Kink instability of a highly deformable elastic cylinder
  When a soft elastic cylinder is bent beyond a critical radius of curvature, a sharp fold in the form of a kink appears at its inner side while the outer side remains smooth. The critical radius increases linearly with the diameter of the cylinder while remaining independent of its elastic modulus, although, its maximum deflection at the location of the kink depends on both the diameter and the modulus of the cylinders. Experiments are done also with annular cylinders of varying wall thickness which exhibits both the kinking and the ovalization of the cross-section. The kinking phenomenon appears to occur by extreme localization of curvature at the inner side of a post-buckled cylinder. 
Apurba Lal Das and Animangsu Ghatak