The Tsai-Wu Criterion is a quadratic, interactive stress-based criterion that identifies failure, but does not distinguish between different modes of failure (Ref. 38). Failure occurs whenever the following condition is satisfied.
The various coefficients Fi and Fij of the Tsai-Wu criterion are defined in terms of known/measured strengths of the composite material.
= Value of σ11 at longitudinal tensile failure
= Value of σ11 at longitudinal compressive failure
= Value of σ22 at transverse tensile failure
= Value of σ22 at transverse compressive failure
= Absolute value of σ12 at longitudinal shear failure
The interaction coefficient F12 can be defined in one of two different ways. If a biaxial failure stress (σ11=σ22=σbiax) is used, F12 is computed as
Otherwise, the interaction coefficient F12 is computed as
where f* is a user-specified constant, 
.
If the Tsai-Wu criterion is selected for analysis, you must specify the coefficient f*. The failure index reported for the Tsai-Wu criterion is
Failure Index = 