Fracture Mechanics 1 theory

Question 1

Why is fracture mechanics better than the classical fatigue approach?

Answer 1

Fracture mechanics acknowledges the presence of cracks. Also it facilitates the safe operation of flawed structural components up to some critical defect size.

Question 2

When does a crack propagate?

Answer 2

When the stress intensity factor (i.e the distribution of stress ahead of the crack rather than the max stress, which is always infinite) reaches a critical value.

K>Kc

Question 3

What is Kc

Answer 3

The critical stress intensity factor or more usually the fracture toughness.

Kc is a material property. It must therefore be determined experimentally for each material of interest.

Question 4

Why is this method only valid for brittle materials?

Answer 4

In a ductile material the crack would continue to grow without reaching catastrophic failure.

Question 5

What is the formula for the Stress Concentration Factor (SCF)?

Answer 5

Kt = σmax/σ

Question 6

What is the formula for σmax at the edge of an elliptical hole?

Answer 6

σmax= σ(2*a/b)

or in terms of the radius of curvature ro

σmax = σ(2*(a/ro)^0.5)