Mechanical Response And Failure Mechanism Of A 2D-SiC/SiC Composite Under Impact Loading

With the rapid development of Aeronautics and Astronautics,nuclear power and other high-tech fields,engineering applications require more and more materials for extreme loads.Ceramic materials,as traditional heat-resistant materials,have gain attention again in recent years as high-temperature materials.In order to avoid the brittleness of traditional ceramic,use continuous fiber to reinforce it is recognized as a good approach.Continuous fiber-reinforced ceramic matrix composites(CFR-CMC,CMC)is now a mature industrial system after 50 years of development.At the same time,researchers are now focused on many kinds of CMC'fabri-cation methods and problems that still existed.Nevertheless,in order to fundamentally solve the problem of CMC,realizing general effect of how continuous fiber-reinforced ceramic matrix composites is not enough.The defects in fabrication process such as voids in matrix,microc-racks,thermal residual stresses play important role in extreme environment loads.This paper focuses on silicon carbide fiber-reinforced silicon carbide matrix composite's anti-impact prop-erties and its Mechanics-Thermo-chemistry mechanism and failure process.Targeting the above objectives,2D-SiC/SiC composite was chosen as research object.We studied the mechanical response and failure process of this specific material at both quasi-static and dynamic loadings.The effects of the high-temperature environment on mechanical response under dynamic loading was also investigated.The main research contents and results are as follows:(1)Using a specular reflection method(SRM)can clearly detail the differences between the transverse fiber bundles,longitudinal fiber bundles,matrix and voids on the surface of a specimen because the density differences of the contents.And the obtained images can be calculated by DIC so that critical deformation data during failure process of components of the specimens can be addressed.(2)Under compression loading,microcracks initiated around the defects including inter-bundle voids and intra-bundle voids.And the crack initiation mechanism can be explained by a yield surface theory around voids in a porous material.Under quasi-static compression,the initiation of microcracks around small voids(Intra-bundle voids)was subdued.While dynamic compression,the initiation of microcracks around large voids(Inter-bundle voids)was subdued.After the microcracks reached the interface layer between fiber and matrix,things became dif-ferent because interface strengthen was higher under dynamic loading than quasi-static loading.(3)Under in-plane compression,the dynamic strength of this material is obviously higher than the quasi-static condition.Under quasi-static compression loading,microcracks initiated along the fiber direction around inter-bundle voids.Due to the weak interface layer,the mi-crocracks can be captured by the interface.The microcracks reflected and propagated many times in the interface,resulting in debonding of the fiber and the matrix.And the debonding situation was the main reinforce mechanism in CMCs.In contrast to quasi-static loadings,mi-crocracks around inter-bundles was subdued.Intra-bundle voids produced microcracks instead of dynamic loading.Strong interface layer made the microcracks propagate into fiber directly which resulted in fiber's rapture.At the macro level,delamination and shear deformation were the causes of failure of 2D-SiC/SiC composites.Under quasi-static in-plane compression load-ing,the interlaminar shear failure was dominant in the failure process,and the macroscopic failure angle of the specimen was small(about 15 degrees),while under the dynamic loading,the shear failure of longitudinal fiber was dominant and the macroscopic failure angle of the specimen was larger than quasi-static loading(about 50 degrees).(4)Under out-of-plane compression,quasi-static microcracks mainly initiated around in-terlaminar voids and propagated in the direction of 19.5°with the loading direction,and when the microcracks propagate into the interface layer,they caused debonding.When the microc-racks met and bridge with each other,macro cracks formed.The angle between the macro cracks and the loading direction was smaller than dynamic loading,and the interlaminar shear strength was lower.Microcracks mainly initiated around the intra-bundle voids and propagated in the transverse fiber bundles under dynamic compression.The microcracks formed in the transverse bundle distributed along 45°with the loading direction.When the microcrack reached the lon-gitudinal fiber bundle,the fiber will be cut off directly because of the interface layer's high strength.The angle between the macroscopic crack and the loading direction of the specimen was about 45°under dynamic loading.In addition,due to the existence of central holes,the shear band tends to exist only in the center of the sample,while under quasi-static loading,the shear band tends to exist on the whole surface.(5)Thermal residual stress exist in 2D-SiC/SiC composites,and matrix in tension and fiber in compression condition;the longitudinal thermal expansion coefficient of the fiber is smaller than matrix;the weakening of the BN interface layer below 1250? includes the interface layer softening and interface layer oxidation.At 1500?,the BN interface layer is oxidized to glass state B₂O₃.The increase of temperature does not affect the failure mechanism of the speci-men under dynamic load.The strength of the interface layer gradually weakens with the rise of temperature:Under in-plane compression loading,the angle between the macroscopic crack and the loading direction of the specimen becomes smaller;Under out-of-plane compression load,when the cracks in the matrix in the transverse fiber bundles of the specimen propagate to the longitudinal fiber bundles,the cracks tend to propagate along the longitudinal fiber bun-dles.When the temperature is low,the fiber fracture mechanism is dominant,and when the temperature is high,the fiber pullout mechanism is dominant.(6)Weibull distribution can be used to describe the strength of the 2D-SiC/SiC composite.Based on damage mechanics,the rate-dependent constitutive model considering that the dam-age accords with Weibull distribution can well describe the shape of stress-strain curve both in-plane and out-of-plane compression,and the fitting curves agree well with the experimen-tal curves.After the modulus,strength and shape parameters of the stress-strain curve of the specimens expressed as the multinomial of temperature,the model above can be used to estab-lish the temperature-dependent constitutive model.The stress-strain curve fitted under 700?,1000?,1500? can successfully predict the stress-strain curve under 1250? and the curve is in good agreement with the predicted results of the model both under in-plane and out-of-plane compression.