| Ultra-high temperature ceramics(UHTCs)have excellent properties such as high melting point,high strength,high temperature resistance,good ablation resistance and abrasion resistance,and have been used as thermal protection materials,and in the key components such as the leading edge,the engine nozzle,the nose cone,the thrust reverser and the thrust splitter of aircraft.However,applications of UHTCs have been limited by their inherently brittleness.When undergoing rapid temperature changes during service,the local thermal mismatch of UHTCs may cause sudden catastrophic failure,due to the nonuniform distribution of temperature.Therefore,studying the mechanical response of UHTCs under thermal shock and exploring their damage mechanism have important practical significance.This thesis mainly carried out the following works:(1)In view of the shortcomings of the current ultra-high temperature test instruments,such as limited maximum testing temperature,low heating speed,we had developed a servo-control testing system using electric heating for testing the mechanical properties of UHTCs at sufficiently high temperature and heating rate.Aiming at the deficiencies in this system,a gas environment control module was further introduced to provide the gas environment under different service conditions.Combined with different loading methods and gas environment,the ultra-high temperature tensile test,creep test and high frequency cyclic thermal shock test in air,vacuum or protective gas can be conducted,which lays a foundation for the follow-up testing work.(2)The cyclic thermal shock tests of Zr B2-20vol% Si C specimens in air or Ar environments at an ultra-high temperature and various heating rates were carried out.The residual bending strength of the specimens were tested.It showed that the larger the heating rate is,the stronger the thermal mismatch and the lower the residual strength of the specimen tested.Because of the healing effect of the oxide layer as well as the borosilicate glass layer on the surface defects,the residual strengths of the specimens tested in air are higher than that tested in argon.(3)The cyclic thermal shock tests of Zr B2-20vol% Si C-15vol% graphite specimens under various preload were carried out in air or Ar environment,and the effects of preload and gas environment on the cumulative damage of the specimens were studied.It is revealed that the residual bending strength of the specimens is affected by different microcracks.Due to the thermal mismatch,it is easy for microcracks to nucleate and grow around Si O2 particles formed by oxidation.There is no borosilicate glass layer to form in the samples tested in argon,so there are many short microcracks nucleated.There are fewer but longer microcracks in the speciments tested in air because of the protective effect of the oxide layer and the transverse growth stress in the layer.It also showed that the residual bending strength of the specimens tested in air is higher than that tested in argon.The oxidation healing under low prestress can effectively slow down the reduction of residual bending strength,while oxidation can aggravate the degradation of mechanical properties of the specimens under large prestress.The decreasing trend of the residual bending strength of the specimens tested in argon is slower than that tested in air.(4)Aiming at the difficulty to obtain the mechanical properties of materials at ultra-high temperature,a new rapid testing method was proposed to test the yield temperature of materials under constant prestress.When heating a material subjected to constant preload in its elastic range at room temperature,plastic deformation would take place as the temperature is sufficiently high.Based on this phenomenon,we suggested that the yield temperature could be measured at a lower even room temperature.The testing method is based on the following concept.Letting a prestressed specimen be subjected to thermal cycles with increasing temperature amplitudes,and measuring the deformation of the specimen before and after each thermal cycle,considering that the variations of thermal strain and elastic strain during heating stage can be compensated by that during cooling stage,if there is residual deformation,it should be ascribed to plastic deformation.Therefore,one can obtain a residual strain-temperature relationship,from which the yield temperature can further be identified.In our work,the temperature corresponding to ??p = 0.2% is defined as the yield temperature Ts(proof temperature)of the tested material under the prescribed prestress.The Ts of pure rhenium sample under some prescribed prestress was measured using the proposed method.For verification,the stress-strain curve of the material at the yield temperature was also obtained,and the comparison between the yield stress in the latter and preloaded stress in the former showed satisfactory agreement,demonstrating the validity of the proposed testing method.This method can further be extended to achieve the stress-strain relationship at high temperature,or the strain-temperature relationship under prescribed prestress.The proposed testing method could achieve the mechanical properties at high temperature by using the measurement of deformation at a lower or room temperature,which could avoid the deformation measurement at high or ultra-high temperature,and should be beneficial to the tests at high or ultra-high temperature. |
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