Research On Microstructures, Mechanical Properties And Coupling Mechanism Of Beetle Elytra

In the dissertation, Morphological structures, mechanical properties, coupling mechanism and geometry of elytra opening and closing from 9 different beetles, which live in various environments (air, water, soil), were investigated.An analysis of the morphological structure indicates that the elytra have a non-smooth surface, containing two kinds of appearances: furrow surface or concave and convex surface, which can reduce adhesion and resistance present in surface contact. SEM photos of elytra sections show that the elytra are hollow and light-weight composite biomaterials, which consist of a compact dorsal side, a hollow section and a ventral side. The dorsal side consists of black compact epicuticle and looser exocuticle formed from some fibre layers complexed with each other in a parallel way, a positive-negative way and a helical way. The elytra's density (in kg/m~3) was tested and shown to be 0.8×10~3～0.9×10~3. The morphology of the Cybister adhesive fore-foot was observed under SEM, and it was shown that lots of micro-discs functioning as a shoe are distributed on its surface and lots of setae are distributed on its marginal side. The maximal normal force is 53.3mN±7.68mN and the maximal tangential force is 213.5mN±33.53mN. It is estimated that the vacuum adsorption of the micro-discs, together with the van der Waals adhesion of the setae, produce the ultimate adhesive forces of a single adhesive foot.Regarding mechanical properties, a Nano-indenter Test and a Tensile Test were carried out on different beetles'elytra to investigate its mechanical properties and tensile intensity. Results show that the mechanical properties of elytra increase gradually from the cephalosome zone to the empennage zone, which presents topological distribution. The hardness and modulus of beetles'elytra were taken from the results of the Nano-indenter and Tensile tests through linear regression and the average values were 0.335GPa±0.130GPa and 6.920GPa±1.461GPa, respectively. Cybister's results were 0.475GPa±0.089GPa and 8.214GPa±0.708GPa, respectively, which is 1.41 and 1.19 times that of beetles'elytra, respectively. Thus, this confirms conclusively that Cybister elytra have a much more excellent performance compared to that of beetles'elytra. Tensile Test results indicate that the distortion of beetles'elytra is largely elastic, while the longitudinal stress to fracture is larger than the transverse one. The stress to fracture of fresh elytra (σ_b) is 169.2MPa～194.5MPa; the specific intensity is 0.20～0.22 (the specific intensity is defined asσ_b toρ). Compared to the Mg-Li alloy (0.16～0.21), fresh elytra have a high specific intensity.Regarding its coupling mechanism, beetles'elytra are conjugated with each other via the cuneal tenon carving into the mortise. The dovetail and mortise are short and thick in the cephalosome zone and become longer and thinner in the empennage zone. There are some spinules oriented in the same way and convex buds emerging into the surface of the dovetail surface. These micro-structures help to lock two elytra tightly together. Coupling of elytra is accomplished via the following two steps: the first is the coupling process of elytra; the other is the locking process of elytra.On geometry, the mark's 3D-tracks marked on the elytra apex were analyzed with special software from beetles'elytra flight videos via a high speed camera to explain the geometry of beetles'elytra opening and closing. Previous research has shown that the geometry of elytra opening and closing in some beetles are a planar rotation around a single axis across the scutellum. The elevated angle of elytra in opening is 30°～60°. Beetles choose an adaptive angle to adequately reduce energy use during flight. This indicates that the coupling mechanism of elytra permits rotation to be easily controlled while the process spends less energy and works more effectively.In conclusion, beetles'elytra are light-weight composite biomaterials with a high ratio of intensity to weight, high stress to fracture and excellent flexibility. The results of this work offer biomimetic reference for designing future light-weight, high intensity composites for use in aeronautics and astronautics industry.