Study On The Model Simplification Methods Of Aerospace Semirigid Shell Structures

Semirigid shell structures have been widely used in launch vehicles for their high specific strength and specific rigidity.The complexity of the launch vehicle model often leads to excessive calculationtime,therefore,it is of great importance to simplify the model.For static analysis,with regard to skin-stringer structures and grid stiffened structures,an optimization method based on equivalent stiffness and a property equivalent method based on asymptotic homogenization theory are proposed respectively.Stiffness error will appear when large diameter skin-stringer structures are converted into beam-shell mixed structures.Adopting size optimization method,not only can the model be updated,but also geometry sizes of the simplified model will be settled.As for periodic grid stiffened structures,the stiffness matrixes of unit cells can be obtained by using improved asymptotic homogenization method(NIAH),using that a simplified model which has the same mechanical properties as the original model's can be got.The static simplified models obtained by the two methods can reduce the computation amount,in the meantime,they have high precision tensile,compressive,bending and torsional stiffness properties.Fordynamic analysis,the dynamic model simplification method which is suitable for slender structures has been improved when applied to solid models.Based on the plane section assumption of the beam theory,it will cause stiffness changes when the displacement of FEM nodes in each cross section is approximated by the motion of centroid of the cross section through the displacement transformation matrix.The model's degrees of freedom will increase by adding the number of cohesion points for each cross section so that the calculation error of the simplified model got by the localized base vectorscan be reduced.Consequently,the simplified model may have high computational efficiency as well as frequency accuracy.In addition,it offers a way to simulate the liquid propellant in a rocket tank's dynamic model.Results of illustrative examples indicate that,while improving the computational efficiency,the displacement error is within 5% and the maximum frequency error is less than 10% between models.The results verifythe effectiveness of thesemethods.