Difference between brittle fracture and fatigue crack growth

[ehabmozart]

My question here is regarding fracture of materials. I have problems in understanding some concepts. Now, for static loading, there is a property called K_IC (fracture toughness). By equating our stress from the static load to this K_IC, we get the maximum length of the crack which does not propagate. Above this length for this static loading, crack start propagating.

q1) What happens if our K becomes larger than K_{_IC}? Does it rupture or the crack starts propagating?

Now, linking this to fatigue loading. In Paris relation, da/dN = C Δ K^m. This is applicable for region II in the crack growth. As seen from the attached figure, region II it has two asymptotes along its sides. Stage I is the ΔK_th. Here this threshold intensity factor is the point at which if we pass this threshold, the crack starts propagating.

q2) Can we link this figure to what is happening in the static failure. In other words, can we use a critical crack length which is common between them?

It would be better if I have a discussion group chat or something since this might save a lot of time and clear any confusion.
Thanks in advance

 

[Nik_2213]

My understanding is one parameter measures the failure of a steadily applied load, eg a notched-bar under increasing tension. The other is the resistance to failure when a lesser, 'working' load is repeatedly applied.

I suppose the classic example is the UK's Comet airliner where fatigue cracking at window corners destroyed several aircraft. This failure mode was proven by ground testing of a complete aircraft. After which, such corners were routinely rounded to prevent concentration of stress. Another engineering lesson bought in blood...

 

[tech99]

My understanding is one parameter measures the failure of a steadily applied load, eg a notched-bar under increasing tension. The other is the resistance to failure when a lesser, 'working' load is repeatedly applied.

I suppose the classic example is the UK's Comet airliner where fatigue cracking at window corners destroyed several aircraft. This failure mode was proven by ground testing of a complete aircraft. After which, such corners were routinely rounded to prevent concentration of stress. Another engineering lesson bought in blood...

And I believe an example of brittle fracture is the habit of the "liberty ships", which were welded construction, of breaking apart in low temperatures even when not highly loaded.