Materials Flashcards
Hooks law states
Extention is proportional to force
In springs k is called the…
Spring constant
A force extention graph starts to curve at…
The limit of proportionallity
What is the elastic limit
The point at which the material is permanently deformed
What is elastic deformation?
When the atoms in a material return to there equillibrium distance after the tension force pulling them apart is removed
What is plastic deformation
When the tension force is removed the atoms dotn return to orignal positions
How to investigate extention
- Spring on clamp w ruler
- adding masses
- can do in parralel and series to show relationship of k like with resistors
Tension in opposite directions on materials are either
Compresive (squash)
Tensile (stretch)
What is fracture stress
- stress pulls atoms apart
- at a point atoms completely sperate and material fractire
- the stress at this point is fracture stress
Calculate elastic strain energy stored in a force extension graph.
Area
Workdone=1/2 Fx
This gives average force
-only works if obeying hookes law
What dimensions of wire should be use when finding youngs modulus?
-very long and thin wire so more extemtion for the same force therefore reducing measurement uncertaintys
Method for findign youngs modulus
- use micro meter to measure diameter at 3 points in wire and take average and assume circular
- clamp wire at one end and put pulley at the other so weights cna be hung
- start w smallest weight that straightens wire
- measure distance between fixed end and marker
- increase the weight in intervals recording the marker each time from above
- calculate stress and strain and plot so gradient is youngs modulus
- make sure not to go past elastic limit
Brittle materials….. w example
break without plastic deformation Eg ceramics (glass ,pottery)
Ductile material….w example
Can be drawn into a wire without losing strength
eg. Copper
Strong materials…. W example
Can withstand high stresses without deforming or breaking
Eg. Steel
Hard materials…
Very difficult to scratch
Eg. Diamond
Stiff materials….. w example
High resistance to bending and stretching
High youngs modulus (measure of stiffness)
Eg. Helmet
Tough materials…. W example
Absorb lots of energy before fracture
Eg.polyethene
What is yield stress
Stress at which plastic takes place w a constant load
Metal structure
- usually crstalline lattice ( atoms arranged in a regular repeating pattern)
- strong electrostatic attraction between positive ion lattice and free electrons making it stiff and tought
- the ions in lattice can move when force is applied so ductile
- when the stress is high enough to cause plastic deformation the planes slip over each other
- if dislocation then lower stress needed
-
What is a dislocation?
Missing atoms in regular structure that allow atoms to slip with a lower stress
What is alloying?
Second metal pins dislocations to increase stress needed for slipping
Making them harder and less ductile
What does polycrystalline mean?
Made up of grains of crystalline structure facing different directions
Structure of ceramics
- Can be crystalline or polycrystalline
- some are amorphous like glass
- All covalently or ionically bonded in a giant rigid structure
- rigid structure means bonds are directional meaning locked in place and cannot slip so hard and brittle
- strong bonds make them stiff
- Cracks spread through when they fracture as force applied to small area gives high stress
What does amphorpous mean?
- randomly arranged (disordered) atoms so no slip plames or mobile disclocationa meaning rare plastic deformation before fracture
- rapid cooling traps ceramics in amorphous state
How does speed of cooling affect ceramics
- rapid cooling traps in amorphous state
- slow controlled cooling can lead to crystalline structure
- in bertween is polycrystalline
Explain crack propagation
- In ceramics cracks lead to fracture
- rigid structures so when force is concentrated at tip of crack neighbouring atoms are pulled apart and then the next two erc
- the crack propagates through until fracture
- very high stress as such small area
What is a polymer? Example
- Long of chain of repeating monomers
- eg natural rubber or man made polyethene
Explain polymer structure?
- covalently bonded so hard to seperate so relatively strong
- Chains are entangled together so can unwravel bu roatating about bonds when stretched making them flexible
What can make a polymer stiffer?
- stronger cross links make bonds harder to rotate so stiffer
- adding cross links means can only stretc hebtween each pne so becomes stiffer
How to carryout Rayleighs oil drop experiment?
-Drop oil onto water and assume it spreads to be 1 molecule thick
-measure surface aread and volume of oil dropper to find thickness
Size of atom=size of oil molecule divided by no of atoms in molecule so gives upper bound
What is scanning tunneling microscope
- scanning tunnelling microscope detects bumps amd dips by keeping a constant current
- 2d images
- has high resoloution so can calculate spacing and size
How do Atomic force microscopes work?
—needle detects contours on sample on an atomic scale
- keeps force constant by an actuator physically moving soecies
- records surface in 3d
- lower resolution
