Title: Integrated hydrobulging of prolate ellipsoids from preforms with multiple thicknesses

Abstract

The integrated hydrobulging of stainless-steel prolate ellipsoids from preforms with two thicknesses was investigated. The produced ellipsoids were closed with two 16-mm-thick closures and had nominal semiminor and semimajor axes of 89 and 125 mm, respectively. The ellipsoidal preforms comprised eight conical segments inscribed inside the target perfect ellipsoid. The four end and middle segments of the preforms had nominal thicknesses of 0.67 and 0.83 mm, respectively. The hydrobulging of these preforms was explored analytically and numerically and was compared with that of prolate ellipsoids with constant thickness. Two nominally identical ellipsoidal preforms were fabricated, measured, and hydrobulged to confirm the theoretical predictions. The results indicated that varying the preform thicknesses is an efficient method of overcoming insufficient hydrobulging of the ends of prolate ellipsoids in other methods. This can be achieved by reducing the thickness because lower thickness results in higher equivalent stress. The fabrication of the ellipsoidal preforms and hydrobulging of the prolate ellipsoids were reasonably accurate and repeatable. The prolate ellipsoids were slightly outbulged, but the deviations from nominal geometry were small. Relatively large deviations were observed at the ends and weld seams of the prolate ellipsoids; these deviations were attributed to imperfections in the preforms. The thickness distributions of the fabricated ellipsoidal preforms and hydrobulged prolate ellipsoids were nearly uniform. Moreover, hydrobulging instability can be effectively monitored by measuring geometric dimensions, such as axial height. High residual stresses and plastic strains were observed in the hydrobulged prolate ellipsoids. This tensile residual stress due to internal pressure may be beneficial for underwater applications; however, this phenomenon requires further investigation. The segmented boundaries are subjected to stress and strain concentrations because of the bending effect and geometric discontinuities.