Fracture Toughness: Stress Intensity Factor Formula

[Altairs]

Reference is to the formula for stress intensity factor :

K_I(mode I) = beta*nominal stress*(pi*crack dimension)^1/2

now the question is that

1.ductile:
if I use the value of fracture toughness K_c in place of K_I and yield strength in place of nominal stress what will the value of crack dimension signify ? Does this mean that both yielding and fracture occurs at same time ?

2.ductile:
if I use the value of fracture toughness K_c in place of K_I and ultimate strength in place of nominal stress what will the value of crack dimension signify ?

3.brittle
if I use the value of fracture toughness K_c in place of K_I and ultimate strength in place of nominal stress what will the value of crack dimension signify ?


please explain. I am getting all these things mixed up.

thx

 

[Altairs]

ANy help here ? :S

 

[Rafa]

I think that you are mixture differents things.
K_I is the stress intensity factor that tells you how your material break if you have a inicial crack.
1.- If you material don't have any crack you can not apply Kic and you use Ultimate streght
2.- If you have a crack, you can not use neither yield or ultimate because is possible that you material breaks earlier.