Research On Fatigue Life Prediction Method Of Shaft Parts Under Combined Excitation

The demand for metal shaft parts is high and their application is extensive.Under actual working conditions,they often experience complex loads that can easily lead to the formation and expansion of fatigue cracks,ultimately resulting in fracture,which can have a negative impact on engineering economics and safety.The stress on shaft parts that experience fatigue failure is often lower than the stress calculated by static mechanics theory,making research into the prediction of fatigue life for shaft parts particularly important both in practical applications and theoretical studies.However,current research is mostly limited to single-frequency vibration fatigue or tension-compression fatigue and torsion fatigue of metal materials,with little research on the prediction of fatigue life under composite vibration load conditions for metal materials or on the crack initiation and propagation mechanisms and fatigue life rules of metal shaft parts.To address these issues,a V-shaped notch was pre-made on the shaft parts and fatigue life prediction research was conducted under composite excitation using a selfdeveloped composite excitation experimental system.The main contents are as follows:(1)Analyzing different fracture types of metal materials and comparing and analyzing the advantages and disadvantages of commonly used fatigue life prediction methods for some metal materials,selecting a suitable fatigue life prediction method for composite excitation conditions.On this basis,the starting position,mechanism,stress intensity factor,and K criterion of fatigue cracks in metal shaft parts under composite excitation loads were studied.The composite fatigue damage model was analyzed with the characteristics of high and low-frequency excitation loads.At the same time,finite element modeling and analysis were carried out on the shaft parts to determine the stress expression rules for the initiation and dangerous locations of fatigue cracks when subjected to excitation loads.(2)Analyze and select a set of local stress-strain method formulas suitable for composite excitation conditions,and calculate the fatigue coefficient related to the 6061 aluminum alloy shaft based on the loading state.In addition,based on the working principle of the composite excitation system,a periodic composite excitation frequency control curve is designed.Based on this,the time-domain amplitude map of the system is obtained during one loading cycle stage,while the nominal stress at the notch tip is also obtained.After extracting the effective nominal stress cycle through the rain flow counting method,the nominal stress spectrum and fatigue life spectrum of 6061 aluminum alloy shaft were obtained.Furthermore,based on Miner’s cumulative fatigue damage theory,the total fatigue damage of 6061 aluminum alloy shaft under one loading cycle was calculated to predict its fatigue life and conduct cyclic loading fatigue verification experiments under composite excitation conditions.(3)The Box Behnken design method in response surface methodology was used to experimentally design the geometric parameters of 7A09 aluminum alloy V-notch.At the same time,the excitation force required for crack initiation was calculated based on the excitation characteristics of the composite excitation system,and the generation of system resonance was indicated.Furthermore,the excitation frequency control curve that promotes the initiation of axial fatigue cracks is adopted,and the excitation frequency is set according to the characteristics of the excitation system.A multiple regression prediction model for fatigue life was established based on the experimental results of fatigue failure obtained,and the reliability evaluation of the experiment and prediction model was conducted using variance analysis and residual analysis methods.The influence and interaction of notch angle,fillet radius,and depth on the fatigue life of 7A09 aluminum alloy shaft were analyzed using response surfaces and contour plots,and fatigue verification experiments were conducted on the fatigue life prediction model.