The Mechanical Principle Of Anti-shock Of Woodpecker Head Under Dynamic Impact

Nature is the best teacher of human beings. In this world, most of inventions are inspired by nature. During the long-term revolution, the livings produced much special materials and stucturces which are better than man-made materials in some fields. Bionics was born during the proceeding of learning from nature, and now it plays a more and more important role in scientific studies especially in multidisciplinary applications. High-speed impact and collision are very common in modern life, and they often result in damage of the structures and material. Consequently, it has important significance to prevent the structures from damage induced from impact and collision. In aerospace industry, a serious vibration of flying aircraft is often induced from a high-speed air flow. A space junk is also a threat to the safety of flying aircraft due to the possible collision. In automobile transportation, crash is almost unavoidable and the passengers have a high probability of injury in the collision. Also in some intense sport competitions, for example rugby game, it is very important to protect the athlete head and brain from impact injury. In recent decades, people have conducted a series of the investigations in preventing high-speed impact and collision, and some of them focused on the anti-shock and energy absorption of structure and materials.Woodpecker is a kind of birds who pecks trunks for living. It can beat trees in high frequency and high deceleration but never gets brain concussion. The special anti-shock ability of woodpecker attracts the attention of many scientists. We set up a computational model of woodpecker pecking in mechanics method, focusing on the anti-shock mechanism, vibration characteristics, energy conversion and absorption of the woodpecker. The dissertation is mainly composed of the following few aspects:In Chapter2, the microstructure of woodpecker skull is first studied. We measured the distribution of the Young’s modulus of the skull structure, observed the inner microstructure of the skull in different positions and discussed the effect of the modulus distribution and microstructure on the stress wave propagation in a one-dimensional viscoelastic bar. Experiment results show that the Young’s modulus of the skull changes in a form of sinusoid along the sagittal plane and coronal plane of the skull, with the maximum value of12GPa and the minimum value of about4GPa. The stress propagation analysis of one-dimensional viscoelastic bar indicates that, the changing of the material parameters will affect the changing and distribution of the peak stress. Analogy and computation results show that the distribution of the material parameter in the skull has the best effect to protect the brain. Observation and computation of the microstructure of skull demonstrate that there are several types of holes in the skull along the impact direction. It is shown that, the rectangle type of holes shows better energy absorption ability than the other types of holes. In Chapter3, the finite element model of the woodpecker head with fine meshes is first established by the reverse engineering method. Then the stress wave propagation in the head is studied. The results show that most of the impact stress goes into the upper beak of the bird. The stress wave is spread effectively by the dome structure of skull and the hyoid around the skull. The viscosity of biomaterials plays an important role in decreasing the stress value. Therefore the stress in the skull and brain are both in a safe level without any damage occurred in head structure.In Chapter4, combining with the stress analysis and spectrum analysis techniques, we studied the vibration characteristics and anti-shock mechanism of the woodpecker head and obtained the stress component of brain in frequency domain. The modal analysis shows that the first thirty modes of the woodpecker head are local vibration occurring within the brain in form of rotating modes or combined rotating modes. Meanwhile there is a large gap between the working, natural and stress response frequencies of the woodpecker head. This enables the bird effectively protect its brain from the resonance injury in translational pecking process. The results also indicate that the application of pre-tension force to hyoid bone can availably increase the natural frequency of woodpecker head with a maximum value of21.3%. This also reduces the possibility of the brain resonance which is consistence with the actual observation of woodpecker shrinking its hyoid before pecking.In Chapter5, the finite element model of a whole woodpecker including the whole body is built. The stress, energy conversion and absorption of the bird during successive peckings are investigated. The results show that except for the collision moment the stress in the body is much larger than that in the head. Therefore the body acts as a cushion to absorb the stress wave energy and reduces the stress value in the head. The energy used for impact is mostly converted into the strain energy in the body and is recycled in the next pecking, thus there is only a small fraction of energy entering into the head. The small strain energy in brain is mainly converted into dissipated energy and results in a little temperature increment. The beaks, skull, dura matter and hyoid bone in the head work together for the protection of brain. A part work of this chapter has been published as a cover story on SCIENCE CHINA Technological Sciences,2014, No.7.