Maximum distortion strain energy theory question. von mises formula?

Where to use the reduced von Mises formula that is a^2 +b^2-ab=(yield stress)^2. And where to use a^2 +b^2-2ab=(yield stress)^2... Is the reduced one

Where to use the reduced von Mises formula that is a^2 +b^2-ab=(yield stress)^2. And where to use a^2 +b^2-2ab=(yield stress)^2...
Is the reduced one used in case of only ductile materials ?

or:Where to use the reduced von Mises formula that is a^2 +b^2-ab=(yield stress)^2. And where to use a^2 +b^2-2ab=(yield stress)^2... Is the reduced one used in case of only ductile materials ?

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