Multi-scale Finite Element Analysis Of Heat Transfer And Mechanical Properties Of 3D Woven Composites

The reinforcement of three-dimensional woven composites(3DWCs)are 3D integral structure with continuous and connected fibers in multiple directions,which greatly improves the structural integrity and reliability of the component.The 3DWCs can fundamentally overcome the shortcomings of the traditional laminated composites,such as delamination or cracking etc,which provides a broad prospect for the application of composite in the main bearing structure.Furthermore,the optimum design of 3DWCs can be achieved by changing the fabric structure parameters based on high degree of designability,which can satisfy weight loss and high load carrying requirements for structural material under the special environment.Meanwhile,the advanced 3DWCs have been considered as promising candidates for thermal structural and thermal protection materials for many high temperature applications,especially in aerospace industries,such as the main bearing structural materials,spacecraft wing joint,the rocket nose cone,heat shields and blades.Therefore,in view of the urgent need of application environment,it is very significant to study and predicate the thermal and mechanical properties of 3DWCs.Aiming at the problems existing in the research of 3DWCs,the research of this thesis focuses on thermodynamic properties of 3DWCs and weavability of 3D woven fabrics.The main contents include:(1)This paper summarizes an extensive experimental and prediction study of thermal conductivities of 3DWCs.Three kinds of innovative 3D woven architectures are examined,including 2.5D angle-interlock,2.5D angle-interlock(with warp reinforcement),and 3D orthogonal woven architectures.The differences of thermal behaviors of 3DWCs in plane and out of plane are assessed by using multi-scale finite element analysis.For the validation of models,the thickness direction thermal conductivity of 3DWCs is measured.It is indicated that the predicted results are in good agreement with the experimental results.The effects of fabric architecture,warp crimp and weave density on the distribution of heat flux and temperature have been discussed in this work,which determined the thermal conductivities of 3DWCs.(2)Based on the transient hot-wire method,the thermal conductivities of 3DWCs are measured.The influence of the orientation of hot wire sensor on the thermal conductivities of 3DWCs is discussed,and the thermal response mechanisms of 3DWCs is analyzed in detail.The results show that the transient hot wire method is very suitable for the measurement of thermal conductivities of 3DWCs,which can reflect the anisotropy of 3DWCs.Furthermore,the main effect of each yarn in the 3DWCs is affected by the orientation of hot wire sensor when other parameters remain identical.(3)A contrastive analysis of thermal response mechanisms of 3DWCs under two measuring methods,namely,the flash method with the surface heat source and the transient hot wire method with the line heat source,are carried out in detail.Based on these investigation,it can be found that the thermal response mechanisms of 3DWCs under different heating modes are different.(4)Based on the multi-scale finite element method,the elastic properties of 3D orthogonal woven composites are predicted.The influence of the extraction method of the unit cell on the continuity of the yarn in the unit cell and the fiber volume fraction of each yarn are discussed.Then the embedded element technique is used to build the macro-scale finite element model of the 3D orthogonal woven composite.The model has given a detailed thought to the influence of material properties superposition and random interception on the cross-sectional area of the yarn,which can improve the prediction accuracy of the finite element model.In addition,the prediction of mechanical properties from the meso scale to the macroscopic of composites can be analyzed step by step scale through the rapid and effective modeling method.(5)The influence of surface sizing treatment on the weavability of high performance fiber is studied,take SiC for example.SiC fibers are easy to fluff and fracture in weaving process due to its brittleness.To investigate the effect of the additive content in sizing agent and the size concentration on the surface and weavability of SiC fibers,the domestic KD-I SiC fibers are sized with waterborn epoxy resin sizing agent through the secondary sizing treatment.By secondary sizing,there is a different enhancement in tensile strength and elongation,abrasion resistance and flexibility allowing a good weavability of the SiC fibers.It is showed that the surface defects of fibers are remedied by the size film,therefore,the strength of fibers is improved and the strength disperse of SiC fibers is reduced.