Module 3 - Section 4 - Interpreting Stress / Strain Graphs

Question 1

Whats a typical ductile material ?

Answer 1

Copper wire

Question 2

What does it means when a material is ductile ?

Answer 2

You can change the shape of ductile materials by drawing them into wires or other shapes - but the most important thing is that they keep their strength when they’re deformed like this

Question 3

How does an stress-strain graph look like for ductile materials ?

Answer 3

A stress-strain graph for ductile materials curves

Question 4

Explain each part of the stress-strain graph of a ductile material

Answer 4

Firstly at the start of the graph, the graph is a straight line and it goes through the origin - this means that the material is obeying hooks law

Then after this the graph reaches a point where the graph is no longer a straight line but starts to bend - at this point the material no longer follows hooks law but will still return to its original shape if the stress were removed - this point is called the limit of proportionality

Then we reach a point called the elastic limit - this is where the material starts to behave plastically - from this point onwards the material will no longer return to its original shape once the stress is removed

Then the final point we see on the graph is called the yield point - here the material suddenly starts to stretch without any extra load attached - The yield point or yield stress - is the stress at which a large amount of plastic deformation happens with a constant or reduced load - so if you just leave the weight or you even reduce the weight after the yield point has been reached - so basically it is the point at which the material starts to stretch a lot without any extra load attached or even with load taken off

Question 5

What is a yield point ( yield stress ) ?

Answer 5

The yield point is the stress at which the material undergoes a lot of plastic deformation ( deformation after the elastic limit ) at a constant or reduced load

Question 6

What does a stress-strain graph look like for a brittle material ?

Answer 6

It is just a straight line that goes through the origin - therefore brittle materials also follow hooks law

Question 7

Why is the stress-strain graph for a brittle material just a straight line through the origin ?

Answer 7

This is because when the stress on a brittle material gets too high - the material snaps

It doesn’t deform plastically ( meaning any deformation after the elastic limit and when you remove the stress - the material will not return to its original shape )

Question 8

What happens if stress is applied to a brittle material ?

Answer 8

When stress is applied to a brittle material - tiny cracks at the materials surface get bigger and bigger until the material breaks completely

This is called brittle fracture

Question 9

What is brittle fracture ?

Answer 9

It is when you apply stress to a brittle material and tiny cracks form on the surface of the material that get bigger and bigger until the material breaks completely

Question 10

What can stronger materials withstand more of ?

Answer 10

Stronger materials can withstand more stress before they break

Question 11

Different materials have different ….

Answer 11

Breaking stresses - the stress at which the material breaks

Question 12

The stronger the material the higher the ….

Answer 12

Breaking stress

Question 13

What are the characteristics of a stiff material ?

Answer 13

A stiff material is difficult to stretch and compress - therefore for a given stress (e.g. 1Pa) a stiff material will have a lower strain ( lower extension ) than a less stiff material

Stiff materials have a large Youngs Modulus

Question 14

Is a stiff material strong ?

Answer 14

A stiff material isn’t necessarily strong and a strong material isn’t necessarily stiff

This means that some stiff materials break under a low stress ( because they are not strong therefore they have a low breaking stress )

This also means that some strong materials are not stiff ( therefore for a given stress they will not have a lower strain than a stiff material )

Question 15

What type of materials are Rubber and Polythene ?

Answer 15

Polymeric materials

Question 16

How are the molecules of Polymeric materials arranged ?

Answer 16

The molecules that make up polymeric ( or polymer ) materials are arranged in long chains

Question 17

According to the stress-strain graph of polythene - how do we know that polythene behaves plastically ?

Answer 17

This is because during the unloading phase - the line does not go back to the original start point of the loading part of the graph ( the start of the graph )

And we could also see that when the material is fully unloaded ( there is no more stress acting on it ) there is still a strain ( an extension ) which means that the material did not return to its original shape

Question 18

What can we say are the properties of Polythene from the stress-strain graph ?

Answer 18

Polythene behaves plastically - applying a stress to it stretches it into a new shape

Polythene is a ductile material - because the stress - strain graph curves

Question 19

How do we figure out how much energy has been converted to heat after the material is unloaded - from the elastic potential energy store ?

Answer 19

The amount of energy converted to heat per unit volume is given by the area between the loading and unloading curves

Question 20

How does a rubber behave due to the stress-strain graph ?

Answer 20

The rubber behaves elastically - because it returns to its original shape once the stress have been removed ( once the material has been unloaded )

Question 21

Why are the loading and unloading curves different for rubber ( stress-strain graph ) ?

Answer 21

This is because the energy released when the rubber is unloaded - is LESS than the energy transferred to the rubber in order to stretch it - this is because some of the elastic potential energy stored in the stretched rubber is converted to heat.