Fatigue 1 theory

Question 1

What are the two approaches to fatigue?

Answer 1

1. Assume a flaw(crack)-free structure.
   
   • Initiation of a flaw is dominant
   • Use S-N Curves and safety factors to account for pre-existing flaws.
2. Assume pre-existing flaws of a known type
   
   • Fracture mechanics for prediction
   • Measurement/monitoring required

Question 2

What are the limitation of the S-N approach

Answer 2

The data contains considerable scatter.

Initiation and propagation not serperate.

Different failure mechanisms based on stress

• Low cycle(slow) - plastic deformation
• High cycle(fast) - purely elasic deformation

Question 3

What are two ways to improve fatigue life?

Answer 3

1. Impose a compressive surface stress (to suppress surface cracks from growing)
2. Remove stress concentrators