Abstract In tube hydroforming, the pressure for forming a small corner is far greater than other positions and thinning or cracking often occurs at the transition region between the corner and a straight side. To reveal the reasons of the local thinning and provide the clues to avoid the defect at the transition region, the stress distribution and deformation pattern of the corner are analyzed by mechanical analysis and numerical simulation. The effect of the friction coefficient on stress distribution is also considered. The material at the transition region much more easily satisfies plastic yielding conditions and produces compressive strain through the thickness. Therefore, severe thinning deformation easily occurs at the transition region. When the ratio of wall thickness to the corner radius is greater than 0.5, the difference of yielding conditions between the straight side and the corner is significant enough to induce serious deformation nonuniformity, even excessive thinning.
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