Dynamics And Fatigue Life Of Three-dimensional Structure Of Dragonfly Wings

Nearly perfect body structures of biology were formed in the millions of years'evolution, survival of the fittest laws in nature made they achieve the best design in structure, form and function. As a skilled fliert, the weight of dragonfly's wings is less than 1-2% of their total weight, but they present superior stability and excellent load-bearing capacity during the flapping flight.The main purpose of bionics is to investigate the structure and characteristic of biology so as to provide new design concept and method for engineering technology. Nowadays, flapping-wing micro air vehicles are used in the military and civilian field, which characters are light mass, good stability and high fatigue life. In this paper, from the bionic point of view, structure and material components were studied by use of microscopy and infrared spectroscopy respectively, three-dimensional models were reconstructed using three-dimensional reverse engineering technology, dynamics and fatigue life of dragonfly wings were investigated by use of the finite element, and wetting properties was analyzed in order to provide technical informations for flapping-wing micro air vehicles.Structural parameters are necessary for establishing finite element model, micro-structural and and material composition are important parameters of the mechanism of surface hydrophobicity. Firstly, macroscopic and microscopic structure and material composition of dragonfly wings were investigated. Mass, length, width, wingspan and aspect ratio parameters of dragonfly wings were measured using stereo microscope and other instruments, and neuration of dragonfly wings were definited using the Comstock-Needham system. Scanning electron microscopy were used to investigate the microstructure. Results showed that there were a series of non-ordered papillate structures on the dragonfly wings. The diameter of column papilla arranged from 66.90 nm to 200.73 nm, spacing varied from 20.00 nm to 650.00 nm, and the average height was about 0.6μm. Geometrical size of drayonfly wings minished along the wing span and wing chord. For example, the maximum thickness of membrane, 4.08um, lied in wing base, and the thickness of wing end was only about 1um. Veins in different positions showed different shapes and sizes, most thier cross section were circular. The infrared absorption spectra pictures showed the main material components of dragonfly wings surface were waxes.Estabilishing geometric model accurately is to ensure that the result of simulation analysis is reliable. In this paper, three-dimensional geometric model reconstructed the geometric model of dragonfly wings using three-dimensional reverse engineering. We could found from the result of precision analysis that the three-dimensional model of dragonfly wings were accurate.Modal analysis is the basic step of dynamics, is the essential step of investigating structure or system, and an important way to prevent structure from resonating with the outside load. Dynamics of three-dimensional structure of dragonfly wings were analyzed. Results indicated that first order natural frequency was much higher than the flapping frequency of dragonfly wings, and which effectively avoided the resonance phenomenon. The first mode is first order bending deformation. Second mode of dragonfly forewings is the twisting, while second mode of dragonfly forewings is bending. Third mode of dragonfly forewings is first-order bending vibration, while hindwing's mode is first-order twisting. Fourth mode is the coupling of bending and twisting vibration. It could be seen from the results that modes of forewing were not same as modes of hindwings in second and third modes. The reasons for that were that structure and aspect ratio of forewing and hindwing were different. Results indicated that modes of forewing and hindwing changed according to the first bending and twisting, second bending and twisting, the coupling of bending and twisting vibration.Transient response and frequency response analysis are to determine dynamic response to any load change with time and frequency, and are one of the important process of structural stability. Transient response analysis showed that at the frequency of 27Hz, the whole wing flapped without local deformation, and displacement response curve and stress response curve of every points were extremely similar. Wing trail had the smallest stress and the largest displacement, but wing base showed the opposite results. Frequency response analysis showed that, because the couple of the natural frequencies and load led to sympathetic vibration, and resulted in the large displacement, the maximum displacement and stress of dragonfly forewing and hindwing occur around the frequency of 60Hz and 50Hz respectively.Fatigue life is an important parameter of evaluating one system