Download - ENGG108 Polymers and Ceramics and Composites
Structure and properties of polymers
What are polymers?
Includes plastics (polypropylene, nylon, PVC, polystyrene….), natural polymers (shellac, amber, cellulose…) and biopolymers (proteins etc).
Mechanical properties vary from one polymer to another, and depend on temperature and processing.
Mostly long-chain hydrocarbon molecules with:
Strong covalent bonds between the molecules along the chain, and
EITHER weak secondary bonds between the carbon chains
OR strong covalent bonds cross-linking adjacent carbon chains
If chains can be aligned, the structure can become partially crystalline.
But frequently the chains are entangled (amorphous)
Tensile deformation of a semi-crystalline polymer
• Elastic deformation:– Amorphous regions elongate
i.e. untangling of the carbon chains
• Plastic deformation:– Crystalline regions align
i.e. orientate in direction of stress
– Cold drawingCrystalline regions slide past each other andChains in the amorphous regions are drawn out
• BUT, to move molecules around (plastic deformation) needs time and temperature (see later)
Force vs. extension for polyethylene stretched at different strain-rates:
(ii)
Mechanical properties of polymers tend to be very sensitive to (i) temperature and (ii) rate of deformation:
PMMA
(i)
Visco-elastic behaviour
Loadcycle
Elasticbehaviour
Above the polymer’s “glass transition temperature” Tg, polymers show a visco-elastic behaviour as the polymer chains only slowly respond to the applied stress:
Viscoelastic
Viscous
PE low density, low strength & impact resistant, Tmelt ~ 120oC
Bottles, toys, film-wrap
PVC high strength, stiff unless plasticizer added, Tmelt~210oC
Flooring, pipes, wire insulation, old LPs
PTFE polytetrafluoroethylene -
high density, low stiffness, Tmelt ~327oC, chemically inert, good electrical insulator, low friction
Seals, pipes, bearings, high-T electronics
PP low density, Tmelt~175oC, cheap Bottles, packaging film, luggage
PS low ductility, cheap Wall tiles, toys, indoor lighting
PMMA medium strength, low ductility, transparent, weather resistant
Lenses, outdoor signs
Popular polymers
polyethylene
polyvinyl chloride
teflon
polypropylene
polystyrene
polymethyl methacrylate
http://video.google.co.uk/videoplay?docid=5994300959507748421&ei=SkL0SrTdPNCr-Aa7nt28BA&q=polymer+history+and+nylon&hl=en#
http://www.youtube.com/watch?v=7nCfbZwGWK8&feature=player_embedded
Polyethylene
HIGH DENSITY POLYETHYLENE
Linear carbon chains
Shorter, stronger secondary bond
Higher strength and density
High levels of crystallinity poss.
LOW DENSITY POLYETHYLENE
Branched carbon chains
Longer, weaker secondary bond
Lower strength and density
Low levels of crystallinity
PVC and polypropylene
PVC
# Very strong dipole (secondary bond)
# Strong, stiff, brittle
POLYPROPYLENE
# Regular side groups~>low density
# Weak secondary bond
# Strength due to entangling caused by side groups
Polystyrene & PMMA
POLYSTYRENE POLYMETHYLMETHACRYLATE
Strong dipole; large side group
Brittle; amorphous
(Further reading: William D Callister Chapters 14 & 15)
Structure and properties of ceramics
Types of ceramics
Glasses
Clayproducts Abrasives
Advancedceramics
Glasses
Glassceramics Fireclay Basic Cements
Structuralclay
productsWhite-wares Silica Special
Refractories
Ceramic materials
Inorganic, non-metallic materials: mostly compounds between metallic and non-metallic elements e.g. oxides, nitrides, carbides etc
Glasses• Glasses
– Non crystalline (amorphous) silicates
– contain other oxides (notably CaO, Na2O, K2O and Al2O3) which influence properties
• Glass ceramics– Fine grained polycrystalline
structure– Formed by heat treatment
of glasses– Better strength and thermal
shock resistance
Refractories and AbrasivesREFRACTORIES
Can withstand high temperatures without melting or decomposing
Used for furnace linings
ABRASIVES
Used to wear, cut or grind away material
Require high hardness plus some toughness and “refractoriness”
Mostly based upon alumina (Al2O3) and silica (SiO2)
Basic refractories based on MgO
Silicon carbide (SiC)Tungsten carbide (WC)Corundum (Al2O3)SilicaDiamond
Clays
Structural clay: bricks, tiles, sewer pipes
Whiteware: tableware, sanitary ware
A “plastic”, earthy natural mineral of alumina and silica found in the ground.
Formed from rocks that were slowly dissolved in water. When the solutions get supersaturated, tiny clay crystals start to grow from the solution.
Cement
Microstructure:
• voids (black)
• hydrated cement (grey)
• unhydrated cement (white)
Produced by mixing clay and lime in proportion and heating together (calcination)
Principle constituent are calcium silicates.
Hardening due to the hydration reactions of calcium silicates (chemical reactions with water) NOT from drying.
Constituent of concrete: added to bind aggregate particles (sand, gravel) to form a composite material.
