Abstract

In this thesis, it mainly focuses on the blank design for the precision stretch forming process of aircraft skin. According to the industry survey and literature review, in the current aerospace industry, the most popular manufacture method for aircraft skin is that the gripper jaws of stretch forming machine hold two ends of the flat blank sheet, stretch and wrap it onto the die to form the desired shape. In this process, the flat blank to be deformed is usually an intact blank sheet with identical thickness. After the deformation, milling process is performed on the curved panel. However, the quality of the curved sheet metal is relatively difficult to be guaranteed since the process of stretch forming would introduce the machining accuracy problem in the milling process.
To improve the quality of the formed part, a new approach is proposed in this thesis for the aircraft skin manufacture. By employing the designed flat blank with pockets pre-machined, the stretch forming process is conducted to form the designed aircraft skin. Comparing with the traditional method that the pockets are machined after the blank deformed, this approach relatively increased the accuracy of pockets positions and shapes and it eliminated the machining difficulty on the curved surface. To study the feasibility and reliability of this method, the commercial tools of CAD software CATIA and FEA simulation software were utilized for research, in which the CAD data of the FEA simulation output was analysed by the CAD software and then input the modified data to FEA again.
To precisely determine whether the shape and position of the pocket features have met the requirement, the major clue is to evaluate the deformed features in X, Y and Z three directions respectively. Once the formed part is coincidence with the correspondence designed features in these three directions, it means that the designed flat blank could be adopted for stretch forming. To consider the deviation in Z direction, it could be converted to the problem of springback value minimization in Z direction. Due to the large number of factors that influence the springback, optimization method is employed to integrate those possible factors and find the optimal solution. For the deviation in X and Y directions, they are studied through the projection of the profiles on the XY plane. By using shape sensitivity method that judging the influence of two designed blanks offset on the formed pockets profiles, the previous designed flat blank is updated. Comparing the results of different methods, the shape sensitivity method showed a result of efficiently decreasing in the iteration numbers of flat blank design modification and reduction in the shapes deviation between the formed pocket profiles and the target pocket profiles.