Section 5 - Materials

Question 1

What is Hooke’s Law?

Answer 1

Extension (ΔL) is directly proportional to force applied (F), given that the environmental conditions are kept constant
k - stiffness constant

Question 2

What is meant by tensile stress?

Answer 2

The force applied per unit cross sectional area Nm^-2

Question 3

What is tensile strain?

Answer 3

A measure of how the material strectcehs: the extension (ΔL) divided by the original length
No units

Question 4

What is the difference between elastic and plastic deformation?

Answer 4

Elastic deformation - when the force is removed the object will return to its original shape
Plastic deformation - after the load is removed the object will not return to its original shape

Question 5

What is breaking stress?

Answer 5

The minimum stress needed to break a material

Question 6

How does stress affect the material at an atomic level?

Answer 6

1. The atoms begin to be pulled apart from one another
2. The stress becomes so great that atoms separate completely and the material breaks - breaking stress

Question 7

What is the Ultimate Tensile Strength?

Answer 7

The maximum stress that the material can withstand - just before the breaking stress

Question 8

How is a brittle material represented on a stress-strain graph?

Answer 8

• Steep gradient, bends slightly then suddenly stops
• The wire undergoes only very small increase in strain beyond the linear section before fracture

Question 9

What is meant when a material is described as brittle?

Answer 9

It doesn’t perform plastically but fractures when the stress reaches a certain value

Question 10

What is the elastic limit?

Answer 10

The force above which the material will be plastically deformed (permanently stretched)

Question 11

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

Answer 11

The work done to deform the material

Question 12

What is Young’s modulus?

Answer 12

Describes the stiffness of a material

Question 13

How do you find the Young’s modulus from a stress-strain graph?

Answer 13

The gradient of the line

Question 14

What does a graph of plastic deformation look like on a force extension graph?

Answer 14

The line is a straight line and then bends across - the unloading line doesn’t go through the origin as the material is permanently extended (stretched)

Question 15

What can a force-extension graph show Hooke’s Law is being obeyed?

Answer 15

When it is a straight line through the origin - force and extension are directly proportional

Question 16

What is the limit of proportionality and what does it look like on a force-extension graph?

Answer 16

The point after which Hooke’s law is no longer obeyed, it is shown by the line beginning to curve on a force-extension graph

Question 17

How is the work done to stretch or compress a material stored?

Answer 17

Elastic strain energy

Question 18

Why are the loading and unloading lines parallel on a force-extension graph for a plastically deformed material?

Answer 18

The stiffness constant (k) hasn’t changed, the force between the atoms are the same when loading and unloading

Question 19

Why isn’t all work done stored as elastic strain energy when a stretch is plastic?

Answer 19

Work is done to move atoms apart, so energy is not stored as elastic strain energy but is dissipate as heat

Question 20

How is dissipation of energy in plastic deformation used to design safer vehicles?

Answer 20

Crumple zones - deform plastically in a crash using the car’s kinetic energy so is less transferred to the passengers
Seat belts - stretch to convert the passenger’s kinetic energy into elastic strain energy

Question 21

Outline the energy changes that occur when a spring fixed at the top is pulled down and released

Answer 21

The work done in pulling down (stretching it) is stored as elastic strain energy, when the spring is released this is converted to kinetic energy which is converted to gravitational potential energy as the spring rises

Question 22

Do stress-strain graphs show the behaviour of a material or a specific object?

Answer 22

Material

Question 23

Where would you find the ultimate tensile stress on a stress-strain graph?

Answer 23

The highest point on a graph, it is the maximum stress a material can withstand

Question 24

What would the stress-strain graph for a ductile material look like?

Answer 24

A ductile material can undergo a large amount of plastic deformation before fracturing

Question 25

What is the equation for energy per unit vol?

Answer 25

1/2 x stress x strain

Question 26

WHat is the yield point on a stress-strain graph?

Answer 26

The stress at which a large amount of plastic deformation takes place with a constant or reduced load

Question 27

What are force-extension graphs used for?

Answer 27

Specific to the tested object and depend on its dimensions

Question 28

What are stress-strain graphs used for?

Answer 28

They describe the general behaviour of a material, as stress and strain are independent of the dimensions