Materials

Materials

1. Describing materials

   • ceramics
     • hard
     • Brittle
     • stiff
   • metals
     • pure metals
       • soft
       • malleable
       • ductile
     • metal alloys
       • harder
   • Polymers
     • glassy polymers
       • replace glass
     • semi-crystalinne polymers
       • tough

2. Stretching wires and springs

   • Hooke's Law
     • F=kx
       • k = spring constant
         • stiffness
   • Force - extension graph
     • linear part
       • elastic deformation
         • energy stored in spring
           • E=1/2kx^2
     • elastic limit
     • non-linear part
       • plastic deformation
     • fracture point

3. stress, strain, and the Young modulus

   • stress = force per unit area
     • fracture stress
       • stress at point of breaking
     • yield stress
       • stress at point of elastic limit
     • 1 Pa = 1N/m^2
   • strain = extension/original length
     • no units (dimensionless)
     • ratio
   • stress-strain graph
     • linear for elastic deformation
   • Young Modulus E=stress/strain
     • units = Pa
     • often very large

4. Choosing Materials

   • selection charts
     • logarithmic
       • powers of 10
     • comparing two different properties

5. materials under a microscope

   • size of atoms
     • measure diameter of oil drop
     • oil onto still water
     • measure radius of spread oil patch
     • h=4(r of drop)^3/3(r of patch)^2
     • uncertainty
       • diameter of oil drop
       • spread not one layer
   • Microscope
     • false colour
     • AFM
     • STM

6. modelling behaviour

   • metals
     • pure metals
       • dislocations
         • shift one by one
         • much less force needed
     • alloys
       • dislocations pinned
   • arrangements
     • crystalline
       • structure
     • amorphous
       • disorder
       • rapid cooling
     • polycrystalline
   • stress concentration
     • crack propogation
       • split atoms one at a time
     • small area at crack
       • high stress
         • toughness = energy needed to extend cracks

7. microscopic strutures/ macroscopic properties

   • ceramics
     • rigid structure
       • covalent bonds
         • bonds strong = stiff
         • atoms cannot slip = hard/brittle
       • directional
   • metals
     • non-directional
       • free electrons
   • stiffness/elasticity
     • metals
       • elastic up to 0.1% strain
     • polymers
       • elastic up to 1% strain
         • rotation of bonds to straighten
         • vulcanisation
           • stiffer
           • add cross links