Meso-structure And Mechanical Behavior Of 2.5D Woven SiC_f/SiC Composites

SiC fiber reinforced SiC matrix(SiC_f/SiC)three-dimensional woven composites have the characteristics of good structural design,low density,excellent mechanical properties,high temperature resistance,good oxidation resistance and excellent interlaminar performance,etc.It has become an ideal candidate structural material for hot-end high-temperature components of aero-engines.Among them,the 2.5D structure is an important member of the three-dimensional woven family,forming an overall network structure through the angle connection layer by layer.However,limited by the particularity of SiC_f and the preparation technology of SiC_f/SiC composites,the current research on the meso-structure and mechanical properties of 2.5D woven SiC_f/SiC composites is still in the primary stage.In this paper,the mechanical properties and damage mechanism of 2.5D woven SiC_f/SiC composites were studied by experimental and numerical simulation methods.The main research contents and conclusions of the thesis are as follows:(1)The second-generation silicon carbide fiber(SiC_f)reinforced 2.5D fabric was used as the skeleton,and the SiC_f/SiC composite was prepared by the precursor impregnation pyrolysis(PIP)process.Micro-CT tomography technology was introduced to scan 2.5D woven SiC fiber preforms and 2.5D woven SiC_f/SiC composites respectively to determine the fiber bundle morphology,pore distribution and size inside the composite.Converting two-dimensional data through three-dimensional images,analyzing fiber bundles and pore parameters inside the material.Then statistically analyze the fiber bundle morphology and pore characteristics.On this basis,a random algorithm for pore generation was proposed.By using Solidworks and ABAQUS software,the idealized geometric model(I-model)without pore defects,the uniform geometric model(U-model)with uniform pore distribution and the non-uniform geometric model(N-model)with real pore distribution were constructed.The results show that the non-uniform geometric model(N-model)with real pore distribution can better reflect the fiber bundle morphology and pore distribution characteristics inside the composite material,which provides data support for the subsequent high-fidelity numerical prediction.(2)Around the 2.5D woven SiC_f/SiC composite material,the universal testing machine is used to study the tensile and bending mechanical behavior of the material in different directions(warp and weft),And through scanning electron microscope and super-depth three-dimensional microscope to obtain damage images and related data.Furthermore,the damage modes of 2.5D woven SiC_f/SiC composites under uniaxial tensile and bending loads in the warp and weft directions were studied,and the failure mechanisms of the specimens in different directions were discussed.The results show that the warp and weft 2.5D woven SiC_f/SiC composites have different damages under tensile load and three-point bending load.The tensile failure of the sample is the result of the coupling effect of tensile stress and shear stress.Warp stretching mainly occurs debonding and slippage along the fiber and the interface,the fiber pullout is obvious,and the fiber pullout length is longer.Therefore,the composite material shows better mechanical properties;The port of the weft specimen is characterized by obvious steps,and the ratio of fiber bundles and fiber filaments is small,and the fiber bundles are mainly brittle fracture.Under bending load,the results show that:weft and warp performance are significantly different,and the bending strength of the latitudinal specimen is larger than that of the longitude specimen.The bending damage modes of 2.5Dwoven SiC_f/SiC composites are complex,in which the warp cracks mainly propagate along the warp and weft contact points,while the cracks of latitudinal specimen mainly occur between weft bundles and eventually lead to bending failure.(3)Based on the fine meso-model,the finite element model of 2.5D woven SiC_f/SiC composites was constructed,and the tensile and bending numerical simulation was carried out by using ABAQUS analysis software.The finite element models of 2.5D woven SiC_f/SiC composites with different pore distributions are established based on meso-scale,namely,the idealized model(I-model)without pore defects,the homogenized model(U-model)with uniform pore distribution and the nonuniform model(N-model)with different pore distribution.The established model was compared with the tensile test to verify the proposed model.The results show that the stress concentration of the material under the tensile load of the numerical simulation mainly surrounds the giant pores,which eventually leads to the damage of the material.Consistent with the mechanism of the final damage along the giant pores in the experiment,the numerical simulation of tensile material stiffness and macroscopic damage pattern are in good agreement with the experimental results.The stiffness and macroscopic damage morphology of the tensile material simulated by numerical simulation are in good agreement with the test results.The elastic modulus of composite obtain from the N-model,U-model,and I-model are 44354.58MPa,42932.23MPa and40477.25MPa respectively,while that of experimental result exhibit 41659.85MPa.Furthermore,the proposed model with non-uniform pores(N-model)can predict the mechanical properties of the composites well,and the simulation accuracy can reach 95%,while the error of the ideal model without pore distribution(I-model)and the model with uniform distribution of small pores(U-model)is 22%and 15%,respectively.Therefore,the proposed N-model can accurately predict the mechanical behavior,full-field stress distribution and damage of 2.5D woven SiC_f/SiC composites.In order to further verify the accuracy of the proposed model with different pore distribution,a finite element model of2.5D woven SiC_f/SiC composites with pore distribution was established based on the full-scale method.The three-point bending loading process of the transverse specimen was simulated,the damage mechanism was analyzed,and the simulation results were compared with the test results.The results show that the numerical simulation of the macroscopic damage pattern of three-point bending is in good agreement with the experimental results,and the finite element model based on the stochastic method can effectively predict the mechanical behavior of the 2.5D woven SiC_f/SiC composite.