Research On Rigid-flexible Coupling Mechanical Characteristics And Bionics Of The Typical Plant Leaves

Fatigue failure is one of three major types of surface damage failure in the engineering field and poses a great influence on the efficient and reliable operation of machinery.The development of technology anti-fatigue design and manufacture are an important project in engineering.Theories,simulations,and experiments methods of fatigue failure have been explored widely.The researches in this area have still been technological bottlenecks for engineering industry due to the complexity of the fatigue failure mechanism,the limitation of approaches and the increasing demand of industrial manufacture for fight fatigue.On account of above problems,application of advanced materials and surface treatment technology are the main solutions.However,surface geometry and physical structure are also essential factors.At the same time,many studies were only focused on the single failure of the surface,and there is still no effective solution for superposition of more factors leaded to the failure phenomenon.The goals of green,environmental,and sustainable social development have attracted researchers to explore the field of bionics by their knowledge and technology.Advanced bionic design concepts have been applied to establish a multi-functional system with excellent crack arrest and fatigue resistance.In the nature,plants have light-weight,high-strength,crack arrest and anti-fatigue functional properties through species competition and natural evolution after millions of years.This provides an important reference for improving the performance of engineering materials.In this paper,typical rigid-flexible coupled monocotyledonous plants with parallel vein(Canna indica L.,Hosta plantaginea Aschers,Rhapis excels and Typha glauca)and reticular vein dicotyledonous plants(Populus alba,Syringa oblate Lindl.and Ailanthus altissima)were used to study the crack arrest and anti-fatigue mechanical properties.The morphological characteristics of the vein distribution and the internal microstructure of the leaves were observed and measured by digital camera,micro computed tomography and scanning electron microscopy.The different positions of cross-sections of the hard phase veins and the soft phase mesophyll were observed and simulated.Then the rigid-flexible coupling mechanical properties of the hard phased veins and the soft phased mesophyll were tested by the in-situ tensile test.The influence of the distribution direction of the hard phase veins of rigid-flexible coupled plants on the tensile properties and deformation damage mechanisms of the leaves was analyzed through microscopy.Based on the rigid-flexible coupling characteristics of the leaf,the strengthening effect of rigid-flexible coupling ratio and hard phase distribution on the substrate surface was studied by finite element numerical simulation.The biomimetic rigid-flexible coupling strengthened surfaces were prepared on the surface of the die steel by laser fusion technology.The influence of the distribution pattern of the hard phase units on the mechanical properties of different substrate materials was studied.In addition,the in-situ tensile test under scanning electron microscopy was used to study the mechanism of micro-mechanical properties and deformation damage mechanism of the bionic specimens of different matrix materials.The main conclusions of this thesis are as follows:(1)Plant leaves are composed of hard veins and soft mesophyll,and there are significant differences between species.The complex network level of venation belongs to the vector balance criterion.The vein sequence branches of the veins showed the shortest path connection.The veins of monocotyledons are parallel to each other and usually perpendicular to the small transverse veins.The stepwise scattering and network distribution of the leaf vein ensure the stiffness which meet the requirement for leaf.(2)The hard phase veins are the branches of the petiole.They presented with porous,honeycomb-like composite structures which are composed of epidermal tissue,parenchyma tissue,vascular bundles and mechanical tissues.The soft mesophyll was a “sandwich” structure composed of epidermis,palisade tissue and spongy tissue.The veins and mesophylls were connected by gentle transition and overall wrapping,which increased the stiffness of the leaf.The morphology,structure and material composition of cross-section of the petiole,vein and mesophyll exhibited gradient variation along the growth direction of the leaf.Numerical simulation results showed that this characteristic was beneficial to the bending and torsion deformation for the leaf,which provided better adaption to the changes of environment and reduced mechanical damage.(3)The rigid-flexible coupling characteristics of the leaf were related to the shape,size,material composition,and the state of the external load.The rigid-flexible coupling ratios of the main veins and mesophyll of the Populus alba,Ailanthus altissima and Hosta plantaginea Aschers leaves showed obvious differences of 15.27,70.17,and 4.26,respectively.The distribution of hard veins had an essential effect on the mechanical properties of the leaves,the larger the diameter of the veins and the more veins parallel to the tensile direction,the higher the maximum tensile load,tensile strength and elastic modulus of the leaves.Compared with the leaves of the reticulate veins,the leaves with the parallel veins had a higher stiffness and exhibited a brittle fracture.The leaves with reticulate veins showed a plastic fracture.In addition,the flexural and compressive tests of the Typha leaves confirmed that the rigid-flexible coupled “sandwich” structure of the leaf could create multiple load paths and redistribute the load when subjected to bending and compression.The “sandwich” structure protected the leaf from causing excessive local stress.(4)The finite element simulation results showed that a certain rigid-flexible coupling ratio existed between the hard phase units and the different matrix materials.The distribution direction of hard phase units had vital influence on the mechanical properties of the material.In particularly,the special branching angles between the venation of hard phase units make the distribution of tensile stress and shear stress on the sample surface more uniform and showed better strengthening effect on the material surface.(5)Based on the principle of rigid-flexible coupling crack arrest and anti-fatigue principle of leaves,laser fusing technology was used to prepare imitation vein hard units on the surface of tempered H13 steel,annealed H13 steel and 45# steel.The unit was divided into two regions of the remelted zone and the heat affected zone.The unit and the substrate were metallurgically bonded.Remelted zone was mainly composed of fine martensitic.Heat affected zone consisted of martensite,incompletely dissolved ferrite and carbide particles.Compared with the substrate material,the average microhardness of the units was higher.For different substrate materials,the micro-hardness of the units increased by 0.37,3.02,and 2.59 times,respectively.(6)Hardened units such as horizontal,vertical and grid stripes were processed on the surface of 45# steel,and the hard unit improved the tensile strength of the material.The reinforcing effect of vertical stripes was the best.The high density distribution of hard units limited the elongation of the material,resulting in a significant decrease in the elongation of the bionic specimen.Hardened units such as horizontal,vertical and grid stripes were processed on the surface of tempered H13 steel,the rigid-flexible coupling between unit and substrate was weak,and the stress transmission during the stretching of the specimen was reduced.Small microcracks and holes were found in the units,which caused stress concentration in the local area of the specimen.The tensile strength of the substrate material was highest and bionic specimens exhibited obvious brittle fracture.(7)Both the plant leaves and the laser bionic surface presented a certain rigid-flexible coupling match.The pin-type unit was processed on the surface of the tempered H13 steel,annealed H13 steel and 45# steel,which provided the materials with a “soft-hard” coupling pattern.The tensile stress in the annealed H13 steel and 45# steel was gradually transferred from the soft phase substrate to the hard phase unit during the in-situ stretching under scanning electron microscope.Before the necking,the substrate on both sides of the unit was fully elongated,and the uniform deformation process of the specimen was prolonged.The unit resulted in deformation of the non-necked region and incrase of the elongation of the materials.The strength between the crystalline grains of the H13 steel matrix was high.The effect of neighboring grains on coordinate deformation was weak.Stress in the grain interface accumulated,which results in the decrease of extending rate of accumulation.