wood,wood macrostructure,wood microstructure, bound water and free water in the wood, mechanical properties

1. Wood
Macrostructure & Microstructure
Praveen P
Department of Materials Science
Mangalore University
Karnataka, India
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2. Wood….
 Most widely used engineering construction material
 Naturally occurring composite material
 Consists complex array of cellulose cells reinforced by a
polymeric substance, lignin and other organic compounds.
 Wood is a heterogeneous, hygroscopic, cellular and
anisotropic material. It is composed of cells, and the cell
walls are composed of micro-fibrils of cellulose (40% – 50%)
and hemicellulose (15% – 25%) impregnated with lignin
(15% – 30%)
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3. Wood….
Natural product with complex structure
Highly anisotropic, not homogeneous
Posses high tensile strength in the
direction parallel to tree stem
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4. Cross section of a typical tree
a. Outer bark layer: Dry, dead tissue and
provides external protection for the tree.
b. Inner bark layer: carries food from the
leaves to all the growing parts.
c. Cambium layer: Tissue layer between the
bark and wood that forms the wood & bark
cells.
d. Sapwood: Light coloured wood which
forms outer part of the tree stem. Contain
some living cells which function for food
storage and carry water from root to leaves.
e. Heartwood: Old inner region of the tree
stem which is no longer living. It is darker
than sapwood and provides strength for the
tree.
f. Pith: Soft tissue at the centre of the tree
around which the first growth of the tree takes
place.
g. Wood rays: Connect the tree layers from
pith to the bark and function is food storage
and transfer of food. 4

5. Classification
Trees are classified in to two major groups
Softwoods (Gymnosperms) and Hardwoods (Angiosperms)
Softwood Hardwood
Tree seed is exposed Seed is covered
Retains its leaves throughout the year Sheds its leaves annually
Evergreen trees Deciduous trees
Physically soft Physically hard
Light colour Dark coloured
Examples: pine, spruce etc Examples: Mahogany, Oak, Teak
etc
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6. Annual growth rings
During each growth season
a new layer of wood is
formed annually around the
tree stem. This layers are
called annual growth rings.
Each ring has two sub
rings: earlywood (spring)
and latewood (summer). In
softwoods the earlywood
has a lighter colour and the
cell size is larger.
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7. Axes of symmetry for wood
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8. Microstructure of wood
Explain the microstructure of wood? Explain the effect of water absorption on the properties of
wood?
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9. Microstructure of a softwood
Three complete growth rings can be seen
Earlywood: larger size cells
Softwood consists mainly of long, thin walled tubular
cells called tracheids.
The length of a longitudinal tracheid is about 3 to 5
mm and and its diameter is about 20 to 80
micrometer.
The longitudinal tracheids constitute about 90%
volume of the softwood.
The large open space in the centre of the cell is
called the lumen and is used for water conduction.
The earlywood cells have a relatively large diameter,
thin walls and a large size lumen.
The latewood cells have a smaller diameter and
thick walls with a smaller lumen.
Wood rays which run in the transverse directions
from the bark to the centre of the tree consist of an
aggregate of small parenchyma cells that are
bricklike in shape and which are used for food
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storage.

10. Microstructure of Hardwood
 Have large diameter vessels for the conduction of fluids.
 The vessels are thin walled structures consisting of
individual elements called vessel elements and are formed in
the longitudinal direction of the tree stem.
 The wood of hardwood trees is classified as either ring-
porous or diffuse-porous depending on how the vessels are
arranged in the growth rings. In a ring-porous hardwood the
vessels formed in the earlywood are larger than those
formed in the latewood.
 In a diffuse-porous hardwood, the vessel diameters are
essentially the same throughout all the growth rings.
 The longitudinal cells in the hardwood tree stem are fibers.
 These are elongated cells with close pointed ends and are
usually thick walled.
 Length of this fibers are about 0.7 to 3 mm and average
diameter is 20 micrometer.
 The food storage cells of hardwood are the ray
(transverse) and longitudinal parenchyma which are brick or
box shaped.
 The rays for hardwoods are usually much larger than for
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softwood, having many cells across their width.

11. Wood-Moisture Relations
Moisture content: Wood is a hygroscopic
material, i.e., it will absorb water vapor
from the atmosphere.
Water may be held in wood in two ways:
Moisture content in wood is expressed as a
percentage of ovendry weight. A moisture (1) bound water, & (2) free water.
content of 50% means that there are 50 • Bound water is held within the cell walls
parts of water to 100 parts of dry wood by adsorption force.
substance by weight. – It is generally believed that bound water
is not in the crystalline regions of the cell
wall, but is adsorbed in the amorphous
regions.
– This has important implications for the
volume changes associated with moisture
changes.
• Free water is not held by any forces and
is situated in the cell cavities know as
lumen. 11

12. Fiber saturation point
The moisture content at which the cell wall is saturated with bound
water & at which no free water is present is called the fiber
saturation point, (FSP).
• The FSP varies from species to species, but it averages about
28% moisture content.
• Addition or removal of water below the FSP has a pronounced
effect on practically all wood properties.
• Addition or removal of water above the FSP has a almost no effect
on any wood properties.
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13. Shrinkage and swelling
• The variation of shrinkage between
different directions can be attributed
to the microstructure of wood.
• The latewood cells dominate
shrinkage since they absorb much
more water and in the tangential
direction there is an unbroken
alignment of latewood.
The greater shrinkage in the tangential
direction causes distortion in lumber with
different orientations.
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14. Effects of Moisture on the Strength of Wood
• The strength of wood is constant above the fiber saturation point.
• Below the fiber saturation the strength of wood increases with decreasing
moisture content. This can be related to where the water is absorbed in the
microstructure.
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15. Mechanical Properties of Wood
Effects of Moisture on the Strength
of Wood
The strength of wood
is constant above the fibre
saturation point.
Below the fibre
The strength of wood is highly saturation the strength of wood
dependent upon direction: increases with decreasing
Tensile strength values in moisture content. This can be
longitudinal:radial:tangential directions related to where the water is
on average are in the ratio of 20:1.5:1 absorbed in the microstructure.
The variation of strength
between different directions can be
attributed to the fine structure of the
wood cells.
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