Study On Damage Failure Mechanism Of SiC/SiC Ceramic Matrix Composites

SiC/SiC ceramic matrix composites,as a new type of high temperature resistant structural material,can well meet the requirements of structural weight reduction and service temperature for hot end components of aerospace vehicles.Understanding and mastering the damage mechanism and mechanical behavior of materials under different stress states is an important theoretical basis for optimizing material preparation process and promoting its practical application in engineering.Composite materials are usually in complex stress state during service.At present,the study of damage mechanics behavior of materials under single axial stress state can not meet the needs of engineering design.Based on this background,taking unidirectional reinforced SiC/SiC CMCs and plain braided SiC-SiC CMCs prepared by second generational SiC fibers and RMI process as the research objects,the damage mechanics behavior of materials under different stress states and micro-damage failure behaviors are systematically studied.Mesoscopic damage mechanism and damage failure process under different stress states is proposed and established.The basic stress-strain behavior and mechanical properties of two kinds of materials were obtained through the tests of unidirectional SiC/SiC CMCs and 2D-SiC/SiC CMCs under various loading forms.Because of their different preform structures,there are obvious differences in initial defects and thermal residual stresses,which directly affect the mechanical properties of materials under different stress states.At the same time,the macroscopical difference shows that the change trend of stress-strain curves of the two materials is different.Based on the macro fracture morphology and micro damage characteristics,the damage failure mechanisms of unidirectional SiC/SiC CMCs and 2D-SiC/SiC CMCs were obtained.Based on the analysis of mesoscopic damage modes under different loads,the normal temperature axial damage failure modes are obtained,which are mainly composed of matrix crack,interfacial debonding and fiber fracture,and the high temperature tensile failure modes of SiC/SiC CMCs based on matrix and fiber oxidation were obtained.Meanwhile,damage factor Dm based on instantaneous modulus change was defined to establish the damage failure process of unidirectional SiC/SiC CMCs and 2d-sic /SiC CMCs.The damage failure mechanism of unidirectional SiC/SiC CMCs and 2d-sic /SiC CMCs at the same stage is also different due to the different reinforced fiber structures in the crack development and failure stage.The fatigue strengthening mechanism of unidirectional SiC/SiC CMCs and 2DSiC/SiC CMCs were analyzed and the fatigue damage model was established in tensiontension fatigue test.During the fatigue loading process of SiC/SiC CMCs,the evolution of meso-damage makes the internal stress redistribute.This process will eliminate the original stress concentration,improve the internal bearing capacity of the material,and then improve the strength of the composite material.On this basis,a fatigue damage model based on modulus change is established,and the parameters of the model are fitted.The predicted fatigue life of the model is well matched with the experimental results.