composites & timber Flashcards
composite
combination of two or more materials
-matrix phase
-reinforcement phase
aim, to share best properties of each wit few of the disadvantages
examples of composites
fibre reinforced polymers wood (natural fibre) concretes particulate composites filler polymers lamellar composites
Rules of mixtures
weighted mean of individual densities matrix -desnsity Pm - volume fraction Vm Reinforcement phase -density Pf -volume fraction Vf
Pc=VfPf+(1-Vf)Pm
specific properties of composites
if weight mas % given,
Vf=(Wf/Pf)/(Wf/Pt+Wm/Pm)
good specific strength may offset high cost in application
polymer matrix composites
fibre types
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
types of glass fibres -composites
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
softwoods
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
hardwoods
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
micro-structure of wood
-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
Anistrophy- wood
properties of wood are different in different directions
highest along the grain
lowest perpendicular fibres run along the grain - can be pulled apart easily
knots - wood
darker circle in the bark
- formed where a branch is
- can be ornamental/aesthetics
- can make it difficult to cut or make holes
cutting wood
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
moisture and drying in wood
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
degradation of wood
part of the ecosystem will degrade over a short period of time <2,5 -fungi -bacteria -insects
preservation of wood
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
fire - wood
timber is combustible, but behavior is predictable
rate 20mm in 30 mins
can be used as a barrier
issues with wood
- some sizes and shapes not easily obtained
- large panels require on large tree
- orientated grains mean beams differ from one another
- distortions
plywood
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
particle board
produced from wood separated into small parts (could be waste)
fragments combined with resin under pressure
tend to be denser, heavier
not aesthetically pleasing
fibreboard
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
glulam
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
