In this investigation, the clamped portion of copper test specimens is allowed to be drawn slightly into the die throat when hydroformed into axisymmetrical shells. Changing the blank holding load results in different draw-in characteristics. In general, the drawing-in action affects the shape of the shell and also lowers the severity of the deformation and surface area of the deformed shell. Although the nominal thickness uniformity of shells formed with draw-in permitted is improved, the actual thickness is less uniformly distributed. Experimental results also show that, at the beginning of the forming process, the infeeding material due to draw-in is unstretched; but as deformation progresses, stretching of the infeeding material becomes necessary for attaining higher polar heights. The findings of this investigation show that care should be taken in interpreting the test results when the bulge test is used as a formability test. The results also have relevance to many current studies on the “limiting dome height” test for sheet metal.

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