n><span class="s2">&quot;The stress tensor is not symmetric, Von Mises &quot;</span>
                             <span class="s2">&quot;stress is based on a symmetric stress tensor.&quot;</span><span class="p">)</span>
        <span class="k">return</span> <span class="n">math</span><span class="o">.</span><span class="n">sqrt</span><span class="p">(</span><span class="mi">3</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">dev_principal_invariants</span><spa