How to estimate crack growth propagation using the maximum tangential stress criterion method?

Hi All  Currently I am running FE modelling using linear elastic fracture mechanics approach to estimate crack growth propagation in a web plate of box girder in an existing bridge deck. The FE model includes 3D solid element. The crack growth direction is expected to follow in a plan perpendicular to the maximum principal stress.  As I know there are many methods can be used to estimate the direction of crack during propagation. My question is about the maximum tangential stress criterion method.   The crack direction is this case is determined from using the following formula  Theta= Cosine-1 (3KII2+(KI4+8KI2 KII 2)1/2 )/(KI2+9KII2) I could not found more details about the notation definitions of this formula. I would be grateful if you could help where I can found more detailed theory about this method.   Regards Amin Abdulmajid