Research On A One-step Fast Algorithm For Bus Rollover Collision Based On Total Strain Theory

With the rapid development of automobile industry,more and more conveniences have been supplied to normal people.Meanwhile,the road traffic accidents frequently occur,and the bus rollover accident is of the most severity,which has become one of the most important society issue that are common concern of the whole society and needed to try hard to be solved.For the research of rollover safety of bus structure collision,there are two manners:bus rollover test and computer simulation analysis.For the sake of long cycle,complex implementation process,high technical difficulty,high cost etc,bus rollover test can not satisfy the increasing fierce market situation.At the same time,most business software of structure collision analysis which mainly adopts the centre difference schemes are based on the explicit integral algorithm of nonlinear dynamic.However,the improvement of computational efficiency is seriously influenced since the computational time is too long,and the analytical result is seriously influenced by the experiences and knowledge level of the engineers.Concerning the similarity of large plastic deformation characteristic of bus rollover collision with sheet metal forming,the idea "one-step finite element method" of sheet metal forming based on total strain theory is introduced into the simulation of bus rollover collision.Relying on the National Natural Science Foundation of China project "Fast Simulation Algorithm of Plate Forming of Large Step Size High Precision Based on Quasi Total Strain Theory",a one-step fast algorithm of bus rollover collision based on total strain theory is proposed.The detailed research is taken from the three sections below:the computational principle of the original algorithm,the research of element model,and the case study of the proposed algorithm.Then the improvement for the original proposed algorithm is carried out from four parts below:the initial solution prediction,the element model of the algorithm,and the iteration efficiency of the algorithm.The final algorithm can be used in the early time of bus structure design.The computational accuracy can be basically guaranteed,and the simulation efficiency can be improved significantly.The rollover safety performance of bus structure can be quickly evaluated,and the develop cycle of bus product can be reduced.Some necessary conditions can be supported for the further study of sensitivity analysis of bus rollover safety,parameter optimization,and the research of topology optimization algorithm.Meanwhile,the use of nonlinear implicit theory can be explored in the collision simulation,and the preliminary basis can be established for the research of more complicated fast algorithm of frontal collision.According to the research thought above,several parts of contents are completed below:First,the establishment of the theory system of the one-step fast algorithm for bus rollover collision based on total strain theory.Based on the characteristic of bus rollover collision process,a one-step fast algorithm for bus rollover collision based on total strain theory is proposed,referring to the one-step fast simulation idea of sheet metal forming.Firstly,introduce the computational principle of the algorithm in detail,which includes the simplification of computation process,the hypothesis of calculation model,the plastic constitutive relations,the prediction of initial solution based on the maximum inertial load and energy conversion principle,the collision contact judgment and modification method,and the equilibrium iteration of generalized imbalanced forces of initial solution etc.Secondly,develop the FORTRAN program for the proposed algorithm,and describe the detailed implementation process of the program.Second,the research on the element model of the one-step fast algorithm for bus rollover collision based on total strain theory.Firstly,wrap modification improvement on the wrap problem of four-node thin-shell element caused of big deformation of partial structure during the rollover collision process of bus structure.Secondly,based on the wrap modification of the element,the space four-node DKT combined thin-shell element model of the one-step algorithm for rollover collision is established by applying the four-node thin plate element base on Discrete Kirchhoff Theory(DKT)and the four-node membrane element.Finally,based on total strain theory,the nodal generalized internal forces of the improved four-node DKT combined thin shell element are computed,which are used in the process of the equilibrium iteration of generalized imbalanced forces of initial solution.Third,the case study of the one-step algorithm for bus rollover collision.Regarding a typical 12-meter bus body section developed by an enterprise as the analytical object,the final deformation pattern of the structure,the deformation value of both pillars of the two closed loops,and the element simulation efficiency are regarded as the evaluation index.The computation result of the one-step algorithm of rollover collision is compared with that of LS-DYNA and rollover test,and the practical value of engineering application of the proposed algorithm system is verified.Four,the improvement research on the one-step algorithm for bus rollover collision.To improve the simulation efficiency of the proposed algorithm one step further upon basically guarantee the computational precision,the improvement research is taken for the original algorithm on the aspects that are initial solution prediction,element model,and iteration efficiency.Firstly,the improvement research on initial solution prediction.A prediction method of initial solution based on the interpolation principle of nodal coordinates of standard template of structure deformation is proposed to improve the original algorithm.The standard template of the deformation of bus body structure before and after is built by analyzing the several results of the structure deformation of bus rollover collision.According to the corresponding mapping relationship of the structure deformation before and after in the standard template,the initial solution needed by the proposed algorithm can be quickly obtained by using interpolation principle of nodal generalized coordinates.The effectiveness of the proposed prediction method of initial solution is verified by carrying out the typical bus body section model mentioned before.The computation efficiency can be improved in some degree with the computational accuracy being basically guaranteed.Secondly,the improvement research on element model.Based on the equilibrium principle of element nodal internal forces and bending moments,a four-node membrane element model with considering the bending modification is proposed with considering accuracy and efficiency.The improved space DKT thin shell element of the original algorithm is replaced by the proposed element model.The element model revises the shortage that the bending deformation and the computation of nodal normal forces can not be considered by normal four-node membrane element.The influence of element bending deformation is simulated without gaining the nodal freedom.The simulation efficiency of the algorithm is greatly improved.The engineering calculation effect of the proposed element model is verified by carrying out the typical bus body section model mentioned above.The computation efficiency can be improved in some degree with some sacrifices of the computational accuracy.Thirdly,the improvement research on the iterative efficiency.Improve Newton-Raphson iteration of the original one-step algorithm of rollover collision by introducing the improved gradient method at first.In the process of the equilibrium iteration of generalized imbalanced forces of initial solution,compute the generalized imbalanced forces of each node and its gradients by defining the "2" norm A of the generalized imbalanced forces.Perturb the generalized coordinates of each node,and look for the descent direction of each generalized imbalanced force gradient.Then the computational result iterates to convergence without computing the stiffness matrices[KT(u)]integral structure.The local perturbation method is introduced into the equilibrium iteration process of the generalized imbalanced forces of the nodes based on the improved gradient method.Perturb the partial nodes of all nodes every time only.The computational efficiency of the proposed algorithm is promoted on step further.The actual application effect of the improved algorithm is verified by carrying out the typical bus body section model mentioned before in the end.The computation efficiency can be improved signaficantly with the computational accuracy being basically guaranteed.Fifth,the verification for the simulation result of the final improved algorithm.Improve the original one-step algorithm of rollover collision with the prediction method of initial solution based on the interpolation principle of nodal coordinates of standard template of structure deformation,the four-node membrane element model with considering the bending modification,accuracy and efficiency,and the improved gradient method with local perturbation method.The typical bus body section mentioned above is regarded as the analytical object,and the actual application effect of the final improved algorithm is verified.And the simulation efficiency is greatly improved with basically guaranteeing the computational accuracy.