3.4 Materials

Question 1

What do tensile forces cause?

Answer 1

Tensile deformation

Question 2

What do compressive forces cause?

Answer 2

Compressive deformation

Question 3

What is Hooke’s Law? (2)

Answer 3

For forces less than the elastic limit of a spring, the extension is directly proportional to the force applied
F = kx

Question 4

What does the force-extension graph of a spring show? (2)

Answer 4

Straight line from origin to elastic limit - elastic deformation
Begins to curve - plastic deformation

Question 5

What is elastic deformation?

Answer 5

The material will return to its original shape when the force is removed

Question 6

What is plastic deformation?

Answer 6

Permanent structural changes mean that the material will not return to its original shape when the force is removed

Question 7

What is the work done in a spring transferred to? (2)

Answer 7

Elastic potential energy

Fully recoverable due to elastic behaviour of spring

Question 8

What does the area under a force-extension graph represent?

Answer 8

Work done

Question 9

How is elastic potential energy calculated?

Answer 9

1/2 Fx

1/2 kx^2

Question 10

What is tensile stress? (2)

Answer 10

Force applied per unit cross-sectional area of wire

σ = force / cross-sectional area

Question 11

What is tensile strain? (2)

Answer 11

The fractional change in the length of the wire

ε = extension / original length

Question 12

What is a ductile material?

Answer 12

One that can be easily drawn into a wire or hammered into thin sheets (e.g. mild steel)

Question 13

What are upper and lower yield points?

Answer 13

The points between which the material extends rapidly

Question 14

What is ultimate tensile strength?

Answer 14

The maximum stress a material can withstand before it breaks

Question 15

What happens, for a ductile material, beyond the ultimate tensile strength? (2)

Answer 15

Necking - the material becomes longer and thinner at weakest point
Snaps at breaking point

Question 16

What is the breaking strength of a material?

Answer 16

The stress value at the point of fracture

Question 17

What is the Young modulus? (3)

Answer 17

The constant ratio of stress to strain within the limit of proportionality
Depends only on material, not shape or size
E = σ/ε

Question 18

What is a hysteresis loop? (2)

Answer 18

The loop formed by the unloading and loading curved on a force-extension graph for a material such as rubber
More work is done stretching the rubber band so the excess energy is released as thermal energy when material is unloaded (area of loop)