5-Choosing Materials Flashcards
What is the definition of stress?
The force per unit cross-sectional area when opposing forces act on a body.
What is the definition of strain?
Extension per unit length due to applied stress
What is the definition of the Young’s Modulus?
Stress / Strain
What is the Young’s Modulus a measure of?
The stiffness
What is the definition of a crystal?
A solid in which atoms are arranged in a regular array
What does Crystalline mean?
A solid consisting of crystals which are arranged randomly.
What does amorphous mean?
Where the atoms are arranged randomly
What is a polymeric solid?
A solid made up of chain-like molecules
What is a ductile material?
A material that can be drawn into wires.
What is elastic strain?
Strain that dissapears when the stress is removed, so the specimen returns to its original length.
What is plastic strain?
When the specimen does not return to its original length after the stress is removed.
What is a dislocation?
An incomplete plane of atoms
What is a grain boundary?
A boundary between crystals in a poly crystalline structure
What is the elastic limit?
Where the deformation becomes plastic, so it can no longer return to its original length
What is Ductile fracture?
Localised thinning which increases the stress as the area is smaller
What is a brittle material?
A material with no plastic flow, which fails by brittle fracture
What is brittle fracture?
Fracture under tension of brittle materials by crack propagation
What is the hardness of a material?
A material’s resistance to plastic deformation
What is the toughness of a material?
How much deformation a material can undergo before fracture.
If a material is stiff, what does it mean?
It is difficult to stretch or bend
If a material is malleable, what does it mean?
The material can be shaped easily
Describe Lord Rayleigh’s oil drop expreiment:
- Measure the diameter and calculate the radius of the oil drop
- Place the drop on still water which has a layer of powder
- Measure the radius of the patch after it has spread about
- Volume of the oil patch=volume of the oil drop
- h= (4r^3) / (3r^2)
What is the number of atoms measure in?
m^-3
What is the equation for the number of atoms?
Density of element / mass of atom
What is the volume of a single atom?
1 / Number of atoms = mass of atom /density
What is the diameter of an atom?
10^-10 5
What is the order of magnitude size of an atom?
The cube root of the volume of a single atom
What does STM stand for?
Scanning Tunneling Microscope
What does AFM stand for?
Atomic Force Microscope
What can AFMs and STMs show?
Individual atoms on the surface of materials and allow atomic diameters to be measured.
Metals are crystalline, what does this mean?
Indivual particles are arranged in a regular pattern over distances many times the spacing between the particles
What are malleability and ductility a consequence of?
Plastic behaviour
What is plastic behaviour explained by?
The movement of dislocations within the crystalline structure.
What does SEM stand for?
Scanning electron microscope
What does Scanning electron microscopes show?
Large scale structures
On what scale can AFMs detect changes?
On an atomic scale
What do AFMs and STMs tell us about?
The arrangement of atoms on the surface of a material.
What don’t AFMs and STMs tell us about?
The arrangement of atoms on the interior of a material.
What technique can show that metals are crystalline?
X-ray crystallography
What is the reasoning for some pure metals being malleable and ductile?
Dislocations
What do dislocations allow layers of atoms to do?
Move one atom at a time
What do dislocations do to the energy required to deform a material?
They greatly reduce the energy required to deform the material
What are the dislocations like in ceramics?
They are not mobile so they cannot move through the material, which makes ceramics brittle
Why are metal alloys less ductile than pure metals?
The different sized atoms pin down the dislocations, making slippages between the layers of atoms more difficult.
What are metal alloys formed by?
The addition of metallic elements that usually have differently sized atoms
When do solids form?
When liquids cool
Is glass crystalline or amorphous?
Amourphous
What does rapid cooling of a liquid result in?
Particles being trapped in an amorphous state, resembling the arrangement of a liquid
What does slow, controlled cooling of a liquid lead to?
A single, pure crystal, which can be arranged in a perfect way
What is a polycrystalline material?
A material that consists of a number of grains all orientated differently to one another, but with regular structure within each grain.
What is the strength of materials affected by?
Tiny cracks and flaws in the structure of a material
Describe what happens to a glass rod that has a tiny crack on its surface:
- The glass becomes strained elastically
- At the tip of the crack, atoms are pulled apart
- The next 2 atoms are pulled apart
- The crack moves through the material like a zip being undone
What is toughness a measure of?
The energy required to extend cracks through a material
What is tougher, a brittle substance or a tough metal?
A tough metal, as more energy is required to extend the cracks in it.
When stress is applied to a crack in a metal, what happens?
The metal deforms plastically in the region of the crack which makes the crack broader, reducing the stress around the crack,
What makes a material strong?
If it has a high breaking stress.
What is tough the opposite of?
Brittle
What are the 3 types of bonds between atoms?
Covalent, ionic and metallic.
What makes the bonds in ionic compounds and ceramic unique?
The bonds are directional
What do directional bonds mean?
The atoms are locked in place and cannot slip, making them hard and brittle
What atoms can form covalent bonds?
Atoms that share electrons
Are covalent bonds directional?
Yes
What type of bonds do metals have?
Non-directional bonds
Why are metals stiff?
Because the bonds are strong
Atoms in metals are ionised, what does this mean?
The free electrons move between the ions
Why are metals ductile?
Because the ions can slip
What does stretching a metal do?
It pulls the bonds apart, causing the gaps to open up a little
What are the array of positive ions bonded by in metals?
Electrons
What is the elastic extensibility of metals?
0.1%
What is the elastic extensibility of polymers?
1%
Why is polyethene very ‘floppy’?
Because it is free to rotate about its bonds
Why is polyethene strong and flexible?
The bonds are difficult to break and they are free to rotate.
What do you do to a polymer to make it less flexible?
Add cross links
What are cross links?
Where polymer chains are tied together at regular intervals along the chain.
What happens when rubber is heated with sulfur?
The sulfur atoms form cross-links with the polymer chains
What does the technique of vulcanisation mean for rubber?
It can be chemically adapted for many uses including tyres.
In unstretched rubber, what do chains meander randomly between?
Sulfur cross-links
In stretched rubber what happens to the chain bonds?
They rotate
What happens when you let go of the stretched rubber?
The chains fold up again and the rubber contracts
What is the plastic extensibility of polymers?
> 100%
