pan class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;</span>
<span class="sd">        returns the von mises stress</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">is_symmetric</span><span class="p">():</span>
            <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</spa