Fatigue failure of metals

Question 1

What are the four distinct phases in fatigue damage process?

Answer 1

• nucleation
• microstructurally-governed crack growth (the field of “short cracks” or “small
  cracks” regime)
• crack growth governed by continuum mechanics through linear-elastic fracture
  mechanics, elastoplastic-, or fully plastic fracture mechanics
• final crack instability

Question 2

What are the different crack growth mechanisms for low-cycle fatigue (LCF) and high-cycle fatigue (HCF)?

Answer 2

• Low-cycle fatigue fracture is generally governed by mechanisms quite similar to those typical of static failure, mainly affected by plasticity phenomena.
• High-cycle fatigue regime is ruled by slow-progressing damage micromechanisms where the crack grows at rates even lower than 1 micro meter/cycle

Question 3

Why do we need to consider the stress intensity factor K when assessing fatigue failure?

Answer 3

Because the sigma-N curves are properly used for materials without defects, so when we are considering defects on material behaviour the refrence parameter is K.

Question 4

What can be assumed about K when the minimun stress becomes compressive in a fatigue cycle?

Answer 4

the K effect could
be taken as negligible

Question 5

Why do we have to consider a deltaK parameter instead of a K parameter in fatigue cycles?

Answer 5

Because of the fluctiation of stress during the cycle.

Question 6

How can thermal fatigue happen?

Answer 6

Rapid thermal changes can produce different temperature fields in material volumes featuring different thermal inertia. When thermal stresses are fluctuating over time, cycling strains can induce fatigue damage

Question 7

What are the different short cracks we can have in a fatigue failure?

Answer 7

• Microstructurally-short cracks: Size in the order of grains or the microstructurally feature that rules its growth process.
• Mechanically-short cracks: size smaller than the geometrical feature that promotes the stress magnification that had induced its nucleation
• Chemically-short cracks: small crack in a chemically aggressive enviroment

Question 8

What are the difference betwee short cracks and long cracks in fatigue?

Answer 8

• Long cracks have an increasing growth rater for increasing deltaK.
• Short cracks may show decreasing or constant trends for an increasing deltaK