The nonlinear characteristics of superelastic phase transition in nitinol alloys mean that the traditional fatigue life theory is difficult to apply. Martensite/mother phase and its role in the mechanism of nitinol fatigue life are still not clearly understood. Fatigue life is still one of the most discussed but least understood aspects of nitinol alloys. The FDA requires that the fatigue life of intravascular stents exceed 400 million times,which means that it is essential to better understand the factors affecting fatigue life and the mechanism of crack germination and growth.
Some aspects of the fatigue life of nitinol alloys do seem to have some rules about them. Surface conditions, inclusions, and plastic deformation do affect crack propagation in stage 1. The crack propagation stage of fatigue is where the most bruises occur.Many factors must be considered when studying fatigue crack propagation in medical applications of nitinol alloys. In Vitro fatigue life testing of nitinol alloys usually involves cyclic loading in strain-controlled or stress-controlled loads. Compared with other alloys, nitinol alloys exhibit excellent fatigue properties at high strain levels. A complicating factor in evaluating the fatigue life of practical applications using laboratory test data is that the actual load conditions in the body may be different average strain, average stress and compliance of the surrounding tissue.
To evaluate the fatigue life of the alloy in terms of alternating, strain amplitude and average strain, the common method is to construct the Goodman diagram. The Goodman relationship is linear.The greater the average strain, the smaller the alternating strain required for fatigue failure. Interestingly, nitinol alloys seem to exhibit a nonlinear Goodman relationship. Other abnormal fatigue behavior related to the average strain is that the fatigue life increases significantly with the increase of the average strain. Fracture surface and cross-section analysis means that these unusual aspects of nitinol fatigue behavior are related to high fault density, internal stress, stable martensite and micro-cracks. Other data also support the importance of microstructure and martensitic/parent relative nitinol fatigue life.