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In flexible die forming processes employing semi-solid or elastic materials as pressure-carrying medium, there exist tangential adhesive stress or friction at medium/blank interface, which may improve sheet formability and has been experimentally verified. However, the mechanism behind this phenomenon has not been clarified yet. In order to fill this gap, analytical investigations were implemented in this paper based on previous studies involving through-thickness shear stress. Concretely, the analytical model of axisymmetric shell bulging process considering tangential adhesive stress and the extended M-K model under non-planar stress state were established at first. Accordingly, the relationship between tangential adhesive stress and through-thickness shear strain was deduced together with the relationship between through-thickness shear stress (strain) and sheet forming limit. On the basis of that, the viscous pressure forming (VPF) process was taken as an example to exhibit the characteristic that forming limit changes with tangential viscous adhesive stress and through-thickness shear stress (strain). It is shown that the tangential viscous adhesive stress has positive correlation with through-thickness shear strain which results in increased sheet formability. Tube bulging tests employing viscous medium with different molecular weight as pressure-carrying medium were conducted to give an experimental evidence of the proposed analytical results. [ABSTRACT FROM AUTHOR]