The displacements and strains of a circular specimen subjected to hydraulic pressure are studied and are compared with theoretical results predicted by previous investigators. By proposing a concept called prolateness, which gives an indication of the deviation of a local surface from a perfect sphere, it is found that the shape of the deformed bulge is not as simple as it has often been thought to be—the prolateness of a bulge is never constant at any deformation stage. Results also show that, between the beginning and the end of the forming process, there are the initial, stable and unstable regimes of deformation.

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