Unit 1.5: Solids Under Stress

Question 1

There are 3 types of solids, name them.

Answer 1

Crystalline, Amorphous and Polymeric

Question 2

What is a crystalline solid?

Answer 2

These solids have short and long range order; there is a regular pattern of atoms.; the particles are arranged in a lattice

Question 3

What is an amorphous?

Answer 3

These solids have no long range order and have an irregular pattern of atoms

Question 4

What is a Polymeric solid?

Answer 4

Polymers consist of very long chains of carbons. The can be both natural (protein, rubber) or synthetic (nylon, polythene)

Question 5

Give an example of each solid

Answer 5

Crystalline: Salt
Amorphous: Glass, Brick
Polymeric: Rubber, nylon

Question 6

What solids will extend under tension?

Answer 6

Crystalline and amorphous

Question 7

State Hooke’s law

Answer 7

Provided, the elastic limit is not exceeded, the extension of a body is proportional to the applied load

Question 8

State the equation associated with Hooke’s law

Answer 8

F=kx

Question 9

What graph does Hooke’s law lead to?

Answer 9

Force-Extension graph

Question 10

On a force-extension graph, what kind of gradient would a stiff material have and a less stiff material have?

Answer 10

If it was a stiff material, it would have a steep gradient and if it was less stiff it would have less of a gradient.

Question 11

On a force-extension graph, what is the area under the graph equal to?

Answer 11

The Work done by the force on the object

Question 11

What is Young’s modulus

Answer 11

E=Stress/strain E=σ/ε

Question 12

What is stress equal to?

Answer 12

Force/area

Question 13

What is strain equal to?

Answer 13

Extension/original lenght

Question 13

On a stress and strain graph what is point ‘p’?

Answer 13

The limit of proportionality

Question 13

What law will a material follow from 0 to point ‘p’

Answer 13

Extension is proportional to load so it follows Hooke’s law

Question 14

What is point ‘e’ on a stress and strain graph

Answer 14

This is the elastic limit; up to this point, the material will return to original shape and size after force is removed.

Question 15

What is point ‘Y1’ on a stress and strain graph?

Answer 15

Y1 is the yield point. At this point large extension occurs with little or no stress as planes of atoms start to slip past each other

Question 16

What is point ‘Y2’ on a stress and strain graph?

Answer 16

In some materials, the material stretches so much that the stress is actually reduced for a while e.g copper.

Question 17

What is the region between points ‘E’ and ‘X’ on a stress and strain graph called?

Answer 17

The plastic region-the material will not return to its original size once it enters this region.

Question 18

What is point ‘X’ on a stress and strain graph?

Answer 18

This is known as the breaking point; this signifies the ultimate tensile strength for most material.

Question 19

What is point ‘X2’ on a stress and strain graph?

Answer 19

Some very ductile materials like copper become copper become narrower and extend rapidly just before breaking

Question 20

What are edge dislocations?

Answer 20

Edge dislocations are where there is an
extra plane in the crystals, plastic deformation occurs when the
dislocations move due to the large stress

Question 21

What are ductile fractures?

Answer 21

Ductile fractures (necking) is where the number of edge dislocations increases and causes the elongation of the metal which increases the stress at the neck

Question 22

Do brittle materials stretch very much?

Answer 22

No. The tend to fracture at a lower stress than expected due a process known as brittle fracture.

Question 23

What is a brittle fracture?

Answer 23

This occurs when cracks in the surface of the
materials magnify the stress at
that point and cause the material
to break.

Question 24

How can brittle fractures be avoided?

Answer 24

To avoid brittle fractures, materials
like concrete and pre-stressed glass are always under
compression to stop cracks opening.

Question 25

What is different about about a stress and strain graph for rubber?

Answer 25

There are 2 curves, stretching and contracting

Question 26

What is the name given to the curve on a stress and strain graph for rubber?

Answer 26

Elastic hysteresis

Question 27

On a stress and strain graph for rubber, is there a greater work done in stretching, or contracting?

Answer 27

As the area under the curve = the
work done, more work is done
when stretching than when
contracting. This means the
extra energy used to stretch is
transferred to vibrational energy in
the rubber molecules.

Question 28

Why will the force exerted on a crystalline material be transmitted equally to each bond?

Answer 28

Because of the long range order

Question 29

When does plastic behavior occur?

Answer 29

When a solid is extended beyond its elastic limit. The atoms in one plane can slip over atoms in another plane, if the forces are great enough.

Question 30

Molten metal cools down when smelted. What can happen in terms of ions and what is it called?

Answer 30

Mistakes happen as the metal ions join the crystal. One in every million atomic planes or so, half a plane of atoms is missed out. This is known as edge dislocation

Question 31

Edge dislocations are the key to what?

Answer 31

Plastic deformation

Question 32

The bonds around the missed plane of atoms are all under strain. These bonds are places of__?

