Abstract: Through the analysis to the tube bulging mechanism with the theory of thin shell, the demarcation related with the tube size and the forming length used to distinguish different tube bulging patterns is found, and the mechanical models of different patterns are given accordingly. All of these provide a reference basis to the application of the thin shell theory in tube forming technology and the accurate prediction to the forming pressure. Research shows that when the bulging length is shorter than certain threshold level associated with the geometric parameters, the support boundary of the bulging area will impact the forming pressure value, and the influences are gradually vigorous along with the shortening of the bulging length. While when the bulging length is longer than the threshold level, the forming pressure will no longer subjected to the influence of the bulging length. The numerical simulation verifies the differences of the two patterns, and confirms that because of the stiff ends assumption of the theoretical model, the calculated value is larger than practical conditions. The typical parts, die structure and technological process matched with the theoretical models are designed, from which the test data of the forming pressure corresponding with the deformation shape are obtained, and by comparing it with the theoretical models, the difference and the causes of errors are discussed.

Key words: tube;internal high pressure forming;bulging;deformation pattern