composites & timber

Question 1

composite

Answer 1

combination of two or more materials
-matrix phase
-reinforcement phase
aim, to share best properties of each wit few of the disadvantages

Question 2

examples of composites

Answer 2

fibre reinforced polymers 
wood (natural fibre)
concretes 
particulate composites 
filler polymers 
lamellar composites

Question 3

Rules of mixtures

Answer 3

weighted mean of individual densities 
matrix
-desnsity Pm
- volume fraction Vm 
Reinforcement phase 
-density Pf
-volume fraction Vf

Pc=VfPf+(1-Vf)Pm

Question 4

specific properties of composites

Answer 4

if weight mas % given,
Vf=(Wf/Pf)/(Wf/Pt+Wm/Pm)

good specific strength may offset high cost in application

Question 5

polymer matrix composites

fibre types

Answer 5

glass
-brittle
carbon fibres 
-graphitic and amorphous regions 
aramid fibres 
-high strength and stiffness, molecules aligned in fibre direction 
-high tensile moduli and strengths 
-poorer performance in compression

Question 6

types of glass fibres -composites

Answer 6

A-glass

• soda-lime-silica
• ~14% Na2O ~10% CaO ~0-2% Al2O ~72%SiO2some insulating fibres

E-glass
-electrically poorly conducting
widely used as reinforcement

AR-glass
-alkali resistant

S-glass
-high strength and modulus

Question 7

softwoods

Answer 7

wood from coniferous or needle-leaved trees
-simpler structure than hardwoods
-cheaper (easily obtained)
-most common wood in construction (650 species, 50 in commercial use)
-eg, spruce, Douglas fir, larch
70% imported to uk

Question 8

hardwoods

Answer 8

from broad-leaved trees 
- slow growing - expensive 
-ege beech, oak, ash , mahogany, balsa wood
colour deepens over time 
more prone to warp, swell or split

Question 9

micro-structure of wood

Answer 9

-cellular, lined up along the length
-a bit irregular, has gaps,effectively porous
hardwoods
-has pores or vessels
-vertically aligned dead cells that carry liquid
softwoods
-dead single celled vertically aligned pipes or tracheids that carry liquid

Question 10

Anistrophy- wood

Answer 10

properties of wood are different in different directions
highest along the grain
lowest perpendicular fibres run along the grain - can be pulled apart easily

Question 11

knots - wood

Answer 11

darker circle in the bark

• formed where a branch is
• can be ornamental/aesthetics
• can make it difficult to cut or make holes

Question 12

cutting wood

Answer 12

anistropic means care needs to be taken when cutting, uneven properties can lead to warping
-plane saw is good in theory, but is susceptible to distortions. fibres are the edges are slightly distorted/bent
-quarter saw is used to achieve best properties
sawing direction can have affect on aesthetics

Question 13

moisture and drying in wood

Answer 13

all wood contains water
- sap when living
when cut
-greenwood 96% water, would distort easily
-drywood 12% which is chemically combined
drying is done in air or in kiln, causes shrinkage
normally 60-20% water

Question 14

degradation of wood

Answer 14

part of the ecosystem 
will degrade over a short period of time <2,5 
-fungi
-bacteria
-insects

Question 15

preservation of wood

Answer 15

nasty stuff
-oil based, creosote, wet environments eg bridges
-waterborne, fixed boned to wood, unfixed capable of being redissolved
-light organic solvent LOSP, combination of fungicides, insecticides
requires maintenance

Question 16

fire - wood

Answer 16

timber is combustible, but behavior is predictable
rate 20mm in 30 mins
can be used as a barrier

Question 17

issues with wood

Answer 17

• some sizes and shapes not easily obtained
• large panels require on large tree
• orientated grains mean beams differ from one another
• distortions

Question 18

plywood

Answer 18

made by joining thin flat layers together
-soaking or steaming might be required
layers are glued together
grains of alternating layer are perpendicular to each other -overcomes tendency to split
not aesthetically pleasing

Question 19

particle board

Answer 19

produced from wood separated into small parts (could be waste)
fragments combined with resin under pressure
tend to be denser, heavier
not aesthetically pleasing

Question 20

fibreboard

Answer 20

wood broken down into fibres - fibres mixed with resin and pressed t bond
medium density fibreboard is most common
not good structurally, sags over time

Question 21

glulam

Answer 21

small rectangular sections easily obtained, not long ones
-technique of sticking small sections together
-beams all aligned in same direction
easily produced in curved form
-useful for construction