Microstructure, elastic modulus, and energy storage properties of portion II in the locust semi-lunar process

A locust stores the energy needed for jumping in its semi-lunar processes (SLPs) at the end of the hind leg femur, and portion II of the SLP plays a key role in the energy storage–release process. In this paper, scanning electron microscopy and transmission electron microscopy tests were conducted on portion II to determine its microstructure. The elastic moduli of different directions of portion II were tested by atomic force microscopy. Test data demonstrate that portion II is a layered structure formed by lamination of composite layers with a thickness of 1.09 ± 0.44 µm and chitin layers with a thickness of 0.10 ± 0.02 µm, and the composite layer is a fiber-reinforced structure. The elastic moduli of the composite layers along the fiber direction and perpendicular to the fiber direction are 11.32 ± 1.09 GPa and 10.36 ± 2.64 GPa, respectively. Furthermore, the influence of the change in the thickness of the composite layer, the volume ratio of the composite layer, and the fiber volume content in the composite layer on the maximum von Mises stress and strain energy of portion II was analyzed using the finite element method. The analysis results show that the strain energy corresponding to the actual parameters of portion II is close to the maximum. Under this premise, the maximum von Mises stress is close to the minimum. This suggests that the actual parameters give portion II almost the largest energy storage and then the longest fatigue life.