| Ceramic matrix composites acting as thermal structural material with integrated function of heat-proof and load-bearing have been widely used in high temperature resistant components of hypersonic vehicles and aero engines.In the service environment,the mechanical properties of the material at high temperature have a great impact on the safety of the structure.In addition,ceramic matrix composites components will inevitably suffer from the threat of impact loads.Therefore,full understanding of their dynamic mechanical behavior at high temperatures is not only of great importance to understand the deformation and failure mechanism of the materials under coupling effect of force,heat and chemistry,but also the basis for structural safety design.However,limited to the experimental technique,it is of great challenge for performing experiments at high strain rate and temperature.In this paper,based on the split Hopkinson bar,an experimental method for measuring dynamic behavior of materials at high temperatures(up to 1600℃)was developed.Meanwhile,the difficulties of high-speed deformation image acquisition and high-temperature speckle fabrication under high-temperature environment were solved.The mechanical behaviors of ceramic matrix composites under different strain rates(quasi-static,dynamic),different temperatures(room temperature to 1600 C),different gas atmosphere(air,argon)and different loading modes(compression,tension)were all studied.The relationships between failure mechanism and temperature and oxidation were clarified.The main research contents and results are as follows:(1)The dynamic compression behavior of uncoated and coated 2D C/SiC ceramic matrix composites under room temperature was studied.The deformation process of the specimens under quasi-static and dynamic loading was captured by a high speed camera.The fracture surfaces of the specimens under different loading conditions was analyzed using a scanning electron microscope(SEM).The results show that the compressive strength of 2D C/SiC composites increased with the increase of loading rate since the interlaminar bonding strength increased and multiple cracks nucleated simultaneously under dynamic conditions.Based on the experimental results,stain rate related constitutive model of the material was also established.The predicted stress-strain curves are in good agreement with the experimental results.(2)An experimental method for measuring dynamic behavior of materials at high temperatures was proposed in this work.First,an experimental system for measuring dynamic compression behavior of materials at high temperatures(up to 1600℃)based on the Hopkinson compression bar was established.The experimental system includes a classical split Hopkinson pressure bar,a heating furnace using Mo Si2 heating source to achieving high temperature,and two piston rods added to complement the double-synchronous assembled system.The heating furnace is equipped with a quartz glass window for observation.Besides,the heating furnace also has a gas inlet to import the protective gas.During the experiments,the specimen can be supported by asbestos and a semi-Alumina ceramic tube.To estimate the thermal conduction of the specimen.The cold contact time(CCT),during which the hot specimen is in contact with the cold bars before being compressed,was measured experimentally based on an on-off circuit.And the finite element method(FEM)was also employed to calculate the thermal conduction of the specimen.Secondly,an experimental system for measuring dynamic tension behavior of materials at high temperatures(up to 1600℃)based on the Hopkinson tension bar was established.During the process of test,the specimen is heated to the predetermined temperature in the high temperature furnace at first.Then the specimen is pushed out from the furnace and achieves assembly with the loading bars by a designed device.The fixture used to fix and push specimens is made of high temperature resistant ceramics.Besides,the problem of image supersaturation at high temperature was solved by suppling and enhancing blue light using light and installing blue light filter in front of the lens.In this way,the ideal high temperature(1600 °C)images were acquired.Meanwhile high temperature speckles made of high temperature adhesive were sprayed on the surface of specimens by using high pressure spraying gun.(3)The compressive mechanical behaviors of 2D C/SiC composites with and without coated at high strain rates and elevated temperatures(20°C ～1600°C)were investigated.In high temperature environment with the protection of argon,the compressive strength of both coated and uncoated 2D C/SiC composites increased with the increase of temperature.And the changes of the residual stress with temperature resulted in the increasement of the compressive strength.At the same time,the high-speed camera was used to record the deformation process of the specimen during loading.The result shows that the damage angle of the specimen also increased with the increase of temperature.In air environment at high temperature the fibers of the uncoated specimens were severely oxidized,which made the compressive strength of the specimen decrease sharply.In contrast,with the protection of SiC coating,the oxidation degree of the coated specimens was reduced.The compressive strength of the coated 2D C/SiC composites increased first,and then decreased,and at last increased again with the increase of temperature.Besides,with the protection of argon,strain rate sensitivity factors of the coated and uncoated specimens decreased as temperature increased.While,in the air environment,the strain rate sensitivity factors increased when the temperature was below 1000 °C,and then decreased as the temperature increased.(4)The tensile mechanical behaviors of 2D C/SiC composites with and without coated at high strain rates and elevated temperatures(20°C ～1600°C)were investigated.In high temperature environment with the protection of argon,the tensile strength of the 2D C/SiC composites increased with the increase of temperature,and then decreased,and finally increased again at temperature above 1200°C.The evolution of the strength with temperature under tensile load was analyzed by considering the influence of the residual stress and interfacial shear strength of the material.The residual stress will influence the cracking strength of the matrix and interfacial shear strength of the 2D C/SiC composites.This leads to a complex temperature effect of the 2D C/SiC composites.In contrast,in the air environment,the cracking of the matrix will aggravate the oxidation effect of the fibers.And when the specimen was loaded in quasistatic condition,the oxidation degree was much higher than that in dynamic condition.In the air environment,the strain sensitivity factor of the 2D C/SiC composites increased sharply with the increase of temperature.While,in the argon environment,the strain sensitivity factor was less affected by temperature. |
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