Simulation Study On Fracture Failure Behavior And Strengthening-Toughening Properties Of Matrix And Interphase Components Of CFCMCs

Continuous Fiber-Reinforced Ceramic Matrix Composites(CFCMCs)has been widely applied in aerospace industry and nuclear industry due to their excellent property such as high specific strength and modulus,low density,high temperature resistance as well as oxidative resistance.CFCMCs show great promise for use in extreme environments,yet challenges remain that require further research and understanding of CFCMCs failure mechanisms.The mechanical properties of CFCMCs are closely related to fibers,interphase and matrix.Nowadays,more attention has been paid to the matrix and interphase since there are lots of methods to improve their mechanical properties comparing to fibers.However,the mechanical properties of matrix and interphase are associated with many factors,and it is hard to quantitatively evaluate the effects of these factors such as microstructure on damage behavior by experiments.Numerical simulation is an effective way to build the quantitatively relationship between microstructure and apparent mechanical properties,which shortens engineering application development cycle and cost by optimizing microstructure.So far,the numerical simulations on CFCMCs mechanical properties study are still limited,such as some microstructural features are not considered in models or the characterization methods are not precise enough.Focusing on these problems,fracture behavior of matrix and interphase are investigated based on Finite Element Method(FEM)in terms of structural mechanics.Characterization parameters such as external work(ALLWK)and damage dissipation energy(ALLDMD)are introduced to quantitatively analyze the influence of microstructural parameters on apparent mechanical properties.Cross scales models are proposed to build the quantitatively relationship between microstructure and apparent mechanical properties.The main research and results are as follow:(1)Voronoi tessellation models are proposed as representative volume element models(RVEs)to describe the polycrystalline ceramic microstructure morphologies,and cohesive zone model(CZM)is used to simulate fracture behavior.Grain size,grain boundary fracture energy,grain boundary defect density and their coupling effects on apparently mechanical properties are investigated.The results show that the grain size and grain boundary defect are intercoupling,while grain boundary fracture energy is weakly coupled with the above microstructure parameters.The mechanical properties could be improved by increasing the grain boundary fracture energy and decreasing the grain size and the grain boundary defect density,respectively.On basis of the above research,the regularity of microstructure parameters on mechanical properties of the matrix can be obtained.(2)A dual-scale model is proposed to investigate the influence of microstructural parameters on polycrystalline ceramic thermal shock resistance based on sequentially thermal coupling and finite-discrete element method(FDEM).Grain size,grain boundary fracture energy,and thermal shock temperature difference are studied and discussed in detail.Damage dissipation energy(ALLDMD),crack patterns,and crack density are introduced to quantitatively analyze the effect.The results show that thermal shock process can be divided into three stages: undamaged stage(stage I),crack initiation stage(stage II),and crack propagation stage(stage III).Grain refinement and increasing grain boundary fracture energy are effective methods to improve thermal shock resistance by extending stage I and stage II.Besides,crack growth rate is mainly relevant to the scale of grain size,and thermal shock temperature difference mainly affect crack initiation temperature difference and crack propagation temperature difference to improve thermal shock resistance.(3)The influences of single-interphase thickness and multilayer interphase with different structure on microstructural mechanical properties of CFCMCs are investigated on example of Si Cf/Py C/Si C,Cf/Py C/Si C and Si C/(Py C-Si C)n/Si C using 3D models.Damage variable(SDEG)and damage dissipation energy(ALLDMD)are introduced to quantitatively analyze the damage and crack deflection of the interphase.The results show that thicker single-interphase,larger n and thicker sublayer will cause a decrease in tensile strength of CFCMCs models.The results of mean SDEG,mean ALLDMD,and SDEG patterns indicate that thicker single-interphase,larger n and thicker sublayer will reduce the damage within interphase since the stress field inner thicker interphase becomes more homogenous,which is not conducive to the improvement of models toughness.Meanwhile,multilayer interphase performs better on crack deflection comparing to single interphase at the same thickness.In addition,the effect of “treated fibers” and “as-received fibers”(“as-received fibers” means the strength of fiber/interphase is weaker than interphase itself;“treated fibers” means the surfaces of fibers are treated and the strength of fiber/interphase is stronger than interphase itself)are also considered.The results show that,for both single-interphase and multilayer interphase,“treated fibers” is better than “as-received fibers” in deflecting cracks,as the former shows a better toughening effect by producing a large amount of damage inside the interphase.