Research On Wear-fretting Fatigue Life Prediction Method Based On Damage Mechanics

Fretting phenomenon is caused by micro slip between contact surfaces,and widely exists in machinery,aviation,microelectronics and civil engineering.Under cyclic loading,fretting causes contact surface wear and fretting fatigue failure prior to plain fatigue failure.Therefore,this dissertation focuses on fretting fatigue of contact surface wear.By decomposing the fretting fatigue failure process into two stages: fretting crack initiation and fretting crack propagation,the fretting fatigue damage model was theoretically deduced and constructed,the crack initiation and propagation under fretting conditions were simulated,the wear of coupling contact surfaces was coupled,the main influencing factors affecting fretting fatigue life were discussed,and the fretting fatigue damage mechanism different from plain fatigue was explored,which ultimately laid the necessary theoretical foundation for the fretting fatigue design method of structural engineering.The main researches of this paper were as follows:1)Based on SWT method in critical plane method and FFD method in fretting fatigue parameter method,fretting fatigue crack initiation was studied.The results showed that SWT method is suitable for the prediction of fretting fatigue initiation life with high stress amplitude,and can predict the initial crack growth angle;FFD method was more suitable for prediction of fretting fatigue initiation life with low stress amplitude.Compared with SWT method,FFD method had higher prediction accuracy and calculation efficiency of crack initiation life.2)Based on the fatigue damage constitutive relation,the influence of the wear morphology of the contact surface was introduced,and the fretting fatigue crack initiation model considering the influence of the wear morphology was established.The real-time update of the wear morphology in the numerical simulation process was realized through the subroutine UMESHMOTION.The results showed that the conservative prediction of the initiation life in the slip zone would be caused if the wear morphology was not considered.Considering the wear morphology,the rate of crack nucleation and damage in slip zone was delayed,and the effect of low stress amplitude was more obvious.The initiation life was negatively correlated with normal stress,tangential stress and friction coefficient.3)By coupling the wear effect of contact surface as additional damage and original damage,the equivalent damage evolution equation was derived based on the irreversible thermodynamic theorem,and the damage equivalent crack initiation damage evolution model considering wear effect was established.The real-time cumulative evolution of fretting fatigue damage in contact area was realized through subroutine USDFLD.The results showed that the dispersion of predicted fretting fatigue crack initiation life decreased after coupling wear and damage.Wear accelerated the accumulation of crack nucleation damage in the partial slip zone,which showed a shorter crack initiation life.The influence of friction coefficient and tangential cyclic stress on crack initiation life was higher than that of normal pressure,and they also had obvious influence on crack initiation location.4)Based on the aforementioned equivalent damage evolution model of wear fretting fatigue crack initiation,the effects of plasticity and wear on fretting fatigue initiation life were coupled.The results showed that both plasticity and wear accelerated the damage accumulation,showing a lower crack initiation life.The effect of plasticity on crack initiation life was greater than that of wear.Increasing cyclic stress led to accelerated damage accumulation of plasticity and wear,and reduced crack initiation life;Increasing the preload slowed down the accumulation of wear damage,but accelerated the accumulation of plastic damage,and finally reduced the crack initiation life;Increasing the friction coefficient led to the reduction of the cumulative damage of plasticity and wear,thus increasing the crack initiation life.5)The total fretting fatigue life was equivalent to the sum of crack initiation life and crack propagation life.The equivalent damage evolution model of wear fretting fatigue was used to predict the crack initiation life,and the crack growth model was introduced.The Paris rule was used to calculate the crack growth life,and the automatic crack growth path analysis combining the SEAM model and the improved MTS method was realized through Python.The fretting fatigue analysis results of cylinder contact,high-strength bolt connection and fan tower flange connection showed that the improved MTS method for crack growth improved the prediction results of crack growth angle,and the crack initiation process combined with wear improves the prediction accuracy of overall fretting fatigue;Fretting fatigue life was negatively related to cyclic stress,friction coefficient and preload,and crack growth path was only related to preload.The crack initiation life played a major role in the total fretting fatigue life;The fretting fatigue control part of the flange connection of the fan tower was the contact surface between the bolt rod and the hole wall,which was located at the middle and lower part of the 0 ° bolt rod on the windward face.The fretting fatigue life here was low due to the unfavorable factors of the contact between the bolt rod and the bolt hole wall.Reducing the friction coefficient between flange contact surfaces or increasing the friction coefficient between bolt rod and bolt hole contact surfaces improved fretting fatigue and increase fretting fatigue life.