Set the Tangent Modulus to 0.0 for a conservative analysis.
Once you use Plasticity in the material model, you need to know another material property: Elongation (at Break).
Elongation is the value of Strain when failure occurs in a simple tensile test.
Without Plasticity, you may have used Ultimate Tensile Strength to calculate a Safety Factor for the design.
With Plasticity, and a zero Tangent Modulus, the stress will not go above Yield, so you need a new Safety Factor.
Safety Factor = Elongation/(Equivalent Total Strain)
A Safety Factor < 1.0 predicts failure.