1. Deformation and structure of solids Flashcards Preview

CHEMMAT 121 > 1. Deformation and structure of solids > Flashcards

Flashcards in 1. Deformation and structure of solids Deck (42)
Loading flashcards...
1
Q

Two modes of failure

A

fracturing and permanent deformation

2
Q

Define stress and give the symbol and units

A

Force/Area, sigma, N/m^-2

3
Q

Define strain and give the symbol and units

A

extension/original length, epsilon, no units

4
Q

Apparatus used to measure stress/strain (two names)

A

Universal Testing Machine, Tensometer

5
Q

(Tensometer) Load cell measure what?

A

Force

6
Q

The movement of the bottom bar in a tensometer represents what?

A

Change in length

7
Q

What is the Lo (original length) of the dogbone samples used in a tensometer?

A

The length between the two nodes (not including the end cylinders)

8
Q

What is the relationship of stress and strain in the elastic region?

A

Stress is directly proportional to strain

9
Q

What is the elastic region also known as?

A

Recoverable strain region

10
Q

What does Poisson’s ratio represent? What is the formula and symbol?

A

How thin a material gets in relation to the extension. Symbol: Nu = strain(x)/strain(z)

11
Q

What is an isotropic material?

A

One that has the same properties in all directions

12
Q

How do you calculate the elasticity (stiffness) of a material? What are the other names of this property?

A

Gradient of a stress vs strain graph, delta stress / delta strain, force required per unit extension. Other names: Young’s Modulus, Elastic Modulus

13
Q

What is the yield point?

A

When plastic deformation starts

14
Q

Plastic deformation does what to the yield point? What is this process called?

A

Makes it higher; the yield stress is higher so the material is stronger. Work hardening.

15
Q

What is necking?

A

A localised reduction in area, the stress/strain graph at that point curves down

16
Q

What is the highest stress on a stress/strain graph called?

A

Ultimate tensile stress

17
Q

After what point does necking start?

A

After the ultimate tensile stress point

18
Q

What is the area under the stress/strain graph proportional to?

A

The energy required to deform it

19
Q

What is the special property of steel’s stress strain graph and what does it look like?

A

Discontinuous yield: the ‘spikes’ after the yield point. (After work hardening there’s no more of it.)

20
Q

Does 0.2% proof stress increase accuracy or reliability or both?

A

Reliability only

21
Q

When can we use a low-ish safety factor?

A

When the load and the environment are known with a high degree of certainty/accuracy

22
Q

How do you get a true stress/strain graph from an engineering one?

A

Calculate the true stress using force/instantaneous area, as the area changes due to ‘necking’

23
Q

Two ways of quantifying ductility

A

%elongation, change in length/original length. %reduction in area change in area/original area

24
Q

What is the repeating unit of a crystal lattice called?

A

the unit cell

25
Q

Body Centered Cubic - how many atoms per unit cell and what is the coordination number?

A

2 atoms per unit cell, coord number 8

26
Q

symbol for unit cell dimension and how to calculate it

A

symbol: a. Use pythagoras and lengths in terms of atomic radii

27
Q

formula for APF, Atomic Packing Factor

A

volume of atoms(in unit cell)/volume of unit cell

28
Q

number of atoms per unit cell and co ordination number of the Face Centered Cubic unit cell

A

4 atoms per cell, coordination: 12

29
Q

atoms in a unit cell and co ordination of hexagonal close packed

A

atoms: 6, coord: 12

30
Q

coordination of a CPS, close packed structure

A

12 (highest)

31
Q

Define interstitial sites

A

gaps between atoms (in a crystal lattice or other giant structure)

32
Q

Define polymorphism

A

materials that can exist in more than one crystal form

33
Q

Describe the polymorphism of iron

A

below 912 degrees C it is BCC, over it is FCC

34
Q

what is the relationship between atomic bond strength and young’s modulus?

A

strong atomic bonds = high young’s modulus

35
Q

slipping in close packed planes vs non close packed planes

A

close packed plane, easier to slip as less distance

36
Q

relationship between slip and ductility

A

higher ductility = easier to slip

37
Q

name of empty atomic space in crystal and impact on strength

A

vacancy, no effect

38
Q

atom of different type replaced in crystal structure and effect on strength

A

substitution, slight increase (usually)

39
Q

atoms in interstitial space are called

A

interstitials

40
Q

another name for planar defects and state the two types

A

dislocations: edge dislocation and screw dislocation

41
Q

explain briefly why slip is easier in dislocation movement

A

only a single line of bonds are broken

42
Q

what brackets do you use to name: vectors, groups of crystallographically equivalent vectors, planes and crystallographically equivalent planes?

A

vectors: [], CE vectors: , planes: ( ), CE planes: { }