Unit 4 - Plastic Deformation

Question 1

what is yield stress

Answer 1

The stress at which a material beings to yield (plastically deform)

Question 2

Yield stress of ceramics

Answer 2

High yield

they do not demonstrate a characteristic yield due to toughness

Question 3

Yield stress of metals

Answer 3

Have lower yields than ceramics and are more ductile.

they have the ability to deform plastically without fracturing due to metallic bonding

Question 4

Yield stress of polymers

Answer 4

Have low yield strengths meaning they are ductile materials

Question 5

ultimate tensile strength

Answer 5

The maximum stress on the engineering stress-strain curve

Question 6

Toughness

Answer 6

The materials ability to absorb energy before fracturing

Question 7

why and when is true stress used?

Answer 7

when strain is larger than 1% some degree of plastic deformation will occur. True stress is more accurate as it take i to account the change in geometry of the sample.

Question 8

In the plastic region how do the true stresses and strain compare to the engineering ones?

Answer 8

true stress > engineering
The current area is smaller than the original
true strain < engineering
Due to the length after strain being greater than the original gauge length

Question 9

How is the safety factor used

Answer 9

So that the maximum working stress < yield stress/ S

Question 10

what is cold working and why is it used?

Answer 10

Loading materials past the yield stress increasing the strength of the material.
It is done to reduce the ductility of a material and is induced during manufacturing

Question 11

What are dislocations and how are they formed?

Answer 11

it is irreversible deformation on a microscopic scale which are created by fluctuations in local stress fields in the material causing a lattice rearrangement..

Question 12

what is ductility?

Answer 12

The ability of the material to deform permanently prior to rupture
usually defined as engineering strain at failure

Question 13

Toughness

Answer 13

The ability of a material to absorb energy and plastically deform without fracturing.
area under the stress strain curve

Question 14

What characteristics determine is a material is tough

Answer 14

it is strong and ductile, the material should be able to withstand high stresses and high strains

Question 15

what is hardness?

Answer 15

Resistance to surface indentation

Question 16

What affect does temperature have on materials ductility

Answer 16

Increased temperature produces greater ductility, it also reduces the yield stress and strain hardening

Question 17

relationship between strength and ductility

Answer 17

Inverse and curved a bit

Question 18

How is toughness related to temperature

Answer 18

there is a transition temperature of Tg. Before this point materials are brittle and have low toughness.
After this point materials are ductile and have high toughness