Materials

Question 1

Describe the relationship between melting point and modulus of a material

Answer 1

They are closely linked, and both increase and decrease with eachother

Question 2

Define crystal

Answer 2

A type of structure caused by the stacking of layers/planes of atoms

Question 3

Which has the weakest bonding between HCP, FCC and BCC?

Answer 3

FCC - most closely packed plane, so bonding isn’t as strong, so the triangle planes can slip past each other

Question 4

Describe the structure of a face centred cubic metal

Answer 4

• Is made up of four atoms
• Has one eighth of an atom on each corner
• Has half an atom on each face
• Is made up of three layers

Question 5

Describe the structure of a body centred cubic metal

Answer 5

• Made up of two atoms
• Has an eight of an atom on each corner
• Has one whole atom in the middle
• Is made up of two layers of atoms

Question 6

Describe the structure of hexagonal close packed metals

Answer 6

• Is made up of a hexagonal prism
• Is made up of two layers of atoms
• Top two layers of seven atoms arranged as a hexagon around a central atom
• Middle layer consists of three atoms, arranged as a triangle.

Question 7

Define crystalline

Answer 7

A material which packs very closes in a very ordinary way

Question 8

What is the atomic packing factor?

Answer 8

Volume of atoms in a unit cell/total unit cell volume (V_s)/(V_c)

Question 9

What is the total unit cell volume (V_c) for a FCC structure?

Answer 9

16R³√2

Question 10

What is the total unit cell volume (V_c) for a BCC?

Answer 10

((4/√3)R)³

Question 11

What is theroetical density formula?

Answer 11

ρ=(n*A_r/V_c*N_a)

Question 12

What is a boundary, how can it be a point of weakness, and how can this be improved?

Question 13

Describe the structures of polymers

Answer 13

• Either amorphous or semi crystalline
• Made of long chains
• Have a carbon chain backbone
• Often have a random arrangement

Question 14

Define elastic modulus

Answer 14

The resistance of a material to elastic deformation

Question 15

Describe the difference between plastic and elastic deformation

Question 16

Define stiffness

Answer 16

Resistance of a structure to elastic deformation

Question 17

Define stress

Answer 17

σ = F/A

Question 18

Describe the difference between engineering stress and true stress

Answer 18

• Engineering stress is force/original area
• True stress is the force/instantaneous area

Question 19

Define strain

Answer 19

ε = ∆l/l

Question 20

Define Poisson’s Ratio

Answer 20

Ration between how far a material is stretched and how thin it gets

Question 21

How do you calculate yield strength from a engineering stress/strain curve

Answer 21

Offset the gradient of the linear section by 0.2%, and where the line cuts the graph is a good estimate

Question 22

Define toughness

Answer 22

Energy required to break a unit of volume of material

Question 23

How can the toughness of a material be calculated from a stress/strain curve?

Question 24

Describe a brittle fracture

Answer 24

Elastic energy - little to no deformation

Question 25

Describe a ductile fracture

Question 26

Describe the difference between strength and toughness

Answer 26

• Toughness is energy required to break something
• Strength is maximum stress something can take

Question 27

Define hardness

Answer 27

Resistance to permanently indenting the surface - restance too plastic deformation

Question 28

Explain how to test a ceramic

Answer 28

• 3 point load test
  
  • Apply a force to a ceramic (two low and on high)
• Use flexual strength - = (3F*L)/(2*w*d²)

Question 29

Describe the points of failures in ceramic

Answer 29

• Little to no plastic deformation
• Porosity is a common point of weakness

Question 30

Describe and explain the four types of failures in ceramic

Answer 30

• Impact/point loading
  
  • Where a sudden force is applied to one point of the ceramic, e.g. hammer
• Bending
  
  • Where a ceramic breaks due to it being bent
• Torsion
  
  • The breaking due to twisting force
• Internal Pressure
  
  • Pressure from inside the ceramic pushing out on the material

Question 31

Describe what happens when testing polymers

Answer 31

After initial deformation, the plastic deformation causes the strain to rapidly increase without little stress change

Question 32

Describe the stages of ductile fractures

Answer 32

• Necking
  
  • Reduction in cross sectional area
• Void Nucleation
  
  • having small voids in the part where necking occurs
• Void growth and coalescence
  
  • Voids grow and become one
• Shearing at the surface
  
  • Cracks can be seen at the surface
• Fracture
  
  • The material breaks

Question 33

Define fatigue

Answer 33

Slow crack growth due to cyclic loading

Question 34

Describe and explain the stages of fatigue

Answer 34

• Crack initiation
  
  • A part becomes damaged as a micro crack forms at a point of high stress concentration
• Slip band
  
  • The initial crack depends on planes of high shear stress and becomes well defined
• Stage ii crack growth
  
  • The well-defined crack grows in a direction normal to the maximum tensile stress
• Ductile fracture
  
  • When the crack reaches a critical length, the remaining cross section cannot support applied forces and the part fails