IB-FEM Numerical Study On Effects Of Defects On The Mechanical Properties Of C/SiC Plain Woven Composites

C/SiC woven composites have many advantages,such as high temperature resistance,oxidation resistance,wear resistance and thermal stability.They have wide application prospects in aerospace and other fields.The mechanical properties of C/SiC woven composites are closely related to their micro and meso structures.Researchers have found that the fabrication defects of C/SiC woven composites,such as voids,micro-cracks and broken fibers,exist in varying degrees.At present,the influence of fabrication defects on the mechanical properties and failure behavior of C/SiC woven composites is still unclear.Therefore,the image-based finite element modeling method of C/SiC woven composites is developed in this paper.Based on this method,the influence of fabrication defects on the mechanical properties of C/SiC woven composites is studied,and the mechanism of the influence of the defects on the failure behavior of the composites is revealed.In this study,we firstly obtain CT digital images of C/SiC braided composite based on a high-resolution CT microscopy system.By optimized image processing and segmentation procedures,we obtained the gray information of the C/SiC textile composite,thus obtained an optimal image binarization segmentation threshold.Then through a series of image processing such as expansion,corrosion,boundary extraction,after the subsequent manual optimization,the boundary information of the digital images is obtained.Introducing the boundary information of CT images into ABAQUS and obtain the CAE model of the C/SiC composite with the defects are included.The multi-scale simulation method is used to transfer the parameters from the C/SiC unidirectional fiber to the C/SiC composite to obtain the material parameters,thus establishing the image-based finite element model of the braided composite.Based on the established image-based finite element method,the effects of different types of defects on the mechanical properties of C/SiC woven composites were studied.The distribution characteristics of stress and strain fields and temperature fields of materials with different types of defects under uniform and gradient temperature fields were analyzed.The calculation results showed that under uniform temperature load,void defects changed the stress state of the middle region of the matrix,resulting in larger tensile stress in the material.At the same time,void defects resulted in the increase of stress level of fibers and matrix in all directions and significant stress concentration within the material.At the same time,the calculation showed that the stress increase caused by the void defect in the material is the most significant,followed by the penetrating defect,and the notch defect has less influence.Furthermore,the damage accumulation and failure behavior of C/SiC composites at room temperature were studied by compiling the user subroutine UMAT for progressive damage analysis of materials,and the influence of defects on the strength and failure mode of C/SiC woven composites was studied.The results showed that defects have a great influence on the failure process.The failure of void-type and notched-type defects gradually diffuse from the tip of the defect to the interface.Finally,the failure of the interface leads to the failure of the whole model,and the adjacent small void defects in the void-type defect model will gradually connect to form a larger void defect;while the penetrating failure is the end.The failure of the model is caused by the gradual diffusion of the interface.Among them,the hole defect model has the highest strength and the penetrating die has the lowest strength.Based on the service environment of C/SiC woven composites,the damage accumulation and failure behavior of C/SiC composites at high temperature are also studied.The results showed that the void defect model firstly broke from the tip and top of the defect,the notched defect first broke from the tip and interface of the defect,and the failure process of the penetrating defect is similar to that of the notched defect.The notched defect model has the highest strength,while the penetrating defect model has the lowest strength.Finally,the in-situ testing experiment of C/SiC composite was designed and carried out based on the CT in-situ mechanical loading system.Evolution behavior of material microstructures under unidirectional pull-up loads was studied,and the effect of internal defects on progressive damage and failure of materials was studied in situ.The experimental results show that the effect of defects on materials and the evolution of defects themselves are analyzed.The experimental results are in good agreement with the simulation results.