Mechanical Properties

Question 1

What is the equation for stress

Answer 1

Stress = force / area

Question 2

What is the equation for strain

Answer 2

Strain = change in length / original length

Question 3

Why do we use stress and strain instead of force and extension

Answer 3

Force and extension look at the dimensions of the sample, where as stress and strain look at the material itself

Question 4

Draw a ductile stress strain graph

Answer 4

Look at example

Question 5

Draw a brittle stress strain graph

Answer 5

Look at example

Question 6

Where is the yield point/strength

Answer 6

Where the straight line begins to curve

Question 7

Draw a proof stress graph

Answer 7

Look at example

Question 8

Name the 3 primary bonds

Answer 8

Ionic
Metallic
Covalent

Question 9

Name a secondary bond

Answer 9

Van der waals
Hydrogen bonding

Question 10

What is the definition of crystaline?

Answer 10

Atoms that have long range order

Question 11

What causes plastic deformation

Answer 11

Slip

Question 12

What allows for slip to occur more easily

Answer 12

Dislocations

Question 13

What is ductility

Answer 13

Amount of permanent/plastic deformation up till fracture

Question 14

What unit is stress measured in

Answer 14

Pa

Question 15

What do u multiple by to get M before a unit

Answer 15

X10^6

Question 16

What do u multiple by to get G before a unit

Answer 16

X10^9

Question 17

What section of the graph does stiffness relate to

Answer 17

Elastic region

Question 18

What is the Young’s modulus equation

Answer 18

E = stress/strain

Question 19

What are the units of Young’s modulus

Answer 19

Pa

Question 20

What is the equation for true stress

Answer 20

True stress = load force/instantaneous area

Question 21

Draw a graph comparing true stress and engineering stress

Answer 21

See example

Question 22

What equation is preferred when measuring ductility

Answer 22

%AR = (original area - final area)/ original area x100

Question 23

What is the term used to describe a material with no long range crystalline order

Answer 23

Amorphous

Question 24

What is the definition of hardness?

Answer 24

How easily a material can be scratched or dented

Question 25

What is a hardness test? (Steps)

Answer 25

Small indented forced into the surface of a material - size of dent created used to quantify hardness

Question 26

What does a big intent mean in terms of hardness for a material

Answer 26

Big indent means the material is soft

Question 27

Why are hardness tests commonly used

Answer 27

Simple, relatively inexpensive, non-destructive, used to infer other mechanical data

Question 28

What does a hardness test indicate

Answer 28

Resistance to plastic deformation

Question 29

Name a hardness test

Answer 29

Brinell hardness Vickers and knoop

Question 30

What is variability (2)

Answer 30

Material properties are not exact quantities Even with the same testing machine, variability of results will be achieved using samples cut from the same block of material

Question 31

What is impact toughness

Answer 31

The ability of a material to absorb kinetic energy up to fracture

Question 32

What are the four factors affecting toughness

Answer 32

- the material - geometry (notches make it more brittle) - rate of loading - temperature

Question 33

Conditions of an impact test

Answer 33

Chosen to represent the most servers situation - low temp - high strain rate - presence of a crack

Question 34

Impact toughness - affects of temperature

Answer 34

A material that is tough at room temp can become brittle at lower temps

Question 35

How is impact toughness tested (5)

Answer 35

- sample has a notch in it - weighted pendulum of known mass is released - hits the sample (will now have known amount of kinetic energy) - samples breaks which requires energy - the more energy used the break the sample, lower the pendulum follow through

Question 36

What is a dipole

Answer 36

An uneven charge distribution

Question 37

What is poissons ratio

Answer 37

Relationship between how much material contracts in one orientation, while being stretched in the other orientation

Question 38

How to account for natural variability of mechanical properties in the real world

Answer 38

Use a safety factor or design factor in the designs

Question 39

How to account for variability of mechanical properties during lab testing

Answer 39

Test multiple identical samples, get an average and standard deviation

Question 40

What occurs during a bending test

Answer 40

Max compression on top surface Max tension on bottom surface

Question 41

What is the neutral axis

Answer 41

Where neither compression or tension happens

Question 42

Why is %AR the preferred method

Answer 42

Gives more consistent/reliable results compared to %EL

Question 43

Why are FCC metals packed so closely together?

Answer 43

They are malleable, so it’s easy to change their shape Because dislocations find it easy to travel in closely packed planes

Question 44

What happens to bonds during elastic deformation

Answer 44

Bonds stretch but no not break

Question 45

What are dislocations?

Answer 45

Imperfections in the crystal structure

Question 46

How do dislocations allow for easier plastic deformation

Answer 46

Fewer bonds need to be broken at once to achieve slip

Question 47

What is the UTS

Answer 47

The maximum stress before the material fails

Question 48

How does changing the length of a cable affect its maximum load

Answer 48

Changing the length would no affect the loading capacity Load relates to stress (stress=force/area) Length would affect the strain (strain=change in length/original length)

Question 49

What is engineering stress?

Answer 49

Defined as force/original area of the sample

Question 50

How will a notch affect a materials toughness

Answer 50

The crack will act as a stress concentrator The material will show brittle behaviour in the presence of any defect

Question 51

Why r I beams used?

Answer 51

- beam theory - compression and tension - material close to the middle is a waste

Question 52

Draw and label a SN curve for a ferrous metal and non-ferrous metal

Answer 52

Draw