Answer 32

Weakness

Question 33

If a small force is put on the side of metal which has an edge dislocation, what happens?

Answer 33

The horizontal bonds are stretched reversibly and the material behaves elastically

Question 34

If a large force is put on the side of metal which has an edge dislocation, what happens?

Answer 34

The already strained bonds around the edge dislocation are stretched even more. At the yield point of the metal, the bonds snap making the dislocation move.

Question 35

If a large force is put on the side of a metal and the yield point has already been passed, what happens to the edge dislocation?

Answer 35

The edge dislocation carries on moving, through snapping and reforming of bonds, until it reaches the edge of the crystal

Question 36

How can metals be strengthened to avoid plastic deformation?

Answer 36

1. Having smaller grains-this restricts movement of dislocations
2. Introducing foreign atom. This creates a point which inhibits the movement of dislocations

Question 37

Glass is an example of what solid?

Answer 37

Amorphous

Question 38

Does glass have a plastic region on a force extension graph?

Answer 38

No, it does not

Question 39

Why are amorphous solids stiff?

Answer 39

They have no regularity in the way their molecules are locked together in the solid. The lack of crystalline structure makes the dislocation slip impossible for amorphous

Question 40

Why are glasses brittle?

Answer 40

Because crack can travel through them easily.

Question 41

How do glasses crack?

Answer 41

A relatively small stress can make a microscopic crack on the surface grow uncontrollably through the solid until it snaps in two

Question 42

How are sheets of glass cut?

Answer 42

Firstly, a small crack is scored on the surface of the glass with a sharp instrument. Then a modest stress is applied to the glass in such a way as to force the crack open. The result is (usually) a clean fracture along the line of weakness defined by the original crack.

Question 43

When cutting the glass, does the initial crack have to be deep?

Answer 43

No. A short but narrow crack can result
in a large local increase in stress. So the material at the crack tip will reach its ultimate tensile stress well before the rest of the material does. The crack will therefore be able to grow even though the average stress on the sample is well below the material’s tensile strength.

Question 44

What is the main factor that dictates the behavior of a polymer?

Answer 44

Cross-links

Question 45

What are cross-links?

Answer 45

Strong covalent bonds that form between overlapping molecules or even between different parts of the same molecule.

Question 46

In a polymeric solid, if the molecules don’t have many cross-links what does that mean in terms of ‘flexibility’?

Answer 46

The molecules can slide over each other quite easily. This makes the polymer stretchy like rubber (latex)

Question 47

How do you increase the number cross-links in a polymer?

Answer 47

Adding impurities such as sulfur

Question 48

What happens if you increase the number of cross-links in a polymer?

Answer 48

The polymer becomes rigid. The latex is then given a new name-‘vulcanite’

Question 49

Cross links are strong____

Answer 49

Covalent bonds

Question 50

Cross links are scarce in natural rubber. What does this mean on a molecular level?

Answer 50

There aren’t enough cross-links to keep the molecules in a tangled, knotted mess.

Question 51

What is the name given to the bonds formed in the tangled mess of molecules in a polymer?

Answer 51

van der Waals forces

Question 52

Are ‘van der Walls forces’ weak bonds or strong bonds?

Answer 52

Weak bonds

Question 53

How are van der Waals bonds formed?

Answer 53

Every time one part of a molecule comes very close to another molecule a van der Waals force can occur – think of it as a weak bond that acts only over very short distances. Many of these are produced between the rubber molecules, that tend
to keep the molecules stuck together.

Question 54

What do van der Waal bonds explain on a force extention graph?

Answer 54

The initial ‘stiffness’ of the rubber material

Question 55

The strength of van der Waal decrease rapidly with distance. After the limit of proportionality, what starts to happen between the molecules?

Answer 55

The molecules begin untangling from themselves and from each other. This is why the plastic region on a stress-strain graph has a large extension with a low stress

Question 56

Once the molecules have been straightened out on a stress-strain graph for polymer, stretching the rubber any further requires a great deal of force. Why is this?

Answer 56

Stretching the rubber any further requires that the strong covalent bonds in between each molecule are lengthened as well as breaking the cross-links. This is much more difficult than unraveling the molecules and so the gradient becomes much steeper.

Question 57

Looking at the force-extension graph for a rubber band, the extension curve has
little extension at the start. Why?

Answer 57

This is because all those cross-links between the molecules are making it difficult to stretch.

Question 58

On a force extension graph for a polymer, once the rubber has been fully stretched the cross links have re-attached at new positions. What happens when the tension is decreased?

Answer 58

it is harder for the molecules to (initially) pass over each other, i.e. rubber band doesn’t contract as easily as expected.

Question 59

Why is there a ‘gap’ in the middle of the hysteresis?

Answer 59

The net effect is that there’s work done internally, a bit like friction. This work done over one extension-contraction cycle is released as heat, and its value is equal to the area between the two curves.