the origin of maturation stress in tension wood · 2018. 6. 5. · physico-chemical changes during...
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Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
The origin of maturation stress in tension wood:
using a wide range of observations to assess hypothetic models
Tancrède Alméras, Bruno Clair, Joseph GrilLaboratory of Mechanics and Civil Engineering
CNRS - Univ. Montpellier (France)
Wood maturation and growth stresses in trees
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Wood maturation stress is set at the end of wood formation
• Induced tension (balanced by compression in inner wood)
• Accumulation of growth stress in the stem
Biomechanical function: optimisation of bending strength
#1
Wood maturation and growth stresses in trees
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Highly pre-stressed wood = reaction wood
• Asymmetric distribution of maturation stress around the stem
• Generates a bending moment
Biomechanical function: stem actuation
#2
Wood maturation and growth stresses in trees
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Technological consequences
• Field of growth stress cracks, warps
• Reaction woods associated to heterogeneity
#3
Consequences of maturation stress at the microscopic level
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Stress appears in cell-walls during their formation
• Detailed mechanism: not clear yet, several hypothetic models …
• Induces a microscopic heterogeneous stress field in the wall
In the tree:
stress in a layer
Cut wood sample:
stress contrast
Between cell-wall layers Between constituents of a layer
• Determines the “initial state” of wood as a material
Pre-stresses may have consequences on wood behaviour !
#4
Maturation stress generation in G-layer tension wood
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Tension wood mechanical design
• Many species: presence of a “gelatinous” layer (G-layer)
• Microfibrils parallel to the cell axis
• Unusual matrix composition
(no lignin, xyloglucans, AGP…)
S1
CML+P
S2
G
What does generate
large axial tensile stress
during its formation ?
#5
Maturation strain (%)
Implication of the G-layer
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Fang et al. 2008 - IAWA Journal
Amount of G-layer is correlated with maturation stress
Clair et al. 2003 - HolzforschungSimilar results:
Yamamoto et al. 2005 - J. Wood Sci.
#6
Implication of the G-layer
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Apparent paradox between structure and function…
• Cellulose is synthesised free of stress
• Crystalline cellulose is not modified after synthesis
• How is tension induced inside crystalline cellulose ???
S1
CML+P
S2
G
CytoplasmG-layer
CSC
#7
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Contraction of amorphous cellulose regions
Okuyama et al. 1994 Annals Forest Sci.
Yamamoto et al. 2004 Wood Sci. Tech.
Bamber 2001 IAWA Journal
#8
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Goswami et al. 2008 - The Plant Journal
Lateral swelling of G-layer and interaction between layers
#9
Clair et al. 2001 IAWA Journal
Clair et al. 2005 IAWA Journal
Selective enzymesAFM topography in water
Goswami et al. 2008 The Plant Journal
Microscopic stress field in tension wood cells
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
#10
Normal >>> Tension wood
Ta
ng
en
tia
lLo
ng
itu
din
al
Macroscopic drying shrinkage
Fang et al. 2007 Wood Sci. Tech.
Wet
Dry
Normal wood Tension wood
Effect of moisture changes
Clair et al. 2003 - Holzforschung
Drying shrinkage of layers
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
#11
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Bowling and Vaughn 2008 American Journal of Botany
Contraction of the matrix due to moisture changes
Presence of AGP
Lower water content
#12
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Mellerowicz et al. 2008 Annals of botany
Formation of microfibril aggregates
Interaction with xyloglucans
#13
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Other mechanisms involving interaction between fibrils
Matrix shrinkage
+ stiff zones
#14
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Other mechanisms involving interaction between fibrils
Active bridges ?
#15
Hypothetic mechanisms
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Other mechanisms involving interaction between fibrils
Matrix swelling
+ stiff bridges
#16
Micro-mechanical model of maturation stress generation
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Embedded network model
Multilayer cell model
Dimensional change in the constituents
Macroscopic stress/strains
Alméras et al. 2005 Holzforschung
#17
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Embedded network model
Multilayer cell model
Microscopic stress/strains
Dimensional change in the constituents
In vivo boundary conditions
Kinetics of layer formation
Microfibril network structure
Stiffness of the constituents
Macroscopic stress/strains
Physico-chemical changes during maturation
Chemical composition
Thickness and MFA of each layer
Stiffness of each layer
Stress induced in each layer
Model parameters and needed data#18
Workshop “Experimental and computational methods in wood micromechanics” May 11-13, 2009
Thank you for your attention…
Embedded network model
Multilayer cell model
Microscopic stress/strains
Dimensional change in the constituents
In vivo boundary conditions
Kinetics of layer formation
Microfibril network structure
Stiffness of the constituents
Macroscopic stress/strains
Physico-chemical changes during maturation
Chemical composition
Thickness and MFA of each layer
Stiffness of each layer
Stress induced in each layer
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