| As the key movement structure of mechanical equipment,the high-bearing and large-drive folding mechanism is widely used in various fields,including aerospace,military,industrial manufacturing and various folding equipment in daily life.With the continuous emergence of new materials and new technologies,the performance of folding mechanisms needs to be further improved,and the application of nanotechnology,smart materials,bionic structures,etc.,has brought more innovation and development opportunities for folding mechanisms.At the same time,with the indepth development of China’s space industry,detectors and aircraft need to complete rapid expansion and accurate operation in space,and high-performance folding mechanisms play a more important role in deep space exploration missions.After millions of years of survival of the fittest,organisms in nature have evolved complex and precise materials and structures according to the needs of their living environment,which bring unique properties to organisms.The excellent mechanical properties of these biomaterials and structures are due to the efficient mechanism of macro-structure control load distribution and micro-structure delay crack propagation.Therefore,studying the mechanism of multistage structure enhancement of mechanical properties of biological components and carrying out related bionic design is a hot spot in the research and development of high-performance mechanical engineering components.The lightweight and high-strength properties of beetle elytra have been widely studied and applied in engineering,but the leg joints of beetle have been seldom studied.The leg joints of beetles are the most commonly used parts that need to bear heavy loads repeatedly.Environmental and survival pressures also promote the evolution of leg joints with excellent mechanical properties.Different environments in which beetles live also lead to differences in the structure and function of their leg joints.Dung beetles,known as the "hercules" of beetles,are known for their ability to push their dung into balls.Its leg joints are cleverly and tightly connected and can withstand the huge load transmitted to the leg joints when pushing objects.Studies have shown that dung beetles can push objects more than 1,000 times their own weight and carry earth more than 400 times their own weight when digging burrows.In the process of pushing the object,the leg is the main force,in which the leg joint between tibia and femur plays a key role.In addition,the structure and function of the dung beetle leg joint are very similar to the bearing connection structure in engineering,and compared with existing mechanical parts,the Dung beetle leg joint achieves excellent balance and optimization in terms of load capacity,lightweight and service life.Dung beetles use limited materials to achieve high mechanical properties far beyond the original biomaterials through sophisticated and ingenious structural design,reflecting the efficient design of nature,providing highperformance biomimetic structural material design basis for materials science and mechanical engineering and other fields.Therefore,exploring the mechanism of multistage structure enhancing mechanical properties of dung beetle leg joints can provide a new bionic idea and inspiration for a new high-performance folding mechanism.In this paper,the leg joint of dung beetles was selected as the research object,and the key factors affecting the mechanical properties of dung beetle leg joints,such as macro and micro mechanical properties,macro and micro structural characteristics,material composition,element distribution,failure process and movement posture,were studied in depth,and compared with other beetle leg joints,the mechanism of multistage structure enhancing mechanical properties of dung beetle leg joints was revealed.Based on this,the bionic joint is designed and the mechanical properties are tested and the failure mechanism is analyzed.At the same time,in order to match the mechanical properties of beetle chitin fiber in nature,epoxy resin was used as the matrix,a complex three-dimensional network was generated by crosslinking with curing agent to improve strength,polycaprolactone induced phase separation to introduce hydrogen bond to improve toughness,low melting point alloy to improve thermal conductivity,and a high-strength and high-toughness shape memory epoxy resin matrix composite with adjustable properties was designed and prepared.The high strength and toughness of epoxy resin matrix composites ensure the excellent mechanical properties of bionic flexion joints,and the shape memory properties lay the foundation for the bionic joints to achieve versatility and intelligence.The bionic flexion joint with multi-stage structure enhanced mechanical properties was designed and prepared by using epoxy resin matrix composite with high strength and high toughness,and its performance and application prospect were studied.Furthermore,based on the surface microstructure of dung beetle leg joint as design inspiration,the micro-structure design of bionic joint surface realizes the self-locking function,which expands the application potential of bionic joint in aerospace and other engineering fields.The specific research content and main conclusions of this paper are as follows:(1)The postural changes of leg joint between tibia and femur of dung beetles under different motion states were studied,and the maximum thrust output of leg joint was determined when the angle between tibia and femur was maximum.The maximum weight that dung beetles can push with their foreleg is 109.11 g,which is 44.35 times their own weight.In the tensile test,the dung beetle’s foreleg failed under the tensile test force of 10.33 N,and the breaking force was 63037.61 times of its leg weight.In the compression test,dung beetle’s foreleg failed under the compression test force of23.33 N,and the breaking force was 101754.77 times of its leg weight.In the compression cycle test,dung beetle leg joint failed after 5712 compression cycles under a compression load of 5263 times of leg gravity,and the maximum compression times of dung beetle leg joint failed after 22525 compression cycles under a compression load of 1446 times of leg gravity.In the nanoindentation test,the elastic modulus of outer epidermis and inner epidermis of hindleg were 35.31 GPa and 28.83 GPa,respectively,and the nanohardness of outer epidermis and inner epidermis were 0.60 GPa and 0.45 GPa,respectively.The mechanical test results show that the leg joint of dung beetle has excellent mechanical properties,which provides an important basis for the subsequent bionic folding structure and material design.