Studies Of Fatigue Crack Initiation Life Prediction Of Rotor Groove Components Based On Back Stress Plastic Work

As one of the core components of a steam turbine,the rotor disc often suffers from fatigue failure due to the stress concentration in the tooth root of the wheel groove during service.Therefore,the prediction of the fatigue life of turbine rotor slots is one of the important contents in the design of turbine fatigue strength.In this paper,in view of the current structure of the energy is becoming more and more attention in the fatigue life prediction method,the fatigue damage evolution characteristics of stress and the plastic work,combined with the fatigue damage analysis of energy consumption,to study based on the stresses in the plastic work energy structure fatigue life prediction model,and applied to the steam turbine rotor wheel groove of fatigue crack initiation life prediction research.In this paper,based on the low cycle fatigue test results of smooth samples of wheel-groove materials under pulsating loads,Cottrell internal stress theory(cyclic stress can be decomposed into back stress and friction stress)was adopted to analyze the cyclic hardening/softening characteristics of wheel-groove materials from the perspective of back stress/friction stress.Based on the fatigue damage energy consumption structure,the plastic strain energy in the cyclic process is decomposed into back stress plastic work and friction stress plastic work.The results show that the plastic work of the back stress fluctuates in a cycle,and its value only accounts for a small part of the plastic strain energy,and is closely related to the strain amplitude.The evolution of plastic work of back stress in the fatigue process is consistent with the change law of material energy storage under cyclic loading.Based on the evolution characteristics of back stress plastic work of wheel groove materials during fatigue process,and combined with the analysis of fatigue damage energy consumption,a prediction energy model of wheel groove fatigue life with back stress plastic work as damage parameter was established by using local strain method.It is applied to the prediction of fatigue crack initiation life of P8 wheel groove components.In order to verify the reliability of the above life prediction energy model,this paper carried out two kinds of low-cycle fatigue tests of P8 wheel-groove simulator with width-diameter ratio,and obtained the fatigue crack initiation life of the tooth root with maximum stress concentration under different loads(corresponding to the crack length of about 0.5mm).By comparing the predicted life and test life of wheel-groove components,it is proved that the prediction accuracy of the prediction energy model of wheel-groove components established in this paper fully meets the engineering needs(the ratio of predicted life and test life is generally within 1.5 times of dispersion zone).In addition,the above fatigue life prediction energy model is compared with the traditional fatigue energy model(based on plastic strain energy and total strain energy).The results show that the fatigue energy model based on plastic work of back stress can significantly improve the accuracy of fatigue life prediction.Because the physical meaning of the model is clear,it is worth paying attention to in the anti-fatigue design of engineering structure.