Fracture II

Question 1

What does fatigue occur in?

Answer 1

Fatigue occurs in structures subject to dynamic and varying stress – metals,
composites, ceramics, plastics…

Question 2

When does fatigue occur?

Answer 2

• Occurs after a long time of cyclic stress

Question 3

What stress levels can fatigue occur at?

Answer 3

Failure at stress levels below yield or tensile strength of the material due to imperfections

Question 4

What are the three stages of fatigue?

Answer 4

• Initiation
• Propagation
• Final fracture (K=Kc)

Question 5

What can be defined from a S-N curve?

Answer 5

Fatigue strength and fatigue life can be defined from an S-N curve

Question 6

What is the fatigue limit?

Answer 6

the stress amplitude at which fatigue
failure does not occur; it is exhibited at low stress amplitudes in some
materials (eg. steel) but not others

Question 7

What do cyclic stresses cause that leads to fatigue?

Answer 7

crack nucleation and growth and crack propagation, leading to fatigue fracture.

Question 8

What are methods that improve fatigue life/reduce fatigue failure?:

Answer 8

1. Shot-Peening
2. Case hardening
3. Mechanical Design (reducing sharp concerns)

Question 9

What can stress concentrations arise from( i.e. sources of stress concentration) ?

Answer 9

• mechanical surface
  damage
• residual stress from
  manufacturing
• corrosion pitting
• sharp corners (design)

Question 10

What happens during shot-peening?

Answer 10

• Residual compressive
  stresses are generated
  on the surface of a
  metal by shooting hard
  particles at it
• Residual compressive
  stresses delay crack
  initiation (may retard
  crack growth)

Question 11

What happens during case hardening?

Answer 11

• Hardens the surface of a metal object
  while allowing the metal underneath
  to remain tough (high Kc)
• A carbon- or nitrogen-rich surface
  layer is created through atomic
  diffusion by exposing the metal to a
  carbon or nitrogen rich atmosphere at
  a high temperature

Question 12

What types of cyclic loading are imposed on
components of a jet-engine?

Answer 12

1. Temperature cycles (engine start-up)
2. Loading cycles (take off, cruise, landing)
3. Blade flutter and vibrations (rotation)

Question 13

When do cracks usually nucleate?

Answer 13

*Cracks usually nucleate in the first 10-20% of
service life

Question 14

Why is there a range in the probability of fatigue failure?

Answer 14

There is variability in number and distribution of defects
within a component due to processing

Question 15

What is the relationship between mean stress and fatigue life?

Answer 15

increasing the mean stress
level leads to a decrease in fatigue life.

Question 16

Name the factors that affect fatigue life:

Answer 16

Mean Stress, Stress Amplitude, Surface Treatment, and Mechanical Design

Question 17

How can fatigue life be improved with mechanical design? Provide an example covered in class.

Answer 17

By incorporating
rounded fillet into a rotating shaft where there is a change in diameter.

Question 18

How do sharp corners impact fatigue failure?

Answer 18

Sharp corners are stress raisers, and can render the component
more susceptible to fatigue failure

Question 19

Where are beach marks found?

Answer 19

Found where crack propagation occurred

Question 20

What is beachmark spacing indicative of?

Answer 20

The rate of crack propagation

Question 21

What is the single largest cause of failure?

Answer 21

Fatigue failure

Question 22

Where do cracks start for fatigue failure?

Answer 22

Cracks start at stress concentration points, usually
on surface – dents, scratches, sharp fillets, keyways, machined threads etc

Question 23

What is the length at which fatigue failure occurs?

Answer 23

• Failure occurs when crack grows to critical length –
  ie. when Kc is exceeded

Question 24

What type of stress is needed for crack nucleation and growth and why?

Answer 24

Tensile stress required – cracks will not form if there are
only compressive stresses present

Question 25

Where can cracks nucleate for fatigue failure?

Answer 25

Cracks can nucleate internally on microstructure discontinuities (eg. porosity, inclusions etc.) or on the surface –dents, scratches, sharp fillets, key-ways, machined threads etc.

Question 26

Why is significant time important for initiation and crack growth?

Answer 26

• This way, cracks can be found by inspection of structures:
  This is why we design structures with critical crack
  lengths that are large and easily detectable.

Question 27

What is the difficulty with internal fatigue cracks?

Answer 27

Internal fatigue cracks can be a lot more difficult to
detect

Question 28

What are beachmarks?

Answer 28

Macro-scale
features usually associated
with the periodic start-up of
machinery; they do not
reflect load cycles, but
indicate rate of crack
propagation

Question 29

What are striations?

Answer 29

Micro-scale
features indicating crack
origin and growth
characteristics; indicative
of load cycles

Question 30

What are the physical features of fatigue failure?

Answer 30

Beachmarks and striations.

Question 31

What is the relationship between mean stress and fatigue life?

Answer 31

Increasing the mean stress
level leads to a decrease in fatigue life.

Question 32

How can striations be observed?

Answer 32

Using electron microscopy

Question 33

What do beachmarks result from?

Answer 33

Interruptions in the stress cycles.

Question 34

What does each fatigue striation correspond to?

Answer 34

The advance of a fatigue crack during a single load cycle.