| Flanging and hemming are two common sheet metal bending processes, which are widely used in automobiles to join inner and outer closure panels for functional requirements, appearance, or safety considerations. During these processes, the sheet metal experiences multi-stage small radius bending deformation yet with insufficient tool constraint. Therefore, the manufacturing defects may arise. Dimensional inaccuracy, surface defects and failures are the critical concerns. The dimensional inaccuracy such as creepage or growing could cause problems in the assembly stage and influence the fitting quality. The surface defects such as recoil and warp could damage the appearance of a product. Wrinkling and tearing cause failures of the process.; In this dissertation, the mechanisms of the hemming process and root causes of the hemming defects have been systematically studied for the first time. The finite element model is developed to quantify the influences of the geometry, material and process variables on the hemming quality indices, creepage/growing, recoil and warp. Creepage/growing is mainly caused by the geometry constraints. Reverse bending, localized buckling and springback are the fundamental mechanisms that give rise to surface warp and recoil. The pre-hemming target ending position is proposed based on a trade-off between stability of creepage/growing and minimization of recoil and warp.; The methodology of hemming process optimization is proposed using finite element simulations and design and analysis of computer experiment method (DACE). By using DACE method, the most representative sampling points can be selected for a computer simulation. Optimization can be conducted on the obtained predictors instead of time-consuming computer simulations.; For the curved flanging and hemming, wrinkling and splitting are important issues that must be addressed. Shrink and stretch flanging with surface contours are studied. Analytical models to predict circumferential strain are derived for both shrink and stretch flanging based on the uniaxial stress assumption, which are verified by experiments. The wrinkling criterion for the convex surface-straight edge flanging is established.; As a summary, the flanging and hemming processes are investigated from mechanistic point of view. Based on the knowledge obtained from this study, a new hemming process design guideline can be established. |
