Structure and Properties

Structure of thermoplastics (e.g polyethene)

• COVALENT BONDING between atoms/within molecules
• SECONDARY BONDING( E.G. VAN DE WAALS, H BONDING, DIPOLE-DIPOLE BONDING) between molecules -> provide strength to material

Structure of thermosets (e.g. epoxy resin)

• COVALENT BONDING between atom/ within molecule
• HIGH NUMBER OF COVALENT CROSSLINKS between polymer chain

Properties due to crosslinks

presence of crosslinks-> no plastic deformation

increase in no. of crosslinks->

• decrease in elastic deformation

• increase stiffness

• increase hardness

• increase strength

• increase brittleness

Structure of elastomer (e.g. rubber(polyisoprene))

• COVALENT BONDING between atoms/within molecules
• LOW NUMBER OF CROSSLINKS between long polymer chain

Structure of ceramics

• IONIC OR COVALENT BONDING between metallic and non-metallic element throughout whole structure

Properties of ionic bonded ceramics vs covalent bonded ceramics

• theoretically:
  ionic: non directional->undergo plastic deformation and slip and low stiffness
  covalent: directional-> cannot slip and high stiffness

• practically:
  ionic: high stiffness and brittle
  covalent: high stifffness and brittle

Definition of hardness

compression strength

• for non-brittle materials
• dimensions of indentation

Name of hardness test

Vichers Hardness Test

Definition of stiffness

resisting elastic deformation

• determined by elastic strain of Youngs Modulus/ elastic modulus

Definition of strength

stress to break/fail material

• stress (Pa)

Definition of tensile strength AKA ultimate tensile strength (UTS)

tensile stress to fracture/fail/neck material

Definition of compressive strength

YIELD STRENGTH(metal &plastic): stress to undergo plastic deformation
(yield stress<UTS stress)

FRACTURE STRESS(ceramics due to no slip): compressive stress to fracture material

Definition of ductility

degree of plastic deformation before fracture

• strain (%)

Definition of toughness

work required to fracture material

• brittle vs tough

Tough material for plastically deformable materials

• slip occurs before fracture
• slip absorbs energy
• make defect tip blunt
• local stress decrease

Tough material for non-plastically deformable materials

low number of defects+not sharp long or deep

• acts as stress concentrations
  -> local stress near defect>>global stress
• local bond break
  -low apparent stress

Definition of fatigue stress

resist failure under cyclic stress

• occur below UTS or yield stress
• affected by amplitude of alternating stress

Definition of corrosion resistance

rate of reaction with environment

Definition of abrasion resistance

avoidance of wear between sliding contact components

Definition of necking

decrease in cross-sectional area at failure point
conservation in volume
-> becomes longer and thinner

Result of necking

global load required to deform decreases

Proof strength

experimental proven stress value at 0.1% or 0.2% strain

Theoretical relationship of fracture stress and stiffness for non brittle material

δ=0.1E

Definition of viscoelasticity

time-dependent elastic behaviour

Definition of strain-rate sensitive

speed of strain affecting stiffness

Definition of isotropic

same properties in all direction

Can ionic bonds shear?

• non-directional bond
• has to slip twice to maintain low energy change distribution
  (one slip will have repulsion between like ions)
• bonds dont break

Can covalent bond shear

• highly directional bond, thus cannot slip

Factor affecting strength of thermoplastics

strength dependent on:

• weak intermolecular forces between covalently bonded molecules

types of intermolecular bonding

Van der Waals
H bonds
Secondary bonding(dipolar interaction)

Definition of dipole-dipole interaction

• a positive portion of a polar molecule is attracted to the negative portion of another polar molecule

• weak intermolecular bond

Definition of a Van der Waals bond

• molecules form temporary dipoles
• form weak intermolecular bonds

Can metallic bonds shear?

• non-directional
• easily slip

Structure of amorphous solid

• short range order over few molecular dimensions
• disordered arrangement of molecules (supercooled liquid)

Structure of crystalline solid

• long range order throughout solid
• ordered arrangement of molecules

Difference in MP of amorphous and crystalline solids

Crystalline: defined MP
Amorphous: undefined MP, soften slowly-> different amount of thermal energy needed to overcome different interactions

Formation of amorphous structure

cool liquid rapidly

• easier to do with molecules, due to more complex structures
  -> metal is unlikely amorphous

Types of material that is amorphous

Metal: assumed always crystalline

Ceramics: most are crystalline

Polymer:

• Thermopolymer: amorphous by default

  -> can change degree of crystallinity by processing

• thermosets & elastomer: amorphous
  -> crosslinks prevents ordering

Process of creating amorphous metal

rotating copper disc process

Process of increasing degree of crystallinity for thermopolymer

• strain polymer until before final failure
• large strain aligned the molecule

Change in properties from amorphous thermopolymer and crystalline thermopolymer

• density increase
• stiffness increase
• yield strength increase
• UTS stress increase
• toughness decrease
• ductility decrease

Definition of glass transition temperature

Temperature where a hard/glassy amorphous solid turns molten