Prediction On Gear Bending Fatigue Damage Considering Residual Stress And Hardness Gradient

High performance gear,as one of the most important basic components,its performance directly determines the dynamic service performance and reliability of the equipment.The gear bending fatigue problem has become increasingly prominent due to the increasing requirement of gear power density,loading capacity and fatigue performance for sophisticated equipment such as high-speed rails,wind turbines,shield tunneling et al.The current gear bending fatigue design method,represented by stresslife(S-N curve)approach,predicts the gear bending fatigue based on the stress/strain-life curve.These methods can not illustrate the evolution of gear damage accumulation and the deterioration of gear material mechanical properties such as elastic modulus.Accordingly,it is difficult to reveal the failure mechanism of gear bending fatigue based on the physical essence.Furthermore,the current widely used S-N formula ignores the effect of residual stress and hardness gradient on the gear bending performance,which results in imprecise of gear bending fatigue life prediction.Considering the limitation of these gear bending fatigue methods,a damage coupled elastic-plastic gear bending fatigue model which taking the effect of residual stress,hardness gradient and gear geometry into consideration is developed based on the continuum damage mechanics(CDM).Based on this simulation model,the gear material performance deterioration is investigated to reveal the gear bending fatigue failure mechanism.The effect of residual stress and hardness on the gear bending fatigue is investigated as well.In addition,the concept of modified stress including residual stress is proposed to unify gear bending stress-life(SN)curves considering the effect of residual stress and surface hardness.Finally,the gear simulation results and modified S-N formula are verified through a series of gear bending fatigue tests.The research work of this thesis can provide theoretical support to modern anti-fatigue design of gear and engineering guidance for the initiative design of gear fatigue performance.The main contents of this thesis are as follows:(1)Based on the CDM,gear fatigue damage equations are proposed considering residual stress and hardness gradient.A damage coupled gear elastic-plastic constitutive is derived.The measurement of gear hardness gradient and residual stress in conjunction with the relationship between gear damage coefficients and gear hardness are utilized to determine the gear damage coefficients.A damage coupled gear elastic-plastic finite element model is finally developed.(2)Stress convergence for the gear bending fatigue damage finite element model is conducted based on different element size.A comparison between simulation gear bending stress and experimental based bending stress is utilized to verify the accuracy of simulation model.The stress-strain response,fatigue damage accumulation and elastic modulus deterioration of gear under cyclic loading were analyzed to reveal the fatigue failure mechanism from the essence of mechanical property degradation.(3)Starting from the stress-strain response and the gear fatigue performance which affecting the gear bending fatigue life to a large extent,the influence of load condition,residual stress and surface hardness on the gear bending fatigue is discussed deeply based on the gear bending fatigue damage model.(4)Considering the limitation that the classics S-N curve is unable to reveal the influence of residual stress and surface hardness on gear bending fatigue in engineering practice,a comprehensive stress-life modified formula including residual stress and surface hardness is proposed.The corresponding parameters are determined based on simulation result.(5)The gear bending fatigue limit and S-N curves for different residual stress and hardness states are obtained through a series of gear fatigue tests.The elasticity modulus before and after fatigue test are measured to verify the deterioration of gear mechanical properties.The effectiveness of gear damage model under different residual stress and hardness states is verified through comparison between simulation results and experimental results including the gear bending fatigue crack initiation position,fatigue life and mechanical properties deterioration.The stress-life modified formula is also verified based on the experimental data.