Sequential Two-level Algorithm For Rib Distribution Optimization On Thin Plates Under Stress And Displacement Constraints

Thin plate is widely used in the fields with high demand for structural lightweight,as aviation,aerospace,automobile,shipping and so on.However,the thin plate has the disadvantages of low structural strength and easy deformation.The structure performance can be greatly improved and the weight can be slightly increased by reasonably placing rib on the plate surface.The rib distribution optimization on thin plates is to obtain a distribution form of rib by reasonable arrangement of rib which can meet the requirements of structural performance with the least material.That is to say,the lightweight design of rib reinforcing is realized at the same time as meeting the requirements of structural performance.The existing methods of rib distribution optimization on thin plates are difficult to solve the problems with a variety of constraints,such as stress and displacement.Aiming at this shortcoming,an optimization method named Sequential Two-level Algorithm with Equal Discrete Distance between 0 and 1 is proposed in this thesis.This algorithm is based on the plate-beam discrete model and the structure optimization method of discrete variable.The analysis of examples shows that the algorithm can effectively solve the problem of the rib distribution optimization on thin plates under stress and displacement constraints.(1)Establish the optimization model of the Sequential Two-level Algoritkhm with Equal Discrete Distance between 0 and 1.Firstly,a height factor of each stiffened element is taken as the design variable,and the continuous interval[0,1]of the design variable is dispersed by equal distance.Then,the local stress constraint conditions is constructed on the Mises Yield Criterion,at the same time,the global displacement constraint condition is constructed by unified constraint function.Finally,with the aim of minimizing the total mass of rib,the optimization model of the algorithm is established.And the model is transformed into a two-stage optimization model based on the idea of solving stress and displacement constraints at different levels.(2)Expound the principle and program realization of the Sequential Two-level Algorithm with Equal Discrete Distance between 0 and 1.Based on ANSYS APDL language,firstly,the finite element plate-beam discrete model is established with the design variables of each stiffened element are initialized,and this model is taken as the initial structure of optimization.Then the one-dimensional search method is used to solve the first level of the algorithm to deal with the local stress constraints.Finally,the relative difference quotient method is used to solve the second level of the algorithm to deal with the global displacement constraints.(3)Solve the key problems and discussing the reasonable value of the parameters in the algorithm.The problem of local optimal solution is solved by introducing the element evolutionary adjustment coefficient in the first stage optimization process,and the automatic optimization subroutine of this parameter is added to the main program of the algorithm to get its most reasonable value.In the second stage optimization process,when the structure and the load are symmetrical,the variable connection technique is used to deal with the irregular distribution of rib after the second stage optimization.Finally,the influence of different step size in search(discrete spacing)of the design variable on the optimization results is discussed.(4)Doing the distribution optimum design of rib for a hydraulic gate with the Sequential Two-level Algorithm with Equal Discrete Distance between 0 and 1.The structural performance and weight loss effect before and after optimization are compared to verify the effectiveness of the algorithm.