fatigue Homa

Question 1

what does fatigue life embrace (3)

Answer 1

nucleation of a crack in a previously crack free material
slow growth of the (sub-critical) crack
Rapid growth when the crack attains a critical length.

Question 2

does fatigue failure require existing defects or cracks for fatigue to occur

Answer 2

no, can be new material but fatigue and fail

Question 3

when fatigue occurs cracks may form where

Answer 3

on crystallographic slip

Question 4

what is a crystallographic slip

Answer 4

large displacement of one part of a crystal relative to another part along crystallographic planes and directions

Question 5

what angle do crystallographic slip occur on and via what stress

Answer 5

45 degrees via shear stress

Question 6

what is cycle loading

Answer 6

continuous and repeated application of a load, leading to fatgiue

Question 7

at what RPM and frequency does fatigue crack grow

Answer 7

3000rpm at 50hz

Question 8

what is the point of S-N curve (2)

Answer 8

It is a measure of the materials performance to resist fatigue under cyclic loading

Question 9

what are the diffrent regimes of fatigue crack

Answer 9

I, II and III

Question 10

what are the different regimes of fatigue crack

Answer 10

I, II and III

Question 11

Crack growth in region I size crack, and atomic

Answer 11

an average crack advance of

less than one atomic spacing per cycle

Question 12

Crack growth in region II and dection

Answer 12

higher crack growth rate compared to regime I via the paris law. and can be detected via ultrasonic or x rays

Question 13

Crack growth in region III

Answer 13

crack growth rate will

accelerate and finally fracture will occur

Question 14

what is the point of using paris law

Answer 14

can predict fatigue life of large cracks

Question 15

what happens under variable amplitude loading

Answer 15

plastic residual stress fields decrease in crack growth rate

Question 16

what size of crack propagates faster short or long crack

Answer 16

short cracks propagate faster

Question 17

in multiaxial loading, most cracks tend to propagate in what region or mode out of mode I, II and III

Answer 17

mode I

Question 18

low cycle fatigue (3)
stress lvels? high or low
does it exceed elastic limit
max life for cycles

Answer 18

high stress levels
exceeding elastic limit
under 100 cycles life

Question 19

high cycle fatigue
, stress yield high or low,
lifetime
3 stages of fatigue

Answer 19

low-stress yield
long lifetime
fatigue stages: crack initiation, stable crack growth and unstable failure

Question 20

what is a Fatigue Striations and its association

Answer 20

an advanced crack - Each striation on the fatigue surface is associated with one
stress cycle for fatigue cracks

Question 21

what regime does fatigue striation require

Answer 21

paris regime

Question 22

what can beach marks and striations indicate

Answer 22

direction and type of the damaging stress occuring.

Question 23

what is LEFM

Answer 23

Linear elastic fracture mechanics

Question 24

what does LBB stand for

Answer 24

leak before break

Question 25

what is LBB

Answer 25

a crack of twice the
wall thickness in length should be
stable at a stress equal
design stress.

Question 26

steps for LBB (5)

Answer 26

1. Material characterisation and stress calculation
2. Determination of critical through-wall crack lengths
3. Calculation of crack opening area
4. Evaluation of leak rate
5. Assessment of results

Question 27

what is the plastic zone at crack tip and explain which zone dissipates more heat in plastic zone, and what equation is it based on

Answer 27

the crack tip is the 2 heart circles, which the broken part is called crack zone, inner circle is strain plane and outer being plane stress.

this circle is based on von mises. as stress is larger, it will dissipate more heat than strain