Buckling of stretched and twisted rubber rods

The coupled nature of the normal load and torque when twisting a cylinder has been a subject of interest to experimentalists and theoreticians since the early measurements of Poynting. When investigating such deformations in soft solids, it is well-documented that sufficient twisting causes a cylinder to deform through a torsional instability. Although many works have investigated these instabilities, their approach often assumes kinking while overlooking the preceding buckling. Here the impact of cylinder aspect ratio and loading conditions on the onset of the buckling are experimentally probed. These measurements are used as a means to examine several criteria for the onset of the torsional instability. It is observed that modeling buckling is essential as the sample becomes shorter. Additionally, cyclic loading measurements show that the buckling point results in a signature path-dependent mechanical response during loading. This result suggests that buckling may be sensed through this mechanical signature. Finally, discussion of the data highlights strategies for probing torsional failure without observing buckling.