Damage Mechanism And Multi-scale Modelling Of Ceramic Matrix Composites

Advanced ceramic matrix composites(CMCs)are ideal high-temperature structural materials for thermal protection structures of high-speed aircraft and hot components of engines.However,the highly no n-uniform composition and structure of this material result in thermal matching differences during the preparation process,leading to various defects with different scales,shapes,and distribution characteristics inside the material.Due to the coupling effect between damage and defects during the loading process,the brittle material exhibits significant nonlinear behavior.The complex damage mechanism,multi-scale and multi-component material characteristics seriously affe ct the effectiveness of CMC mechanical behavior and damage failure mode characterization.In order to ensure the accurate evaluation of the mechanical properties of CMC materials and the rapid prediction of the structural service performance,the typical 2D woven SiC_f/SiC ceramic matrix composite(SiC fiber reinforced SiC matrix)is studied in this paper.The research works on multi-scale damage constitutive modeling,damage nonlinear behavior prediction and experimental verification,and data-driven failure model construction of SiC_f/SiC composites are mainly carried out to improve the prediction ability of damage failure behavior of ceramic matrix composite.Aiming at the multi-scale characteristics of components and defects in SiC_f/SiC composites,at the micro-scale,mechanical performance testing of SiC fiber tows was carried out,and a two-parameter Weibull distribution function was introduced to describe the statistical distrib ution of SiC fiber monofilament strength,and the local elastic modulus of SiC matrix was characterized usin g nanoindentation testing.At the meso-scale,the damage nonlinear behavior of fiber bundles was studied through uniaxial tensile tests of SiC _f/SiC mini-composites.The analysis showed that the nonlinear mechanical re sponse in tension mainly comes from micro damage mechanisms such as matrix microcrack propagation and interface debonding.At the macro-scale,uniaxial tensile and compressive mechanical tests were conducted on 2D woven SiC_f/SiC composites,and the tensile and compressive mechanical behaviors s howed nonlinear and linear elasticity,respectively,indicating that 2D woven SiC_f/SiC composites have the unilateral crack closure and strength-differential effect.In addition,the fiber distribution characteristics,matrix micropores and micro-cracks in the composite were observed using Scanning Electron Microscopy(SEM)and Micro Computed Tomography(Micro-CT).Aiming at the nonlinear damage and diversity of failure modes of SiC _f/SiC composites,considering the pr obabilistic fracture of SiC fiber tow at the micro-scale,the damage variable related to the distribution of SiC fiber fracture strength is introduced to characterize the progressive failure behavior of fiber tow.A progressive damage model for SiC_f/SiC fiber bundles was established at the meso-scale,and the failure modes of fiber failure and matrix cracking were characterized by the damage variables of the fiber s and matrix within the fiber bundles.A damage constitutive model considering multiphysical mechanisms has been developed for SiC matr ix in SiC_f/SiC composite,which takes linear elasticity,microcrack damage and inelasticity as the basic deformation mechanisms.By defining microcrack density as an internal variable to capture the damage effects induced by microcracks in SiC matrix,and incorporating inelastic mechanisms that consider unilateral and strength-differential effects into the damage constitutive model of SiC matrix,the nonlinear response caused by microcrack damage in SiC matrix can be described.Based on the established multi-scale damage constitutive models of SiC_f/SiC composite,unit cell models were constructed at the micro-scale and meso-scale for finite element method(FEM)simulation under different loads.The applicability of the corresponding scale damage constitutive models and the effectiveness of the FEM models were verified through mechanical tests at each scale.Using the validated FEM models,the effects of SiC fiber volume fraction and microcrack defects in SiC matrix on the mechanical behavior of fiber bundles were analyzed,and the damage failure modes of SiC _f/SiC composites at the micro-scale and meso-scale were predicted.Based on this,a unit cell model with pores was constructed at the m eso-scale,and the influence of pores on the mesoscopic damage and failure modes was analyzed.Finally,the influence mechanisms of micro SiC fiber volume fraction,m eso SiC matrix porosity,microcrack density and interface strength on the mechanical properties and damage failure modes of SiC_f/SiC composites were systematically studied.Based on the damage failure study of SiC_f/SiC composite under uniaxial stress state,the validated mesoscopic unit cell model was used to predict the effects of radial and orthogonal biaxial loading paths on the failure point of SiC_f/SiC composite.The effects of micro SiC fiber volume fraction,meso SiC matrix porosity and microcrack density parameters on the failure envelope of SiC_f/SiC composites were analyzed under biaxial stress state.Subsequently,the failure points generated under differ ent scale feature parameters were used as the database for e Xtreme Gradient Boosting(XGBoo st)training,and a data-driven failure model was constructed.The trained XGBoost model was used to accurately predict the overall trend of the failure envelope of SiC_f/SiC composites under different scale characteristic parameters,and it was compared with the formal failure criteria(Tsai-Wu,Hashin and the fitted failure criteria based on FEM).The results indicate that the SiC_f/SiC composite data-driven failure model is reliable in constructing damage failure criteria related to mic ro and meso characteristics.