Fatigue 2 theory

Question 1

What are the three stages in the fatigue failure of a component?

Answer 1

1. Initiation: where a surface crack develops on an otherwise smooth component
2. Incremental Propagation: where the crack propagates stably by a very small amount during each cycle; normally at right angles to the principal tensile stress direction
3. Unstable failure : where the component separates, either by reaching a limit load or by a fracture-toughness-determined mechanism

Question 2

What is the formula for Paris’s Law?

Answer 2

da/dN = C(ΔK)^m

Question 3

What is the formula for the calculation of the number of cycles required to propagate a crack from an initial size to a final size?

Answer 3

Integrate Paris’s law, take 1/C out of the integral, upper limit of integral is af, lower limit is ai.