Failure Mechanism And Multi-scale Simulation Of The Fatigue Behavior Of Ceramic Matrix Composites With Complex Preform

At present,Ceramic Matrix Composites(CMCs)has been tentatively applied in aviation and aerospace and shows bright prospects.The CMCs used in engineering is usually of a complex preform.This kind of composites presents very different damage modes at different scales,and its final failure is an outcome of the synergistic progressing of those damages.Accordingly,the present study will focus on the failure mechanism and multi-scale simulation of CMCs with complex preform under fatigue loading by the analysis of needled CMCs.The mechanical testing of three kinds of CMCs,C/SiC minicomposite,needled unidirectional C/SiC composite and needled 2D C/SiC composite,have been performed.The properties of C/SiC minicomposite under monotonic tensile loading and loading-unloading,the tensile-tensile fatigue behavior of needled unidirectional C/SiC composite and needled 2D C/SiC composite have been achieved.A fatigue strengthening behavior was found in needled 2D C/SiC composite,and the effects of fatigue stress level and cycles on the residual strength were studied.The fracture surfaces of the three kinds of CMCs have been studied using Optical Microscope(OM)and Scanning Electron Microscope(SEM).The failure procedure of C/SiC minicomposite was analyzed referring to the damage modes of unidirectional fiber reinforced CMCs.The fatigue properties of needled unidirectional C/SiC composite and the fatigue strengthening behavior of needled 2D C/Si C composite were discussed by comparing the fracture morphology and damage modes.Then,a model based on the wear-in/wear-out mechanisms is developed to characterize the residual strength vs.the number of fatigue cycles for needled 2D C/SiC composite.As the nonuniform evolution of fatigue damage in CMCs with complex preform,the minicomposite inside would suffer a complex loading process.In order to analysis the behavior of unidirectional fiber reinforced CMCs under complex loading process,a micromechanical model based on the frictional slipping of interface has been developed.The proposed micromechanical model is able to achieve the evolution and distribution of slip zones by the friction law of interface which is the main difference from traditional shear-lag model.By applying the new model,the characteristics of interface slipping were studied and the stress-strain response under complex loading process were simulated.Comparing to the testing results of unidirectional SiC/CAS composite and C/SiC minicomposite,it was found that the proposed model can well predict the stress-strain response and fatigue life of unidirectional fiber reinforced CMCs.For needled C/SiC composite which is of a complex preform,a multi-scale mechanical method has been developed.The RVE of the composite was chose using XCT technology.The property of the basic component of RVE,the minicomposite,was described by the micromechanical model.Accordingly,the stress-strain curve of needled C/SiC composite was simulated using nonlinear finite element method.Moreover,the fatigue life of needled C/SiC composite was predicted by introducing fatigue failure criteria into the multi-scale model.By comparing to the testing results,it was found that the multi-scale model can well predict the stress-strain response and fatigue life of needled C/SiC composite.