Research On The Mechanical Properties Of Beetle Forewings And Biomimetic Fully Integrated Honeycomb Plates

The characteristics of beetle forewings (also called "elytra")include a high degree of optimization, and light weight and high strength, and thus the beetle forewings are one of the ideal models for biomimetic objects composed of lightweight functional materials,which has caused the extensive concern at home and abroad. Therefore, based on the review of the progress of the biomimetic application research on beetle forewings and the study on the shear and edgewise compressive menchanical properties of honeycomb plates, firstly,this project performed an empirical study on the queries about the structures and the mechanical properties of beetle forewings, and hereby established the biomimetic models of beetle forewings which this project should adopt. Secondly, the experimental study on the shear mechanical propterties of short basalt fiber reinforced polymer (BFRP) composites was producted, providing the mechanical characteristics of BFRP under the conditons of different fiber lengths and volume fractions, which would establish a foundation for finite-element analysis of the shear mechanical properties of BFRP honeycomb plates. And on this basis, finally, the shear and edgewise compressive mechanical properties of biomimetic fullly integrated honeycomb plates (FIHPs) manufactured by BFRP were actually measured. Therefore, the present sutdy mainly performed three parte of research work (Part ?-?) as follows:Part I Verifying study on the structures and mechanical properties of beetle forewings1. Establishment of the biomimetic models of the structures of beetle forewingsFor the report that the bio-inspired structure "most similar to the structure of elytra of Cybister beetle forewings" had more excellent mechanical properties", the microstructures of beetle forewings were directly observed, and the comparative analysis of compressive mechanical properties of the related biomimetic models was performed using the finite element method in this chapter, verifying that the trabeculae of Cybister are also solid and there are not the transverse tubes in its core layer, and verifying the fact again that Cybister owns the 3D structural characteristics belonging to the normal beetle forewings, and then verifying that the models of biomimetic honeycomb plates based on whether hollow-core or solid-core trabeculae biomimetic models possessed better compressive mechanical properties. Therefore, this project adopted FIHP which was extracted by Chen JX based on the structures of beetle forewings to perform the corresponding study.2. Characteristics of the tensile mechanical properties of fresh and dry forewings of beetlesFor the report "there is no obvios difference" of tensile yield strains between the fresh and dried forewings of Cybister, based on the observation of microstructures of many beetle forewings and the tensile experiment of the fresh and dry forewings, this chapter demonstrated the characteristics of their tensile mechanical properties. The results revealed obvious differences in the tensile fracture morphology and characteristic of tensile mechanical properties of fresh and dry forewings of Cybister and Allomyrina dichotoma.There occurred a viscous, flow-like, polymer matrix plastic deformation on the fracture surface of fresh forewings of the two beetles, with the morphology soft and many fibers being pulled out, whereas on the dry forewings, the tensile fracture surface was straightforward and there were no features resembling those found on the fresh forewings.The fresh forewings exhibited a greater fracture strain than the dry forewings, which was caused by the relative slippage of hydroxyl inter-chain bonds due to the presence of water in the fibers and proteins in the fresh forewings. The phenomenon of sudden stress drops caused by the fracturing of the lower skin because the lower skin fractured before the forewings of A. dichotoma reached their ultimate tensile strength was proposed for the first time, and the reasons and mechanism underlying this phenomenon were also investigated.Part II Mechanical charateristics and applied basic research of BFRPFor obtaining the basic properties of BFRP composites and then analyzing the BFRP honeycomb plates and establishing a foundation for generalizing the applications of basalt fiber, the shear mechanical properties of BFRP plates with the fiber length (Lf) of 3, 6 and 9 mm were emphatically measured based on the improved V-notched rail shear device and test method in this part.1. Shear test method for and mechanical characteristics of BFRPThe shearing failure cracks of BFRP can be classified into three types, namely, main cracks, coupling cracks and micro-cracks. The micro-cracks, which originate from micro-slippage at the interfaces between the short fibers and the epoxy resin, initiate prior to the main cracks. The existence of a critical value of the fiber volume fraction (Vf) was proposed, above which a sample possesses an elastic-plastic deformation capacity by virtue of the initial micro-slippage at the fiber/matrix interfaces. Furthermore, a higher Vf gives rise to a stronger elastic-plastic deformation capacity. The report of the shearing mechanical properties and other basic material attributes of BFRP with Lf of 3 mm were presented,providing the popularity conditions that Vf must not be less than 15%, and greater than 30%as much as possible when using BFRP with Lf of 3 mm.2. Characteristics of the shear mechanical properties and the influence mechanism of BFRP1) The first critical loads on their shear load - displacement curve are stable at a fixed interval, and the internal reasons that the same interface slip conditions happened were proved in term of