Determination Method And Experiment Verification Of Knockdown Factor For Large Scale Cylindrical Shell

With the development of heavy-lift launch vehicles,the diameter and diameter-thickness ratio of primary structure are significantly increased,and imperfection-sensitivity also increases.This makes the accurate prediction of Knockdown Factor(KDF)directly affect the lightweight design of cylindrical shell.In this dissertation,the measurement and expression methods of geometric imperfections were introduced,then developed two kinds of finite element model reconstruction method,and the numerical analysis methods of axial load capacity was systematically studied based on different imperfection approximation method.Finally,the correctness of the numerical analysis methods were verified by three experiments,especially the correctness and advancement of the Worst Multi Perturbation Load Approach(WMPLA).The main works of this dissertation are given as follows:(1)The measurement and expression methods of geometric imperfections were introduced.Due to the limitations of the viewing angle,the morphology of a cylinder must be measured several times,and then spliced as a whole.This paper described a variety of measurement methods,and compared the splicing error.The results shown that the multi-lens shooting method has a higher measurement accuracy.In addition,this paper proposed a geometric imperfection extraction method based on quadratic transformation,and introduced a imperfection expression method based on double Fourier series.(2)Two kinds of finite element(FE)model reconstruction methods were compared.The geometric imperfections can be introduced by means of FE model modification method based on scatter points and Fourier series function.But the scatter points method involves two-layer traversal of grid nodes and data scatter points,so it is very time consuming.While the Fourier series method only involves a traversal of the grid node by fitting the data scatter points into Fourier series,thus greatly improving the computational efficiency.(3)The load capacity of cylindrical shell was analyzed by a variety of imperfection approximation methods.Taking 1m diameter cylindrical shell as an example,the load capacity was calculated numerically considering eigenvalue mode imperfection,measured imperfection,Single Perturbation Load and Worst Multi Perturbation Loads.Compared with the perfect model or theoretical solution,the load capacity obtained by imperfection approximation methods has a large reduction.Among them,the result obtained by WMPLA was relatively accurate and robust.Although the calculation result of measured imperfection approach was accurate,measured imperfection approach cannot be used in the design stage of cylindrical shell because measured imperfections are uncertain imperfections.(4)Three experiments were carried out to verify the correctness and advancement of different imperfection approximation methods.For 1m diameter cylinder,a set of high precision test configuration system was designed.The measurement system was constructed using electrical measurement equipment,optical measurement equipment,et al.And buckling tests of cylindrical shells with the initial manufacturing imperfections,single plastic dimple,worst multi perturbation load were planned and carried out.Test results verified the correctness of numerical results,and confirmed the correctness and advancement of WMPLA.