# Tresca criterion in terms of invariants

Hi,

The Tresca Critrion is given in the form of non continuous equations:

Max(½|σ12|,½|σ23|,½|σ31|) = k

How did they come up with the invarient equation

f(J2,θ) = 2√J2 * sin(θ+⅓π)-2k, θ from (0 to 60)