Study On Propagation Behavior And Model Of Short Fatigue Crack In Nickel-based Alloy Inconel 690

Before fatigue fracture occurs,mechanical components will undergo a multi-scale damage evolution process including crack initiation,short crack propagation and long crack propagation.The short crack propagation stage consumes most of the fatigue crack growth life.On the one hand,because short fatigue crack is greatly affected by the microstructure of the material,classical fracture mechanics based on continuum mechanics is difficult to describe the growth behavior of short crack.On the other hand,the current models used to predict the growth behavior of long crack and no longer suitable for short crack stage.Therefore,an in-depth study of the growth behavior of short fatigue crack and its influencing factors are of great significance for establishing the prediction model of the growth rate of short fatigue crack.For this reason,this article takes the nickel-based alloy Inconel 690 as the research object,and uses advanced in-situ fatigue test method to study its short crack growth behavior.The main content and research results are as follows:(1)The growth behavior of Inconel 690 short fatigue crack is studied by in-situ fatigue experiments.First of all,the crack growth rate of Inconel 690 fluctuates within the range of 10^-9?10^-12m/cycle during the short crack growth stage.As the crack length grows,the crack growth rate will experience a non-monotonic process that first decreases and then increases.Secondly,short fatigue crack is sensitive to the microstructure.The cracks originate in the persistent slip zone,and the crack propagation path is affected by the slip band and expands in a zigzag shape.Periodic crack deflection and bifurcation lead to the non-monotonicity and volatility of the short crack growth rate,which shows the excellent microstructure anti-cracking performance of the alloy.Finally,the barrier effect of the grain boundary on the short fatigue cracks of Inconel 690 is obviously,and more cycles need to be consumed at the grain boundary.And secondary cracks are one of the reasons that lead to non-monotonic changes in the crack growth rate.(2)Through the analysis of the short crack growth behavior,the Inconel 690short fatigue crack growth model is established.First of all,the crystallographic orientation of the grains around the crack path is obtained by electron backscatter diffraction,and the short crack propagation of Inconel 690 is analyzed with the crystallographic model.It is found that the cracks propagate along the slip plane in the crystal.At the grain boundary,the slip plane with the smallest twist angle among adjacent grains will be selected to continue to expand,and the larger the twist angle,the more obvious the obstruction of the grain boundary.Secondly,a model has been established to describe the growth of short crack after encountering grain boundaries through the increment of unit cycle dislocation density,which can be used to calculate the number of stagnation cycles.And a binary crack growth model for predicting fatigue short crack growth life is proposed.Finally,an attempt is made to use neural network to study the Inconel 690 short fatigue crack growth model.The results show that this method can better describe the nonlinear change trend of the crack growth rate in the short crack stage.