chapter 8

Question 1

How many times of flatulating time modes are possible

Answer 1

1) sinusoidal time dependence . Amplitude is symmetrical about the mean zero stress level. Alternating from maximum tensile strength to minimum compression stress.

2) Repeated Stress cycle maximia and minima asymmetrical relative to the zero stress level.

3) stress level may be very random

Question 2

Higher the magnitude of the stress, the smaller the number of cycle the material is capable of sustaining (T/F).

Answer 2

TRUE

Question 3

What is fatigue limit

Answer 3

Limiting stress level below which fatigue failure will not occur. This represents the largest value of flatulating stress that will not cause failure for infinite number of cycle

Question 4

what is fatigue strength

Answer 4

strenght of a material after 10^7 cycle

number at which failure occurs for specified number of cycle

Question 5

what is fatigue life

Answer 5

number of cycle required to fracture a sample at a given load

Question 6

Where is fatigue damage initiated at

Answer 6

fatigue damage is initiated at imperfections such as welds screws/ threads, porosity machine surface finish

Question 7

Can a ductile material break in brittle fashion

Answer 7

If the material breaks, a ductile material can break in brittle fashion due to the repeated tension 2 compression, the fracture surface has ridges/marks on it.

Question 8

what enhances fatigue life

Answer 8

-eliminating sharp corners i.e remove stress concentration

-compressive residual stress i.e shot peening

-surface treatmean-diffusion case hardening

Question 9

what is creep

Answer 9

plastic deformation of material over long time at elevated temperature

Question 10

Describe 3 graph region for creep

Answer 10

1. Primary continuously decreasing creep rate that is slope of the curve decreases with time means material experience an increase in creep
2. secondary: rate is constant plot becomes linear. That is often the stage of creep longest duration
3. tertiary creep: acceleration of rate and ultimate failure termed rupture and results from microstructural/ or metallurgical

Question 11

what happens when stress or temp increases

Answer 11

1) instaneous strain at the time of stress application increases
2) steady state creep rate increases
3) rupture life decreases

Question 12

what do ductile metal exhibit before fracture

Answer 12

substantial plastic deformation with high energy absorption

Question 13

what is ductility quantified as

Answer 13

elongation % and % reduction in area

Question 14

what is ductile function of for materials

Answer 14

function of material temperature

Question 15

Process is very slow and such crack length is said to be stable which means it resists any extension unless there is an increase in applied force (True/False)

Answer 15

True

Question 16

Brittle crack may spread extremely rapidly with very little accompanying plastic deformation these are also called unstable cracks. Crack propagation once started continue spontaneously without an increase in magnitude. (T/F)

Answer 16

True

Question 17

flaws and crack always exist under material. These flaws are determined to fracture the strength because applied strength may be NOT quantified or concentrated at the tip ( True / False)

Answer 17

False there should be no not

Question 18

Does magnitude of crack decreases distance away from crack tip.

Answer 18

yes it does

Question 19

what is stress raiser

Answer 19

ability to increase stress and effect of stress raiser is more significant in brittle than ductile

Question 20

what is ductile to brittle transition related to

Answer 20

it is related to the temperature dependence of the material impact energy

Question 21

what does ductile fracture look like

Answer 21

dull and fiber

Question 22

what does brittle fracture look like

Answer 22

shiny texture

Question 23

Where does fatigue failure occur

Answer 23

this type of material failure occurs below the material yield strength, because of cyclic load(i.e e repeating loading).

Question 24

What happens over time in fatigue failure

Answer 24

the damage in the material accumulates over time and failure occurs in a brittle fashion, even if the material is normally ductile

Question 25

The more tensile the average stress is the shorter the life span of the component is (T/F)

Answer 25

True

Question 26

What is fatigue ?

Answer 26

It is type of failure which happens below the material yield strength because of cyclic loading

Question 27

The damage in the material accumulates over time and failure occurs in brittle fashion, even if the material is normally ductile (T/F)

Answer 27

True

Question 28

which one is better average compressive stress or average tensile stress?

Answer 28

Average compressive stress is better than average tensile stress

Question 29

Explain what happens when the average stress is more tensile ?

Answer 29

The more tensile the average stress is the shorter the life span of the component is. To estimate the fatigue behavior of a material we need to load it for a certain number of cycles. until material fails.

Question 30

why does a ductile material break in a brittle fashion?

Answer 30

Ductile material break in a brittle fashion due to the repeated tension 2 compression. The fracture has ridges and marks on it

Question 31

what has major effect on creep?

Answer 31

stress and temperature have major effect on creep