from: 2013-11-06 Created: 2013-11-06 Last updated: 2017-12-06Bibliographically approved
2. Short compression testing of multi-ply paperboard, influence from shear strength
Open this publication in new window or tab >>Short compression testing of multi-ply paperboard, influence from shear strength
2016 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 31, no 1, 123-134 p.Article in journal (Refereed) Published
Abstract [en]

The influence of the through-thickness shear strength profiles on the short span compression test was examined. This was done both with experiments and finite element simulations on five industrial produced paperboards. It was concluded that the short span compression test is governed by in-plane stiffness and through thickness delamination The delamination damage was in turn dependent on the local transverse shear strength and in-plane stiffness gradients. Furthermore, it was concluded that the pre-delamination mechanisms were elastic. Finally it was possible to alter the results from the test by altering the shear strength of the paperboard; this should be done uniformly over the entire middle ply of the board if an increased SCT value was what was sought after.

Place, publisher, year, edition, pages
AB Svensk Papperstidning, 2016
Keyword
Paperboard, Elastic-plastic properties, In-plane compression, Shear
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-184987 (URN)000371253400014 ()2-s2.0-84961639787 (Scopus ID)
Note

QC 20160407

Available from: 2016-04-07 Created: 2016-04-07 Last updated: 2017-11-30Bibliographically approved
3. Investigation of sample-size effects on in-plane tensile testing of paperboard
Open this publication in new window or tab >>Investigation of sample-size effects on in-plane tensile testing of paperboard
2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 2, 295-304 p.Article in journal (Refereed) Published
Abstract [en]

The impact of sample size on in-plane strain behavior in paperboard was investigated, with the aim to explore the differences between local and global properties in paperboard, and try to pinpoint the mechanisms behind such differences. The local properties are of interest in converting as well as for future 3D forming of paperboard. It is important to identify differences in behavior between local and global properties since most paperboards are evaluated against the latter. The methods used for evaluation were tensile tests in controlled environment and speckle photography. The results show that there is a difference in strain behavior that is dependent of the length to width ratio of the sample, that this behavior cannot be predicted by standard tensile tests and that it depends on the board composition. The speckle analysis revealed that the behavior is a result of the activation of strain zones in the sample. These zones are relatively constant in size and therefore contribute differently to total strain in samples of different size.

Keyword
Size dependency, Paperboard, In-plane, Tensile test, Speckle photography
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-104883 (URN)10.3183/NPPRJ-2012-27-02-p295-304 (DOI)000315696000018 ()2-s2.0-84865202355 (Scopus ID)
Note

QC 20121114

Available from: 2012-11-14 Created: 2012-11-14 Last updated: 2017-12-07Bibliographically approved
4. Thermographical analysis of paper during tensile testing and comparison to digital image correlation
Open this publication in new window or tab >>Thermographical analysis of paper during tensile testing and comparison to digital image correlation
2016 (English)Report (Other academic)
Abstract [en]

The thermal response in paper has been studied using thermography. It was observed that an inhomogenous deformation pattern arose in the paper samples during tensile testing. In the plastic regime a pattern of warmer streaks could be observed in the samples. On the same samples digital image correlation (DIC) was used to study local strain fields. It was concluded that the heat patterns observed by thermography coincided with the deformation patterns observed by DIC. Due to the fibrous network structure paper has an inhomogenous microstructure, called formation. It could be shown that the formation was the cause of the inhomogenous deformations in paper. Finite element simulations were used to show how papers with different amount of homogeneity would deform. Creped papers, where the strain at break has been increased, were analysed. For these paper it was seen that an overlaid permanent damage was created during the creping process. During tensile testing this was recovered as the paper network structure was strained.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. 23 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0595
Keyword
Paperboard, Elastic-plastic properties, In-plane heterogeneity, Thermography, Digital image correlation, Formation, Finite element method
National Category
Paper, Pulp and Fiber Technology
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-185911 (URN)
Note

NQC 2016

Available from: 2016-04-28 Created: 2016-04-28 Last updated: 2016-04-29Bibliographically approved
5. Stiffness Heterogeneity of Multiply Paperboard Examined with VFM
Open this publication in new window or tab >>Stiffness Heterogeneity of Multiply Paperboard Examined with VFM
2016 (English)Conference paper, Oral presentation with published abstract (Other academic)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-185914 (URN)
Conference
SEM XIII International Congress and Exposition on Experimental and Applied Mechanics
Note

NQC 2016

Available from: 2016-04-28 Created: 2016-04-28 Last updated: 2016-04-29Bibliographically approved

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