"Natural" composites - timber and wood products

Question 1

What type of composite can wood be classified as?

Answer 1

Wood can be classified a type of natural composite material:
- small scale structure - strong fibres in a softer matrix (a polymer - polymer composite)
- larger scale structure - contain porosity, like a foam or sponge (a polymer-air composite)

Question 2

What is softwood?

Answer 2

Wood from coniferous or needle-leaved trees
-> simpler structure than hardwoods
-> generally cheaper - more easily obtained and worked with

Question 3

What is hardwoods?

Answer 3

Woods from broadleaved trees. Generally slow growing and more expensive.

Question 4

What are some common species of softwood?

Answer 4

• spruce
• Douglas fir
• larch

Question 5

What percentage of softwood is imported into the uk?

Answer 5

70% imported to UK

Question 6

What are some common species of hardwoods?

Answer 6

• beech
• oak
• ash
• mahogany -> tropical hardwoods are often evergreens
• balsa wood -> grows very quickly hence high porosity

Question 7

What are some cons to hardwoods?

Answer 7

• colour deepens over time
• more prone to warp, swell or split

Question 8

What is the microscope structure of wood?

Answer 8

On a microscope scale the structure is cellular, mostly lined up along the length, effectively a porous material

Question 9

What is the microscopic structure of hardwood (oak)?

Answer 9

Notes pores or vessels
Vertically aligned dead cells that carry liquid

Question 10

What is the microscopic structure of softwood (pine)?

Answer 10

Dead single celled vertically aligned pipes or tracheid’s carry liquid

Question 11

What is an anistropy?

Answer 11

The properties of wood are different in different directions. They are highest along the grain (lowest at right angles to the grain) “fibres” run along the grain

Question 12

What is a knot?

Answer 12

Formed when a branch is enclosed within the growing truck, can be ornamental, can be defects

Question 13

What are some anisotropic properties?

Answer 13

Question 14

Cutting wood?

Answer 14

Underlying anisotropic structure means care needs to be taken when sawing planks (can get uneven properties and warping). Quarter sawn used to get best properties. Sawing direction affects the appearance of wood appearance (hence other orientations may be used)

Question 15

What method of sawing wood gives the best properties?

Answer 15

Quarter sawn used to get best properties

Question 16

All wood contains water, when does sap contain water?

Answer 16

When living

Question 17

All wood contains water, when does Greenwood contain water?

Answer 17

Greenwood (as cut) - very significant

Question 18

All wood contains water, when does drywood contain water?

Answer 18

Drywood - 12% water which is chemically combined

Question 19

Where is drying done?

Answer 19

Drying is done in air or a kiln - causes shrinkage and changes properties -> in use water content will vary with relative humidity.

Question 20

Without proper treatment how long will wood last?

Answer 20

Without proper treatment wood may degrade over a relatively short period (0 to 25 years)

Question 21

Why will wood degrade?

Answer 21

• Fungi
• Insects
• Bacteria

Question 22

What are some preservative treatments available?

Answer 22

• Oil-based - e.g. creosote particularly for wet environments; bridge timbers
• Waterborne - Fixed (bonded to wood), unfixed (capable of seeing redissolved)
• Light organic solvent (LOSP) - Combination of fungicides and insecticides

Question 23

Timber in fire:

Answer 23

Timber is combustible but behaviour is predicable. Typical rate is 20mm in 30 minutes - effective barrier to fire for this time

Question 24

What are some properties of wood?

Answer 24

Strength
-> affected by density, drying and by defects present
-> strength across the grain may be 1/20th of that along the grain
Fracture
-> wood does not crack easily - it is tough
-> pulling apart of fibrous strength absorbs energy

Question 25

Why is it better to use wood products?

Answer 25

Wood has many advantages but some sizes and shapes cannot easily be obtained (e.g. large panels require old trees), oriented grain means that properties are variable from one beam to the next. A solution is to use wood to produce other materials - wood products.

Question 26

What is plywood?

Answer 26

Made by joining thin, flat layers together
- such layers are called veneers
- they are produced by rotary cutting of small trees
- soaking or steaming may be needed
The layers are glued together. The grain of alternate layers is orientated perpendicular to each other, which helps to overcome the tendency to split.

Question 27

What is plastic boards?

Answer 27

Produced from wood separated into small fragments -> may be waste from wood machining
Fragments combined with resin under pressure to make a board -> depending on fragment size produce:
- chipboard
- strandboard - strandboard flakes are oriented in the plane of the sheet.

Question 28

What is a fibreboard?

Answer 28

Wood is broken down wood to its fibres by grinding. The fibres are then:
- mixed with resin
- pressed at temperature to bond
medium-density fibreboard (MDF) is the most common variety, however, it is not suitable for structural application. Sags over time under static loads.

Question 29

What is Glulam?

Answer 29

Small rectangular sections of wood are easily obtained, but larger beams are not. GLUe LAMination is the technique of sticking these small sections together
-> ends of the beam are finger-jointed
-> beams are aligned in the same direction
Laminates are easily produced in curved form
-> useful for construction
-> Members 40+ m long by 2.5m wide have been made

Question 30

What is cross-laminated timber?

Answer 30

Cross-ply laminated board product. Bonded together with adhesive pressure applied during bonding. Can be used to build multi-storey structures used in structures.