^{i am performing thermal stress analysis of ceramic tube and i would like to apply von-mises stress criterion to set my own stress limit, as it is said below in solidworks help.}

The maximum von Mises stress criterion is based on the von Mises-Hencky theory, also known as the Shear-energy theory or the Maximum distortion energy theory.

In terms of the principal stresses s_{1}, s_{2}, and s_{3}, the von Mises stress is expressed as:

s_{vonMises} = {[(s_{1} - s_{2})^{2} + (s_{2} - s_{3})^{2} + (s_{1} - s_{3})^{2}]/2}^{(1/2)}

The theory states that a ductile material starts to yield at a location when the von Mises stress becomes equal to the stress limit. In most cases, the **yield strength** is used as the stress limit. However, the software allows you to use the **ultimate tensile or set your own stress limit**.

s_{vonMises} ≥ s_{limit}

Yield strength is a temperature-dependent property. This specified value of the yield strength should consider the temperature of the component. The factor of safety at a location is calculated from:

Factor of Safety (FOS) = s_{limit} / s_{vonMises}

**Pure Shear**

In the case of pure shear t, von Mises stress can be expressed as: s_{vonMises} = (3)^{1/2} t

Failure occurs if: t_{max} = 0.577 s_{yield}

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Just set up a new material with the approapriate properties. You might want to consider using P1 for your criteria as brittle materaisl tend to fail du to tensile stresses.