Fracture Behaviour Of UHTC With Surface Crack Under Thermal Shock

As ultra high temperature ceramic (UHTC) with high temperature resistance, thermal conductivity, strength and chemically stable at high temperature, it becomes one of the most prospective materials for application in ultra high temperature and reactive atmosphere, spaceflight, which has the potential to be used in extreme environments including those associated with hypersonic flight, atmospheric re-entry, rocket propulsion, key parts including the nose cones, shape leading edges and warm end of the engine. However, the preparation, processing and assembly will result in a lot of material internal or surface defects, mainly in the form of surface crack defects, will directly influence the reliability and the application of such materials. It is necessary to research on the material surface crack defects, especially for the thermal shock performance of material. Under this background, this work is focus on the influencing factors of material thermal shock damage behavior and damage mechanism with ZrB2+20%SiC+10%AlN (ZS10A)with Vickers indention, several aspects of problem for thermal shock are investigated as following:Equipment by fast local heating of UHTC was established, and it provides a simple and easy method for characterizing heating-up thermal shock property of UHTC. ZrB2+20%SiC+10%AlN were characterized by the new equipment. Results show that in the rapid thermal shock conditions, thermal shock failure behavior is related to the heating rate. We change the input current for different heating rate, 200℃/s ,500℃/s,800℃/s,and find there is a linear relationship between the heating rate and the temperature of critical damage. In addition, the thermal shock temperature and stress distribution of specimens during the process is obtained by Abaqus, in order to further characterization of sample failure mechanisms, and provide reliable data for performance assessment in practical application.Thermal shock damage behavior is mainly investigated by studying the surface crack extension and residual strength of the material under thermal shock. The surface crack is characterized by Vickers indention, and the transient thermal shock process is simulated with the quenching method. Thermal shock property of UHTC was investigated by comparison of surface crack length changes before and after thermal shock and residual strength of the material after quenching-bending test. Result shows that the crack growth increases with sample thickness and initial crack length, and the trend of crack growth will be influenced by quenching temperature difference, re-heating in different temperature and the same temperature. The critical temperature and residual strength of the material are decreased with the increase of sample thickness and initial crack length. The study reveals those factors influencing the thermal shock behavior of the material. According to the classical heat conduction equations, establish quasi-static stress model of quenching process. Then thermal shock model considering dynamical behavior and crack propagation model were established. According to the features of surface defects, introduce oval surface crack model and dynamical thermal stress model, calculation formula of dynamic thermal stress intensity factor of crack is obtained. Crack resistance of UHTC can be calculated by dynamical thermal stress, the dynamic thermal stress intensity factor and equation of ZrB2+20%SiC+10%AlN. Results show that dynamical thermal stress is much higher than quasi-static one, so effect of dynamical thermal stress should be considered during quenching test. The stress intensity factor provides driving force for crack propagation, the relationship between it and crack resistance is the key to predict whether crack extension, the calculation results and experimental results are basically identical, considering the effects of the size of material, the relationship between the specimen thickness and thermal shock damage is calculated to predict crack propagation.