Experimental Observation of Different Instabilities When Stretching and Twisting Rubber Rods

Stretching and twisting a cylindrical rod is an attractive measurement for probing the failure strength of materials under shear loadings. Making measurements of material strengths in this manner is important to mapping the envelopes needed to develop failure criteria that are critical to advancing our physical understanding of cavitation and yielding in soft solids. Rubbery crosslinked polymer networks represent a class of materials that are highly extensible before failure. This extensibility leads to rubber rods often displaying torsional instabilities before failure, which precludes the ability to map a failure envelope. This work uses a combined tension and torsion tester to probe the instabilities that occur in a stretched and twisted rubber rod before failure. By examining both the load and torque that develop during deformation, it is found that stretched and twisted cylinders undergo both a buckling transition as well as a “knotting” ring transition. The onset of the instabilities is compared to the predictions of established theory. The ability to separate these two behaviors is found to be sensitive to the aspect ratio of the cylinder. Additionally, by imposing cycles of stretching and twisting, the instabilities are found to cause path-dependent loadings which display different forces and torques.