Structural Characters, Nanomechanical Behaviors And Biomimetic Analysis Of Dragonfly Membranous Wing

Biological structures and configurations are always impressive due to their multiple functions and ability to adapt to their living surroudings. Bionics is to imitate the principles of biosystem and to create corresponding techniques or make the artificial system possess the characteristics of biosystem. The main idea of bionics is to learn from natural system and to be used for technical innovation. Natural biomaterials are not only constructional materials but also functional materials. The research on the relativity between the structure and performance or function of natural biomaterials is the basis of the development of biomimetic structures and materials. Insect wings have the optimized properties in structures, functions and materials through the evolution over millions of years. Insect wings are highly specialized flight organs for flight. Dragonfly wings possess great stability and a high load-bearing capacity during flapping flight, glide and hover although their mass is less than 1-2% of their total body mass. The wings of dragonflies are mainly composed of veins and membranes, a typical 2-dimensional composite material in micro- or nano-scale.The wings of three kinds of dragonflies were examined. The dragonfly Pantala flavescens Fabricius with medium-sized yellow body belongs to Odonata, Anisoptera, Aeschnidae, and Anax. The male and female are with the same morphological characteristics. The dragonfly Anax parthenope julius Brauer with strong capacity of flight belongs to Odonata, Anisoptera, Aeschnidae, and Anax. The male is a little brighter than the female with the same color and stripes. The dragonfly Sympetrum striolatum Charpentier belongs to Odonata, Anisoptera, Libellulidae, and Sympetrum. The mass and the morphological parameters of the three kinds of dragonflies were measured by an electronic analytical balance, a stereomicroscope and an image analysis system. The parameters include the body mass, wing mass, body length, body width, wing span, wing length and wing area. The vein system of dragonfly is very complex. The venation of the three kinds of dragonflies was steady and similar according to the Comstock-Needham System. The vein combination is occurred in Odonata. The main veins and the branches cross at right angles on the dragonfly wings. The main veins act as beam and bear greater loads than the branches. The angles are about 120 degrees between two branches in the middle and edge parts of the dragonfly wings since the branches bear loads alike.It was found by stereoscopy that at low magnification there are some microtrichias on the veins of the three kinds of dragonflies. The membranous cuticle is composed of cross-like fiber structures and the density of the cross-like fiber structures differs from that of the surrounding materials. The cross-like fiber structures bracing the membranes diagonally are able to increase the bending and torsion resistance of the neighboring veins. The wing membrane is very homogeneous and the cross-like fiber structures are disappeared if the wax layer on the wing membranes removed. It is indicated that the cross-like fiber structures are not a part of the membrane.Three typical cross sections of the dragonfly wings were selected to investigate the geometrical morphology. Dragonfly wings are not smooth or simple cambered surfaces. The cross-sectional profile of the wing has an irregular corrugation and the wing changes flat at the wing tip. The longitudinal veins are situated on the highest points and the lowest points, and connected by the cross veins and membranes are to form a dimensional truss structure. The corrugation can enhance the warping rigidity and the flexibility.The geometrical morphologies of the cross sections on different positions of the veins are different. The veins consist mainly of hollow tubes transporting hemolymph and there are tracheae and nerves in the cavities of veins. The veins is about 20-200μm and the membranes is about 2-5μm in thickness. The wing membranes are composed of a two-double integument with nerves inside. The wings of dragonfly are composed of cuticle, a fibrous biomaterial, whose mechanical properties range from very stiff to flexible, depending on its chemical composition. The cuticle consists chemically of two main components: chitin-a crystalline polymer, embedded in fibrillar matrix. This matrix material contains numerous structural proteins and lipids, and the cuticle is covered with a thin waxy layer.The nanomechanical behavior was investigated. The holding time and the loading rate were selected 20s and 53μN/s by the method of test optimization. In nanoindentation experiment, 6 indentation measurements were done in an area of 0.075×0.01mm and then took the mean value as the nanomechanical parameter of this position. It was shown by contrasting the nanoindentation properties of the living dragonfly Pantala flavescens Fabricius with the fresh one on the corresponding parts that the reduced modulus and the hardness of the living one are a little higher than the fresh one. The reduced modulus and the hardness of the dragonfly Anax parthenope julius Brauer are maximum on the corresponding parts among the three dragonflies, related to the large somatotype.The digital measurements of the dragonfly wings were carried out by a 3D laser scanner after the wings treated with dye check agent. Using reverse engineering software, Imageware, the scanning data point groups of the dragonfly wing were processed, including of deleting error points, smoothing the scanning data by Gaussian filter and reducing the data by chordal deviation method. Based on the shape features of the dragonfly wing, the boundary curves were picked up by Circle-Select Points from the scanning data point groups. The 3-dimensional models of the dragonfly wing were reconstructed with the boundary curves and the scanning data point groups.The scanning data point groups of contours of the three typical cross sections were obtained by using three cross sections which were normal to the scaning plane to intercept the scanning data. The coordinates of the scanning data of the three typical cross sections were obtained with the software, Imageware. The curves of the discrete points were plotted using Matlab and several fitting function methods were attempted. The curves fitted well were selected to act as the contours of the cross sections of the dragonfly wing. The fitting curves were evaluated by the coefficients of R2 and the residual sum of squares.The finite element software was used to simulate the dragonfly wing. The veins were simulated by pipe20 with 2 nodes and the membranes by shell43 with 4 nodes. The influence of geometrical nonlinearity was taken into account but material nonlinearity, and the models were assumed in the elastic range. The finite element models of dragonfly wing were simulated with structural statics and the deformation, the stress and the strain under the uniform load, the bending moment and the torque were analyzed. The dragonfly wing is a dimensional truss structure with excellent structural rigidity and deforms only a little as a whole under loads. It was shown that the grid structures of the dragonfly wing deforming together at the boundaries of veins and membranes have excellent integrity.The models of the biomimetic sheet and the grid structure of veins imitating the dragonfly wing were set up. The bearing capacity of the model of the biomimetic sheet was decreased by 6 times. The deformation, the stress and the strain under the bending moment were analyzed. The membranes under loads can bring tension to reinforce the grid structure of veins, so the membranes and the veins can work together. The membranes can lessen the vertical deflection to ensure the stability of the dragonfly wing during flapping flight. The maximum deformations of the grid structure of veins and the model of the dragonfly wing are about the same.Three structural models imitating the dragonfly wing were designed learning from the basic units of dragonfly wing. The three structural models are the interleaving grid structure of rectangle, the grid structure of hexagon and the combined grid structure of polygon built on the basis of the features of the dragonfly wing. The three models deform as a whole under the concentrated load and the maximum deformation is 5.632mm for the interleaving grid structure of rectangle, 11.179mm for the grid structure of hexagon and 8.888mm for the combined grid structure of polygon. The deformation of the interleaving grid structure of rectangle is the minimum, and the combined grid structure of polygon take second place and the grid structure of hexagon the maximum. The interleaving grid structures of rectangle with great rigidity are to form among the main veins to resist the large deformations and avoid the damages to the wing. The grid structures of hexagon with great elasticity are to form among the branches to make the wing some flexibility. The combined grid structures of polygon are optimal with fine structural rigidity and flexibility.The areas surrounded by a hexagon are the maximum, and the areas of a rectangle take the second place and a pentagon the minimum under identical perimeter. The grid structure of hexagon can surround the maximum areas with the least material among the three polygons. The grid structures of hexagon on dragonfly wing are with the minimum material consumption to form the maximum areas. The utilization of materials on dragonfly wing is optimal over millions of years'evolution.