Inflation of plastic tubes: Yielding and strain hardening affect the mode and growth of the bulge instability

Tubes made of elastomeric materials, e.g. rubber hoses, show a bulge instability during inflation wherein a portion of the tube inflates to a larger diameter than the rest of the tube. We conduct an experimental study of, polyethylene tubes undergoing inflation. These tubes develop a non-axisymmetric bulge which evolves to a nearly hemispherical "bubble" on one side of the tube, and eventually ruptures by a circumferentially-aligned crack. This behavior is completely different from the bulge instability of elastic materials such as rubber hoses. Quantification of the strain field shows that the initial bulging is almost completely circumferential, but the axial strain increases rapidly as inflation proceeds and "catches up" to the circumferential strain, thus giving a nearly hemispherical blister. Polyethylene is well-known to show yielding and yielding and strain hardening, and these two factors are implicated in the non-axisymmetric growth of the bulge. Finite element simulations are conducted to illustrate how simple models of constitutive behavior can explain the inflation behavior.