2) Materials In Domestic And Industrial Applications

Question 1

What is elastic behaviour?

Answer 1

The ability of a material to spring back to its original shape and size after being stretched, squashed or otherwise distorted.

Question 2

Refer to page 129 for the stress-strain curve

Question 3

What is elastic limit (yield strength)?

Answer 3

Highest tensile stress with full elastic recovery

Question 4

What occurs past the elastic limit?

Answer 4

Permanent plastic deformation

Question 5

What is ultimate tensile strength (UTS)?

Answer 5

The highest STRESS a material can sustain just before it breaks.

Question 6

What kind of materials don’t have a yield point or plastic deformation?

Answer 6

Brittle materials such as concrete or glass they only have UTS

Question 7

What is plasticity?

Answer 7

Materials experiencing a permeant change in shape without completely breaking up, this occurs past the elastic limit.

Question 8

How do materials such as metals become permenantly deformed?

Answer 8

When the stress exceeds the elastic limit, the crystalline structure of metals allows layers of atoms to glide over each other. When stress is removed, the material stays deformed and has a PERMANENT SET.

Question 9

What does the amount of plasticity depend on in metals?

Answer 9

The microcrystalline structure of the metal, this varies between materials.

Question 10

What is 1 structure to describe metals?

Answer 10

They are brittle, due to showing little or no plasticity.

Question 11

What is a plastic that behaves similarly to metals?

Answer 11

Nylon

Question 12

What is creep?

Answer 12

A slow version of plastic deformation.

Question 13

What increases creep?

Answer 13

Temperature

Question 14

Why might creep eventually lead to failure?

Answer 14

If components no longer fit or if they neck in.

Question 15

What is fatigue?

Answer 15

Fatigue is caused by repeated cycles of loading and unloading which causes gradual hardening and crack growth.

Question 16

When does eventual fatigue failure occur?

Answer 16

At stress levels well below the normal UTS value.

Question 17

What is ductility?

Answer 17

The ability to be shaped by plastic flow under tension.

Question 18

What can you do to ductile materials?

Answer 18

Drawing into rods, wires or tubes
Deep drawing of sheet materials into bowls, cans, etc.

Question 19

What does ductility depend on?

Answer 19

Temperature, if cooled sufficiently even metals become brittle.

Question 20

What is brittle failure?

Answer 20

Small cracks growing rapidly, and break right across the item

Question 21

What is brittle failure visible by?

Answer 21

Under x-ray

Question 22

Why does brittle failure occur?

Answer 22

It occurs when stress becomes concentrated at the tips of tiny cracks and imperfectons that exist in the material

Question 23

What is malleability?

Answer 23

The ability to be shaped by plastic flow under compression

Question 24

What can you do with malleable materials?

Answer 24

Rolling/hammering into sheets
Cold forming by stamping/pressing

Question 25

Give an example of a metal that is malleable but not ductile

Answer 25

Lead

Question 26

Give an example of plastic materials that are malleable

Answer 26

Clay

Question 27

What is elastic hysteresis?

Answer 27

The difference between the strain energy required to generate a given stress in a material, and the material’s elastic energy at that stress.

Question 28

Why does elastic hysteresis occur?

Answer 28

Due to internal friction:
Both stretching and recovery take time
The force is higher during loading
Work turns into heat

Question 29

Materials that undergo elastic hysteresis, are useful for and why?

Answer 29

Cushioning shock, and dampening of oscillations because they absorb energy

Question 30

The area enclosed by the hysteresis loop equals..?

Answer 30

Energy lost due to interal friction