Testing and looking inside material Flashcards
Definition of strong
Takes a large stress to break
Definition of hard
Resistant to dents and scratches
Definition of tough
Can undergo a lot of plastic deformation after passing its elastic limit before breaking
Definition of brittle
Can only undergo little plastic deformations after passing its elastic limit before breaking
Definition of stiff
Is difficult to bend
Definition of malleable
Can easily be hammered or pressed into shape
Definition of ductile
Can easily be drawn into a wire
Units of strength
Pa
Units of hardness
Pa
Units of stress
Pa
Units of UTS
Pa
Units of toughness
Jm^-2
Equation for young modulus
stress/strain
Equation for stress
Force / area
Equation for strain
extension / length
Equation for UTS
maximum load / original area
Equation for the energy stored in a spring
Work done = 1/2ke^2
How do you find the energy stored in a spring from a force extension graph?
It’s the area under the line
Definition of elastic
A material that is able to regain its shape after being distorted
Definition of plastic
A material that has a low elastic limit and beyond that limit it doesn’t regain its original length/shape
Definition of yield strength
The amount of stress at which a material begins to deform plastically
What does Hooke’s Law say?
Force is proportional to extension, as long as the elastic limit is not exceeded
What is the equation linked to Hooke’s Law?
Force = spring constant X extension
In Hooke’s Law, what does the spring constant show?
Shows how much force it takes to stretch a material (larger spring constant = stiffer material)
What does crystalline mean?
That particles in a material are arranged in a regular pattern
What does polycrystalline mean?
That within each grain of a material there is an ordered structure, but the orientation of each grain is random/disordered
What does amorphous mean?
That a material has a disordered internal structure and a random orientation of each grain
Definition of a lattice
A regular, repeated 3D arrangement of atoms/molecules in a crystalline solid
Do bigger or smaller grains give a less efficiently packed material and why?
Bigger - because the grains can’t get as close to each other and so have bigger gaps in between
Are metallic bonds directional or non directional?
Non-directional
How strong are metallic bonds compared to ionic and covalent?
Similar to ionic, weaker than covalent
Why do cracks blunten in metals?
- Because they are ductile so the crack ‘stretches’ sideways.
- And the layers absorb energy so extra would be needed to affect another layer
Why are metals tough?
Their atoms are forced tightly together by their sea of electrons. The -ve mobile electrons keep the +ve ions in place.
Why are metals stiff?
They have strong electrostatic attraction between the sea of electrons and ion lattice, forming strong metallic bonds
What is UTS?
Ultimate tensile strength = the max stress a material can withstand before breaking
Definition of yield stress (not yield strength)
The point at which a large amount of plastic deformation takes place with a constant load
How does a material behave past its elastic limit?
It behaves plastically - would no longer return to its original shape once the stress was removed
Why and how are alloys made?
- Use atoms from a dif. metal to fill in dislocations
- Increases stress needed to cause a slip
- Makes the metal harder and less ductile
What type of lattice do metals usually form?
A crystalline lattice
What makes a metal stiff?
The electrostatic attraction between the ion lattice and free electrons form strong metallic bonds, making it hard to bend/stretch
What makes a metal tough?
The strongly bonded lattice structure
What makes a metal ductile?
The ions being able to move in the lattice when a force is applied
What makes a metal hardER?
Alloying: fill dislocations in a metal with atoms from a dif. metal to make it harder for planes to slip as they require more stress
What is alloying?
Filling dislocations in a metal with atoms from a dif. metal so that a greater stress is needed for planes to slip to strengthen a metal
What happens to a metal’s microstructure when a force is applied to it?
The interatomic spacing between ions increases uniformly during elastic deformation. Once the stress is large enough for plastic deformation, the planes slip over each other
What makes a metal need less stress than normal for its type to cause slipping?
If it has dislocations (is an imperfect metal)
What type of lattice do ceramics form?
Mostly crystalline or polycrystalline, but sometimes amorphous
What makes a ceramic more likely to have an amorphous structure and why?
If the molten ceramic is cooled very quickly; the atoms don’t have time to arrange regularly
What features of a ceramic’s microstructure make them easy to fracture?
Their random atomic bonding means the lattice has no slip planes or mobile dislocations
What makes a ceramic stiff?
Their strong bonds between atoms
What type(s) of bonds do ceramics form?
Ionic or covalent
What type of bonds to polymers make?
Covalent bonds
What is a polymer?
A molecular chain made up of repeated monomers
Polymer chains are often entangled, what makes them flexible?
The chains can be unravelled by rotating the monomers about their bonds when you pull them
What are the 2 things the flexibility of a polymer dependent on?
- How easily the monomers rotate about their bonds
- The strength and number of bonds between the chains
Give an example of two materials together which make a composite material and name the composite material
e.g. steel + concrete make reinforced concrete
steel + glass make safety glass
Give an example of two materials together which make a composite material and name the composite material
e.g. steel + concrete make reinforced concrete
steel + glass make safety glass
