Study On Gap Coordination And Shim Compensation In Aircraft Assembly

The assembly gaps between mating interfaces of aircraft parts and components are inevitably produced due to manufacturing errors and assembly deformations.Shims are usually used to fill the assembly gaps to achieve integral joint structure and better load capacity in aircraft assembly.The foundational theory and technogy support are rare for gap control and shim design,thus it is difficult to achieve automatic gap coordination and compensation in digital assembly system.In this dissertation,the gap control method,shim design algorithm,mechanics performance comparison and shimming software development are researched systematically.The recent development of digital assembly technology is comprehensively presented.The main contents,methodology and implication of gap coordination,shim compensation and mechanics investigation are analyzed and expounded.A gap control method based on posture evaluation is proposed,and two-step alignment strategy is applied to initially align the components and then slightly adjust their relative posture to control the gaps.The mathematic definition of small posture transformation(SPT)is given,and then the formula of gap increment with respect to the SPT is deduced.Taking the component posture as variables,and the gap tolerance as constraints,the optimization model for multi-region gaps is established.By SVD and PHR algorithms,the optimal posture is solved to redistribute the gaps initiatively.A method for shim design and optimal selection is proposed for non-uniform gaps in aircraft assembly.Based on seed point and region growth algorithms,the local gap region is segmented,and the section line of the region is extracted by point cloud slicing.Taking the section line as gap profile,the shimming schemes are designed with the constraints of hole margins,shim specifications and gap values.By using Delaunay triangulations,the component surface is reconstructed from local point cloud,and then the finite element(FE)model of the joint structure is established with Johnson-Cook and Traction-Separation material models.Under the tensile tests,the stress,stiffness and damage of the joints with different shimming schemes are analyzed.The selection guidance for optimal shimming scheme is finally given by considering both the mechanics performance and installation convenience.In order to comprehensively study the shimming effects on mechanics performance of shimmed joints,the joint structure of aluminum specimens and various shims are designed.Conducting tensile and damage tests on the joints,the strain distribution,secondary bending,initial stiffness and damage status are obtained.By comparing the experimental results with FE calculation,the FE model is validated.Using the FE simulation method,the stress and damage of joints without shims and with various shims are analyzed under four typical gaps(0.2mm,0.6mm,1.0mm and 1,4 mm).This work explores how the shims affect the mechanics performance of the joints,and may help the manufacturers to optimize the shimming process further.A digital shimming software based on process template is developed.The scanning system,data transfer chain and sofware requirements are introduced.By extracting relative process information from nominal aircraft model,the template is defined and organized as a structured set combined of shimming demands and specifications.Anaysing and comparing the scanning data with shimming templates,the kernel functions of gap-tolerance judgement,gap visiualization,shim design and ER data structure design are realized.Applying Object-Oriented methd(OOM),the shimming software is developed,which improves the data analysis ability and aircraft assembly automation.Finally,the summary of the whole work in this dissertation is given,and some valuable topics to be researched further are also discussed.