Fatigue 1

Question 1

What is fatigue?

Answer 1

• Failure of a component after repeated loading at levels lower than the static failure load.
• Approx. 90% of metallic failures are due to fatigue.
• Fatigue failure occurs suddenly (a > ac) and is brittle-like in nature.

Question 2

What are the stages of fatigue?

Answer 2

(1) Crack initiation: a small crack forms at some point of high stress
concentration (e.g. scratches, dents, threads)
(2) Crack propagation: crack advances incrementally with each stress cycle
(3) Final failure: occurs very rapidly once the crack has reached a critical
size.
Cyclic loading, to microstructural surface discontinuities, to microscopic cracks, to coalescence of micro cracks into a big crack (striations), to stable crack growth (beach marks), therefore rapid failure

Question 3

What is fatigue/ endurance limit?

Answer 3

the largest value of repeated stress that will not cause failure for an infinite number of
cycles.

Question 4

What is fatigue strength?

Answer 4

the stress level at which failure will occur for some

specified number of cycles

Question 5

What is fatigue life?

Answer 5

number of cycles to cause failure at a specified stress level