| As one of the advanced materials developed in recent years,3-Dimensional braided C/C composites have many advantages in terms of the good inter-layer performance,excellent fatigue resistance,relative high strength and temperature resistance,as well as good anti-fraying ability and ablation performance,and as a result,they has been widely used in many engineering fields such as aeronautics,astronautics,naval crafts,biological,and medical instruments manufacturing nowadays.However,restricted by the limitations of current fabrication method,experiment equipment,and detection capability,the research on the static and fatigue performance of 3-D braided C/C composites at high temperature are far from mature.In addition,the high complexity of the micro-structure and fatigue failure mechanisms of braided composites,as well as the unique behavior at high temperature bring difficulties to the mechanical modeling for 3-D braided C/C composites.Therefore,to conduct static and fatigue tests for C/C composites at high temperature,and to develop the prediction model applicable at high temperature,are significant for the theoretical research and engineering application for C/C composites.In this dissertation,the tensile and tension-tension fatigue tests for 3-D braided C/C composites at high temperature has been conducted,and the static and fatigue performance of unidirectional(UD)and the 3-D braided C/C composites at high temperature have been investigated.Based on the micro-structure of the material from the perspective of meso-scale,stiffness,strength and fatigue models for 3-D braided four directional C/C composites have been proposed to predict the mechanical properties and fatigue life of C/C composite plate specimen at high temperature.The main contents of this dissertation can be summarized as follows:(1)A method to investigate C/C composite material properties at high temperature has been presented.Since the mechanical properties of C/C composites would change with regard to the ambient temperature,a predictive model considering the effect of temperature for the mechanical properties of C/C composites was proposed in this article.By comparing the prediction results and experimental data,the present model has been proved to be able to describe more accurately the relationship between temperature and the properties of C/C composites in comparison with other models.Besides,a mechanical degradation model due to high temperature oxidation with the incorporation of the time and rate of oxidation has also been established,the model was verified on the basis of the static tensile experiment for the UD laminates at 700 ℃ and 900 ℃.Finally,a rule of equivalence for the properties of UD laminate and fiber tows at high temperature was deduced,the results show that the same mechanical properties can be achieved between UD laminates and fiber tows with the same fiber volume fraction under same working condition,which allows for the measurement of the mechanical properties of the components of C/C composites.(2)A stiffness prediction method for 3-Dimensional braided C/C composites at high temperature was proposed based on the meso-scale finite element(FE)analysis.The braided C/C composite plaque can be divided into three types of cells from meso-scale perspective,namely inner cell,surface cell and corner cell,and the corresponding micro-structure models and FEA models were thereby established by introducing the cross-sectional shape as well as the orientation of the yarn.Based on the application of the periodical boundary condition considering the temperature effect and the introduction of the effect of cropping width,a stiffness prediction model for C/C composites considering the influence of temperature,oxidation rate,oxidation time,and cropping width has been proposed.Both the prediction results for the cropped/uncropped specimen at room temperature(RT)and 700℃ have errors lower than 10%,indicating the present model is accurate and reliable.(3)On the foundation of cumulative damage analysis,a temperature-associated strength prediction model for 3-Dimensional braided C/C composites has also been proposed,the model considers the influence of the rate of oxidation,oxidation time,cropping width,and interface strength.The 3-D Hashin criteria were implemented to identify the failure modes of finite elements,and the corresponding stiffness degradation was accomplished accordingly.Lastly,the relationship between thee cropping width and the mechanical properties of the material has also been established,and the prediction has been made for the cropped/uncropped 3-D braided C/C composites at both room temperature and 700℃,the errors of the prediction are below 10%.(4)The residual stiffness and residual strength model for the C/C fiber bundle.From the perspective of the stiffness and strength degradation,the fatigue damage was quantified with the consideration of temperature,the rate of oxidation,and strain level.The experimental data were fitted on the basis of the present models,and good results could be achieved,which indicating that the present model can be applied to describe the stiffness/strength degradation of C/C fiber bundle at both room temperature and high temperature with or without oxidation.(5)A high-temperature residual stiffness model for 3-D four directional braided C/C composite material was proposed.In the present model,the inner braiding angle as a function of the number of cycles was introduced,and the impact of temperature,rate of oxidation,and strain level were also being taken into consideration.Based on the present model,curve fittings were achieved for the experimental residual stiffness data,and the experimental parameter pertaining to the inner braiding angle change with respect to the number of circles can thus be acquired,which can be utilized to calculate the working length and pitch length of the specimen at 700℃.The model has been verified by comparing the simulation and experimental data.(6)A fatigue life prediction model of 3-D C/C braided composites at high temperature has been proposed.Under fatigue loading,the braided angle was adjusted based on the preceding model,the residual stiffness/strength models and 3-D Hashin criteria were utilized to accomplish the abrupt and gradual degradation for the element.The validation of the model is achieved through the comparison between the predicted results and the test data.(7)Tensile tests for UD and 3-D four directional braided C/C composites were conducted at 700℃.Based on the data of UD laminates,the aforementioned equivalency theory,and the experimental results of UD plaque at high temperature,the properties of the matrix and fibers of the material at 700℃ is obtained.(8)single fiber push-out test for UD laminates at room temperature has been conducted to obtain the interfacial shear strength(ILSS)of the C/C composite materials.According to the relationship between the ILSS and temperature,the ILSS of UD laminates at 700℃ was predicted.Besides,the thermal expansion coefficients for the components of C/C UD composite material below 1000℃ was also measured.(9)Tension-tension(T-T)fatigue tests for C/C UD composites were conducted at 700℃.Based on the experimental data of UD composites,the equivalency theory,and the test results of UD composites at high temperature,the residual stiffness/strength and experimental parameters of the fiber tow were obtained.Meanwhile,T-T fatigue tests for 3-D four directional braided C/C composites were also carried out,experimental results including the fatigue life,stiffness reduction,hysteresis loop,and residual strength at 700℃ were obtained. |
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