C6 Failure

Question 1

type of failure

2 types

Answer 1

1. ductile
2. brittle

Question 2

elobrate

ductile

Answer 2

• occur at high temp
• show plastic deform
• has 5 cup-cone fracture stage

Question 3

elobrate

brittle

Answer 3

• occur at low temp
• not showing any deform
• break without warning
• when look at microscope can see thin line sample fail

Question 4

what are

stages in cup-and-cone fracture

Answer 4

1. necking
2. cavity formation
3. coalescence cavities form crack
4. crack propagation
5. fracture

Question 5

the sharp tip in sample crack can causing

Answer 5

it propogate crack

Question 6

how to stop crack growth

Answer 6

blunt the tip

Question 7

crack growth condition

Answer 7

K ≥ Kc = Yθ√(πa)

Y=material behaviour, a=area

Question 8

Formula for: θ (y-axis) vs a max (x-axis)

Answer 8

θ design < Kc / Y(√πa max)

Question 9

Formula for graph: a max (y-axis) vs θ (x-axis)

Answer 9

a max < (1/π) [Kc / Y(θ design)]^2

Question 10

for both graph: (a max vs θ) & (θ vs a max)

the area under graph means

Answer 10

no fracture

Question 11

for both graph: (a max vs θ) &

the area above graph means

Answer 11

fracture

Question 12

ductile brittle transition test

Answer 12

charpy experiment test

Question 13

charpy experiment test conduct at diff temp

Answer 13

• to the left (low temp): brittle
• to the right (high temp): ductile
• impact energy = (max - min) / 2

Question 14

Type application of failure

Answer 14

1. Fatigue
2. Creep

Question 15

what is fatigue

Answer 15

• predict max cycle
• repeated cycling load

Question 16

what is creep

Answer 16

• predict max time
• time dependent deform due to constant load at high temp

Question 17

low cycle fatigue

Answer 17

small no of cycle
* high load
* plastic & elastic deform

Question 18

high cycle fatigue

Answer 18

large no of cycle
* low load
* elastic deform (N>10^5)

Question 19

type of fatigue behaviour :

Answer 19

fatigue limit Sfat : no fatigue is S < Sfat

Question 21

Fatigue S-N curves

Answer 21

above graph: unsafe
below graph: safe

Question 21

type of stress for fatigue test

3 types

Answer 21

1. axial (tension-compression)
2. flexural (bending)
3. torsional (twisting)

Question 22

mean stress, σm

Answer 22

(σmax + σmin) / 2

Question 23

stress amplitude, σa

Answer 23

(σmax - σmin) / 2

Question 24

stress range, σr

Answer 24

σmax - σmin

Question 25

stress ratio, R

Answer 25

σmin / σmax

Question 26

Factors affect fatigue life

• magnitude of stress
• quality of stress

what is solution

Answer 26

• polish surface
• introduce compressive stress into surface area
• case hardening
• optimize geometry
• avoid internal corners

Question 27

environmental effect

thermal fatigue solution

Answer 27

• change design
• use material with low thermal expansion coefficient

Question 28

environmental effect

corrosion fatigue solution

Answer 28

• decrease corrosiveness of medium
• add protective surface coating
• add residual compressive stress

Question 29

improve fatigue life

Answer 29

1. impose a compressive surface stress ( to suppress surface crack growth
2. remove stress concentrators

2 method : shot peening & caburizing

Question 30

prediction of creep rupture lifetime

Answer 30

T(20 + log tr) = L

T= temp,K . tr= time rupture/failure . L= function of applied stress

Question 31

materials that resilent to creep

resilient = tahan

Answer 31

• stainless steel
• refractory metals (contain element high m.p. eg;Nb, Ta)
• superalloys (Co, Ni based)