Research on deformation and stress in hydroforming process of an ellipsoidal shell without constraint

Abstract

In order to the study the deformation of die-less hydroforming of ellipsoidal shell under the condition of without restricting the elongation of short axis, experimental research on hydroforming of an ellipsoidal shell with the initial axis length ratio 1.8 was carried out to analyze the effect of internal pressure on the variation of shell shape, volume, and wrinkling. The results show that as the internal pressure increases at the early stage of deformation, the short axis elongates; correspondingly, the volume varies slightly; wrinkling occurs on the equatorial plane when the internal pressure is up to 1.0 MPa, and it becomes severe when the internal pressure is 2.2 MPa; as the internal pressure continuously increases, the long axis begins to shrink rapidly, but the shell volume has a very tiny change within the range of internal pressure 2.2–3.5 MPa; when the internal pressure is 3.8 MPa, the wrinkling is eliminated completely, and the shell volume suddenly increases; when the internal pressure is up to 5.5 MPa, the equatorial plane is no longer shrinking but expanding and the out-of-roundness approaches zero; finally, when the internal pressure is 6.8 MPa, the ellipsoidal shell with the axis length ratio 1.06 is obtained, and the variation of long axis, short axis, and volume are −2.9, 64.6, and 52.7 %, respectively. Simulation was also carried out simultaneously to analyze the stress locus of typical points. It has indicated that the compressive stress in the circumferential direction is the reason of wrinkling occurring; accordingly, the tensile stress in the circumferential direction is the reason of wrinkling disappearing.