Abstract: An ellipsoidal shell with double generating lines is designed as the pre-form shell, which can avoid the wrinkling occurrence on the equatorial zone during hydro-forming of ellipsoidal shells with the axis length ratio exceeding . In order to analyze the stress distribution and deformation during hydro-forming, experimental research along with simulation of ellipsoidal shells with the axis length ratio 1.5 and 1.7 are carried out. The mechanism of avoiding wrinkling is pointed out by simulation. The sheet materials on the equatorial zone suffer from double tension stress, while the weld seam suffers from latitudinal compressive stress at early stage of deformation. The reason is that the initial dihedral angle between lateral petals would be gradually flatten, which brings a moment and the compressive stress on the weld seam. However, it is not sufficient for causing wrinkling. Stress locus of typical points is illustrated, and the expanding of plastic deformation and thickness distribution are analyzed. Plastic deformation first occurs on the pole and then expands towards equatorial line, and the sheet materials are deformed prior to the weld seams along latitudinal direction. The maximum thickness is located on the pole due to server deformation, accordingly the thinning of the two shells is 10.4% and 16.3% respectively. The sound ellipsoidal shells can be obtained.

Key words: ellipsoidal shell, hydro-bulging, hydro-forming, spherical shell