calculates the first Piola-Kirchoff stress

        Args:
            def_grad (3x3 array-like): deformation gradient tensor

def piola_kirchoff_1(self, def_grad):
        """
        calculates the first Piola-Kirchoff stress

        Args:
            def_grad (3x3 array-like): deformation gradient tensor
        """
        if not self.is_symmetric:
            raise ValueError("The stress tensor is not symmetric, \
                             PK stress is based on a symmetric stress tensor.")
        def_grad = SquareTensor(def_grad)
        return def_grad.det*np.dot(self, def_grad.inv.trans)