| Turbine rotor is the core component of the turbocharger,which is subjected to a large load at high temperature and speed working condition,and it is prone to fatigue failure.Due to the addition of BNNT,the plasticity and toughness of BNNT/Si3N4 composite ceramic have been improved remarkably,and its anti-fatigue ability is also excellent,so it is considered to be the best alternative material for conventional turbine materials.In this dissertation,the BNNT and BNNT/Si3N4 composite ceramic turbine were fabricated firstly,and then the ratcheting fatigue behavior and life of the turbine was analyzed.At last,the dynamic characteristics and ratcheting fatigue crack propagation behavior of the turbine were researched by the finite element software such as ANSYS and ABAQUS.In the preparation of BNNT by high energy ball milling,amorphous boron powder and NH3 were used as raw materials and CNT as template,and high purity BNNT/Si3N4 was successfully prepared.The high energy ball milling method greatly reduces the preparation cost of BNNT,because of the advantages of simple operation,low energy consumption,short time and low experimental requirements.Considering the similarities of fatigue behavior between ceramic and metal materials,the ratcheting fatigue life mathematical model was established which including some new factors,based on the expression of fatigue life of metal materials.And the mathematical model provides theoretical support for calculating the fatigue life of ceramic turbine blades under centrifugal loading.The ratcheting fatigue crack initiation life Ni and crack propagation life Np of the composite ceramic turbine blade are calculated according to the established mathematical model.The results showed that the threshold value of ratcheting fatigue damage of the composite ceramic turbine blade is 140000r/min,which is better than that of pure Si3N4 ceramic.The ratcheting fatigue life of BNNT/Si3N4 turbine is higher than that of the Si3N4 turbine at the same speed,and the increase rate is about 40 percent.It is found that the ability of resisting crack initiation and preventing crack propagation has significantly increased because of the addition of BNNT by comparing the calculation results of ratcheting fatigue life of two different ceramic turbine blades.It established the simplified 3D model of turbine and then analyzed its modal,stress,temperature field,crack stress intensity factor and crack propagation behavior.The simulation results showed that the maximum stress region is located at the root of blade suction surface and the results coincides with the actual situation.The BNNT/Si3N4 composite ceramics has better mechanical properties of low density,low Poisson ratio,high elastic modulus and dynamic balance performance,so that the composite ceramic turbine has less stress at the same speed.In addition,the crack stress intensity factor and crack growth rate of BNNT/Si3N4 composite ceramic turbine is lower than Si3N4 turbine.Based on the above research,it is proved that the ability of the composite ceramics to resist crack initiation and prevent crack propagation is considerably improved. |
PDF Full Text Request