Study On Fatigue Crack Growth Theory And Applications Model Of Nickel-based Superalloy

Superalloys due to its excellent mechanical properties are widely used in aviation,aerospace,ships,vehicles,industrial gas turbines and other industrial sectors.Superalloys are used to make aircraft engines,space shuttle main engines,diesel engines,turbo internal combustion engine,gas turbine,metallurgical rolling mill furnace pads,engine exhaust valve seats and other components are exposed to the hot end of fatigue loading.This paper takes the most widely used nickel-based superalloys as the research object and analyzes the experimental data of fatigue crack growth under constant amplitude loading.For the fatigue crack growth rate and the stress intensity factor range relationship,at home and abroad made many related models.For the relationship between the fatigue crack growth rate and the stress intensity factor,the researchers at home and abroad have put forward a lot of relevant models.In this paper,the widely used Paris formula,Walker formula and the modified Paris formula are used to study fatigue crack propagation theory of alloy and application of the model according to the the fatigue crack growth test data of more than 20 kinds of nickel-based superalloys.The main research contents and conclusions are as follows:1.Firstly,a brief introduction to the research background of metal fatigue crack propagation,research history and current the situation of research and development of nickel-base superalloy is given.Then the research progress of fatigue crack growth model,fatigue crack growth theory and numerical simulation are summarized.2.In this paper,the fatigue crack growth test data of nickel-based superalloys under different experimental conditions,such as temperature,stress ratio,loading frequency,environmental factors and the geometric size of the specimen,etc.,are widely collected.Comparative study on test data show that the influence of service temperature on the fatigue crack growth rate is the most,and general higher temperature can make the nickel-based superalloy da/dN value increased by an order of magnitude;the influence of stress ratio,loading frequency and environmental factors on da/dN is obvious,while the effect of the thickness of test piece and other geometrical factors of test piece on da/dN value is relatively difficult be observed.The change of nickel content had no significant effect on the fatigue crack growth rate da/dN value of nickel-based superalloys,which is a nickel-based superalloy in the study area of nickel content,but the heat treatment process had significant effect on the da/dN value.3.Respectively Paris formula,Walker's formula and the modified Paris formula were used to regression analyze the fatigue crack growth test data of nickel-based superalloys.The results show that corresponding to different temperature,stress ratio,loading frequency,environmental factors and specimen geometry,the material constants of the fatigue crack extended model varies in a certain range.Moreover,these changes under action of different influence factors have their regularity.The selection of the theoretical model of fatigue crack propagation has little influence on the material constants,but has great influence on the numerical value of fatigue crack propagation rate.And,in the use of Paris formula for analysis,using different numerical methods have a greater impact on the value of material constants.4.The theoretical model calculation and experimental data comparison shows that the fitting precision of Paris formula is related to the discrete nature of the test data itself.Walker formula applicable to describe the nickel-based superalloys fatigue crack propagation behavior and the value of the three material parameters of Walker formula is within a certain range.The influence of temperature on the material parameters of Walker formula has a certain rule.The modified Paris formula used in this paper,for the nickel-based superalloys under the condition of high temperature,has high precision at low stress intensity factor range of fatigue crack growth rate.In the high stress intensity factor range,the predicted value of the theory is low.By summarizing the research of each model,this paper puts forward a general modified Paris formula,which can accurately predict the fatigue crack growth behavior of the nickel-based superalloys under the condition when there are a lot of experimental samples.