1. Deformation and structure of solids

Question 1

Two modes of failure

Answer 1

fracturing and permanent deformation

Question 2

Define stress and give the symbol and units

Answer 2

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

Question 3

Define strain and give the symbol and units

Answer 3

extension/original length, epsilon, no units

Question 4

Apparatus used to measure stress/strain (two names)

Answer 4

Universal Testing Machine, Tensometer

Question 5

(Tensometer) Load cell measure what?

Answer 5

Force

Question 6

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

Answer 6

Change in length

Question 7

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

Answer 7

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

Question 8

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

Answer 8

Stress is directly proportional to strain

Question 9

What is the elastic region also known as?

Answer 9

Recoverable strain region

Question 10

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

Answer 10

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

Question 11

What is an isotropic material?

Answer 11

One that has the same properties in all directions

Question 12

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

Answer 12

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

Question 13

What is the yield point?

Answer 13

When plastic deformation starts

Question 14

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

Answer 14

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

Question 15

What is necking?

Answer 15

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

Question 16

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

Answer 16

Ultimate tensile stress

Question 17

After what point does necking start?

Answer 17

After the ultimate tensile stress point

Question 18

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

Answer 18

The energy required to deform it

Question 19

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

Answer 19

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

Question 20

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

Answer 20

Reliability only

Question 21

When can we use a low-ish safety factor?

Answer 21

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

Question 22

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

Answer 22

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

Question 23

Two ways of quantifying ductility

Answer 23

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

Question 24

What is the repeating unit of a crystal lattice called?

Answer 24

the unit cell

Question 25

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

Answer 25

2 atoms per unit cell, coord number 8

Question 26

symbol for unit cell dimension and how to calculate it

Answer 26

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

Question 27

formula for APF, Atomic Packing Factor

Answer 27

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

Question 28

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

Answer 28

4 atoms per cell, coordination: 12

Question 29

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

Answer 29

atoms: 6, coord: 12

Question 30

coordination of a CPS, close packed structure

Answer 30

12 (highest)

Question 31

Define interstitial sites

Answer 31

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

Question 32

Define polymorphism

Answer 32

materials that can exist in more than one crystal form

Question 33

Describe the polymorphism of iron

Answer 33

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

Question 34

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

Answer 34

strong atomic bonds = high young’s modulus

Question 35

slipping in close packed planes vs non close packed planes

Answer 35

close packed plane, easier to slip as less distance

Question 36

relationship between slip and ductility

Answer 36

higher ductility = easier to slip

Question 37

name of empty atomic space in crystal and impact on strength

Answer 37

vacancy, no effect

Question 38

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

Answer 38

substitution, slight increase (usually)

Question 39

atoms in interstitial space are called

Answer 39

interstitials

Question 40

another name for planar defects and state the two types

Answer 40

dislocations: edge dislocation and screw dislocation

Question 41

explain briefly why slip is easier in dislocation movement

Answer 41

only a single line of bonds are broken

Question 42

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

Answer 42

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