Design and optimization of loading trajectory for S-skin stretch forming process by simulation

Stretch forming is one of the primary methods in skin forming process. Uniform strain distribution and springback are main factors which affect the precision of air skin. In the article the stretch forming process based on S-skin was analyzed. Firstly the parameters ranges of the loading trajectory were designed through the analytic method. Secondly the initial loading trajectory was optimized through finite element numerical simulation. The optimization processes was performed through FET software integrated with the optimization arithmetic. The motion parameters of jaw and machine's instructions were selected as design variables. Optimization mathematics model was set up which objective is to reduce springback and improve the strain distributes uniform degree. During optimization the maximum main strain and thickness thinning rate of elements were restricted in permissive range. The forming degree of each stage was rational distributed, and the reasonable loading trajectory was founded. The result shows that the reasonable loading trajectory is including pre-stretch, wrap, press and after stretch. After optimization the strain distributes uniformly and the maximum main strain is between 3%∼5%. The maximum stretching rate which appears in the shoulders area is less than 6%. In the concave area in which the insufficiency deforming can be occurred easily the strain achieves about 3%, and the deformation is enough. After optimization the unloading springback is decreased distinctly. The average springback of all elements is 0.47mm which reduces 30% compare with before optimization. The result meets the manufacture requirement.