Fatigue Life Prediction And Verification Of TC4 Central Hole Plate Strengthened By Cold Expansion

Titanium alloy material has low density,high strength and good corrosion.It is increasingly used in the aerospace industry.Due to the serious stress concentration near the notch under flight loads,the fatigue problem of the notched titanium alloy structure is more likely to occur.Therefore,the fatigue performance of the notched structure has been the focus of attention in theory and engineering.The cold expansion can effectively improve the fatigue performance of the notched structure by producing the residual compressive stress near the notch.Based on the above reasons,it is important to study the fatigue performance of the TC4 central hole plates strengthened by cold expansion.In this thesis,the fatigue properties of TC4 central hole plate under constant amplitude and spectrum loads are studied.The main contents of this thesis contain the following three parts.1)Fatigue tests under constant amplitude and spectrum loads.The fatigue data of TC4 material,unstrengthened and strengthened central plates was obtained by the fatigue tests under constant amplitude and spectrum loads.These tests provide important data to verify the method for predicting the fatigue life of strengthened central hole plates.It can also serve as an important reference for the design and fatigue performance analysis of the notched structure.2)Residual stress analysis and fatigue life prediction of strengthened TC4 central hole plates under constant amplitude fatigue loads.Strategies for modelling the cold expansion with reaming process by using three dimensional finite element method were extensively studied.And the compressive residual stress distribution near the strengthened hole was obtained.Furthermore,the finite element method was used to obtain the stress field near the strengthened hole under the constant amplitude loads,and it was determined that the entrance face is the most likely location of fatigue failure.Combined with the critical distance method,the fatigue lives of titanium central hole plates with 4% expansion were predicted.The ratio between the predictive and experimental results is within 0.5 to 2,indicating that the prediction model is feasible and effective.The research provides a methodology to obtain the residual stress field and to quantitatively evaluate the fatigue performance of titanium specimen strengthened by cold expansion and reaming process.It can be also used to optimize the cold expansion and reaming process to achieve desired fatigue performance.3)Prediction and verification of fatigue life of strengthened TC4 central hole plates.After obtaining the fatigue data of the strengthened central hole plates under the constant amplitude loads,their fatigue life under spectrum loads can be predicted by using fatigue cumulative damage theories.The Manson criterion and the Dolan criterion were used here to establish the nonlinear cumulative damage model.Improved parameter ? in Manson criterion and parameter d in Dolan criterion were determined by a iterative algorithm with the fatigue life of the strengthened material at a certain stress level under spectrum loads.The fatigue lifes of the strengthened material at other stress levels were predicted by using the improved ? value and d value.The improved values overcome the shortcomings of traditional parameters,which can not consider the strengthened effect by cold hole expansion.Compared to the prediction results from Miner criterion,traditional Manson criterion and traditional Dolan criterion,the accuracy of the iterative model is greatly improved.The ratio of predictive and experimental results is between 0.5 to 2,which demonstrates the iterative model is feasible and effective.The results provide an important reference for predicting the fatigue behavior of strengthened structures under flight loads.