the state of fiber/matrix interface bonding; 2) the shear strength,deformation and energy dissipation capacity will increase with the increase of Vf, but when Lf is longer, their growth speeds slow down, whereas Lf =3 mm (the shortest), all its indexs above almost increase at a constant speed; 3) the shear elastic modulus of specimens will reach their maximum values when Vf is 25% (Lf = 3 mm), whereas for the specimen with the fiber length of 6 or 9 mm, the longer Lf is, the faster growth speed of shear elastic modulus is. The failure mode of the specimens with Lf of 6 and 9 mm is mainly debonding between the fiber and matrix; when Vf is high, the fibers can easily bundle, which reduces the adhesion between the fibers and matrix. In addition, the main failure mode of the 3-mm-long fiber specimens is fiber pull-out; when Vf is high, the short fibers that are not pulled out significantly increase the deformation ability.Part ? Mechanical charateristics and applied basic research of FIHPs1. Study on the shear mechanical properties of core layer structure of FIHPsThe shear failure of the core layer structure of FIHPs is mainly caused by the debonding failure of fibers in the middle surface of the core layer, which belongs to the material interface failure, and there is no stripping failure of the interface between the core layer structure and the upper and lower laminations, indicating that such biomimetic structure has excellent and integral mechanical properties. FIHPs imitated in this chapter have yet to be perfected, and hereby the method in which the fiber surface should be pretreated was proposed. According to the biomimetic technique of the distribution of the continuous fibers around the trabeculae of beetle forewings, the laying structure of fibers in the honeycomb walls and the sealing edges can be optimized for improvement, which will point out the research direction for improving the shear mechanical properties of the core layer structure of FIHPs.2. Experimental study on the edgewise compressive mechanical properties of FIHPsFIHP has edgewise compressive failure mechanism with the main characteristics as the compression failure of imperforate panel, has obviously plastic characteristics, and doesn't appear obvious brittle failure like panel buckling and so all. Because there is no panel-core cement face, FIHP doesn't express the failure type of panel buckling, which states that it has good integrality, at the same time, we found that FIHPs with Lf of 9mm obtain the best shearing modulus of elasticity and good plastic deformation. The simple calculating formula of FIHP's edgewise compressive bearing capacity was obtained.The innovations of this paper are as follows:(1) That trabeculae of Cybister are also solid was confirmed, and that there are not the transverse tubes in its core layer was confirmed as well, verifying the fact again that Cybister owns the 3D structural characteristics belonging to the normal beetle forewings through observing the structures of the core layer of Cybister forewing in terms of vertical and horizontal directions. The difference between the tensile mechanical properties of fresh and dried beetle forewings was experimentally verified, and the main reasons why there is the difference between them were analyzed. And the paper first presented that there existed a phenomena of sudden stress drop caused by the fracture of lower skin because the lower skin fractured in advance before the forewing of Allomyrina dichotoma was reaching its ultimate tensile strength, and then investigated the reasons from the macro- and microscopoic angles, respectively.(2) The shearing mechanical properties and other basic material attributes of BFRP with the fiber lengths of 3, 6 and 9 mm were presented, and it was founded that the elastic-plasticity will increase with the increasement of Lf and Vf. Meanwhile, the relationship between three types of shearing failure cracks of BFRP and fiber distribution,and there were the close relationships between the elastic-plasticity, the types of shearing failure cracks and the micro-slippage at the interfaces between the short fibers and the epoxy resin, were proven.(3) The FIHPs with Lf of 3, 6 and 9 mm and more perfect core structures were manufactured using the improved manufacture process of FIHPs, and the shear failure modes and mechanical properties of FIHPs with the sealing edge were investigated using experimental and finite element analysis method: the shear failure of the core layer structure of FIHPs is mainly caused by the debonding failure of fibers in the middle surface of the core layer, which belongs to the material interface failure, and there is no stripping failure of the interface between the core layer structure and the upper and lower laminations,indicating that such biomimetic structure has excellent and integral mechanical properties.(4) The tests of edgewise compressive mechanical properties of FIHPs were performed. FIHPs are primarily characterized by compression failure of the non-perforated surface when bearing an edgewise compressive load, and the FIHPs clearly exhibit the features of plasticity, and the buckling failure mode of the upper or lower laminates was not observed. Because there is no panel-core cement face, and there didn't occur the buckling failure mode of laminates, the BFIHPs possess the ideal integrity. At the same time, we found that FIHPs with Lf of 9mm have the best shearing modulus of elasticity and good plastic deformation. The menchanical properties of FIHPs were verified, and the simple calculating formula of FIHPs'bearing capacity was presented as well,which would provide the optimized material component parameters for generalizing the application of the basalt fibre using beetle-forewing biomimetic technology.