Midterm Questions 2

Question 1

Difference between macro-vickers and micro-vickers hardness test?

Answer 1

macro-most frequently used for rapid routine hardness measurements. Indenting force between 5 to 3000kgf

micro-used with: materials that have coatings, surface hardness, or hardness in different phases of mulit phase material. diamon pyramid with small force 10 to 1000gf

Question 2

What are marcohardness test methods, and microhardness test methods. Compare them in terms of load applied.

Answer 2

Macro- Rockewell, Brinell, Macro-Vickers, range from 5-3000 kgf. loads must be above 1kgf

Micro- Micro-Vickers, Knoop- range from 10 to 1000gf

Question 3

What are the basic hardness test considerations?

Answer 3

sample size
cylindrical samples
sample thickness
scales
Gage R & R

Question 4

List 4 precautions for Rockwell test.

Answer 4

1. the indenter and anvil should be clean and well seated
2. the surface to be tested should be clean, dry smooth and free from oxides
   3 the surface should be flat and perpendicular to the indenter
3. the spacing between indentations should be three to five times the diameter for the indention

Question 5

What is the reason of that hardness values of cylindrical surfaces need correction factor?

Answer 5

will give low readings, with error depending on the curvature, load, indenter and the hardness of the material. indentor will not be going through the sample throughly. may be taking measurements from the side etc

Question 6

Explain in detail why there are different techniques to calculate toughness value of a sample? Example: tensile test, impact test? Why these two toughness values are different?

Answer 6

Generally toughness is the amount of energy by the material for a material to fail. It can be measured different ways depending on how energy is applied

IMPACT CHARPY -testing of fracture upon rapid loads (impact). how resistive sample is to impact. high strain. measure of energy absorbed can be compared to other samples for relative toughness. measured energy

TENSILE or static toughness which would be a tensile test. low strain. area under stress vs strain diagram. this test give more idea into mechanical properties`

Question 7

Explain schematically the crack propagation of ductile and brittle materials?

Answer 7

ductile- microvoids… draw

brittle- transgranular (through grain )or (along grain boundries) intergranular fracture…..

Question 8

Impact energy is affected by several factors? List 4 of them and explain why they affect impact energy?

Answer 8

notches- they create a point of weakness and are more likely fracture upon impact

temperature- some materials become more brittle due to temperature transition temp. ex steel become brittle when cold.
strain rate

ductility/brittleness-if something isnt as ductile or has high brittleness it will absorb less energy. It fractures sooner

microstructure- the more ordered the packing of the strucutre is the more energy it can indure before fracture

Question 9

What is the primary function of Charpy test?

Answer 9

to measure a materials ability to resist high rate loading. used for qualative and comparison results. Also studies temperature ductile to brittle transition

Question 10

Explain why some materials behave brittle at low temperatures?

Answer 10

they undergo a phase for ductile to brittle transiton under temperature change. BCC. main because of its structure it cannot allow for as many slipping of atom like FCC. and thus when energy is lowered it become brittle.

At higher temperatures the impact energy absorbed is relatively large since the fracture is ductile.
As the temperature is lowered, the impact energy absorbed drops as the fracture becomes more brittle.

Question 11

What are the parameters that can be calculated using Tensile test?

Answer 11

Modulus of Elasticity
Yield Strength
Ultimate Tensile Strength
Failure Strain
Reduction in Area at fracture
True Stress and True Strain

Question 12

What is Hook’s law? Write down the formula, and show where it is valid on the stress-strain diagram.

Answer 12

This law states that within the elastic limit, the Stress is proportional to the strain.
sigma=Ee

show youngs modulus. slope of elastic deformation

Question 13

What are the factors influencing properties of polymers

Answer 13

molecular weight
degree of polymerization
molecular strucutre

Question 14

What are the 4 molecular structure of polymers? Write the relationship between material’s strength and different molecular structure?

Answer 14

linear
branched
cross linked
network

the material become stronger with more crosslinkage. therefore linear is the weakest and network is the strongest.

Question 15

Define crazes.

Answer 15

are regions of very localized plastic deformation, which lead to the formation of small and interconnected microvoids.

Question 16

What are the advantages and disadvantages of polymers?

Answer 16

advantages-
good electrical insulation, good thermal insulation, high specific strength, can be recycled, low density, can be used in composite applications

disadvantages-
low toughness, less resistance to abrasion, can produce hazardous fumes when burned, in general cannot be used in high temperature applications

Question 17

What are the three phases that a composite has, and what are their role in the composite?

Answer 17

matrix/bulk phase- accepts the load over a large surface area

reinforcement- its stiffness increases strength of composite

interphase- this is where it is essential to have interfacial bonding keep the composite together.

Question 18

Give 3 example for the materials that can be used as matrix phase and reinforcement phase/

Answer 18

-matrix
polymers
metals
cermaics

reinforcments
glass
carbon
boron
cermamic

Question 19

Define laminate composite

Answer 19

materials formed in layers called lamina. the lamina has the same/ different fiber orientation/ different materials. these are then stacked together to make laminate.

Question 20

What is the reason of using plies in different directions for some of the laminate composites?

Answer 20

increases strength. load be applied to more dirrections without the composite failing.

Question 21

List 3 factors that affect the strength of a composite.

Answer 21

fiber concetration
distrubution
amount of defect.