Dear Engmostafa Kalil, the definition is in this link.

http://bechtel.colorado.edu/~willam/constitutivemodel.pdf

you will find the definition here http://bechtel.colorado.edu/~willam/constitutivemodel.pdf/page 4

Hello Engmostafa,

I like this definition (but there are others):

http://www.21pw.com/dictionary/Willam-Warnke-yield-criterion_71233

Best wishes

In the criterion, how to calculate shear transfer co efficient for open and closed crack

In general the open and close shear coefficients are between 0 to 1.0. And the values depends on the performed test and in most simulations they are taken as 0.3 and 0.8 for bending tests.