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Transverse anisotropy in softwoods: Modelling and experiments
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
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 [en]
Softwood, Cellular solid, Anisotropy, Deformation mechanisms
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-3988ISBN: 91-7178-385-7 (print)OAI: oai:DiVA.org:kth-3988DiVA: diva2:10271
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
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

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Citation style
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