Sem 2 - Composites Flashcards
Describe Pearlitic steel structure
microstructure of alternating cementite (Fe3C) and ferrite layers (α).
α is soft and ductile
Fe3C is hard and brittle
Generally describe Large particle strengthened composites
particulate phase is harder and stiffer than the matrix
particle tend to restrain movement of the matrix
its large because the particle interactions are not at atomic or molecular scale
Describe carbon black reinforced rubbers
Added to vulcanised rubber to enhance toughness and abrasion resistance.
The particles must be evenly distributed and form a strong adhesive bond with the matrix.
Tyres have about 15-30vol% of carbon black.
Advantages and disadvantages of cement
it is a pulverized material that develops binding force due to a reaction with water
It can be poured and set at room temperature even under water
Disadvantages:
relatively weak and brittle
tensile strength is only one tenth of the compression strength
large structure experience considerable thermal expansion and contraction with temperature
water can get into external pores and cause cracking in cold weather
Why is steel used in reinforcing concrete
they have similar thermal coefficients
it does not corrode in cement environments
relatively strong adhesive bonds forms
Describe dispersion strengthened materials
The matrix-particle interactions lead to strengthening to occur on the atomic level
The matrix bears a major portion of the load and particles hinder the motion of dislocations
Plastic deformation is restricted so yield and tensile strengths are improved
Factors which have a significant affect on strength in a composite material
- arrangement and orientation of fibres relative to each other
- fibre concentration
- distribution
2 stages of the stress strain graph of reinforced composite materials
Stage one
fibres and matrix deform elastically and this is linear
Stage two
the matrix yields and deforms plastically, fibres continue to stretch elastically
Tensile strength of the fibre is greater than the matrix
Onset of composite failure begins as fibres start to fracture corresponds of strain
How does composite failure differ from regular materials
It is not catastrophic as not all the fibres don’t fracture at the same time. The matrix stays intact, so the fractured fibres are still embedded in the matrix and are capable of sustaining a diminished load
What is transverse strength influenced by
Properties of the matrix and fibres
fibre-matrix bond strength
presence of voids
why are randomly orientated fibre composites used?
in applications with multidirectional stresses
reinforcement given is around 20% of an aligned composite but its the same in all directions
fabrication costa are lower and production rates are more rapid than continuous fibre reinforced composites
Describe the fibre phase of whiskers
V thin crystal strands with extremely large length to diameter ratio
virtually flaw free as it had a perfect crystalline structure
strongest known materials
They are rarely used as they are very expensive (graphite, silicon carbide and aluminium oxide)
describe the fibre phase of fibres
can be polycrystalline or amorphous
have small diameters and are usually polymers or ceramics
examples glass carbon boron aluminium oxides
Describe the fibre phase of wires
large diameter
used in the wire wound high pressure hoses
examples : steel molybdenum tungsten wire used in radial steel
Describe the matrix phase in fibre reinforced composites
the phase binds together the fibres and acts as a medium to transmit applied stress and distribute to fibres
Matrix is ductile
Elastic modulus of the fibres»_space; matrix
Protects fibre from mechanical abrasion and chemical reactions
separates fibres to stop crack propagation
