structure of woods

STRUCTURE OF WOODS MATERIAL TECHNOLOGY

7740 Ravi Bhivra

7741 Aniket Devadiga

Outline

» What it is like at…?

» Types of woods…

» Structure of wood in detail.

» Stiffness of wood.

» Strength of wood.

» Research paper’s.

What it is like at “CELLULAR LEVEL” ?

• The basic unit of wood structure is the plant cell, which is the smallest unit of living matter capable of functioning independently.

• The cell has many functions, such as the manufacture of proteins, polysaccharides and mineral deposits. A plant cell varies in diameter from 10–100 μm.

• The main difference between the plant and animal cell is that plant cells have a cell wall outside the plasma membrane, which is 0.1 to 100 μm thick.

• This makes the cells rigid, among other effects prohibiting the locomotion typical of animals.

What it is like at “CELLULAR LEVEL” ?

http://www.doitpoms.ac.uk/tlplib/wood/structure_wood_pt1.php

http://www.doitpoms.ac.uk/tlplib/wood/wood_strength.php

What it is like at “MACROSCOPIC LEVEL” ?

• Wood has extreme anisotropy because 90 to 95% of all the cells are elongated and vertical (i.e. aligned parallel to the tree trunk).

• The remaining 5 to 10% of cells are arranged in radial directions, with no cells at all aligned tangentially. http://www.doitpoms.ac.uk/tlplib/wood/structure_wood_pt2.php

• The diagram shows a cut through of a tree trunk:


It’s Complete Structure

• The structure of the tree trunk has now been discussed at both the cellular and macroscopic scale.

• At the level of the complete structure, there is a further point of interest: the tree is prestressed due to wind which makes it necessary for the trunk to have two different type of material properties.

• The center of tree trunk is in compression, and the outer layers are in tension.

• The stressing is achieved as the inner sapwood shrinks as it dries and becomes heartwood.

• As the heartwood has lower moisture content it is better able to resist compression.

It’s Complete Structure



Types of wood

• There are basically two types of woods:-

1. “SOFTWOOD”

2. “HARDWOOD”

SOFTWOOD

• PINE

» ASH

• HICKORY

» BEECH

• BIRCH

» CEDAR

• REDWOOD

» HEMLOCK

• FIR

» SPRUCE

HARDWOOD• OAK

» MAPLE

• MAHOGANY

» CHERRY

• WALNUT

» ROSEWOOD

• TEAKhttp://www.hoovedesigns.com/woods.html

http://www.hoovedesigns.com/woods.html

The structure of wood “SOFTWOOD"

• A scanning electron micrograph of a softwood specimen is shown next.

• Softwoods consist mainly of long (3 to 5 mm) cells called trachoids which are about 20 to 80 x 10-6m.

• This structure mainly eliminates the need for vessels as the transport of the fluids are conducted by the rays.

• This in term increases the chances of softwood to undergo warping

• This is mainly due to the ability of the whole soft to transport water thus allowing it to get moisturised

The structure of wood “HARDWOOD”.

• Hardwoods consist mainly of two kinds of cells: Wood Fibers and Vessel Elements

• Wood fibers are elongated cells which are similar to trachoids except they are smaller, only 0.7 to 3 mm long and less than 20 x 10-6m in diameter, and they do not serve for fluid transport in the living tree.

• The vessel elements do serve for fluid transport in the living tree, and they can have a wide range of sizes.

• Due to this arrangement of fluid transport hardwood can easily resist warping and making it brittle as the trachoidsare small reducing its shock absorbing capacity.

RAYFIBERS

VESSELS

Structure of wood in a “Broader sense”

Earlywood or spring wood

• Formed during spring season.

• Formed early in a year.

• Consists of xylem tissues with wider vessels.

• Produced more in amount.

• Less dense.

• A broad zone of wood.

• Not as strong as late wood.

Latewood or autumn wood

• Formed during winter season.• Formed after the early wood.• Consists of xylem elements with

narrow vessels.• Produced less in amount.• More dense.• A narrow zone of wood.• Stronger than early wood due to

larger volume of wall materials.

Structure of wood in “DETAIL”

Earlywood & Latewood.

Structure of wood along its “FIBERS”

Why isn’t wood not used on a large scale though being easy to machine?

• Longitudinal direction: parallel to the long axis of the stem.

• Radial direction: perpendicular to both the growth rings and the long axis of the stem

• Tangential direction: tangent to the growth rings.

http://www.ce.berkeley.edu/~paulmont/CE60New/wood.pdf



Stiffness of wood

The stiffness of wood can be measured using a simple threepoint bend test as shown below:

http://www.doitpoms.ac.uk/tlplib/wood/wood_stiffness.php


Stiffness of wood

• The width (w) and height (h) of wood samples are measured, and the specimens are placed in a three-point bend testing apparatus with the height of the wood oriented vertically in the apparatus. The distance (L) between the two supports is also measured.

• The deflection of the middle of the beam, as a function of load on the pan of the apparatus, is measured to calculate the stiffness.

Stiffness of wood

• The resulting load (m) – displacement ( ) curves on loading and unloading for (a) balsa, (b) Scots pine and (c) greenheart are shown below

Stiffness of wood



Stiffness of wood

CALCULATION

Using the equation for the deflection of a material under symmetric threepoint bending:

The Young’s modulus for each sample is calculated from:

Strength of wood

• The strength of wood can also be determined by the very same method of “three point bend test”.



Strength of wood

• The width (w) and height (h) of wood samples are measured, and the specimens are placed in the three-point bend testing apparatus with the height of the wood orientated vertically in the apparatus.

• The distance (L) between the two supports is also measured.

• The wood samples are again loaded in 100 g increments.

• If the micrometer needle continues to move after a 100 g load has been added to the pan, the reading is allowed to stabilise before further mass is added.

• The mass on the pan is increased in this way until the sample fails.

• At this point the load and deflection of the sample before failure are noted.

Strength of wood

• By following this method and repeating for three samples of balsa, Scots pine and greenheart the following results were obtained:

Strength of wood

• To calculate the strength the following equation is used:

Any Questions.. ?

Thank You

structure of woods

Engineering