Interference Riveting Induced Residual Stress In Aircraft Lap Joints And Its Influence On Fatigue

The fatigue reliability of riveted lap joints is very important to the structure safety of the aircraft.With the rapid development of aviation industry in China,many aircrafts defined as a new generation are being developed which requires higher quality of the structure and advanced riveting technology.With years of development,the automatic drilling-and-riveting technology was widely adopted rather than the early manually drilling and riveting,which results in more efficient production with greatly improved production quality.However,this technology didn't find its place in Chinese aircraft manufacturing industry because we are in short of sufficient basic knowledge regarding the mechanical properties of the riveted structure and the relating process parameters for our aero materials.To promote the application of automatic drilling-and-riveting technology in China and improve the general level of our aviation manufacturing industry,a comprehensive study of residual stress characteristics induced by interference riveting in aircraft lap joints and its influence on fatigue were presented in the thesis.Major works and advanced achievements are summarized as below:(1)For a comprehensive understanding of the riveting mechanics,several models concerning the relationship between the squeeze force and the driven head dimension were built according to different material flow analysis methods,whilst,in particular,the mechanical model concerning the rivet/hole deformation under the mutual influence of squeeze force and initial fit tolerance was deduced.By carrying out this part of work,theoretical riveting parameters can be given for a force-controlled riveting process.Besides,a quantitative prediction can be made on the influence of the squeeze force and initial fit tolerance on the changed stress characteristics.(2)2D axisymmetric finite element(FE)models were built with the explicit finite element code to analyze the stress/strain in the holed sheet relating to different driven head dimensions and different initial misfit characteristics.Experimental test of micro strains were carried out on the faying surfaces of the riveted sheets which validated the FE results.In addition,one of the theoretical relationships between the squeeze force and the driven head dimension was revised for a good agreement with experimental results.By carrying out this part of work,it demonstrated that a reasonable design of the driven rivet head dimension can create some compressive residual stresses in hoop direction surrounding the hole in the sheet,and the level of the residual stress can be improved by increasing the squeeze force within a certain range.However,the design of initial interference fit between the rivet and the hole might result in an increased tensile residual stress somewhere away from the hole surface.(3)3D finite element models were employed to analyze the stress characteristics in three-row riveted lap joints,while by invoking a user defined element(UEL)during the normal FE analysis the stress intensity factors were calculated for crack propagation analysis which directly takes into account the influence of riveting induced residual stress and the secondary bending effect.By carrying out this part of work,it explained the front shape of the fatigue crack observed in the experiment.Besides,it was found that the plasticity-induced fatigue crack closure phenomenon is influenced by not only the stress ratio but also the crack length.(4)By conduct a series of experiment,fatigue properties including crack initiation and propagation characteristics,fracture characteristics and fatigue lives of riveted lap joints were studied consequent to different riveting process parameters.In addition,the Elber's model of crack closure was validated for riveted lap joints in the presence of residual stress that preexist in the structure based on an equivalent fatigue life method.By carrying out this part of work,it showed the influence rule of different riveting forces,different initial fit tolerances and different rivet dimensions on the fatigue properties of the lap joint,while new crack initiation sites were observed in the sheet other than the fretting induced crack initiation site on the faying surface.Moreover,on the basis of residual stress analysis,the fretting mechanism in the riveted lap joint under secondary bending was proposed to be one of the main factors that account for the change in fatigue lives resulted from different squeeze forces(or different interference level).