G491 2 - Designer Materials

Question 1

Stress

Answer 1

Force applied divided by the cross-sectional area it is applied to. We use the Greek letter sigma, σ, for Stress.
Stress = Force/Area. (Units of stress: Nm-2 or Pa). There are several types of stress e.g. Tension (stretching), compression, torsion (twisting).

Question 2

Elastic deformation

Answer 2

A temporary change of shape that occurs when a stress is applied. When the stress is removed the object/material returns to its original shape (without loss of energy).

Question 3

Plastic deformation

Answer 3

A permanent change of shape that occurs when a stress is applied.

Question 4

Strong

Answer 4

A large STRESS is needed to cause failure.
(Note some materials have 2 values for strength as there are 2 types of failure: yielding and breaking/fracture)
(Units of strength: Pa)

Question 5

Strength

Answer 5

The maximum stress a material can withstand before failure.
(Note some materials have 2 values for strength as there are 2 types of failure: yielding and breaking/fracture).
(Units of strength: Pa)

Question 6

Weak

Answer 6

Opposite of strong. i.e. small stress results in failure.

Question 7

Toughness

Answer 7

When some materials are stressed past their elastic limit, they do not immediately break but undergo considerable plastic deformation before breaking. In so doing, they absorb a large amount of energy before they break. (Units of toughness are usually J/m2 )
Toughness is defined as either:
a) The energy required to create new surface area. Units in this case are J/m2
b) The energy absorbed per unit volume. Units in this case are J/m3

Question 8

Tough

Answer 8

A tough material:

• Needs a large amount of energy to break it.
• Undergoes a large amount of plastic deformation before breaking (usually)
• Resists crack propagation (= passing through).

Question 9

Brittle

Answer 9

Brittle is the opposite of tough.
Brittle materials do not undergo much plastic deformation.
They break soon after their elastic limit.
Cracks pass easily through the material and fracture tends to be sudden and catastrophic.
Brittle materials can be strong.

Question 10

Hard

Answer 10

Resistive to dents or scratches. (Units of hardness: Pa)

Question 11

Ductile

Answer 11

Can easily be drawn into a wire.

Question 12

Malleable

Answer 12

Can easily be hammered or pressed into a shape.

Question 13

Resistance (R)

Answer 13

x

Question 14

Conductance (G)

Answer 14

x

Question 15

Resistivity (ρ)

Answer 15

x

Question 16

Conductivity (σ)

Answer 16

x

Question 17

Conductors

Answer 17

x

Question 18

Insulators

Answer 18

x

Question 19

Semiconductors

Answer 19

x