Conditional Equation of Hill Yield Surface

f_{2} : \frac{{σ_{∥}}^{2}}{{f_{m ∥}}^{2}} + \frac{β \times σ_{∥} \times σ_{⊥}}{f_{m ∥} \times f_{m ⊥}} + \frac{{σ_{⊥}}^{2}}{{f_{m y}}^{2}} + \frac{γ \times {τ_{x y}}^{2}}{f_{m ∥} \times f_{m ⊥}} - 1 = 0

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