<span class="p">,</span> <span class="n">def_grad</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;</span>
<span class="sd">        calculates the second Piola-Kirchoff stress</span>

<span class="sd">        Args:</span>
<span class="sd">            def_grad (3x3 array-like): rate of deformation tensor</span>
<span class="sd">        &quot;&quot;&quot;</span>

        <span class="n">def_grad</span> <span class="o">=</span> <span class="n">SquareTensor</span><span class="p">(</spa