Research On Prediction Of Damage Failure And Fatigue Life For C/C Composites

Carbon/Carbon (C/C) composites have the advantages of high temperature performance andexcellent mechanical properties. It can be used as both high temperature structure materials andfunctional materials. However, researches on mechanical properties such as strength and fatigue life ofthree-dimensional and four-directional braided C/C composites are behindhand, because ofcomplexities of their microstructure. Such restriction has limited C/C composites for further application.Therefore, it is necessary to analyze fatigue damage modes of C/C composites through theexperimental research and to build models for predicting fatigue life of them, which can provide atheoretical support for their widely use. The main work done in this dissertation includes the followingfive parts:(1) Experimental research on mechanical properties and fatigue characteristics of carbon fiber andC/C composites. Unidirectional and three-dimensional and four-directional braided C/C composites areprocessed. The tensile strength and tension-tension fatigue behaviour of carbon fibers yarn are tested.The tensile strength of carbon fibers is tested. The experimental research for tensile strength andtension-tension fatigue behavior of unidirectional C/C composites in the longitudinal, transverse andin-plane-shear directions is conducted. The tensile strength and tension-tension fatigue behaviour ofthree-dimensional and four-directional braided C/C composites in the longitudinal direction is tested.The fiber-matrix interfaces strengths of unidirectional C/C composites and fiber yarn-matrix interfacesstrengths of three-dimensional and four-directional braided C/C composites are tested. Theexperimental study provides a data support for the theoretical predictions on fatigue characteristics ofC/C composites.(2) The model for predicting tension-tension fatigue life and residual strength of unidirectionalC/C composites is studied. Based on the random crack core theory and model for predicting thelongitudinal tensile strengths of unidirectional composites, the Weibull model describing residualstrength distribution of single fiber and the residual strength model of fiber-matrix interfaces is broughtin for building the tension-tension fatigue life and residual strength prediction model of unidirectionalC/C composites in the longitudinal direction. The tension-tension fatigue life and residual strength ofunidirectional C/C composites can be predicted by using the model.(3) The research on microstructure model and stiffness prediction model of three-dimensional andfour-directional braided C/C composites. Elliptic with trimming is assumed to describe thecross-section of yarns, a new microstructure model of3D braided composites is established. The new model truly reflects the jamming pattern between yarns and its cross-section variation along the axial ofthe yarns. Based on the stiffness volume average method and flexibility volume average method,introducing the microstructure model which is established in this paper, a model to predict stiffness isthen established. The calculated simulation values of the geometric characteristics and elastic constantsof braided composites well agree with the measured values. The numerical result indicates theeffectiveness of the model.(4) The model for predicting tension-tension fatigue life of three-dimensional and four-directionalbraided C/C composites is studied. The criterion of yarn fracture after fiber-matrix interfacialdebonding and the Tsai-Hill criterion considering the axial tension and shear stress are extended fromstatic load to fatigue load. The new extended criteria, as well as matrix failure criterion are adopted asthe fatigue failure criteria of the three-dimensional and four-directional braided composites. Based onthe theory of the fatigue life and residual strength prediction model of unidirectional C/C composites,fatigue life prediction model of three-dimensional and four-directional braided C/C composites is built.Compared with the experimental results, the fatigue life prediction model is reasonable.(5) The progressive damage method of predicting the strength and fatigue life ofthree-dimensional and four-directional braided C/C composites is studied. The Hashin-type failurecriteria are introduced into the method to detect damage for diverse damage modes, the criteria of thestructure catastrophe and the material property degradation rules are also established. Based on the theresidual strength degradation model and the residual stiffness degradation model of unidirectional C/Ccomposites, a progressive damage method of predicting the strength and fatigue life ofthree-dimensional and four-directional braided C/C composites is established by using the finiteelement method. The geometrical model considering the jamming pattern between yarns is applied inthe finite element method model coupled with the periodical displacement boundary conditions. Anexcellent agreement is found between date obtained from this study and the experiment.