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Physics > Mechanical properties of materials > Flashcards

Mechanical properties of materials Flashcards

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1
Q

Brittle

A

Breaks suddenly without deforming plastically, tend to shatter

Ceramics, glass

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2
Q

Ductile

A

Can be drawn into wires without losing its strength

Copper

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3
Q

Strong

A

Can withstand high stress without deforming or breaking

Steel

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4
Q

Hard

A

Very resistant to cutting, indentation and abrasion

Diamond, cutting tools (needs to be harder than what its cutting)

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5
Q

Stiff

A

High resistance to bending and stretching

Safety helmets

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6
Q

How do we measure stiffness?

A

Young modulus

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7
Q

Tough

A

Really difficult to break, can absorb a lot of energy without breaking

Polythene

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8
Q

What is the limit of proportionality?

A

Point where a material stops obeying Hooke’s law but would still return to its original shape when the force is removed

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9
Q

What is the elastic limit of a material?

A

Point when the material starts to behave plastically (won’t return to original shape once forces are removed, its permanently stretched)

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10
Q

What is the yield stress of a ductile material?

A

When the material suddenly starts to stretch without any load (lots of plastic deformation)

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11
Q

What is Hooke’s law?

A

That the extension of a material is directly proportional to the force applied to it

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12
Q

What is the equation for Hooke’s law?

A

F=kx

k is the spring constant/stiffness of an object

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13
Q

What is the unit of k?

A

Nm-1

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14
Q

What do tensile forces do to a spring?

A

They pull on the spring at both ends so stretch it

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15
Q

What happens beyond the elastic limit?

A

The material will stretch further for a given force

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16
Q

What is plastic deformation?

A

Atoms in the material move position relative to one another and when the load is removed, the atoms dont return to their original positions

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17
Q

How do you calculate stress? units

A

stress = tension / cross-sectional area

Nm-2 or Pa

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18
Q

What is stress?

A

The force applied per unit area

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19
Q

What is strain?

A

The extension divided by the original length

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20
Q

How do you calculate strain? units

A

extension / original length

no units

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21
Q

What is the fracture stress?

A

The amount of stress required to break a material

(when the atoms are separated completely)

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22
Q

What is the effect of stress on a material?

A

Pulls the atoms in a material apart

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23
Q

What is the ultimate tensile strength?

A

The maximum stress a material can withstand before breaking

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24
Q

What is elastic strain energy?

A

The energy stored in a stretched material

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25
Q

What is the area under a force - extension graph?

A

Work done in deforming a material

26
Q

How do you calculate elastic strain energy (work done in stretching a material)?

A

E = 1/2 Fx

E = 1/2 kx^2

27
Q

Why isn’t all the energy when the forces are removed from a material that’s exceeded its elastic limit released?

A

Some work is done separating the atoms so this energy isn’t stored as elastic strain energy

28
Q

What is true of stress and strain up to the limit of proportionality?

A

They are directly proportional

29
Q

What is Young modulus?

A

A measure of stiffness, the ratio of stress to strain

E = stress/strain (Nm-2 or Pa)

30
Q

When finding the Young modulus of a wire what are the ideal dimensions of the wire?

A

Long and thin so that it extends more for the same force which reduces the uncertainty in the measurements

31
Q

What do you need to find before you can start the young modulus experiment and how do you find it?

A

Cross sectional area of the wire

Use a micrometer to find the diameter of the wire in three places and take an average

32
Q

How can you reduce random errors when finding the Young modulus of a wire?

A

Use a thin marker on the wire and always look directly above the marker and ruler when measuring the extension

33
Q

What is the gradient of a stress - strain graph?

A

Young modulus

34
Q

What does the area under a stress - strain graph represent?

A

Elastic strain energy per unit volume

35
Q

Why is copper ideal for electric wires?

A

Ductile, high electrical conductivity and keeps its strength when stretched

36
Q

Why do we use steel beams in bridges?

A

Very strong so can withstand large forces (like lots of cars) without bending or breaking

37
Q

Describe the structure of metals?

A

Crystalline structure with delocalised electrons making them good electrical and thermal conductors

38
Q

What are the properties of metals?

A

Stiff - strong metallic bonds
Tough - Strongly bonded lattice structure
Ductile - when force is applies the ions move

39
Q

Describe elastic deformation in metals

A

When force is applied the interatomic spacing increases uniformly

40
Q

Describe plastic deformation in metals

A

The layers slip over each other

41
Q

What happens to the amount of stress required to make ions slip when there are dislocations?

A

Stress needed is less

42
Q

What does alloying do?

A

Pins down dislocations within a metal which increases the stress needed to cause slipping so it makes them harder and less ductile

43
Q

Examples of ceramics

A

Pottery, bricks and glass

44
Q

How are ceramics made?

A

Melting certain materials and then letting them cool

45
Q

What is the arrangement of atoms in ceramics?

A

crystalline, polycrystalline or amorphous

46
Q

What is a polycrystalline structure?

A

When there are many grains (regions) of crystalline structure

47
Q

What is a grain?

A

A group of atoms which line up in the same direction

48
Q

What is an amorphous arrangement?

A

There’s no overall pattern, the atoms are arranged randomly eg glass

49
Q

What increases a ceramics chances of being amorphous?

A

If a molten ceramic is cooled quickly

50
Q

Why are amorphous structures brittle?

A

There are no slip planes or mobile dislocations so they dont deform plastically before they fracture

51
Q

What makes ceramics stiff?

A

Ceramics are either ionically or covalently bonded in a giant rigid structure so have lots of strong bonds

52
Q

Why do cracks spread through brittle materials when they fracture?

A

The applied force acts on a small area (tip of the crack) meaning the stress is high

53
Q

What is a polymer?

A

A molecular chain made up of monomers

54
Q

What is the bonding in polymers?

A

Covalent

55
Q

Why are polymers strong?

A

Covalent bonds makes them hard to seperate

56
Q

Why are polymers flexible?

A

The polymer chains are often entangled and can be unravelled by rotating about their bonds when you pull them

57
Q

What makes a more rigid polymer?

A

Lots of strong cross linking bonds

58
Q

What was Rayleigh’s oil experiment and what did it find?

A

Calculating atomic size

Olive oil was realised one drop at a time into a tub of water until it just covered the entire surface, assuming it spreads as much as it could the thickness would be the size of a molecule of oil

If you know the surface area of the bath then thickness of a molecule = volume of oil dropped / surface area

Thickness of an atom is thickness of an oil molecule divided by no. atoms in an oil molecule

59
Q

What methods do we now use for measuring size of atoms and atomic spacing?

A

X-ray crystallography

STM (Scanning tunneling microscopes)

SEM (Scanning electron microscopes) and AFM (Atomic force microscopes)

60
Q

How does X-ray crystallography work?

A

Firing electrons at a sample and using their diffraction patterns to investigate atomic spacing and structure

61
Q

How do Scanning Tunnelling Microscopes work?

A

They have a very fine tip which a voltage is applied to, electrons from the samples surface tunnel from the surface to the tip and cause a current to flow.

The tip is moved across the sample surface and the height of the tip is adjusted to keep the current constant so any small bumps or dips in the surface are detected.

An STM has such a fine resolution that individual atoms can be resolved and their size and spacing measured

62
Q

How do Scanning Electron Microscopes and Atomic Force Microscopes work?

A

Measure atomic sizes by building up an atom by atom image of the surface on a computer screen. By knowing the magnification of the image on the computer screen and the size of the blobs representing each atom, atomic size can be calculated

Physics (16 decks)

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