Mechanics unit 5 - Fracture under stress deck 1

Question 1

What are the 3 factors which are important in determining if a material will fail ?

Answer 1

1. The magnitude of the applied load
2. The rate of speed at which the load is applied
3. The number of times the load is applied

This is a bit like fatigue fractures

Question 2

What additional factor can lead to failure of a material ?

Answer 2

Wear and corrosion of the material

Question 3

What test is usually used to produce a stress-strain curve of a material ?

Answer 3

A tensile test

Question 4

Describe how a tensile test is done and how the strain and stress can therefore be calculated

Answer 4

During the test a gradual or stepwise increase in tensile load is applied to the material until the material fractures.

• The change in length in the material is measured throughout the test and divided by the original length to give the strain
• The stress can be calculated using the applied tensile load and the cross-sectional area of the material

A stress-strain curve can then be plotted using this information

Question 5

On a stress-strain curve what is the stress at the fracture point called ?

Answer 5

The fracture/ ruputure strength

Question 6

Describe the phenomenon of necking

Answer 6

This is where a materials cross-sectional area is reduced during loading such that the material appears to reduce in strength

Question 7

Why is the maximum calculated strength of a material ultimate strength of a material higher than the rupture/ fracture strength of material?

Answer 7

Due to necking - the specimen appears to carry less stress when it is calculated using the original cross-sectional shape rather than the now true reduced cross-sectional shape of the material

==> a material will fracture when it is subjected to a load greater than its ultimate strength

Question 8

From application of a steady load what are the 2 ways in which a material can fracture and state the key difference between them?

Answer 8

• Ductile fracture - fracture occurs with necking
• Brittle fracture - fracture occurs without necking

Question 9

What do ductile fractures occur after ?

Answer 9

Considerable plastic deformation, characterised by necking

e.g. the handles of a shopping bag when overloaded will quickly stretch before breaking

Question 10

Describe the specific stages of a ductile fracture

Answer 10

1. Application of a tensile load
2. Formation of microscopic voids (small holes) at the centre of the bar - due to high stress causing the separation of the metal at the grain boundaries
3. As the stress increases the microscopic voids grow and connect with one another creating larger cavities
4. Eventually the actual metal to metal contact area within the bar is reduced so much due to the microscopic voids that fracture occurs

Question 11

What also contributes to a ductile fracture along with the applied tensile load ?

Answer 11

Deformation due to shearing

Question 12

What are the 4 characteristic features of ductile fracture ?

Answer 12

• Necking
• Flat granulated central portion and Shear lip - giving the fracture a cup and cone appearance

Question 13

How do brittle fractures occur ?

Answer 13

The occur without any plastic deformation (no necking or elongation)

Question 14

What are the 3 key features characterising the appearance of a brittle fracture ?

Answer 14

1. The fracture surface is flat and perpendicular to the applied fracturing load
2. It has a granular appearance
3. It has a chevron pattern

Question 15

What scenarios may result in a ductile material undergoing a brittle fracture ?

Answer 15

1. If it has been exposed to fatigue loading (repeated loading and unloading)
2. If the material contains a crack or notch
3. Decreasing the temperature and increasing the strain rate

Question 16

Do SAQ 2&3&4

Question 17

What are stress concentrations within a material and what can they be shown by?

Answer 17

They are points at which the level of stress is greater than the average level of stress within the material

They can be indicated using stress trajectories

Question 18

What are high levels of stress indicated by within a material ?

Answer 18

A high concentration of stress trajectories

Question 19

What are changes in shape such as notches or holes that result in stress concentrations known as ?

Answer 19

Stress risers

Question 20

What do the points of highest stress within a material represent ?

Answer 20

The weakest points within a structure and therefore the points which are likeliest to fail

Question 21

What sort of changes in shape should be avoided when desiging an object and why?

Answer 21

Sharp changes in shape - as the shaper the change the higher the stress concentration

Question 22

Describe what fracture propagation is

Answer 22

This is the development of a fracture from the tip of a crack or notch:

• The stress concentration will be highest at the tip of the crack/ notch
• A fracture will then propagate/ spread from the tip

Question 23

What can be built into a material to hault fracture propagation ?

Answer 23

Smooth holes can be built into the structure to blunt a propagating (spreading) crack so that the stress is no longer concentrated at one sharp point

Question 24

Do SAQ 5

Question 25

In stuff just covered we looked at steady loading, but now we will look at impact loading, define what this is and do SAQ 6

Answer 25

Impact load = a sudden and intense blow

Question 26

What test is used to test a structures resistance to impact loading ?

Answer 26

Charpy impact test

Question 27

Describe how the charpy impact test allows you to calculate a structures resistance to impact loads

Answer 27

1. A heavy pendulum is released from known height
2. It then strikes and breaks the test specimen and continues up to its peak swing
3. The height reached at the end of the swing is lower than its original height because energy is absorbed as the specimen is fractured
4. The height difference is measured and allows the change in potential energy (EP) to be calculated
5. The change in EP is equal to the energy absorbed by the specimen called the impact energy

Question 28

What type of material is tougher a ductile or brittle ?

Answer 28

A ductile as it can absorb more energy

Question 29

What is the ability of a material to absorb energy dependent on ?

Answer 29

Temperature - as temp increases the materials ability to absorb energy increases as it changes from a brittle to more ductile material

Question 30

Do SAQ 8