Hill Criterion

Calculates the ply failure index according to the Hill (Tsai-Hill) theory.

${\text{F}}_{\text{index}}=\frac{{\text{σ}}_1^2}{{\text{X}}^2}-\frac{{\text{σ}}_1{\text{*σ}}_2}{{\text{X}}^2}+\frac{{\text{σ}}_2^2}{{\text{Y}}^2}+\frac{{\text{τ}}_{12}^2}{{\text{S}}^2}$

Where, the failure parameters $X$ and $Y$ depends on the considered quadrant of the coordinate plane, being $\text{X}=X_T$ or $\text{X}=X_C$ if ${\text{σ}}_1\ge 0$ or ${\text{σ}}_1<0$ and the same for $Y$ .