1.5 Solids Under Stress

Question 1

What are the three different types of solids?

Answer 1

Crystalline, Amorphous, Polymeric

Question 2

Crystalline

Answer 2

Atoms are arranged in a regular array (lattice). There is long and short order. They can be made from a single crystal or be polycrystalline

Question 3

Amorphous

Answer 3

Atoms arranged randomly such that they have no long range order, but may have short range order

Question 4

Polymeric

Answer 4

Made up of long chain-like molecules. There is no long range order between molecules but there is order within each molecule

Question 5

Hooke’s law

Answer 5

The tension in a spring or wire is directly proportional to its extension from its natural length, provided the extension is not too great. F = kx

Question 6

Spring constant

Answer 6

Is the force per unit extension

Question 7

Stress, σ = … / …

Answer 7

F
A

Question 8

Strain, ε = … / …

Answer 8

ΔL
L

Question 9

Young’s Modulus, E = … / …

Answer 9

Stress, σ
Strain, ε

Question 10

The work done in a deforming solid is equal too …

Answer 10

the area under a force extension graph

Question 11

On a Stress-Strain graph for brittle materials the point P, is called …

Answer 11

The Limit of proportionality

Question 12

On a Stress-Strain graph for brittle materials the point E, is called …

Answer 12

The Elastic Limit

Question 13

Brittle Material

Answer 13

A material with no region of plastic deformation. Under tension, it breaks by brittle fracture, caused by crack propagation

Question 14

What happens for the first section of a Force-Extension graph for rubber?

Answer 14

The stretching of the rubber means the breaking of cross-link bonds, this makes it stiff, which means it requires more force per unit of extension

Question 15

What happens for the second section of a Force-Extension graph for rubber?

Answer 15

Stretching further only involves breaking weak Van Der Waals forces between the long chain molecules, this makes it less stiff, which means it requires less force per unit extension

Question 16

What happens for the third section of a Force-Extension graph for rubber?

Answer 16

Stretching even further now means trying to stretch the actual long chain molecules which is not easy due to the strong covalent bonds within these molecules, this makes it very stiff, which means it requires exponentially more force per unit extension until it eventually breaks

Question 17

What are the two points about a Stress-Strain graph for brittle materials that can be noted?

Answer 17

1. The material doesn’t stretch much before fracturing
2. The material pretty much obeys Hooke’s law until the fracture

Question 18

Dislocations

Answer 18

Are small gaps of atoms missing in the lattice array of a crystalline solid, it allows the material to extend further then normal buy gradually moving the each of the dislocations to the edge of the material. This leads to a part of the Force-Extension graph having small increases in force but larger increases in extension.