ass="sd">&quot;&quot;&quot;</span>
<span class="sd">        returns the principal invariants of the deviatoric stress tensor,</span>
<span class="sd">        which is calculated by finding the coefficients of the characteristic</span>
<span class="sd">        polynomial of the stress tensor minus the identity times the mean</span>
<span class="sd">        stress</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span