Hows does Abaqus calculate the Stress Intensity Factors?

Dear All,

I have been calculating the Stress Intensity Factors, KI - KIII with Abaqus and they compare very closely with close form solutions, for different mesh densities and element types. The question I have is: would anyone know which method does Abaqus use to calculate Ks? The Abaqus Theory Guide only describes how the J-Integral is calculated.

Thank you in advance for your help,

-Francesco