Study On Small Crack Propagation Behavior And Life Prediction Of FGH96 Powder Superalloy

Powder superalloy has become the preferred material for advanced engine turbine disk and baffle because of its uniform microstructure,small grain size and no macroscopic segregation,so it has good durable creep and fatigue properties.The life prediction of powder superalloy turbine disk includes initiation life prediction,small crack propagation life prediction and long crack propagation life prediction.The small crack propagation characteristics are different from those of long cracks,and there are few studies on the small crack behavior of powder superalloys.In this paper,the small crack growth characteristics and life prediction of powder superalloys are studied.This paper taking FGH96 powder superalloy as the research object,the fatigue small crack under the in situ fatigue test method and the fatigue small crack growth rate test method are studied,the crack propagation process data are obtained,and the crack initiation characteristics are analyzed.The fracture characteristics of different stages of cracks are observed and analyzed,the crack growth rate and stress intensity factor range are calculated,and the propagation life is calculated by using different small crack expression models.The fatigue small crack tests were carried out by using the high temperature in situ fatigue test and the test methods specified in HB7705-2001,and their propagation life was predicted.The results show that the prediction effect of effective crack length model on crack propagation life of in situ specimen is worse than that of HB7705-2001 small and medium crack stress intensity factor expression on crack propagation life of unilateral notched specimen.The crack growth rate analysis of high temperature in situ fatigue test and fatigue small crack growth rate test shows that the crack growth rate increases at first and then tends to be stable with the increase of crack length.The crack growth rate fluctuates greatly in the range of low stress intensity factor and is dispersed.The characteristics of fatigue crack initiation and propagation are as follows: one or more crack sources are produced at the root or scratch of the notch at high temperature,and the propagation of these small cracks is independent for most of the time.Some cracks stop to propagate to form non-propagating cracks after a period ofpropagation,some of them converge with other cracks in the process of crack propagation,and the propagation path of fatigue small cracks is more tortuous.After crack initiation,there are different ways of propagation and analysis of various propagation modes,but in the process of analysis,the propagation path of crack should be characterized and its physical connotation should be reflected.In order to predict the propagation life of SENT3 specimen,SENT3 specimen propagates to both sides after initiation of crack,and the predicted propagation life is close to that of specimen by using the crack data with sufficient propagation.This method is an analysis along the direction of crack propagation,which is of physical significance,and the one side of the crack length passes through one side of the specimen and the whole specimen is broken.Under the condition of high temperature test,the fatigue small crack growth rate is different from the long crack growth rate,and the stress intensity factor of the small crack is in the range of And crack growth rate Take the logarithm respectively and find out With That first increases,then gradually stabilizes,and finally tends to have a long crack.However,the long crack growth rate almost shows a stable upward trend.In the same The growth rate of small crack is higher than that of long crack,the material shows obvious small crack effect,and it is more obvious near the threshold value,and the stress intensity factor of small crack is lower than the threshold value of long crack stress intensity factor.Expansion can still occur.The main reason for the difference of crack growth rate between small crack and long crack under the same stress intensity factor amplitude is that the crack closure degree of small crack and long crack is different.For the prediction of small crack growth life,effective crack length must be used to drive the difference of crack growth rate between small crack and long crack.Instead of nominal stress intensity factors..After modifying the range of stress intensity factor of small crack by using NEWMAN plastic induced closure model,the small crack will be obtained.And Taking logarithm,it is found that the extension law can be expressed in the form of Paris formula,that is.