n.analysis.elasticity.stress.Stress.piola_kirchoff_1">[docs]</a>    <span class="k">def</span> <span class="nf">piola_kirchoff_1</span><span class="p">(</span><span class="bp">self</span><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 first Piola-Kirchoff stress</span>

<span class="sd">        Args:</span>
<span class="sd">            def_grad (3x3 array-like): deformation gradient t