Mats lecture 13 & 14

Question 1

What is the modulus of resilience (Er) graphically?

Answer 1

-The area contained under the elastic portion of a stress strain curve

Question 2

What is the definition of Ductility?

Answer 2

-Is the ability of a material to be permanently deformed “without breaking when a force is applied”

Question 3

List the common measures of ductility

Answer 3

-The percent elongtion
-The percent reduction in area

Question 4

What is the difference between engineering stress and true stress?

Answer 4

Engineering stress: force acting on initial area
True stress: Force acting on current area

Question 5

What is the difference between engineering strain and true strain?

Answer 5

engineering strain: displacement over initial strength
True strain: displacement over current length

Question 6

In other words, what is engineering strain?

Answer 6

The amount that a material deforms per unit length in tensile test

Question 7

what is engineering strain also known as?

Answer 7

Nominal strain

Question 8

In other words, what is true strain?

Answer 8

-It equals the natural log of the quotient of current length over the original length

Question 9

What happens when engineering strain is small?

Answer 9

Engineering stress is around true stress

Question 10

In what materials is engineering strain normally small during elastic deformation?

Answer 10

-in most of the metals

Question 11

In what region do most engineering designs operate?

Answer 11

-in the elastic region

Question 12

At what point are the equations that relate true and engineering stress and strains valid till?

Answer 12

-Valid up to the necking point

Question 13

Is the yield strength apparent in metals?

Answer 13

NO
-The transition is very gradual

Question 14

how much is the offset yield strength for metals?

Answer 14

0.2%

Question 15

What happens to metals undergoing plastic deformation?

Answer 15

-Their strength increases
-Their strength increases to a maximum (the tensile strength), after which necking leads to fracture

Question 16

What is the definition of plastic strain

Answer 16

-the permanent strain that results from plastic deformation

Question 17

What is spring back?

Answer 17

-When the load is removed from a plastically deformed material, there is a recovery called Spring Back

Question 18

What does ductility mean in the context of plastic deformation?

Answer 18

-Ductility is a measure of how much plastic deformation a material can withstand
-It is simply the strain to failure

Question 19

Describe the mechanical responses of polymers and what they depend on

Answer 19

-Can range from brittle to Viscous
(depends on both structure and temperature)

Question 20

Are ceramics suitable for standard tensile tests?

Answer 20

No
-because they are brittle
-They are tested in compression or in bending

Question 21

List the drawbacks to the tensile test

Answer 21

-It requires machining to prepare specimens
-It is destructive

Question 22

What does the hardness test measure?

Answer 22

-measures the resistance to penetration of the surface of a material by a hard object

Question 23

List the main material groups from least hard to most hard

Answer 23

-Polymer materials are exceptionally soft
-metals and alloys have intermediate hardness
-Ceramics are exceptionally hard

Question 24

What are the most common hardness tests?

Answer 24

-Rockwell test
-Brinell test

Question 25

Describe what happens in a Brinell hardness test

Answer 25

-A hard steel sphere (10 mm in diameter) is forced into the surface of the material
-The diameter of the impression is used to calculate the HB number

Question 26

Describe the Rockwell hardness test

Answer 26

-Uses a small diameter steel ball for soft materials and a diamond cone, for harder materials
-The depth of penetration of the indenter is automatically measured by the testing machine and converted to a Rockwell hardness number (HR)

Question 27

Describe what the Nanoindentation is

Answer 27

-method used to measure local properties of materials
-The imposed load and displacement are continuously measured with micronewton and sub-nanometer resolution