KINK RESISTANCE
For the most part, nitinol wires cannot be kinked. To some extent, this design property stems from the increased elasticity cited above, but it is even more a result of the shape of the stress-strain curve. When strains are locally increased beyond the plateau strain, stresses increase suddenly. This causes the incremental strain to partition to the areas of lower strain, instead of increasing the peak strain itself. Thus kinking, or strain localization, is prevented by creation of a more uniform strain than would be realized in a conventional elastic-plastic material.
The first applications to take advantage of this feature were angioplasty guidewires, which must be passed through tortuous vessel paths without kinking. Even very small permanent bends in the wire cause "whipping" and destroy its ability to be steered. The ASDOS procedure described earlier, for example, employs nitinol guidewires to place the catheters correctly. There can be little doubt that nitinol has played a key role in the success of angioplasty procedures.