or structure, fatigue life analysis is an important demand of structural design or optimization. MSC.Fatigue was used to analyze the mechanical response of dragonfly wings, and based on the result of stress response, fatigue life of dragonfly wings were analyzed using Miner linear cumulative damage theory. It could be seen from the simulation that fatigue life of dragonfly wings were more than 105.3, belonging to the high-cycle fatigue life.Self-cleaning is one of factors of system or structure maintaining stability. The wettability of dragonfly wings was researched using the optical contact angle measuring instrument. Results indicated that dragonfly wings had self-cleaning effect, and different species of dragonflies had different wetting properties. Based on the self-cleaning structure, the relationship between stability and dynamics of dragonfly wings was analyzed by use of the finite element method. Once contaminated, natural frequencies of dragonfly wings reduced accordingly, and changed with the pollution location and mass. Studies showed that self-cleaning structure of dragonfly wings guaranteed the mass stability of wings, avoiding the change of dynamics of dragonfly wings.The hydrophobic mechanism is exploring the secret of self-cleaning properties of dragonfly Wings, and one important step in bionic project. In order to the hydrophobic mechanism, this paper dealed dragonfly wings surface with organic solvents. Because non-smooth papilla structures are one part of the upper epidermis, the main ingredients of which are soluble in organic solvents, and the dissolution rates increases with the time. Moreover, different organic solvents have different effect on the non-smooth mastoid structures. Microstructure and material composition of treated and untreated dragonfly wings surface were studied using scanning electron microscope and infrared spectroscopy. Non-smooth mastoid structures treated by organic solvents were destroyed more or less. Compared to other solvents, chloroform solution had greatest effect on the non-smooth structures, followed by benzene, heptane, methanol, n-hexane, ethanol acetone. Non-smooth structures of dragonfly wings treated by chloroform were removed completely. Treated by benzene for two hours, 50-60 percents of non-smooth mastoid structures were dissolved. Treated by any organic solvents of enzene, heptane, methanol, n-hexane, ethanol acetone, non-smooth papilla structures happened to change. As the main chemical constituent of endocuticle was chitin, which was not soluble in organic solvents, endocuticle of dragonfly wings was not damaged. After treated by chemical reagents, wettability of dragonfly wings varied. The more non-smooth papillaes were destroyed, the more hydrophobicity of wings falled. When non-smooth papillaes were completely destroyed, contact angle of dragonfly wings reduced to 95.8±3.69°, which was close to the intrinsic contact angle (95°). The infrared absorption spectra pictures of treated dragonfly wings surface showed that the main material components of treated dragonfly wings surface were chitin and protein. From this, we could achieve conclusion that self-cleaning characteristic of dragonfly wings was the coupling of nano-scale non-smooth papillas and the waxy layer.In order to make the structure used in the engineering, it is necessary to simplify the structure of dragonfly wings and establish the bionic model. Based on the results of modal analysis, this article optimized and elected the bionic models which have the same dynamic parameters with primary dragonfly wings, and analyzed transient response and harmony response of bionic models. Results indicated that veins had an important role in the dynamics of structure. Compared to the original models of dragonfly wings, natural frequencies of bionic models fell. Longitudinal veins can maintain that dragonfly wings have reasonable flexible deformation, and make dragonfly can use aerodynamics, and reduce resistance. Compared the transverse veins near the leading edge, transverse veins near the trailing edge took less effect on the dynamics of the whole dragonfly wings. In the same boundary conditions and stimulated load, bionic models had the same dynamic response as the primary dragonfly forewing. From the harmony response analysis of dragonfly hindwing, we could see that local vibration took place at the high frequency. In conclusion, bionic models and primary dragonfly wings still had the same dynamic parameters. It could be seen from the fatigue life simulation of bionic models that although the fatigue life was somewhat smaller than the original model, the smallest cycle number N was greater than 10 5, belonging to the high-cycle fatigue and meeting the demand of dragonfly'flight. Results further verified the reliability of bionic models and provide the theoretics datas for the development of flapping-wing micro air vehicles.