Study On Fatigue Crack Propagation Of TC4-DT Titanium Alloy Electron Beam Welded Joints

In the new generation of aero-engine compressor structure,the use of weldment of titanium alloy disc to shaft has become a common measure to reduce the weight of the structure.The welded joints of disc and shaft are often subject to centrifugal force,axial force and torque.Once a micro-crack appears,it will grow in some mixed mode.Researches on the fatigue crack propagation behavior of titanium alloy welded joints are very important in evaluation of the fatigue life of aircraft engine disk and shaft welded structure.The main contents of this thesis are listed as follows:1.In this paper,the mode I crack propagation tests were carried out for the investigation of crack propagation behavior of TC4-DT welded joints,and the crack growth curves of the weld zone,heat affected zone and base metal zone were obtained.The finite element method was applied to calculate the stress intensity factor.Combined with the experimental data,the Paris equations of the mode I crack in weld zone,heat affected zone and base metal zone were established,so as to the comparison of fatigue crack behavior under different loads.The failure mechanism of crack propagation in different zones of welded joint was analyzed by analyzing the fatigue fracture.2.In order to study the mixed mode fatigue crack propagation rules of TC4-DT welded joints,the modified Arcan fixture was used to test the fatigue crack growth.The crack propagation angle and the crack propagation life of the mixed mode crack in weld zone were calculated.We found:(a)the theoretical results of crack propagation angle based on the maximum circumferential stress criterion and energy density criteria are mainly consistent with those from experimental results;(b)the mixed mode crack growth rate are higher than the mode I crack growth rate under the same equivalent stress intensity factor.In this paper,the macro and micro morphology of the mixed mode fatigue fracture surface was measured and the failure mechanism of the crack was analyzed.3.The fatigue crack propagation of welded joints in different zones and mixed mode crack propagation in weld zone were simulated based on XFEM.It is found that the errors are within acceptable limits according to the comparison between the XFEM and experimental results.