1.5 Solids under stress

Question 1

What will happen when a force is applied to a wire?

Answer 1

It will extend. Increasing the force will increase the extension up to a point where the string becomes permanently deformed or stretched.

Question 2

What does Hooke’s law state?

Answer 2

The tension in a spring or wire is proportional to its extension from its natural length provided the extension is not too great

Question 3

What is the equation for Hooke’s law?

Answer 3

F = kx

Question 4

What is the equation for work in relation to force and extension?

Answer 4

W = 1/2 Fx

Question 5

What is the equation for energy stored in a spring?

Answer 5

W = 1/2 kx^2

Question 6

What is the equation for Young Modulus?

Answer 6

E= σ / ε (stress/strain)

Question 7

How do we find stress?

Answer 7

σ = F/A (force/cross sectional area)

Question 8

What are the units of stress?

Answer 8

Nm-2 or Pa

Question 9

How do we find strain?

Answer 9

ε = Δl / l (extension/original length)

Question 10

What is the unit for strain?

Answer 10

It doesn’t have one

Question 11

What are the units for Youngs Modulus?

Answer 11

Nm-2 or Pa

Question 12

What is elasticity?

Answer 12

The ability of a material to resist deformation and return to its original size and shape after an applied stress has been removed

Question 13

What is the elastic region of a stress strain graph?

Answer 13

A material will have linear elastic behaviour up to the proportional limit where it will still behave elastically but in a non linear way.

Question 14

What is the Yield point of a stress strain graph?

Answer 14

Its elastic limit, at the end of the elastic region of the graph

Question 15

What is the plastic region of a stress strain graph?

Answer 15

Beyond the yield point a material will no longer return to its original length. It is now permanently stretched.

Question 16

What is ultimate tensile strength?

Answer 16

The amount of stress a material can endure before it breaks

Question 17

What is the area under a stress strain graph and what is the equation?

Answer 17

EPE stored per unit volume.
1/2 σε = EPE / V

Question 18

When does necking occur?

Answer 18

When a material is stretched beyond its ability to behave as an elastic.

Question 19

What is necking?

Answer 19

As a material stretched and extends it must maintain the same volume so it becomes thinner. Eventually the material will ‘neck’ as an area of the material becomes thinner more quickly

Question 20

What are the three main classifications of a solid?

Answer 20

1. crystalline
2. amorphous
3. polymeric

Question 21

What order does Crystalline have?

Answer 21

Long and short range order

Question 22

What order does Amorphous have?

Answer 22

No long range order. Possible short range order.

Question 23

What order does Polymeric have?

Answer 23

Order within molecules. No long range order between chains

Question 24

What arrangement does Crystalline have?

Answer 24

Lattice of regularly repeated patterns of individual atoms

Question 25

What arrangement does Amorphous have?

Answer 25

irregular arrangement of individual atoms

Question 26

What arrangement does Polymeric have?

Answer 26

Long chain molecules randomly arranged throughout material

Question 27

What are some examples of Crystalline material?

Answer 27

Sodium chloride, Diamond, Graphite and Quartz

Question 28

What are some examples of Amorphous material?

Answer 28

Glass, Porcelain, Brick and Cement

Question 29

What are some examples of Polymeric material?

Answer 29

Polyethene, Polyester, Protein and Teflon

Question 30

In plastic deformation, in which direction do new bonds form?

Answer 30

To the left

Question 31

What happens to dislocations when a solid is under stress?

Answer 31

They are pushed out to the right until they’re on the end

Question 32

What are the main ways of strengthening a metal?

Answer 32

1. Alloys
2. Work Hardening
3. Polycrystalline grains

Question 33

How do alloys strengthen a metal?

Answer 33

Metals can be made stronger by introducing ‘foreign’ atoms into the metal lattice. The foreign atom prevents the movement of dislocations by acting as additional barriers for edge dislocations.

Question 34

How does work hardening strengthen a metal?

Answer 34

By squeezing, bending and shearing the material dislocations are created. As more dislocations are created it eventually becomes more difficult for dislocations to form. As dislocations move through the material they meet and are unable to pass each other. Because they are unable to move the material becomes harder.

Question 35

How do polycrystalline grains strengthen a metal?

Answer 35

By heating metals up past their recrystallisation temperature and then rapidly cooling, it forms smaller grains. Smaller grains mean dislocations have less distance to travel which prevents further motion of dislocations when they meet a boundary.

Question 36

Where does brittle fracture occur?

Answer 36

In amorphous solids like glass

Question 37

What are the main ways to strengthen brittle materials?

Answer 37

1. Reinforcing the material
2. Pre-stressing the material

Question 38

What is a reinforced material?

Answer 38

A combination of two materials - one that is strong in tension and the other strong in compression

Question 39

What is a pre-stressed material?

Answer 39

A material that shatters into small crystal shards and not large sharp ones