Under certain conditions it is possible to calculate the stress experienced at any point within a material, using simple engineering theory. Specifically, if the stress is not too great, the material acts under what is called the elastic regime and elastic fatigue fracture occurs. In such a situation, the stress felt at the tip of a crack can be calculated, and the crack opening force deduced.
Using these methods it is possible to determine the rate of crack propagation. However, the material will not always follow elastic behaviour, and plastic effects must be taken into account. J-integral methods deal with such a situation.