Inelastic effects in bulge formation of inflated polymer tubes

When a long elastomeric tube such as a rubber hose is inflated, it can exhibit two limiting behaviors: homogeneous expansion maintaining a cylindrical shape or localized bulge formation. The latter case can then evolve into a “bulge propagating instability” wherein two regions with a large difference in the degree of inflation coexist. Existing theory that assumes hyperelastic behavior can capture both homogeneous inflation and bulge propagation behaviors quantitatively. This talk will describe experiments with polyurethane tubes which show more complex behavior where the tubes inflate in an irregular fashion, sometimes with multiple bulges, which are not explained by current theories. We examined the role of inelastic deformation, viscoelasticity, and strain-induced damage on inflation behavior. We show that both one bulge and multiple bulges may appear depending on the flow rate. The peak pressure prior to bulge growth decreases as flow rate reduces, and unexpectedly, bulges grow slowly over few-minute timescales, not abruptly. Strain quantification by digital correlation analysis of inflation videos shows that different regions of the tube have up to 100% differences in strain. We also show that tubes are left permanently deformed after deflation suggesting plastic deformation. Theoretical predictions assuming that the tubes are hyperelastic overestimate the pressure at which bulges should first appear. Tensile creep experiments show that the tubes respond very slowly to changes in stress, and hence the bulge formation lags behind the pressure within the tube at any instant. It is this slow mechanical response of the tube material that underlies the complex inflation behavior.