12. Fracture and Fatigue

Question 1

What causes fatigue?

Answer 1

dynamic and varying stress

Question 2

Define Fatigue limit

Answer 2

the stress amplitude below which fatigue failure does not occur

Question 3

Is fatigue failure common?

Answer 3

yes, it is the single largest cause of failure

Question 4

Cyclic stresses cause ____ ____ and ____ ____

Answer 4

crack nucleation

crack growth

Question 5

Failure occurs when the crack grows to a ____ ____

Answer 5

critical length

Question 6

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

Answer 6

Beachmarks

Question 7

____: micro-scale features indicating crack origin and growth characteristics; indicative of load cycles

Answer 7

Striations

Question 8

Define shot-peening

Answer 8

the creation of Residual compressive stresses on the surface of a metal by shooting hard particles at it

Question 9

What is the purpose of shot peening?

Answer 9

Residual compressive stresses delay crack initiation (may retard crack growth)

Question 10

Define case hardening?

Answer 10

hardening the surface of a metal object while allowing the metal underneath to remain tough (high Kc)

Question 11

How do you case harden a material?

Answer 11

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