The Tsai-Wu failure criterion is very similar to the Hoffman criteria, but has different value for the coefficient associated with the interaction of the direct stresses. In the Hoffman criterion it is the same as the coefficient associated with the direct longitudinal stresses. In the Tsai-Wu criterion, F 12 is a term that should be determined from a test which induces a combined state of stress. Due to the difficulty of configuring such test, some references suggest setting the value of the term to zero. Tsai recommends an specific relationship with other coefficients:
F12 =-0.5.(F11 .F22 )0.5
For the Tsai-Wu criterion:
FI=(F11 .σ1 2 + F22 .σ2 2 + 2.F12 .(σ1 .σ2 ) + F66 .σ12 2 ) + F1 .σ1 + F2 .σ2