Study On Optimization Methods Of Pre-joining Processes For A Class Of Skin-stringer Panel Structures

Panels composed of skins and stringers are used extensively as important parts of aircrafts.Panel assembly processes mainly include two stages:pre-joining and drilling-riveting.The pre-joining is a fixed connection that makes the skin and stringers joined on fixture by hundreds of fasteners.In the pre-joining stage,there are large clearances between mating surfaces after positioning and clamping of the skin and stringers due to manufacturing errors,etc,and these clearances greatly impact on the quality of drilling and riveting for panels.The pre-joining has two contributions:determining the relative positional relationship between the skin and stringers and reducing the initial clearances among the mating surfaces of panel to meet the conditions of drilling and riveting.Fasteners of pre-joining after completion of drilling and riveting for panels need to be removed,and then these places are riveted supplementarily.Presently,the pre-joining processes are entirely determined by the experiences of workers,which cause the improper number,location,and sequence of pre-joining points;the poor suppression effect of clearances for panels and the huge manual workload.In theory,the three-dimensional finite element method can be used to make the pre-joining processes reasonably,but the number of pre-joining points and their candidate points are too large,leading to time-consuming,difficult to adapt to engineering application.To solve the above-mentioned problems,employ the method of division of assembly unit to simplify a whole panel into dozens of units,orienting the suppression and flow mechanism of clearances for panels under pre-joining forces,and research the method of pre-joining processes optimization for the panel from three aspects:the fast pre-joining processes optimization for units based on single shipset of panel clearances,the fast pre-joining processes optimization for units based on the method of clearance statistics,and the process correlation caused by clearances flow between units.This will provides solution for improving the quality and efficiency of pre-joining.The specific contents include the followings.In the beginning,this paper elaborates the situation of panel assembly technologies at home and abroad,investigates the automatic drilling and riveting technology for panels,and points out that the pre-joining is one of the difficulties of achieving automatic drilling-riveting for panels and assuring quality of drilling-riveting.And the causes of initial clearances and their adverse impacts on drilling and riveting are revealed by analyzing the pre-joining assembly flow.In the pre-joining process,the skin and stringers are of large size,low stiffness,easy deformation by force,and complex mechanical mechanism.Traditional pre-joining processes often adopt a redundant number of pre-joining points,which lead to large amount of workload and high comprehensive cost.In order to determine the number,location,and sequence of pre-joining points reasonably,study on the change regulation of the clearances for panels under the action of the pre-joining forces,propose a research frame for pre-joining process,oriented to the suppression and flow mechanism of clearances for panels.The pre-joining processes determined by the experiences of workers lead to the different parameters and the unpredictable effects of suppression effect of clearances for panels,etc.And the three-dimensional finite element method is applied to make sure the pre-joining processes for the whole panel,resulting in too many process combinations,time-consuming,and so on.To solve the aforementioned problem,a fast pre-joining processes optimization method for units to suppress the panel clearances effectively is presented,by employing the method of division of assembly unit and the Saint-Venant principle to simplify a whole panel into dozens of units,applying the stiffness of key points to substitute the full stiffness of panel,and considering the variation of stiffness of panels in pre-joining processes.This proposed method may solve the problem of making sure pre-joining processes for units of single shipset of panel.However,with the increase of the production shipsets of aircraft,clearances of each panel are measured and their processes are determined,difficult to adapt to the requirement of high-efficiency assembly in production.Meanwhile,because the manufacturing errors,positioning errors and so on for every shipset of panel are different,these lead to the different initial clearances of panels and the inconsistent pre-joining processes.To solve the above-mentioned problem,based on the above-proposed method,consider the difference of initial clearance distributions of panels,a method of establishing a unified pre-joining process for units of many shipsets of panels is proposed.Firstly employ the random Bezier curves and the method of inferring population with a sample to calculate the random residual clearances after pre-joining quickly;subsequently,based on the evaluation criterion of processes and the constraints,propose a fast pre-joining processes optimization method for units based on the method of clearance statistics.These two proposed methods have been verified by experiments.These examples have shown that:1.These two proposed models can satisfy the small and large sample cases of pre-joining for panels,respectively,and not only save lots of computation time,but also make the panels reduce the number of pre-joining points heavily;2.The method based on the method of clearance statistics not only could be well applied to establish a unified pre-joining process for many shipsets of panel units,but also has saved much measurement workload of clearances.The pre-joining processes for units can effectively suppress the local clearances of panels.However,for the pre-joining processes of the whole panels,the problem of process correlation caused by the clearances flow for panels among units,should be globally considered.To suppress the clearances of panels and avoid too large residual clearances for local units causing crinkling,this paper proposes a pre-joining process method for the whole panels,by considering the clearances flow between units based on the pre-joining processes optimization for units.Firstly take the clearances of panels not retained inside as an aim,choose the clearances flow between units as a constraint,and then establish the process correlation model between units;subsequently,build up the internal process model for a unit to optimize the number,location and sequence of pre-joining points;finally,build up a pre-joining process method for panels including the process correlation between units,based on two aforementioned models and their constraints.The effectiveness of this method has been verified by a computation example.This example has demonstrated that:1.The pre-joining affection of the current unit will lead to that the initial clearance of the adjacent unit has a slight increase,and this will not directly affect the initial clearance of other units;2.Due to the consideration of the clearance flow between units,the proposed method can solve the problem of the pre-joining sequences between units and the determination of processes of adjacent units under the influence of current pre-joining.Finally,the content of this research is summarized,and the further studies of future are also discussed.