Characteristics Of Metals

Question 1

Engineering Stress is calculated by…

Answer 1

Dividing the force acting on the chosen object, by the cross sectional area of said object that is perpendicular to the force.

Question 2

Engineering Strain is calculated by…

Answer 2

Dividing the change in length of an object by the original length of said object.

It is usually given as a percentage, so then multiply by 100%.

Question 3

Young’s Modulus is…

Answer 3

The gradient of the portion of a Stress/Strain graph before the object reaches its yeild stress.

Question 4

Young’s Modulus is calculated by…

Answer 4

Dividing Engineering Stress by strain. Rise/Run.

Question 5

What is resistivity?

Answer 5

Resistivity is a material property that determines how much resistance it has.

Question 6

How do you calculate resistivity?

Answer 6

You take the resistance of a known peice of material, multiply it by the cross sectional area perpendicular to the flow of electricity and divided by the length of the material.

Resistivity = (Resistance x Area) / Length

Question 7

What is conductance?

Answer 7

Conductance is simply the inverse of resistivity. It is calculated as 1 / Resistivity.

Question 8

What is cold work?

Answer 8

Cold work is the process of plastically deforming a material in order to increase the yield strength and hardness of the material by introducing more dislocations.

Cold work is measured as a percentage.

Question 9

How is cold work calculated?

Answer 9

(Change in cross sectional area / original area) x 100%

Question 10

Why do strength increases from cold work level off?

Answer 10

Because the material eventually reaches a capacity for dislocations. New dislocations become much less likely to form.

Question 11

What is the porportional limit of a material?

Answer 11

The porportional limit describes the point at which the graph of a materials stress-strain changes from linear to a curve. Technically this describes the exact moment that a material goes from elastic to plastic deformation.

Question 12

Why is the porportional limit not used?

Answer 12

The porportional limit of a material is incredibly precise and so very difficult to measure exactly. So instead we use Yield Strength.

Question 13

What is 0.02% yield strength?

Answer 13

Since the porportional limit is so difficult to define, engineers instead make a line on the stress-strain graph that is parallel to the young’s Modulus, and starts at 0.02% strain.
The point at which this parallel line and the original stress-strain graph meet is known as the yield point. And the stress at that point is called the yield stress of the material.

Question 14

What is Poisson’s Ratio?

Answer 14

The Poisson’s Ratio of a material is the ratio between how much it elongates in the direction of an applied force, versus how much it shrinks in the directions perpendicular to that force.

v (Poisson’s Ratio) = - (€x (strain in a perpendicular direction) / €z (strain in the direction of the force) )

Question 15

What is Hooke’s law?

Answer 15

Hooke’s law describes the relationship between the stress and strain of a material when the stress is tension and before the porportional limit.

® (Engineering Stress) = E (Modulus of Elasticity) x € (Strain)

Question 16

What is tensile strength?

Answer 16

After the porportional limit of a material is reached the stress-strain graph begins to curve. The stress necessary to continue plastic deformation continues to increase until it reaches a maximum point. This point is the tensile strength. A material subjected to this amount of stress will begin to “neck”.

Question 17

What is “Necking”?

Answer 17

A ductile material placed under enough tension stress will eventually begin to form a small constriction or “neck” at some point in the object. This “neck” narrows more than the rest of the material and as such all further deformation is confined to this neck. Fracture will ultimately result at the neck.

Question 18

What is Ductility?

Answer 18

Ductility is a mechanical property that describes how much plastic deformation a material can sustain before fracture. Ductility is expressed as either “Percent Elongation” or as “Percent Reduction in Area”?

Question 19

How is the “Percent Elongation” measurement of ductility calculated?

Answer 19

%EL (% Elongation) = (lf (Length Final) - lo (Length Original)) / lo (Length Original) x 100

Question 20

How is the “Percent Reduction in Area” measurement of ductility calculated?

Answer 20

%RA (% Reduction Area) = (Ao (Area Original) - Af (Area Final)) / Ao (Area Original) x 100

Question 21

What does the term “Grain” refer to? (Crystal’s)

Answer 21

A grain is a single crystal structure. Metals and other crystals are generally formed of a collection of imperfect crystals attached together.

Question 22

Do metals cool into single crystals?

Answer 22

No.

Question 23

What does the term “Equiaxed” refer to?

Answer 23

A crystal structure that is Equiaxed is made of of crystal grains of relatively equal size on every axis. (roughly spherical).

Question 24

What is “Recrystallisation Temperature”?

Answer 24

It is the temperature at which a 50% Cold Worked. Etal will just fully recrystallise in one hour.

Question 25

Would a metal that was cold worked to 10% take longer or shorter than an hour to recrystallise at its Recrystallisation temperature?

Answer 25

Shorter. The driving force of crystallisation is the energy stored in the metal. Increased cold work increases that energy stored in the grain boundaries.

Question 26

Grain size after Recrystallisation depends on what?

Answer 26

The amount of cold work the material is subjected to before recrystallisation. More cold work increases stored strain energy, which increases nucleation leading to more small grains.

Question 27

What does the term critical cold work refer to?

Answer 27

The amount of cold work that must be done before recrystallisation is possible. If less cold work than this is done then recrystallisation will be impossible.

Question 28

How does increasing the temperature of recrystallisation affect the process? (of recrystallisation)

Answer 28

It reduces the time necessary for recrystallisation to occur.

Question 29

What is annealing?

Answer 29

The process of heating a metal in order to regain a materials ductility after cold work has been done.

Question 30

What are the three stages of annealing?

Answer 30

1st. Recovery
2nd. Recrystallisation
3rd. Grain Growth

Question 31

What does the term hot work refer to?

Answer 31

Any work that is done to a metal above its recrystallisation temperature.

Question 32

Does most hot work take place at the recrystallisation temperature?

Answer 32

No, most hot work is done at a much higher temperature (though still below melting point) in order to reduce the time needed for recrystallisation.

Question 33

What are the two types of diffusion? (that we are interested in).

Answer 33

Vacancy diffusion and interstitial diffusion.

Question 34

What letter denotes activation energy?

Answer 34

Q.

Question 35

What does diffusion in metals require in order to occur?

Answer 35

Energy above the activation energy.

Question 36

What is an Absolute Temperature?

Answer 36

A temperature measured in Kelvin.

Question 37

What is the Arrhenius Equation?

Answer 37

Rate = A e^ (-Q / R T)
A = PreExponential Coefficient
Q = Activation Energy
R = Universal Gas Constant (8.314)
T = Absolute Temperature (in K)

Question 38

What is diffusivity?

Answer 38

How easily a material is able to diffuse.

Question 39

What kind of atoms use interstitial diffusion?

Answer 39

Small atoms relative to the size of the atoms in the crystal structure.

Interstitial diffusion requires atoms to move in the interstitial spaces between atoms, so they must be small.