BMEE209L: Module 4 (UPDATED)

Question 1

Define stress

Answer 1

Force per unit area

Question 2

Define strain

Answer 2

How much deformation has occurred with that applied force

Question 3

Elastic Deformation

Answer 3

The linear part of the stress strain curve; at this stage when the force is removed the material will revert back to its original shape

Question 4

Yield Strength

Answer 4

A point on the stress-strain curve above which any additional stress will cause permanent deformation

Question 5

What happens when the materials reaches Ultimate Strength point in the stress-strain curve?

Answer 5

After that point, less stress is needed as the material begins to decrease its cross-section, through a process called necking, which continues until the material fractures

Question 6

Poisson’s Ratio

Answer 6

• (Lateral Strain/Longitudinal Strain)

Question 7

Modulus of Resilience

Answer 7

1/2 x yield strength x strain at yielding

Question 8

Tensile Toughness

Answer 8

Energy absorbed by a material prior to fracturing

Question 9

Ductility

Answer 9

Amount of deformation a material can withstand without breaking

Question 10

Tensile Strength/Ultimate Strength

Answer 10

UTS = Fmax/A0

Question 11

How to determine 0.2% offset yield strength

Answer 11

when a yield point isn’t sharply defined:
1) Start at strain = 0.002
2) Draw a line parallel to linear region of curve
3) Point of intersection is the yield strength

Question 12

Modulus of elasticity

Answer 12

Young’s Modulus = stress/strain

Question 13

True stress strain curve vs Engineering stress strain curve

Answer 13

True stress strain curve will always keep increasing while the engineering stress strain curve will drop during necking

Question 14

True stress

Answer 14

σt = σe(1+εe)

Question 15

True strain

Answer 15

εt = ln(1+εe)

Question 16

Effect of temperature on stress strain curve

Answer 16

High temperature: curve shifts downwards and more to right becomes soft and ductile

Low temperature: curve shifts up and more to left
becomes hard and brittle

Question 17

Brinell Hardness

Answer 17

BHN = 2P/piD(D-sqrt(D^2-d^2))

where P = load
D = diameter of ball indenter
d = diameter of indentation

Question 18

Rockwell Hardness

Answer 18

N - h/S

N = scale factor
h = depth of indentation
S = scale division factor

Question 19

Lower strain rate means material is

Answer 19

ductile

Question 20

Higher strain rate means material is

Answer 20

brittle

Question 21

Among BCC and FCC metals which have ductile to brittle transition temperatures

Answer 21

BCC (iron, tungsten) metals do but FCC(aluminium, copper) metals do not

Question 22

Fracture toughness

Answer 22

K = fσsqrt(pia)

Question 23

Fatigue

Answer 23

Lowering of strength or failure of a material due to repetitive stress which may be above or below the yield strength

Question 24

Stages of Fatigue

Answer 24

1) Crack formation
2) Crack Growth
3) Fracture

Question 25

Name the axes of an S-N curve

Answer 25

Y-axis: stress range X-axis: number of cycles before failure

Question 26

Endurance limit

Answer 26

The material can be cycled below this stress range forever without fatigue

Question 27

Creep

Answer 27

Time dependent permanent deformation at a constant load or stress and at high temperatures

Question 28

Stages of Creep

Answer 28

1) Primary Creep - creep rate decreases with time 2) Secondary Creep - creep rate remains constant 3) Tertiary Creep - creep rate accelerates

Question 29

Strengthening by grain size reduction

Answer 29

Grain boundaries block dislocation motion, so smaller grain size means more boundaries so fewer dislocation motion

Question 30

Hall-Petch Equation

Answer 30

σ y = σ0 + kd^-1/2 σ y = yield strength σ 0 = friction stress k = strengthening coefficient d = grain size

Question 31

Solid Solution Strengthening

Answer 31

Add impurities that distort the lattice making it more difficult for impurities to move

Question 32

Strain hardening

Answer 32

Do plastic deformation as it introduces more dislocations thus making the dislocations harder to move