Solids under Stress

Question 1

What is tension?

Answer 1

The magnitude of the force that a stretched object exerts on whatever its end are attached to.
This is equal to magnitude of the force which is applied to the ends of an object to keep it stretched

Question 2

What is extension?

Answer 2

The increase in the length of an object when put under tension

Question 3

When is a material elastic?

Answer 3

If it returns to its original shape and size if the tension is removed

Question 4

What is the elastic limit?

Answer 4

The point at which the deformation ceases to be elastic

Question 5

What is Hooke’s law?

Answer 5

Provided the elastic limit is not exceeded, the extension of a spring is directly proportional to the tension
F=kx

Question 6

What is the spring constant?

Answer 6

The force per unit extension of the spring, the stiffness of the spring

Question 7

What is stress?

Answer 7

The tension per unit cross-sectional area

Question 8

What is strain?

Answer 8

The tension per unit length due to the applied stress

Question 9

What is the Young modulus?

Answer 9

E - the ratio of stress to strain for a material in the Hooke’s law region - usually in the value of around 10-200 GPa

Question 10

How do you calculate the work done in stretching out a spring?

Answer 10

F=0.5kx^2

Area under a tension-extension graph

Question 11

Define crystalline

Answer 11

consisting of crystals;regular array of particles (usually ions)

Question 12

What is a crystalline material?

Answer 12

long range, regular (unit cell repeated)
Solid with a long range order
particles in a regular arrangement called a lattice

Question 13

Define polycrystalline

Answer 13

consisting of a large number of interlocking crystals

Question 14

What are polycrystalline metals?

Answer 14

Most frequent crystalline engineering materials

Lattice particles in metals are spherical metal ions arranged hexagonally in the lattice planes

Question 15

Why are particles arranged hexagonally?

Answer 15

This gives maximum possible number of near neighbours so the potential energy of the hexagonal lattice is the lowest possible
This is the same as the plane below, so each ion is in contact with 12 near neighbours (maximum possible)

Question 16

Why do crystal planes in a metal have random orientations?

Answer 16

A piece of metal is made up of lots of crystals which all started to form seperately from molten state

Question 17

What is a metal?

Answer 17

A condensed material (solid or liquid) in which the atoms have lost one or more electrons to become positive ions, which are held together by the released ‘delocalised’ electrons

Question 18

What are amorphous materials?

Answer 18

short range, irregular
Materials with no long range order, because when they were cooled they didn’t have time to assume a crystal arrangement before they lost the ability to move

Question 19

What are ceramics?

Answer 19

Not usually formed from molten state
Partly crystalline with an amorphous matrix
Consist of molecules of metals and non-metals, with either covalently or ionically bonded

Question 20

What is a polymer?

Answer 20

A material comprising large molecules which consist of many repeat units

Question 21

What is a monomer?

Answer 21

A molecule which can combine with other molecules to form a polymer
Has one or more double bonds

Question 22

A material is plastic if…

Answer 22

when the stress is removed the material is permanently deformed

Question 23

What is a ductile material?

Answer 23

A material which can be drawn into a wire

Question 24

What is the elastic limit?

Answer 24

The stress at which the deformation ceases to be elastic

Question 25

What is the yield point?

Answer 25

The point on a stress-strain graph at which a large increase in strain occurs for little or no increase in stress

Question 26

Yield stress

Answer 26

The stress at the yield point

Question 27

What might you need to label on a stress-strain graph?

Answer 27

Limit of proportionality
Elastic limit
Yield point
Yield stress
Breaking stress
Breaking point

Question 28

What is an edge dislocation?

Answer 28

an additional part-plane of ions in a crystal
if a large enough force is applied (stress exceeds yield stress) the dislocation will move irreversibly and the crystal will suffer permanent deformation

Question 29

Easy ductile metal to draw a stress-strain graph for

Answer 29

Steel

Question 30

How can edge dislocations form large deformations?

Answer 30

Large stresses can cause a crystal plane to snap at a point of weakness (missing ion) producing two edge dislocations, which migrate in opposite directions, making the crystals elongate in the direction of the stress

Question 31

What can strengthen and stiffen a material?

Answer 31

Foreign (impurity) atoms, grain boundaries and other dislocations impede the movement of edge dislocations

Question 32

What is ductile fracture?

Answer 32

Breaking that occurs when a ductile material is stressed to a breaking point. Involves plastic deformation and necking
As the stress reaches the breaking stress, more edge dislocations are generated and migrate, causing the elongation. A decreases so stress increases, causing more dislocations in a runaway process

Question 33

Are brittle materials elastic?

Answer 33

YES

Question 34

What are reasons for brittle materials having no mobile edge dislocations?

Answer 34

They are amorphous (no regular lattice) eg glass
They are ionically or covalently bonded eg ceramics
They are metallic but have very small crystals with large numbers of impurities eg cast iron

Question 35

Why are materials brittle under tension?

Answer 35

They have microscopic cracks in the surface and no mobile edge dislocations to relieve the stress

Question 36

How are brittle materials used in engineering?

Answer 36

Propagation of cracks must be avoided

In concrete and brick structures the brittle material is always under compression so the cracks do not open up

Question 37

How is pre-stressed concrete made?

Answer 37

Pouring concrete around steel rods under tension, and the rods are slackened off when the concrete has cured, putting the concrete under compression

Question 38

How is pre-stressed glass manufactured?

Answer 38

A rapid cooling of the surface and a later cooling of the core puts the surface under compression, so a tension can be applied without the crack-bearing surface being put under tension.

Question 39

What is elastic hysteresis?

Answer 39

When a material such as rubber is put under stress and then relaxed, the stress-strain graphs for increasing and decreasing stress do not coincide but form a loop

Question 40

In a hysteresis loop what does the area between the graphs represent?

Answer 40

Energy dissipated in moving - manifests itself as random vibrational energy of rubber molecules

Question 41

What happens in elastic hysteresis?

Answer 41

First it straightens out the molecules - no bonds are stretched. Then it stretches the bonds. When it’s relaxed, natural vibrations of molecules tangles up the long chains again

Question 42

What is vulcanisation?

Answer 42

Introducing cross-linkage between molecules or different parts of the same molecule

Question 43

Polymeric

Answer 43

long chain molecules (no order between, only

within molecules)