Resistance curve in fracture fracture mechanics

P.F.C

Hi everybody,

Dealing with crack propagation and crack instability, I have to work with JR-curves.

I would like to know how is computed a JR-curve (JR vs Δa (crack propagation)) and what is the proper way to use it in order to determine the critical crack length.

What happens if the the load is not the critical load ?

Thanks in advance for your help.
Best regards.

PFC