Assembly Deviation Modeling Via ANCF And Optimization Of Fixture Scheme For Large Scale Curved Thin-walled Structure

The large scale curved thin-walled structure constitutes the skeleton of aerospace products such as launch vehicle.It is faced with great challenges in dimensional accuracy during assembly.Due to the large flexibility and geometric nonlinear characteristics of the thin-walled structure,the selection of fixture positioning scheme has great influence on its stiffness,which further affects the assembly deviation.However,the selection of fixture scheme depends on many factors such as part geometry and its initial deviation,which makes the work time-consuming and laborintensive.Moreover,the relation between fixture scheme and assembly deviation is unknown,so that the ‘trial and error' method is usually applied for the assembly procedure,which obstructs the efficiency and quality in practical application.Therefore,the optimization of the fixture scheme during assembly process has become the key to the assembly quality and efficiency of aerospace products.In this paper,the assembly deviation model of the curved thin-walled structure is established considering the influence of the clamping constraint and initial deviation.The correlation between clamping constraint and assembly quality is revealed.Optimization of fixture scheme for structures with various diameters is conducted using the PSO algorithm and FEM method,which provides guidance for the fixture scheme design of large scale curved thin-walled structure and is of great significance to improving the assembly precision.The main contents are listed as below:(1)Assembly deviation model of curved thin-walled structure considering the influence of clamping constraintsThe assembly deviation model for different fixture schemes and initial deviation is established,and its accuracy is guaranteed by a curved element based on ANCF and continuum mechanics.The calculation model of assembly deviation is established based on the quasi static hypothesis after deducing equilibrium equations and system constraints under ANCF.(2)Effects of clamping constraints and other factors on the assembly deviationThe effects of clamping constraints,part geometry,initial deviation and part dimension on the assembly quality of thin-walled structures are investigated by numerical simulation.The deviation of scheme II is greater and the nonlinearity of deviation with various geometric parameters is more obvious.The influence of constraints in different directions on assembly quality is various.Deviation mode affects the sensitivity of the assembly quality to fixture scheme;initial deviation value and part dimension affect the changing rate of deviation.The impact of different structural parameters on deviation is various.The deviation of hemispherical shell is smaller than cylindrical shell with less nonlinearity.(3)Optimization of fixture scheme for large-scale curved thin-walled structuresPSO algorithm and finite element method are both introduced to optimize the layout of fixture for curved thin-walled structures.The assembly deviation analysis is performed based on the optimized scheme to verify the effectiveness of the optimization.The influence of structure dimension on the selection of fixture scheme is studied.The optimal number of constraints increases with the part dimension,and the effect of optimized scheme on the reduction of strain energy decreases.When the diameter of thin-walled structure reaches 10 m,the optimized scheme cannot reduce the maximum deviation.However,the overall deformation of structure is significantly improved.