Jiro Sakamoto,School of Mechanical Engineering, Kanazawa University, Kakuma-machi,Kanazawa, Ishikawa, 920-1192, JAPAN
Yasuhiro Endo, Graduate School of Kanazawa University
Eichiro Kinosita, Institute of Natural and Environmental Technology, Kanazawa University

Key Words: Plant Fruit, Burst, Impatiens, Finite-Element Analysis, High-Speed Video Camera, and Optimization

Bursting of plant fruit is very interesting phenomenon in viewpoint of plant biomechanics, because it is mechanical phenomenon directly related to reproduce of the plant. If a kind of plant has fruit, which can burst powerfully and scatter seed widely, it has the advantage to expand breeding grounds of the plant in natural selection. Many types of plant fruit probably have been optimized mechanically and structurally in evolutional process. We have studied on mechanics of bursting fruit dealing with impatiens as a research subject. Strain energy is stored in impatiens fruit due to swelling just before bursting. Mechanical stress of the fruit occurs in self-equivalent condition at that time. Then, stored strain energy of the fruit is released in very short time while bursting. Quickly and large motion of the fruit pericarp in a certain direction is advantageous to throw the seed far away. The bursting motion and deformation of fruit depend strongly on their mechanical stress just before burst. We analyzed the pre-burst stress generated in a pericarp of impatiens by using finite-element method based on its deformation history taken by high-speed video camera, in previous study. We treated one pericarp in the study, but impatience fruit is composed of five pericarps. In this study, finite-element model of the total fruit composed of five pericarps was created, and then swelling and bursting simulation of the fruit was performed. That is, we gave assuming temperature distribution on fruit surface of the FE model to generate thermal stress simulating swelling stress. Burst motion simulation of the fruit performed by releasing a pericarp boundary after swelling. We considered an optimization problem to maximize top velocity of the pericarp tip with varying swelling stress distribution as design variables under stress constrain. Optimal burst motion of the fruit was obtained by solving the problem, and compared with the real fruit motion. Optimality of the stress distribution was discussed.