The theory of plasticity also states that the material
satisfies the von Mises yield condition
where is the deviatoric part of the stress . Derive an expression for in terms of the normal to the yield surface
The von Mises yield function is
We can alternatively write the yield function as
in which case the following equations take a slightly different form.
Therefore,
The deviatoric part of is
Therefore,
Now,
and
Therefore,
The derivative with respect to is
Let us use index notation to find the derivatives. In index notation,
Hence, the components of the second-order tensor are
Therefore,
Similarly,
Hence, the components of the second-order tensor are
Therefore,
Plugging the above results into the expression for the derivative
of we get
Hence, we get
The required expression is