Materials Practicals

Question 1

What does the sequence of metallurgical grinding and polishing do?

Answer 1

Removes marks and levels and cleans the specimen surface. Polishing removes the artifacts of grinding.

Question 2

What is chemical etching and why does it work?

Answer 2

A manufacturing process that removes metal in order to produce parts in a desired shape.

Question 3

What does etching time depend on?

Answer 3

The etch rate of the acid

Question 4

What is a grain boundary?

Answer 4

A 2D defects in materials crystal structure. It’s the interface between two grains.

Question 5

Why do grain boundaries appear dark under a reflected light microscope?

Answer 5

Because the light is scattered and reflected from the sample surface.

Question 6

Define Hardness

Answer 6

Materials ability to resist plastic deformation

Question 7

Define Stiffness

Answer 7

The extent to which a materials resists deformation in response to an applied force (rigidity)

Question 8

Define Toughness

Answer 8

A materials ability to absorb kinetic energy without fracturing

Question 9

What is Yield Stress?

Answer 9

The stress at which a material starts to deform plastically

Question 10

What is 0.2% Proof Stress?

Answer 10

The point at which a particular degree of permanent deformation occurs in a test sample

Question 11

Why do some steels show discontinuous yielding?

Answer 11

Because of higher amounts of interstitial carbon atoms

Question 12

What is the UTS?

Answer 12

The maximum stress that a material can withstand before fracture.

Question 13

What is the difference between true stress and engineering stress?

Answer 13

True stress takes into account the change in cross sectional area a specimen will undertake as it is places under stress.

Question 14

What does an impact test measure?

Answer 14

Toughness

Question 15

What does a notch do in an impact test?

Answer 15

Controls the location at which the specimen fractures. By creating a structural weakness

Question 16

Why does modulus measurement during a tensile test require an extensometer?

Answer 16

Because the specimens extension is required when calculating young’s modulous

Question 17

How would you measure fracture energy from a tensile test?

Answer 17

Find the area under the curve of a stress strain curve to failure

Question 18

For a given material, what are the effects on YP and UTS of increasing the testing temperature in a tensile test?

Answer 18

Yield point and UTS occurs at lower stress levels

Question 19

What is the effect on Charpy test data when the test temperature is decreased below room temperature for mild steel and copper?

Answer 19

Steel shows a ductile to brittle transition, copper doesnt

Question 20

How do you measure hardness and what precautions are necessary to obtain accurate results?

Answer 20

Measuring the depth of an indentation made by a diamond vickers hardness tester.

Question 21

In a 0.2wt%C plain carbon steel, what are the two microstructures present at room temperature?

Answer 21

Pearlite and Ferrite

Question 22

What is the name of the microstructure consisting of ferrite and cementite lamella?

Answer 22

Pearlite

Question 23

How does a dislocation move?

Answer 23

When the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge

Question 24

Define a substitutional solid solution and its effect on hardening

Answer 24

A mixture of two types of atoms in which one atom can replace the other type of atom. This increases hardness

Question 25

Define an interstitial solid solution and its effect on hardening

Answer 25

A solid solution in which the solute atoms occupy positions between the atoms in the structure of the solvent. This increases hardness

Question 26

What is work hardening?

Answer 26

Increasing hardness by cold working

Question 27

What is annealing?

Answer 27

heating and slow cooling in order to remove internal stresses and increase toughness

Question 28

How is hardness affected by increasing the C content in a plain carbon steel?

Answer 28

Hardness is increased

Question 29

What is a typical inclusion in mild steel?

Answer 29

oxides and sulfides

Question 30

Give a size range for a typical non metallic inclusion

Answer 30

1-10µm

Question 31

What is the mechanism of brittle fracture?

Answer 31

– Little or no plastic deformation
– Catastrophic
– Usually strain is < 5%.