Investigation Of Mechanical Properties And Material Paramters Identification Of 3D Six-directional Braided Composite

Three-dimensional six-directional braided composite is a good over-all structure,which is added fixed yarns to the three-dimensional four-directional woven fabric preform in the longitudinal direction and width direction respectively.Besides high specific strength and stiffness,it shows excellent performances in terms of out-of-plane mechanical properties、shear resistance and structure designability,as a result,it has a broad application prospect on aviation and space industries. There are only several literatures about three-dimensional four-directional braided composite, and their concerns almost were how braid angle or fiber volume fraction affects on material tensile property and connection joints property, which is needed in the stage of material design and usage,but as a typical orthotropic and dual extension-compression elastic modulus material,performance differences in different directions and in various load cases are also should be considered. Besides, the stiffness prediction of three-dimensional six-directional braided composite is lack of combination of micro-configuration and consideration of different behaviour in tension and compression.Moreover, on one hand,there are facing difficulties including large amount of work,inefficiency and high cost in mechanics performance testings of three-dimensional six-directional braided composite when these testings are enlightened by the testing standards of fiber reinforced laminated plates. On the other hand, it has certain difficulty in horizontal comparison between different types of testings because of different specimen shapes and the decentralization of composite material’s performance datas.So,it is necessary to develop a method to test many mechanical property parameters in only one test. Based on these considerations above,three aspects research work will be done in this thesis for the existing problems.Firstly,tests in several load conditions and in different directions were conducted to three-dimensional six-directional braided T300/TDE86 composite with low braiding angle and medium fiber volume fraction.They were axial tensile test、three different directions’ compression tests and in-plane shear test,corresponding modulus,strength,defected and failure modes were obtained,and horizontal comparison and reasons of the similarities and differences between these tests’ results were analyzed in detail.Secondly,the orientation of yarns in different directions of original material and material with its resin burnt off was observed,as a result,the geometric microstructure and characteristic parameters were determined,as well as twisting and contact consituation between different yarns. Full size geometric analysis model in thickness direction was built under certain assumptions and under the consideration of material’s macro-size, whose mechanical property parameters of fiber bundle and matrix were from the existing literatures.The model to predict stiffness was then established based on the assembling stiffness method. The elasticity constants of the material in different directions were obtained,which coincided well with experimental results except compressive property in thickness direction and in-plane shear properties. So the method of this chapter was accurate.Finally,The error of strain field of digital image correlation and finite element modelling using specimen with a central hole was analyzed,then a platform was set up to optimize the design variables(the paramters of in-plane mechanical properties),bound variable(reaction force) and optimization objective(error of strain field) using Downhill Simplex Method.The convergence results were in good agreement with the corresponding experimental test data.So the method of elastic parameters inversion for three-dimensional six-directional braided composite was feasible.