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Micromechanics of softwoods in the transverse plane: effects on cell and annual ring scales
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
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: urn:nbn:se:kth:diva-9604ISBN: 978-91-7415-181-7 (print)OAI: oai:DiVA.org:kth-9604DiVA: diva2:126704
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
List of papers
1. Elastic deformation mechanisms of softwoods in radial tension: Cell wall bending or stretching?
Open this publication in new window or tab >>Elastic deformation mechanisms of softwoods in radial tension: Cell wall bending or stretching?
2008 (English)In: Holzforschung, ISSN 0018-3830, Vol. 62, no 5, 562-568 p.Article in journal (Refereed) Published
Abstract [en]

Radial softwood modulus ER is typically twice as high as the tangential modulus ET. The reason for this is unclear, although cell geometry is likely to contribute. The established hexagonal honeycomb model for prediction of ER is based on a cell wall bending mechanism only. If cell wall stretching also takes place, the dependence of ER on relative density will be different. If experimental data for ER as a function of relative density show deviations from cell wall bending predictions, this may indicate the presence of cell wall stretching. A SilviScan apparatus is used to measure density distribution. A procedure by means of digital speckle photography is then developed for measurements of local ER within the annual rings of spruce. Comparison is made between experimental data and the two expected density dependencies from cell wall bending and from stretching. The hypothesis of cell wall stretching as a contributing mechanism is supported based on the observed linear dependence of ER over a wide density range.

Keyword
Softwoods, Cell walls, Deformation mechanisms
National Category
Wood Science Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-9595 (URN)10.1515/HF.2008.082 (DOI)000258963400011 ()2-s2.0-48649105298 (Scopus ID)
Note
QC 20100827Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2010-08-30Bibliographically approved
2. A two-phase annual ring model of transverse anisotropy in softwoods
Open this publication in new window or tab >>A two-phase annual ring model of transverse anisotropy in softwoods
2008 (English)In: Composites Science And Technology, ISSN 0266-3538, 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.

Keyword
Wood, Microstructure, Anisotropy, Cell model, Micromechanics
National Category
Wood Science Mechanical Engineering Dentistry
Identifiers
urn:nbn:se:kth:diva-9600 (URN)10.1016/j.compscitech.2008.06.022 (DOI)000261017300027 ()2-s2.0-56949103498 (Scopus ID)
Note
QC 20100825Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2010-11-19Bibliographically approved
3. Method to determine the transverse shear modulus (GRT) of softwoods using full field strain measurements in off-axis compression
Open this publication in new window or tab >>Method to determine the transverse shear modulus (GRT) of softwoods using full field strain measurements in off-axis compression
(English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835XArticle in journal (Other academic) Submitted
National Category
Wood Science Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-9601 (URN)
Note
QS 20120326Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2012-03-26Bibliographically approved
4. The single cube apparatus for shear testing: Full-field strain data and finite element analysis of wood in transverse shear
Open this publication in new window or tab >>The single cube apparatus for shear testing: Full-field strain data and finite element analysis of wood in transverse shear
2009 (English)In: Composites Science And Technology, ISSN 0266-3538, Vol. 69, no 7-8, 877-882 p.Article in journal (Refereed) Published
Abstract [en]

The design and analysis of wood structures require accurate data for shear properties, where transverse shear in particular has been neglected in the past. The single cube apparatus (SCA) was applied to transverse shear of Norway spruce (Picea Abies), due to the importance of this species in wood structures, such as glulam, and also its allegedly low value of GRT . Full-field strain data and FEA were used to analyze the potential of the method. The presence of a large central region of homogeneous and close to pure shear strain was confirmed. The SCA method is therefore a strong candidate for improved shear test procedures in wood and other materials, where porosity (gripping problems), heterogeneity on mm-scale and polar orthotropy (annual ring curvature) may cause particular difficulties. In contrast to many other shear test studies, the accuracy of the present GRT data is supported by documented large and homogeneous specimen stress- and strain-fields in almost pure shear, direct measurements of strain field, and careful stress analysis based on FEA.

Keyword
Wood, Mechanical properties, Anisotropy, Elastic properties, Digital speckle photography (DSP)
National Category
Wood Science Mechanical Engineering Dentistry
Identifiers
urn:nbn:se:kth:diva-9602 (URN)10.1016/j.compscitech.2008.11.013 (DOI)000266380700001 ()2-s2.0-64849092622 (Scopus ID)
Note
QC 20100827. Uppdaterad från accepted till published.Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2010-08-30Bibliographically approved
5. Functional gradient effects explain the low transverse shear modulus in spruce: Full-field strain data and a micromechanics model
Open this publication in new window or tab >>Functional gradient effects explain the low transverse shear modulus in spruce: Full-field strain data and a micromechanics model
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).

Keyword
Wood; Mechanical properties; Elastic properties; Finite element analysis (FEA); Digital speckle photography (DSP)
National Category
Wood Science Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-9603 (URN)10.1016/j.compscitech.2009.06.025 (DOI)000271369900031 ()2-s2.0-70349411525 (Scopus ID)
Note
QC 20100830. Uppdaterad från manuskript till artikel (20100830).Available from: 2008-11-19 Created: 2008-11-19 Last updated: 2010-08-30Bibliographically approved

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