3.4 Materials [Custom]

Question 1

What is tensile deformation?

Answer 1

Tensile forces act away from the centre of the spring in both directions, and will stretch it out. This is known as extension.

Question 2

What is compressive deformation?

Answer 2

Compressive forces act towards the centre of the spring in both directions, causing it to compress / shorten.

Question 3

State Hooke’s law

Answer 3

For a material within its elastic limit, the force applied is directly proportional to the extension of the material

Question 4

When is Hooke’s law no longer obeyed?

Answer 4

Past the limit of proportionality.

Question 5

In the formula F = Kx, what does K represent?

Answer 5

‘K’ is the force constant / spring constant of the material. The larger it is, the stiffer the material is. It can only be used within the elastic limit of the material.

Question 6

What are the force extension characteristics of a metal wire?

Answer 6

It obeys Hooke’s law, and shows elastic deformation until its elastic limit.

Beyond the elastic limit, it experiences plastic deformation.

A permanent extension can be observed from the graph.

Question 7

What is elastic deformation?

Answer 7

Materials that undergo this will return to its original shape when the force being applied is removed.

Question 8

What is plastic deformation?

Answer 8

Materials that undergo this experience permanent deformation, and do not return to original shape when the force being applied is removed.

Question 9

Describe the force extension graph of rubber.

Answer 9

Doesn’t experience plastic deformation, but it doesn’t obey Hooke’s Law.

It has a hysteresis loop.

Question 10

Describe polyethene and its force extension graph.

Answer 10

It is a polymeric material.

It doesn’t obey Hooke’s law.

It experiences plastic deformation.
(when any force is applied to it, making it very easy to stretch into new shapes.)

The loading curve is similar to rubber. The unloading curve is a straight line downwards and slightly backwards, revealing a massive permanent deformation.

Question 11

How can you calculate the force constant of springs in series?

Answer 11

1/k[1] + 1/k[2] + 1/k[3] = 1/k[Total]

Question 12

How can you calculate the force constant of springs in parallel?

Answer 12

k[total] = k[1] + k[2] + k[3]

Question 13

What does the area under the line on a force extension graph represent?

Answer 13

The work done to deform the material. It is stored as elastic potential energy, and released when the material is allowed to return to its original length.

Question 14

What is a hysteresis loop?

What does it represent?

Answer 14

The area between the loading and unloading curves.

It represents the energy required to stretch the material out, which was transferred to thermal energy when the force was removed.

Question 15

Describe how can we use a force extension graph to calculate the elastic potential energy stored in a material?

Answer 15

The area under the line is a triangle. It can be calculated using [area = 1/2 x base x height]

On the graph, this will be E = 1/2 Fx
Because F = kx, we can use
E = 1/2kx^2

Question 16

How is work done stored during plastic deformation?

Answer 16

WORK IN PROGRESS

Question 17

Define tensile stress. What is it measured in?

Answer 17

The force applied to a material per unit cross-sectional area.

Measured in Nm^-2, or Pascals (Pa)

Question 18

Define tensile strain. What is it measured in?

Answer 18

The extension or compression of a material per unit of its original length.

It is dimensionless, and so it has no unit. It is sometimes written as a percentage.

Question 19

Define the young modulus of a material.

Answer 19

The ratio of stress to strain. It is a measure of the material’s stiffness, independent of shape and size of the material.

Question 20

On a stress-strain graph, which part is the young modulus?

Answer 20

The gradient (within the straight line section).

Question 21

What does the young modulus depend on?

Answer 21

Only the material. (it is completely independent of shape and size)

Question 22

What is a yield point?

Answer 22

A point where there is rapid extension / rapid increase in the strain with little increase in stress.

Question 23

Describe the stress-strain graph of a brittle material.

Answer 23

Elastic behaviour is shown until the fracture point. There is little to no plastic deformation, and the loading and unloading curve are the same.

Question 24

Describe the stress-strain graph of a elastic material.

Answer 24

It can endure a lot of tensile stress before breaking. There is no plastic deformation, but the unloading curve is different to the loading curve, as some energy has been lost as thermal energy.

Question 25

Describe the stress-strain graph of a ductile material.

Answer 25

It can easily be hammered into thin sheets or drawn into wire. They generally experience elastic deformation until their elastic limit, then undergo plastic deformation before reaching their UTS and fracture point.