Further assessment of the plastic instability of thin-walled tubes in double-sided hydro-bulging process with verification experiments

Abstract

An interesting process of double-sided tube hydro-bulging was proposed to provide a beneficial three-dimensional stress state in the deformation zone of the tube, so as to delay the occurrence of bursting on the tube. Previously proposed forming limit theory models with consideration of through-thickness normal stress found that the external pressure could lead to a deferred occurrence of plastic instability, but it has not been confirmed experimentally. In this article, the tensile plastic instability of tubes under double-sided pressures is further assessed based on the classical plastic instability theory. However, it is seen that the occurrence of plastic instability has not found delay when high external pressure was exerted on the outside surface of tube simultaneously, which is opposite to the previous forming limit models. In addition, an experiment investigation about double-sided tube hydro-bulging is conducted to verify the theoretical results, and the experimental results show that the external pressure has hardly any influence on forming limit strain of the 2A12 aluminum alloy tube before the occurrence of necking. Moreover, the contradiction between our results and the previously proposed forming limit theory models is that the previous models ignored the thickness item in the equilibrium equation.