(2)The macro and micro morphological characteristics and material composition of the dung beetle leg joint were analyzed.The joint was divided into two parts,the tibial part was a large spherical joint head with two half-spherical dimples on both sides;The leg segment is a joint socket with two hemispheres raised on both sides,and the two hemispheres cooperate with each other to form the dung beetle leg joint.The surface of the joint of the dung beetle’s leg has a scale-like,brush-like and spiky microstructures that are anti-fouling and wear-resistant.The section of the tibia and leg of dung beetle’s leg joint showed a gradient structure from the outside to the inside,and the outer skin was a solid block structure.The inner epidermis is a cross-layered structure with an angle of 90 ° between two adjacent fiber layers.At the same time,the crystallinity of chitin in the leg joints of dung beetles is high,and there are small Angle grain boundaries and large Angle grain boundaries.The main components of dung beetle leg joints are α-chitin and protein,and the main elements are C,N and O.The multi-scale structural characteristics and material composition of dung beetle leg joints were studied and analyzed,which laid a foundation for further revealing the complex interaction mechanism between macro-structure,micro-structure and material composition.(3)Through in-situ mechanical tests and digital image correlation tests,it is proved that the hemispherical connection of dung beetle leg joint can transmit load to leg exoskeleton when it is subjected to load,thus avoiding catastrophic damage at the weak joint.The hardness gradient structure of the exoskeleton of dung beetle leg joint and the cross-layered arrangement of chitin fibers in the inner epidermis can effectively prevent the formation and expansion of cracks,and the nanoscale crystal structure directly improves the mechanical properties of chitin fibers.The mechanical properties of dung beetle leg joints are enhanced by hemispherical connection-chitin fiber crosslayering-macroscopic-microscopic multistage structure of nanocrystal structure.The biomimetic model based on the design of dung beetle leg joint has an increase of 81 %and 48 % in tensile fracture force and 59 % and 38 % in compressive fracture force,respectively,compared with the long-axis connection model and the traditional bearing connection model.Through digital image correlation test and finite element simulation,it was verified that the failure mode was the same as that of dung beetle leg joint,and the stress was concentrated in the side wall.The mechanical properties of the long-axis model and the through-axis model are poor.The stress is concentrated at the joint when the force is applied and breaks from the joint when the failure occurs.(4)Based on the structure and mechanical properties of chitin fiber of dung beetle exoskeleton,a high-strength and high-toughness shape memory epoxy resin matrix composite material with similar mechanical properties to chitin fiber is designed and prepared.Epoxy resin is used as the matrix material and polyamide curing agent is used to cross-link to generate three-dimensional network to improve strength.Polycaprolactone induced phase separation to produce a large number of hydrogen bonds to improve toughness,low melting point alloy to increase thermal conductivity to improve shape memory performance,and high strength and high toughness shape memory epoxy resin matrix composite was designed and prepared.Among them,the best epoxy resin polycaprolacton-low melting point alloy composite has the tensile strength of 34.95 MPa,elongation at tensile break of about 3.6 %,bending strength of234.79 MPa,elongation at bending break of about 7.0 %,and has the function of variable stiffness.Its tensile strength can vary from 34.95 MPa at 25 °C to 0.27 MPa at80 °C,and it has excellent shape memory performance,with a shape recovery rate of43.65 °/s,shape recovery rate of 97.60 %,and shape fixation rate of 99.40 %.The shape memory epoxy resin matrix composite prepared from chitin fiber distribution of dung beetle leg joint exoskeleton exhibits high mechanical properties and excellent shape memory properties,which provides the basis for multi-functional and intelligent bionic flexion joints.(5)Based on the multi-stage structure of dung beetle leg joint and the high-strength and high-toughness shape memory epoxy resin matrix composite material,the bionic exoskeleton with gradient hardness and the high-performance bionic folding joint with multi-stage structure were successfully prepared.The tensile strength and flexural strength of biomimetic exoskeleton are 56.46 % and 43.38 % higher than those of unstructured bulk materials,respectively.The bionic joint effectively improves mechanical properties by means of hemispherical connection,cross-layered arrangement of high-performance composite blocks,three-dimensional network structure and multistage structure of hydrogen bonds.At the same time,the bionic intelligent joint is designed by micro-structure on the surface of the bionic joint,which realizes the function of rotation in the direction and self-locking in the direction of reverse.Finally,the bionic intelligent joint is installed on the satellite model solar panel as the key rotating part of the folding mechanism. |
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