Multi-scale Modelling Of Mechanical Behaviour Of Plain Weave Ceramic Matrix Composites

As candidate structural materials,plain weave Ceramic Matrix Composites(CMCs)have been developed for use in the hot structures of high thrust-weight ratio aerocraft engine,rocket motors,thermal protection structure of flignt vehicle,nuclear reactor and emergency brake system.The failure of plain weave CMCs is due to the damge of constituent materials in microstructure,which increases the necessity of developing a micro-meso-macro multi-scale modelling approach of mechanical behaviour of this materail for their applications.For the above purposes,the researches were performed on the plain weave SiC/SiC(siliconcarbide fibre reinforcement)and C/C-SiC(carbon fibre reinforcement))composites in this thesis:(1)The axial tensile,compressive,in-plane shear and high-temperature tensile tests of plain weave SiC/SiC composite were carried out and the mechanical behaviour of material under various loading conditions were obtained.Based on SEM observations of fracture surfaces,the corresponding damage mechanisms were analyzed.Meanwhile,the tension-shear and compression-shear damage coupling effects in the fibre directions of material subjected to off-axial loads were elucidated.The damaged mechanical behaviour of material at room temperature and 1200? were analyzed comparatively,the influence of high temperature oxidation on material's fracture strength were also compared and analyzed.Experimental results show that the damge coupling effects accelerate the damage evolution of materail and make the failure mode change from axial tensile or compressive failure to the coupling failure modes.Furthermore,the material has high fracture toughness and the weak interface result in the longer pullout length of fibre at 1200? in the oxidation environment.(2)Multi-scale modelling of plain weave SiC/SiC composites under tensile and compressive loadings along the warp direction and in-plane shear loadings was performed.By observations and analysis of complicated microstructure and mesostructure of plain weave SiC/SiC composites,the comprehensive geometry informations were obtained in order to predict the mechanical properties of constituent materials and establish the unit cell model.The mechanical properties of constituent materials were implemented in a user-defined material subroutine USDFLD through the FE software Abaqus.The periodic boundary conditions were applied in order to ensure that both the displacement and stress are continuous on the boundary surfaces.The non-linear stress-strain responses and damage evolution of unit cell under different loading conditions were simulated,as well as the failure mechanisms were analyzed.Although the stress-strain curves differ from the test results,the key parameters are in good agreement.Thus,the correctness of the multi-scale model has been verified.(3)A 2×2 unit cell model of plain weave C/C-SiC composite was built based on the formulation of Binary Model.The thermo-mechanical coupled constitutive equation and steady-state heat conduction equation were given.The thermo-mechanical properties were assigned to effective medium and tow elements.Then,the coupled temperature-displacement steady-state analysis of plain weave C/C-SiC composite was performed by using thermally coupled elements through a temperature difference between the upper and lower faces,and consequently,the composite through-thickness thermal conductivity degradation with uni-axial straining was predicted,the internal heat transfer mechanism was analyzed.Then,on the basis of stochastic description of tow paths in plain weave CMCs,a statistical analysis method of tow feature parameter based on reference period method was established.Then,this method was utilized to calculate the statistical properties of in-plane and out-of-plane deviations of warp and weft tow paths.The stochastic 1D tow representations in a 2×2 unit cell binary model of plain weave C/C-SiC composite was generated using the Python script.The effects of stochastic tow waviness on stiffness and strength of plain weave C/C-SiC composite were discussed.The predictions have good agreements with test results,thus,the accuracy and high efficiency of the Binary Model have been verified.This work is helpful for reveal the mechanical behaviour of plain weave CMCs under different loading conditions.It can also help to solve the problems about multi-scale modelling and enrich the mechanical analysis models.The test and analysis can provides the effective analysis methods for optimizing the preparation process so as to promote them for engineering applications.