Softening effect on thick-walled tube inflation and bulging instability

Inflatable tubes often consist of rubber-like materials that exhibit stress softening over cyclic loading, referred to as the Mullins effect in the literature. A theoretical model is proposed to study the stress softening effect on the inflation of a thick-walled hyperelastic tube and the onset of an axisymmetric bulging. The three studied pre-loads (tension, torsion, inflation) create different softening distributions along the tube wall thickness. The resulting predicted bulging instability onset fluctuates, exhibiting a memory-like effect of the load type and intensity. A finite element analysis validates the theoretical analysis and evaluates the consequence of pre-softening loads on the post-bifurcation evolution. The theoretical analysis provides new elements for the design, control, and failure prevention of elastomer tube elements under inflation.