Hardwood
and Softwood
Trees are divided into two classes: hardwoods and softwoods. The
hardwoods such as Teak, birch, maple have broad leaves. The terms
"hardwoods" and "softwoods" are not directly associated with the
hardness or softness of the wood although in most cases hardwoods are
actually harder and tougher than softwoods. In general softwoods
originate from cone-bearing trees and hardwoods from trees that have
their seeds contained in a seed-case.
Sapwood
Sapwood plays and important part in a tree's living process. In general
only the last few outside layers of sapwood are alive. The rest of the
sapwood carries moisture from the roots to the leaves and store food for
the tree. It usually have a higher moisture content than the heartwood.
Heartwood
During the life of the tree, sapwood gradually changes into heartwood.
As it does so, it becomes less permeable. Since moisture movement is
retarded considerably, heartwood dries more slowly than sapwood.
Heartwood usually surface checks and honeycombs more readily than
sapwood, hence requires milder drying conditions. It is usually darker
in color and also more resistant to decay than sapwood.
A : Cambium layer inside of inner bark
B : Inner bark
C : Outer bark
D : Sapwood
E : Heartwood
F : Pitch
G : Wood rays
Wood and Moisture
All wood in growing trees contains considerable quantities of water. It
is present in two forms, i.e. (a) Free-water in the cell cavities and
(b) Bound-water bounded to the cell walls. Most of this water should be
removed in order to obtain the satisfactory service from the wood in
use. The removal of free-water is much easer than that of bound-water
from the wood and there is no change in dimension or in strength
properties by removing free-water.
The wood is said to be in a state of Fiber Saturation Point (FSP) when
no free-water is present and the cell wall is saturated with
bound-water. In this state, moisture content in wood is between 25% and
30% and removal of water below fiber saturation point (FSP) results
dimensional change (shrinkage) and increase in many of the strength
properties.
Because of its hygroscopic nature wood is always seeking to equilibrate
its vapour pressure with that of the environment. That is wood always
try to maintain its Equilibrium Moisture Content (EMC) which correspond
to the environment. It is the unique characteristic of wood that expends
when it absorb moisture if its EMC is higher than its existing moisture
content, whereas contracts or shrinks when it loses moisture if its EMC
is lower than its existing moisture content.
Anisotropy of Wood
As wood possesses a complex fiber-composite structure, it varies in its
most properties with the directions, called anisotropy. Moreover its
difference in structure occurs not only between different species but
also between trees of the same species grown in different environments
or different parts of a single tree. This results undesirable variation
in its properties as a material.
The figure below shows three principal axles of wood with respect to
grain direction and growth rings.
The three principal axles of wood with respect to grain direction and
growth rings.
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WOOD