At room temperature, most ceramics fracture before any plastic deformation …… --> catastrophic BRITTLE fracture
Deformation of Ceramics
PLASTIC deformation can occur (eg at high temps), but it is difficult as most ceramics are either:
(i) ionic crystals e.g. MgO, NaCl, ZnS etc - dislocations cannot easily move, or
(ii) covalent crystals e.g. silcates, aluminates (rock etc), WC etc with VERY strong bonds between atoms, or
(iii) non-crystalline (amorphous) e.g. fused/vitreous silica glass, - dislocations don’t exist. Plastic deformation is by viscous flow of atoms (like a very viscous liquid)
Strength determined by porosity levels.
For metals, the compressive strength is basically the same as the tensile yield strength. However, CERAMICS are much stronger in compression than in tension, as they are often full of micro-cracks (causing failure in tension). Hence tend not to use the tensile test (gripping ceramic test specimens without breaking them is also difficult)! Instead often use a standard 3-point bend test to measure elastic (Young’s) modulus, and rupture strength (stress at failure):
F
SiC 345 100-1100Al203 360 300-700Glass 70 100(steel 200 200-600)
Rupture stress(MPa)
Young’s modulus (GPa)
Concrete: compressive strength of concrete typically 15-40 MPa (c.f. a tensile strength of only 1-4 MPa), but only achieved after ~4 weeks of curing.
Tension/Compression Testing of Ceramics
World consumption of hydrocarbons (left column), metals, polymers, building materials and C-fibre composites:
Fig 20.1 in Ashby
Structure and properties of composite materials
from Ashby
Materials……….
Tyres are also composites: rayon cloth, steel bands and nylon belts all set in a matrix (binder) of rubber
Bundle of fiberglass
+ plastic
CFRP tail of an RC helicopter
Fabric made of woven carbon filaments
+ plastic
http://science.discovery.com/videos/how-its-made-bicycle-frames.html
What is a composite material?
Two or more individual materials (metal, polymer or ceramic) combined.
Frequently two phases (Matrix & Dispersed reinforcement)
Examples:FRP (fibreglass – glass fibre reinforced polymer)CFRP (carbon fibre)i.e. Ceramic fibres bonded together (reinforced) by a polymer resin
Principle of combined action:composite can exhibit combined properties of constituents, and allows property trade offs.
Example:Polymer (ductile, weak and flexible) + Ceramics (strong, stiff but brittle) Strong, stiff and tough composite.
(Ceramic fibres/particles strengthen & stiffen polymer matrix. The Matrix protects brittle fibres from damage.
Mechanical propertiesFor continuous-aligned long fibre-reinforced composites stressed in the longitudinal direction:
Elastic deformation…..
Summary: for continuous aligned fibre reinforced composites in the longitudinal direction:
Stage II – plastic deformation of matrix; elastic deformation of fibres
Stage I – elastic deformation of fibres and matrix• Modulus of composite:
Ec = EfVf + EmVm
where V is the volume fraction
• Strength of composite:
c = fVf + mVm
Example
• What is the modulus of a FRP containing 40% glass fibres modulus 69GPa in a resin modulus 3.5GPa?
Ec = EfVf + EmVm
Ec = 69 * 0.4 + 3.5 * 0.6
Ec = 27.6 + 2.1
Ec = 29.7 GPa
Ans: 5.6 GPa
Ec
Concrete• Aggregate particles (usually
60-80%) act as (cheap) filler to reduce the amount of cement (expensive)
• Fine particles of sand fill spaces between gravel
• Sufficient cement required to coat aggregate particles and bond them together.
• Water:
• Weak and brittle in tension, so increase strength by (steel) reinforcement
• Good bond between steel and concrete, and similar thermal expansion
Prestressed concrete:• Steel reinforcement stretched
before concrete is poured.• Release tension once concrete
has set Concrete placed into compression.
• Concrete now able to withstand tensile forces
Too little ~> incomplete bondingToo much ~> excessive porosity.Both ~> reduced strength
• a heterogeneous, hygroscopic, cellular and anisotropic material. • composed of fibres of cellulose (40% – 50%) and hemicellulose (15% – 25%) impregnated with lignin (15% – 30%).
WOOD
A diffuse-porous hardwood (Black walnut), showing vessels ("pores"), rays (white lines) and annual rings
Black locust end grain, showing the ring-porous structure.
Standard static-bending, compression, tension and shear testing used…. generally conducted at 12% moisture content and at 20°C.
Strength valueSitka
spruceHoop pine
Density [kg.m-3] 432.5 520.6
Static bending stress at elastic limit 42 MPa 56 MPa
Static bending rupture stress 72 MPa 90 MPa
Static bending modulus of elasticity 10 GPa 13 GPa
Compression parallel to grain:maximum crushing strength
38 MPa 48.7 MPa
Compression strength perpendicular to grain
5.6 MPa -
Tension strength parallel to grain[= modulus of rupture ]
72 MPa 90 MPa
Tension strength perpendicular to grain 0.9 MPa -
Mechanical testing of wood
e.g. soft-woods for structural parts of aircraft:
Parallel to grain
Density
Strength
Stiffness
Fracture Toughness