Bulge formation during internal pressure testing of viscoplastic tubes

The burst test is one of the most common experiments for measuring the mechanical behavior of a tubular sample and can identify a variety of properties, including the constitutive behavior of the material and the pressure capacity of the tube. Due to the complexity of the sample behavior, care must be taken in interpreting experimental data. It is well known that materials with mechanical behavior independent of strain rate experience a bifurcation at the point of maximum pressure, in which a localized bulge forms at a certain axial location on the tube. As this bulge grows, the pressure in the tube decreases, and the non-bulged portion of the tube does not experience any further plastic deformation. In contrast, this study demonstrates that for tubes with viscoplastic behavior, controlled plastic strain can continue throughout the entire sample long after passing the point of maximum pressure. Finite element simulations and experiments revealed that for viscoplastic zirconium alloy tubes, small non-uniformities in tube geometry and temperature lead to gradual, stable bulge formation that begins in the early stages of plastic deformation. The numerical simulations and experimental results are used to compare the gradual formation of this stable bulge to the sudden bulge formation that occurs at the point of maximum pressure for a tube whose mechanical behavior is independent of strain rate.