Research On Damage Accumulation Of Ceramic Matrix Composites Under Typical Loading/Unloading Load

Ceramic matrix composites with many excellent properties,such as lightweight,high tensile,corrosion resistance and high temperature resistance,have been widely used in aerospace and other engineering fields.The material that is in the course of service often encounters various working conditions such as cyclic loading,and the various damages it receives continue to accumulate until it fails.In order to give full play to the superior performance of ceramic matrix composites,the accuracy of the prediction and characterization of the mechanical behavior of C/Si C composites under cyclic loading/unloading are inextricably linked with structured applications.Therefore,the study of the mechanical properties of materials under typical loading/unloading has important theoretical significance and engineering application value.In this work,the quasi-static uniaxial tensile and cyclic loading/unloading tests of the material at room and elevated temperature are carried out,respectively.The tensile stress-strain relationship of the material is simulated and predicted from the macro and meso angles.The Kuo-Chou shear lag theory and fracture mechanics method are used to describe the mesoscopic stress field of the material under different interface debonding and slip conditions.The hysteresis loop of the material is simulated.Finally,the predicted stress-strain relationship is compared with the test results to verify.The specific research work includes:First,an experimental study was conducted on the mechanical behavior of sutured 3DC/Si C braided composite under uniaxial stretching and loading/unloading conditions.The macroscopic mechanical behavior of this material under tensile and loading/unloading was studied.The fracture of the sample was scanned by an electron microscope to analyze the damage process and failure mode of its internal structure.A meso mechanism was established for the impact of loading and unloading behavior on the subsequent bearing capacity of the material.The meso-mechanism of the influence of the material's subsequent bearing capacity and the influence of temperature on the interface are compared to analyze the strength change of the material at room and elevated temperature.Results have shown that the three-directional fiber bundle introduced by the stitching process destroys the integrity of the material structure,and the loading/unloading behavior dissipates the internal energy of the material to resist deformation,affecting the subsequent load-bearing capacity of the material,and the temperature affects the material interface,which tensile strength increases as the temperature increases.Secondly,from the macro and meso scales,the tensile stress-strain relationship of the sutured 3D-C/Si C braided composite was simulated and analyzed.On the macroscopic scale,the methods based on the unloading modulus and residual strain changes and the model of Weibull strength distribution are used to make predictions;on the microscopic scale,for obtaining the tensile stress-strain relationship of the overall structure,the material was simplified to a 0°-90°-90°-0° orthogonal layer structure,and the stress-strain responses of the0°-layer and the 90°-layer are analyzed using AC theory and a simplified shear lag model,respectively.The results obtained by macro and meso methods were compared with the test data,and shows that the macro and meso methods are suitable for the simulation of the tensile stress-strain relationship of the material,but the results of the meso method are closer to the test results.Finally,the Kuo-Chou shear lag theory is used to analyze the mesoscopic stress field of the simplified 0°-90°-90°-0° orthogonal laminated plate structure mode under the four interface debonding and slip conditions.Combining with the fracture mechanics method,the stress-strain relationship between 0°-layer matrix cracking,90°-layer lateral cracking,fiber-matrix interface with debonding,partial/complete debonding and slipping is described,and the hysteresis loop of the material is simulated and compared with the test results to verify the accuracy.The results show that the prediction under a single damage mode cannot describe the hysteresis loop that is generated by the combined effect of multiple damage modes,which needs to be combined with multiple damage modes for analysis.The simulation fits well with the test results.