Module 3 - Mechanical properties of matter

Question 1

What is meant by stress

Answer 1

Force applied per unit cross-sectional area

Question 2

What is meant by strain

Answer 2

Extension per unit length of material

Question 3

What is the unit for strain

Answer 3

None

Question 4

What is the equation for stress

Answer 4

σ = F/A

Question 5

What is the unit for stress

Answer 5

Pascals

Question 6

What is the equation for strain

Answer 6

ε = ΔL/L0

Question 7

What is the relationship between stress and strain

Answer 7

• The greater the stress (force per unit area) the greater the strain (the extension per original length)

Question 8

What is the proportionality between stress and strain and explain the constant

Answer 8

• σ α ε
• σ = kε
• This is a constant called the Young Modulus, which is a measure of a materials stiffness

Question 9

What is the equation for Young Modulus in 2 main ways

Answer 9

• E = σ/ε
• E = FL/Ax

Question 10

What is the unit for Young Modulus

Answer 10

Pascal, usually GPa

Question 11

What is meant by stiffness

Answer 11

The extent to which a material under stress experiences strain

Question 12

What is meant by the Young Modulus

Answer 12

The tensile stress per unit tensile strain

Question 13

What is the gradient of a stress strain graph

Answer 13

The Young Modulus

Question 14

What is the area under the stress strain graph within the limit of proportionality

Answer 14

• The energy density: The work done per unit volume to stretch a material

Question 15

What is the UTS

Answer 15

The ultimate tensile strength, the maximum stress a material can withstand before fracture

Question 16

In what region can you use the youngs modulus

Answer 16

The elastic limit

Question 17

What is the Limit of Proportionality in terms of stress and strain

Answer 17

The value of the stress beyond which the stress is no longer proportional to the strain

Question 18

Describe the plastic and elastic region in terms of removal of stress

Answer 18

• Elastic: where the removal of stress will return the material to its original length
• Plastic: Removal of the stress will not return the object to its original length

Question 19

What is the yield point

Answer 19

The point at which the strain increases with a small increase in stress

Question 20

What is meant by a brittle material

Answer 20

A material that:
- Doesnt plastically deform
- low strain for large stress

Question 21

Describe the stress/strain graph of a brittle material

Answer 21

Y = mx, where the loading and unloading curves are the same

Question 22

Describe the stress strain graph for rubber

Answer 22

• Hysterisis loop
• No plastic deformation

Question 23

What is meant by a ductile material

Answer 23

• Materials that have large plastic regions and so can be drawn into wire

Question 24

What is a polymeric material

Answer 24

A material consisting of long chain molecules, that experience high strain before breaking

Question 25

What is meant by a tough material

Answer 25

- A material that needs a large energy to break (greater area under stress strain graph)

Question 26

What is meant by hard

Answer 26

A measure of the difficulty of scratching or denting a material

Question 27

What is meant by a strong material

Answer 27

The extent to which a material resists breaking under stress

Question 28

Describe the behaviour of rubber under stress

Answer 28

The stiffness of rubber gradually decreases as it is initially stretched. When the polymer chains have uncoiled fully, the rubber becomes stiffer.