Research On Fatigue Life Prediction Of Peening Titanium Alloy Under Spectrum Loading

In the study of the aircraft structure about safety,the fatigue life of the aircraft has been a hot research.Good fatigue fracture performances are important properties of the materials used in aircraft,but also an important basis for damage tolerance design of aircraft.Titanium alloys have good properties such as high specific strength,low density,corrosion resistance,heat resistance,so they are widely used in the aviation field,and one of the main metallic materials used in aircraft.The purpose of this thesis,is to study the fatigue crack life prediction of shot peening Titanium alloy under spectrum loading.The research work of this thesis can be divided into three parts:1)Finite element simulation of shot peening residual stress field.Taking into account complicated working conditions of shot peening,a single-shot model and a multi-shots model were used to simulate a residual stress field with 100% coverage,and consider effect of shot peening sequence.By comparison between Finite element simulation and experimental results,the method as used herein,can simulate the shot peening residual stress field of Titanium alloy.2)Fatigue life prediction of Titanium alloy under spectrum loading.Based on the fracture mechanics and small crack theory,this thesis predicted fatigue life of shot peening Titanium alloy SENT specimen under spectrum loading.In fatigue life prediction of specimens under Mini-Twist spectrum,rainflow counting method was used to convert test spectrum into calculation spectrum,and Newman closure model was used to analysis crack growth.Comparison with experimental results showed that the method used was effective when predicting the fatigue life of Titanium alloy under Mini-Twist spectrum.3)Research on some parameters in fatigue life prediction.The parameters to be studied include incomplete residual stress field data,variable constraint factor and stress relaxation coefficient.Research indicates that: the influence of incomplete residual stress field data on fatigue life prediction depends on the depth of residual stress field,and a shallower residual stress field will cause a bigger effect on prediction.Lower limit value of the variable constraint factor has a great influence on life prediction results.The residual stress relaxation phenomenon under fatigue loads has an important influence on fatigue life of the material,and the choice of an appropriate relaxation mode is significant for the accuracy of life prediction.