Research On Defects And Loading Trajectory Optimization In Stretch Forming Process Based On Distributed Displacement Loading

Manufacturing is the pillar industry of the national economy, its technical level is a very important symbol to measure a country’s comprehensive national strength. Skin parts are usually used in the aviation industry, and theirs main forming method is stretch forming. In recent years, with the increasing pursuit of consumers on product performance and personalization, the demand on the surface quality and forming accuracy also become higher. So the stretch forming process should be developed to the trend of high efficiency and flexibility. In the traditional stretch forming process, rigid gripping jaw was used and the forming defects such as wrinkle, crack, severely local deformation and low forming accuracy may usually occur when the parts with complicated shape are formed.The stretch forming process based on distributed displacement loading is a new method for forming three-dimensional surface parts, the stretch load is applied at a series of discrete points on the two ends of sheet metal. The gripped sheet metal can vary with the die surface, if loading trajectories of the discrete points are reasonably designed, and satisfied forming results can be obtained with small transition zone length, then the material utilization can be improved. In this paper, traditional stretch forming process and stretch forming process based on distributed displacement loading were comparatively analyzed, and wrinkle, crack, deformation uniformity and forming precision of formed parts were discussed respectively. And experimental research of precision of formed surface was also made.The main contents and conclusions of this paper are as follows:1. The characteristics and deformation processes of sheet metal in traditional stretch forming and stretch forming process based on distributed displacement loading were analyzed. Finite element model and material model were built based on ABAQUS, the contact settings between the various components and boundary treatment in simulation were also introduced. The contact states of sheet metal and die corresponding to different transition zone lengths in traditional forming process had been discussed, and the results indicated that increasing the transition zone length can make sheet metal and mold contact better. By numerical analyzing the wrinkle, crack defect in two different forming processes, it can be drawn that the forming defects such as stress concentration, crack and wrinkle may usually occur, because rigid gripping jaw was used in traditional stretch forming process. If loading trajectories of the discrete points are unreasonably designed in the stretch forming process based on distributed displacement loading, the same defects will occur and even worse forming results can be obtained. The basis of loading trajectory design was given by exploring the influence of the sheet width on the longitudinal strain in curved surface forming process. And the analytical expressions of displacements at each discrete load point were given for any given target profiles. To investigate the wrinkle, crack, deformation uniformity and forming precision of formed parts corresponding to different forming processes, four different loading modes were designed.2. The theory of dispersion instability and concentration instability of sheet metal were studied, and the strains when instability occurs were given in different conditions. The influence of anisotropy of thick on the deformation stability was discussed, and the plastic stress-strain relationship and the strains when instability occurs were also obtained, then the FLD was drawn. By numerical analyzing on sheet metal forming performance, FLD obtained by simulation was shown. The danger zone of crack was determined by studying deformation process of the sheet metal. To analyzing the safety margin from crack, the strains of danger zone of crack on the different formed parts were extracted. It can be founded that minimum elongation and varying displacement loading mode, can effectively suppress the generation of crack defects, and present a better ability to form surface parts than traditional and equal elongation loading mode. In addition, the crack tendency of sheet metal corresponding to the parts with different curvatures was comparatively analyzed, and the results showed that the greater of the curvature of the sheet is, the larger of probability of crack will be. Therefore, in the actual production, in order to effectively avoid crack defects, the loading trajectory should be designed reasonably.3. The criterion of the wrinkling defects was given, by studying the theory of compression instability. The contacting process of sheet metal and die corresponding to positive and negative Gaussian curvature curved surface were analyzed comparatively, in traditional stretch forming and stretch forming process based on distributed displacement loading. Through the numerical analyses on the stress of winkling of spherical and saddle-shaped part formed with different processes, it can be concluded that if the loading trajectory is designed reasonably in forming process based on distributed displacement loading, sheet metal deformation process can be controlled, and transverse compressive stress can be reduced, compression buckling can be avoided, so as to inhibit the generations of wrinkling defects.4. Based on the numerical simulation study on deformation uniformity and forming accuracy of parts formed with four different loading trajectories, it can be founded that the deformation uniformity of the part formed with traditional loading mode is the worst, the forming error is the largest; while, the deformation uniformity and accuracy of the part formed with varying displacement loading mode are the best. Through the numerical analyses on the forming processes of spherical and saddle-shaped parts with different number of loading points, it was concluded that the more discrete loading points are, the more uniformly deformation distribution of the sheet metal will be. The uniformity of the sheet metal deformation and forming accuracy were used as the test index, and the orthogonal test was design for optimization of the loading trajectory, the correctness of the optimization results had also been verified.5. The measuring apparatus of three-dimensional surface was introduced specifically, and specific process of surface measurement was given, in addition, the issues that require attention in measurement process were also analyzed. Different types of curved surface were formed with variable displacement loading mode, it can be easily founded that the experimental surfaces are smooth and have no wrinkling defects. The portable 3D measuring instrument was used to scanning the shapes of test pieces. Through analyses on the forming errors, it can be concluded that if the loading trajectory design was reasonable, high accuracy can be kept when different surfaces were stretch formed based on distributed displacement loading. Then, stability and reliability of the forming process were validated.