Biaxial Failure Mechanism And High Temperature Tensile Properties Of Needled C/C-SiC Composite

Needled C/C-SiC composite has become the key material in the aerospace industry and national defense fields,due to the excellent high temperature mechanical properties and resistant ablation properties,such as high specific strength,high specific modulus,excellent wear resistance,thermal shock resistance and chemical stability,etc.The complex structure of needled composite materials and the uncertainty of the microstructure of needling regions pose the challenge to the mechanical modeling and analysis of materials.Under the high temperature and complex loads,the needled C/C-SiC composite exhibit the complex failure mechanisms and temperature dependence of the material properties.Therefore,the failure criteria at room temperature and high temperatures should be established to provide a theoretical basis for the structure design and strength evaluation of the C/C-SiC composite.In this paper,the complex loads and high temperature experiments for the C/C-SiC composite material were conducted,the macroscopic mechanical characteristics and failure mechanisms of the material were analyzed,the mechanisms of the microstructure and the temperature dependence of the material performance were studied,the prediction method for the strength of the material at room temperature and high temperature was developed,and failure criteria for needled C/C-SiC composite at room temperature and high temperature were established based on their failure modes.Firstly,the microstructures of the needled preform were observed,the molding process of the C/C-SiC composite was introduced,and the characteristics of the main micro defects after the molding were analyzed.The basic mechanical properties experiments and the complex loads experiments including biaxial tensile,tensilecompressive,biaxial compressive experiments were condcuted,in which the room temperature complex loads samples were designed,the stress calculation method of the central measuring area was provided,and the macroscopic mechanical behaviors of the material were obtained.The failure mechanisms of the composite materials under different loading conditions were studied by analyzing the failure morphology of specimens.Secondly,combining the microstructure characteristics of the material and the size of the specimens,the calculation model of the acupuncture C/C-SiC composite was established,in which the distribution of the needling regions was considered.Based on the failure mode analysis of microstructure of the composite materials,reasonable failure criteria for constituents were selected,and the strength properties of components were determined.The calculation model was used to analyze the failure process of the needled C/C-SiC composite under uniaxial and biaxial tensile loads.The calculated results were in good agreement with the experimental results,and the influence of the distribution of acupuncture on the dispersion of the mechanical properties of the composite was discussed.Thirdly,the high temperature tensile experiments of needled C/C-SiC composite were carried out.The change law of the macroscopic tensile properties of the material with the temperature was analyzed.The failure mode was analyzed to determine the mechanism of the microstructure of the material failure mode with the change of temperature.Based on the influence mechanisms of C/Py C interface and SiC matrix on the tensile strength properties of high temperature,the variation of material properties of needled C/C-SiC composite with temperature was obtained,and the uniaxial and biaxial tensile strength of the materials under high temperature conditions were predicted by combining with the calculation model.Finally,it is found from the failure morphologies for uniaxial compression,biaxial compression and double axial tension-compression that the composite material was mainly subjected to double direction shear failure,and the shear failure angle of the material under the different load ratio loads changed in a range.Based on the failure mode,a three parameter twin-shear strength criterion considering hydrostatic pressure was established.The influence of the undetermined parameters on the criterion also was analyzed.Typical composite failure modes were presented in the uniaxial and biaxial tensile experiments.The applicability of several typical composite strength criteria to the biaxial tensile failure of needled C/C-SiC composites was studied in this paper.It is found that the Tsai-Wu strength criterion was suitable for predicting the macroscopic biaxial tensile failure of mater ials at room temperature and high temperatures.In this paper,the correlation mechanisms between microstructure,damage failure mode and macroscopic properties of needled C/C-SiC composite were studied.The mechanical behavior of the composite under high temperatures and complex loads were predicted.The material strength criteria were established to powerful guide the engineering application of needled C/C-SiC composite.