Micro-structure and mechanical properties of the piths of sunflower

The pith of sunflowers is a kind of natural cellular material. In this paper, through scanning electron microscope (SEM) and optical microscope, a sunflower's pith with compound structure and morphology was studied. The single cell shape deduced is a tetrakaidehedron. The compressive yield stress and modulus of the pith of the sunflower were tested. The effects of the cell shapes and structures of the pith on their mechanics properties and deformation mechanisms on compress have been studied and discussions about their reasons are made on the basis of the different models. The results show that the change of cell shape, from a normal tetrakaidehedron to the elongated one, is the main reason why the compressive modulus in the elongated direction increases 3 to 4 times than others, and the compressive yield stress over 2 times.