Why woodpecker can resist the impact

Zhang, Zhe 2019, Why woodpecker can resist the impact, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Why woodpecker can resist the impact
Author(s) Zhang, Zhe
Year 2019
Abstract Head injuries are taken as an increasingly prevalent cause of death and severe physical disabilities all over the world. The majority of head injuries result from a sudden impact such as car crash, sports accident and falls. A large number of experimental and numerical studies have been carried out for protecting the human brain from tremendous trauma during impact. Differing from vulnerable human, in nature, the woodpeckers exhibit remarkable head impact injury resistance. Because the woodpeckers feed themselves by drumming into the tree trunk and utilizing their long tongue to catch the inside insects, they need to beat the wood at a speed of 6-7 m/s, a deceleration up to 1,200 g, a pecking rate of 20-25 times per second, and a frequency of 12,000 times per day on average, which occur without cerebral concussion or brain injuries. The investigation into the anti-shock mechanism of a woodpecker's head will be beneficial for the prevention of human brain injuries. Unfortunately, the research on the impact-resistance of such complex biological organs remains challenging because of the difficulties in obtaining accurate models and simulating pecking process precisely.

Initially, with the application of micro-computed tomography (micro-CT) scanning technique, the computed tomography data of a woodpecker's head is gained. Then, the image segmentation method is demanded to separate such complicated raw data into well-defined structures. However, owing to the limitations of existing numerical techniques and computer resources, an efficient and clear separation method is still highly desirable for three-dimensional computed tomography data. In order to overcome the insufficiencies of previous approaches, we propose a novel reaction diffusion-based level set method. It reports a framework that enables to process images by rendering the structures as the zero-level contour of a level set function whose value is constrained to a narrow band ranging from -1 to 1. A cost function that is constructed by summing the fitting energy for extracting the local intensity and the diffusion energy for regularisation is minimised within a framework of optimisation. To avoid the re-initialisation of level set function and accelerate the convergence when updating the level set function, a reaction-diffusion technique is developed to replace the upwind algorithm by the finite element analysis. Numerical examples demonstrate that clear and smooth profiles can be generated within a few iterations by setting the time step 100-fold greater than the allowable value in accordance with Courant-Friedrichs-Lewy stability condition. In contrast to the conventional upwind algorithm, the proposed method exhibits better convergence without falling into local minima. By using this robust algorithm, a distinct interface model of a woodpecker¿s head that contains complete structural information is obtained.
Next, the iso2mesh, an open-source mesh generator and MATLAB-based processing toolbox, is used to discretise the interface structure into a finite element entity model with around 1.1 million hexahedral elements. Moreover, through strictly following the accurate anatomical features, a woodpecker¿s long hyoid is replaced with four curved cylinders which are connected by two spherical hinges used to restrict the translation and facilitate the rotation, and two revolute hinges serving as the rotation adaptors for restraining the rotation angle and direction. In addition, the encephalocoele is filled with viscoelastic brain tissues. Thereafter, the heterogeneity of this complex biological structure was fully considered via categorizing materials into 53 types in accordance with the intensity of raw data, ranging from 200 HU to 5,400 HU. In accordance with the existing experimental data of several components on woodpeckers' head and the relationships between image intensity, Young's modulus and density, through determining intensity ranges of multiple components, the material properties for each element can be obtained. With the purpose of investigating the roles of individual components in the sophisticated non-impact-injury system, a hyoid-removed model is established to compare its impact response with that of the real model. This comparison reveals that the hyoid can restrain the opposite velocity and maintain structural stability, especially after impact. On the other hand, a set of models with diverse lengths of external rhamphotheca and internal bony structures are built up to estimate the effects of beaks. Numerical results show that the distinctive anatomic structure of upper and lower beaks can reduce impact force and enhance structural crashworthiness.

Thereafter, according to the differences in the intensity values, the internal bone model was separated from the complete head model. The numerical results of the modal analysis on both models demonstrate that the natural frequency of complete head is far larger than the woodpecker's working frequency that is between 20-25 Hz, while the skull bone structure can suffer from resonance injury. In addition, it is found that applying the pre-tension force on the hyoid tip is capable of increasing the natural frequency of the head. Meanwhile, the larger such pre-tension forces, the higher the natural frequencies of a woodpecker's head. Furthermore, via applying the mean pecking force to the beak tips within a duration of 0.8 ms, all relatively high magnitudes of stress component are over 1000 Hz that is far larger than both the natural and working frequencies.  The large differences among the natural, working and stress response frequencies is a kind of the woodpecker's protective mechanisms for preventing the brain from sustaining resonance injury.

In the future, we will apply three-dimensional printing technology to produce a woodpecker's head structure with corresponding biological material substances for further testing our findings related to such extraordinary impact-resistance system. In addition, by combining the topology optimisation with shock absorption mechanism, we would like to design impact-related injury resistant devices, such as helmets and car bumpers to protect the human race from impact-related severe brain damages.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Construction Engineering
Keyword(s) Biological structure
Image segmentation
Level set method
Reaction-diffusion equation
Impacting simulation
Material definition
Computational models
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