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A two-phase annual ring model of transverse anisotropy in softwoods
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.ORCID iD: 0000-0001-5818-2378
2008 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 14, 3020-3028 p.Article in journal (Refereed) Published
Abstract [en]

Transverse anisotropy in softwoods is an important phenomenon of both scientific and industrial interest. Simple one-phase hexagonal honeycomb cell models for transverse moduli of softwoods are based on cell wall bending as the only deformation mechanism. In the present study, a two-phase annual ring model is developed and includes both cell wall bending and stretching as deformation mechanisms. The proportion of cell wall bending and stretching for different cases is analysed and the importance of stretching is confirmed. A two-phase annual ring model is presented based on fixed densities for earlywood and latewood. Such a model is motivated by the large difference in density between earlywood and latewood layers. Two-phase model predictions show much better agreement with experimental data than predictions from a one-phase model. Radial modulus is dominated by bending at low density and by stretching at high density. For tangential modulus, bending is more important at all densities.

Place, publisher, year, edition, pages
2008. Vol. 68, no 14, 3020-3028 p.
Keyword [en]
Wood, Microstructure, Anisotropy, Cell model, Micromechanics
National Category
Wood Science Mechanical Engineering Dentistry
Identifiers
URN: urn:nbn:se:kth:diva-9600DOI: 10.1016/j.compscitech.2008.06.022ISI: 000261017300027Scopus ID: 2-s2.0-56949103498OAI: oai:DiVA.org:kth-9600DiVA: diva2:126670
Note
QC 20100825Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Micromechanics of softwoods in the transverse plane: effects on cell and annual ring scales
Open this publication in new window or tab >>Micromechanics of softwoods in the transverse plane: effects on cell and annual ring scales
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Transverse mechanical properties of wood are important in many practial applications and an interesting scientific subject. A very low transverse shear modulus has been identified in spruce, which causes large strain concentrations in wood structures. In this thesis, experimental characterization of local density variations as well as local strain fields are carried out using the SilviScan apparatus and digital speckle photography, respectively. This is combined with micromechanical modeling based on hexagonal wood cells in combination with finite element analysis. Problems addressed include the moduli in the transverse plane, including variations at the scale of individual annual rings. The relative importance of cell wall bending and stretching deformation mechanisms is analysed as a function of wood cell geometry, relative density and direction of loading (radial, tangential and shear). Transverse anisotropy is also analyzed, including its dependency of earlywood and latewood characteristics. The wood cell shape angle variation and density effects are sufficient to explain transverse anisotropy in softwoods (no ray effects), and the influence of earlywood/latewood ratio is explained. As a practical test method for shear modulus measurements, an off-axis compression test with full-field strain determination is proposed. The advantage is a simple fixture and large region of representative strain required for a heterogeneous material such as wood. As an alternative, the single cube apparatus (SCA) for shear tests is evaluated. The SCA is used to determine the shear strain distribution within the annual rings. Based on the density distribution of the shear test specimen and a micromechanics model, a finite element model is developed, and predictions are compared with the measured shear strains. The agreement between predicted and measured shear strains at the annual ring scale are remarkably good. It shows that the low GRT of spruce is due to the low earlywood density and the large cell wall bending deformation resulting from shear loading. Furthermore, it illustrates the need for improved understanding of annual ring scale effects. For example, fairly low transverse global loads will lead to lage local shear strains.

Abstract [sv]

