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Functional gradient effects explain the low transverse shear modulus in spruce: Full-field strain data and a micromechanics model
Laboratory of Structural Function, Research Institute for Sustainable Humanosphere, Kyoto University.
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.ORCID iD: 0000-0001-5818-2378
2009 (English)In: Composites Science And Technology, ISSN 0266-3538, Vol. 69, no 14, 2491-2496 p.Article in journal (Refereed) Published
Abstract [en]

An important failure mechanism in glulam beams is cracking caused by out-of-plane transverse loads. It has been demonstrated that the low transverse shear modulus G(RT) in spruce contributes to large transverse strain inhomogeneities due to the annual ring structure in combination with shear coupling effects. In the present study, improved understanding of annual ring effects is achieved by the development of a micromechanical model. It relates the functional density gradient in spruce annual rings to shear modulus GRT. The geometrical basis is a hexagonal cell model, and in shear it is demonstrated to deform primarily by cell wall bending. Full-field strain measurements by digital speckle photography (DSP) show very strong correlation with predicted shear strains at the annual ring scale. Predictions are obtained by implementation of the micromechanics model in a finite element (FE) model developed for the single cube apparatus shear specimen. The low GRT of spruce is due to the strong dependence of GRT on relative density rho/rho(s)(G(RT) proportional to (rho/rho(s))(3)). This is particularly important in spruce. Even though average density is typically quite high, the functional gradient structure includes local densities as low as 200 kg/m(3).

Place, publisher, year, edition, pages
2009. Vol. 69, no 14, 2491-2496 p.
Keyword [en]
Wood; Mechanical properties; Elastic properties; Finite element analysis (FEA); Digital speckle photography (DSP)
National Category
Wood Science Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-9603DOI: 10.1016/j.compscitech.2009.06.025ISI: 000271369900031Scopus ID: 2-s2.0-70349411525OAI: oai:DiVA.org:kth-9603DiVA: diva2:126674
Note
QC 20100830. Uppdaterad från manuskript till artikel (20100830).Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2010-08-30Bibliographically 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

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