Study On Pre-joining Process And Riveting Deformation In Automated Drilling-Riveting Of Aircraft Panel

Automated riveting machines are widely used in the digital assembly process of aircraft panels.Before the automated drilling-riveting process,the skin and stringers are temporarily fastened by a certain number of pre-joining fasteners on the dedicated fixture to determine the relative positions of the parts and to reduce the initial clear-ances between the mating surfaces.At present,the determination of the pre-joining schemes mainly relies on worker’s experience.Lack of theoretical guidance usually leads to excessive number and inappropriate positions of pre-joining holes,leading to low assembly efficiency and unstable residual clearance control.Meanwhile,exces-sive clearances and low rigidity will easily lead to vibrations during the drilling and riveting process.In the subsequent automatic drilling and riveting stage,rivets create a pattern of circumferential compressive stress fields,and a long line of such stress patterns usually causes the dimensional growth of the assembly,primarily in the lon-gitudinal direction,and can cause an elongated part to deflect along its length.The overall distortion of the riveted structures affects the size control of the panel assem-bly and the coordination with other components,and may lead to the repair process and the potential forced assembly in the subsequent assembly process.This paper makes an in-depth theoretical study on the above process planning problems during the automatic drilling-riveting process of aircraft assembles,and provides relevant theoretical support and solutions for improving the quality and efficiency.The main research contents of the dissertation are as follows:In Chapter 1,the domestic and foreign development of aircraft digital assembly technology is introduced,and the research status of the key techniques in the pre-joining and automated riveting processes are discussed.Based on the needs in practical engineering,the significance and content of the research are expounded.In Chapter 2,taking the distributions of the initial clearances and the stiffness matrices as inputs,an equivalent gap assembly model is proposed on the basis of the mechanism of variations during the pre-joining process.By retaining the geometric and mechanical information of the key features of the skin panel,the calculation of residual clearances is transformed into the minimization of the potential energy of the concerned equivalent model represented with key features only.Subsequently,with the consideration of the residual clearances,the permissive clearances and the number of the pre-joining holes,the pre-joining schemes are optimized using the Pare-to-optimal based genetic algorithm,providing workers a standardized operation in the panel assembly process.In Chapter 3,aiming at the influence of the pre-joining schemes on the dynamic characteristics of the skin-stringer panel assemblies,an analytical study is perfonned to analyze the modes of single pre-joining unit under different pre-joining schemes.The geometric shape of the equivalent beam element is established according to the equivalent gap assembly,and the mechanism of the pre-joining connection is imple-mented through the impedance coupling method.Afterwards,the natural frequencies of the pre-joining units can be obtained while the transfer matrix of the local struc-tures approaching zeros.The influence of residual clearance and local contact stiff-ness on the dynamic characteristics of the system and the relationship between the external excitation force and the pre-joining scheme selection under the common ma-chining parameters are discussed,which lays a certain theoretical foundation for the further study of the drilling and riveting process planning for the aircraft panel.In Chapter 4,based on the local-global method,a prediction method for the de-formations of the riveted panel assembly.A simplified analytic study is performed to analyze the deformations of single rivets with the consideration of the barrel-like shape of the driven head and the through-thickness variations along the rivet shank.Afterwards,the displacement field around the connection area have been calculated for the development of a local equivalent unit.The simplified model is modeled with temperature and thermal expansion coefficients with corresponding unit radius,and embedded into the global-level model for the prediction of the overall distortions of riveted sheets.Finally,experimental tests are designed and carried out for the valida-tion of the proposed method on an automatic riveting machine developed inde-pendently.Finally,in Chapter 5,the research content of the dissertation is summarized,and the advanced topics to be researched further are also discussed.