Action FP0802

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