Transversella mekaniska egenskaper hos trä är viktiga i många praktiska tillämpningar och är av vetenskapligt intresse. Gran har exemplevis mycket låg transversell skjuvmodul, vilket leder till stora lokala töjningskoncentrationer i trästrukturer. I den här avhandlingen utförs experimentella mätningar av densitetsfördelning och lokal töjningsfördelning med hjälp av SilviScan utrustning (röntgen) och digital speckelfoto grafi (DSP). Det kombineras med mikromekanisk modellering med hexagonala cellmodeller som utgångspunkt, ibland i kombination med finita elementberäkningar. Transversella moduler bestäms liksom töjningseffekter på skalan individuella årsringar. Den relativa betydelsen av böjning och sträckning av cellväggen analyseras som funktion av relativ densitet och belastningsriktning (radiell, tangentiell och skjuvning). Stor andel böjdeformation ger låg modul och proportionerna mellan de båda mekanismerna styr graden av anisotropi. Transversell anisotropi analyseras därför, inklusive dess beroende av karakteristiken hos vårved och sommarved. Formvinkeln på vedcellen och inverkan av densitet är tillräckliga för att förklara graden av anisotropi (utan inverkan från märg- strålar). Inverkan av förhållandet mellan mängden vårved och sommarved på anisotropin analyseras särskilt. En enkel tryckbelastningsmetod (“off-axis metod”) används för att bestämma transversell skjuvmodul hos trä. Metoden kombineras med DSP. Fördelen är en enkel fixtur i kombination med det stora område av ren skjuvdeformation som uppstår i provstaven. Som ett alternativ utvärderas också en metod baserad på kubiskt prov (SCA). Metoden används för att bestämma lokala skjuvtöjningar på skalan individuella årsringar. Baserat på densitetsfördelningen i provet och en mikromekanisk modell så utvecklas en finita element-modell. Den utnyttjas för att beräkna lokala skjuvtöjningar. Jämförelsen mellan beräkningar och uppmätta skjuvtöjningar ger enastående god överensstämmelse. Det visar att den låga skjuvmodulen för gran orsakas av låg densitet i kombination med att böjning av cellväggarna dominerar som deformationsmekanism. Det illustrerar också att vi behöver förbättra vår förståelse för deformationsfält på årsringsnivå. En praktisk konsekvens är t ex att relativt låga globala laster ger upphov till mycket hög lokal skjuvdeformation.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. x, 21 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2008:64
National Category
Wood Science Mechanical Engineering Dentistry
Identifiers
urn:nbn:se:kth:diva-9604 (URN)978-91-7415-181-7 (ISBN)
Public defence
2008-12-18, Hörsal Flodis (F3), KTH, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20100830Available from: 2008-12-04 Created: 2008-11-19 Last updated: 2010-08-30Bibliographically approved
2. Transverse anisotropy in softwoods: Modelling and experiments
Open this publication in new window or tab >>Transverse anisotropy in softwoods: Modelling and experiments
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Transverse anisotropy is an important phenomenon of practical and scientific interest. Although the presence of ray tissue explains the high radial modulus in many hardwoods, experimental data in the literature shows that this is not the case for pine. It is possible that anisotropy in softwoods may be explained by the cellular structure and associated deformation mechanisms.

An experimental approach was developed by which local radial modulus in spruce was determined at sub-annual ring scale. Digital speckle photography (DSP) was used, and the density distribution was carefully characterized using x-ray densitometry and the SilviScan apparatus. A unique set of data was generated for radial modulus versus a wide range of densities. This was possible since earlywood density shows large density variations in spruce. Qualitative comparison was made between data and predictions from stretching and bending honeycomb models. The hypothesis for presence of cell wall stretching was supported by data.

A model for wood was therefore developed where both cell wall bending and stretching are included. The purpose was a model for predictions of softwood moduli over a wide range of densities. The relative importance of the deformation mechanisms was investigated in a parametric study. A two-phase model was developed and radial and tangential moduli were predicted. Comparison with experimental data showed good agreement considering the nature of the model (density is the only input parameter). Agreement is much better than for a regular honeycomb model. According to the model, cell wall bending dominates at both low and high densities during tangential loading. In radial loading, cell wall stretching dominates at higher densities.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 9 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2006:30
Keyword
Softwood, Cellular solid, Anisotropy, Deformation mechanisms
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-3988 (URN)91-7178-385-7 (ISBN)
Presentation
2006-06-08, S40, Farkost och flygteknik, Teknikringen 8, Stockholm, 13:15
Opponent
Supervisors
Note
QC 20101119Available from: 2006-05-22 Created: 2006-05-22 Last updated: 2010-11-19Bibliographically approved

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