# Novel Bionic Design Method for Skeleton Structures Based on Load Path Analysis
## 1 Introduction  
The skeleton structures have been widely used in vehicles, aerospace, aviation and civil engineering because of their universality and foundation. As the lightweight is becoming an inevitable trend in these fields, the skeleton structures can usually be designed as thin-walled structures, including thin plates, shells and slender members [1, 2]. Biological structures have excellent mechanical performance after hundreds of millions of years of evolution, which provided structural engineers with a lot of creative ideas for addressing the lightweight demands, such as bamboo, honeycomb, waterlily, bone and so on [3, 4]. If the biological characteristics are transferred to the engineering structures in the design stage, a lot of creative ideas for structural engineers will be provided to solve the lightweight demand. Therefore, structural bionic lightweight design has become a research hotspot [5, 6]. In recent years, many achievements applying biological structures to engineering equipment to achieve lightweight have been carried out [7]. [PERSON] et al. [8] designed an anti-collision beam vehicle with functional gradient features according to the human tibia porous gradient with good bearing and impact resistance, which effectively improved the crashworthiness and energy absorption performance of the anti-collision beam. [PERSON] et al. [9] applied the growth law of the waterlilyand cactus to the cross beam of gantry machining center by analyzing the distribution law of the main and sub branch veins. The advantages of the bionic model were verified by simulation and experiment. [PERSON] et al. [10] applied sandwich structure to the layout of ribs for micro-EDM machine tool, which improved structural bearing performance. These achievements are based on the full understanding of the functions and working conditions of parts and then the bionic prototypes with high similarity in structure are selected by human intuition and transferred to engineering structures [11]. However, for those complex structures, especially the skeleton structures, such as automobile frame, steering knuckle, aircraft landing gear, control arm etc., it is difficult to find a bionic prototype with high similarity in structure. For example, the car frame requires high collision resistance [12], but it is a typical multi-branch skeleton structure with multiple load paths, and it is difficult to find a bionic prototype with similar features. In order to expand the application of bionic design, some scholars [13; 14; 15] took natural branching systems as bionic objects to study the optimum morphology and growth mechanism. A bio-adaptive growth algorithm is proposed and the growth law of stiffeners is obtained by numerical calculation, which skillfully avoids the selection of bionic prototype. However, it is only suitable for the layout optimization of stiffeners in the box structures. Some scholars [16; 17; 18] combined the theory of the solution of inventive problems (TRIZ) with bionic design and used the conflict matrix to guide the selection of bionic prototype, which improves the efficiency to a certain extent. But it is still limited to parts with similar structures or functions to bionic prototypes. In addition, some scholars try to guide the bionic design by topology optimization. [PERSON] et al. [19] used the topology optimization to get the load transfer skeleton of the aircraft bracket and then applied the porous structure to the load transfer skeleton, the lightweight aircraft bracket is obtained by additive manufacturing (AM). [PERSON] et al. [20] applied the porous structure of biological microstructure to the bionic design and introduced the topology optimization to solve the boundary problem of the bionic model. Although topology optimization can reduce human subjectivity, its calculation process is relatively complex and the obtained load transfer skeleton often has many branches and holes [21; 22], which makes selection of bionic prototypes more difficult.