M. R. Mitchell

Fatigue and durability of Nitinol stents

Nitinol self-expanding stents are effective in treating peripheral artery disease, including the superficial femoral, carotid, and renal arteries. However, fracture occurrences of up to 50% have been reported in some stents after one year. These stent fractures are likely due to in vivo cyclic displacements. As such, the cyclic fatigue and durability properties of Nitinol-based endovascular stents are discussed in terms of an engineering-based experimental testing program. In this paper, the combined effects of cardiac pulsatile fatigue and stent-vessel oversizing are evaluated for application to both stents and stent subcomponents. In particular, displacement-controlled fatigue tests were performed on stent-like specimens processed from Nitinol microtubing. Fatigue data were collected with combinations of simulated oversizing conditions and pulsatile cycles that were identified by computer modeling of the stent that mimic in vivo deformation conditions. These data are analyzed with non-linear finite element computations and are illustrated with strain-life and strain-based constant-life diagrams. The utility of this approach is demonstrated in conjunction with 10 million cycle pulsatile fatigue tests of Cordis SMART Control((R)) Nitinol self-expanding stents to calculate fatigue safety factors and thereby predict in vivo fatigue resistance. These results demonstrate the non-linear constant fatigue-life response of Nitinol stents, whereby, contrary to conventional engineering materials, the fatigue life of Nitinol is observed to increase with increasing mean strain.

Cyclic deformation, fracture, and nondestructive evaluation of advanced materials

This book covers the proceedings of Cyclic Deformation Fracture and Nondestructive Evaluation of Advanced Materials. Topics covered include: the cyclic deformation and fracture of advanced metallic, ceramic and polymeric monolithic, and composites, as well as methodologies for nondestructive evaluation of these same material systems. Phase changes occurring at high temperature and their influence on smooth and notched fatigue behavior; contributions of matrix cracking, fiber bridging and pull out, and their effects of crack propagation are explained. Experimental methodologies and techniques for these difficult to test materials as well as short/long fatigue crack propagation threshold behavior; modeling of crack growth resistance in ceramic and ceramic matrix composites.

Fatigue to Fracture: An Informative, Fast, and Reliable Approach for Assessing Medical Implant Durability

This paper compares the differences between the “test to success” and “test to fracture” paradigms for medical implant durability assessment. It recognizes that test to success can be one of the tests within test to fracture, in addition to a series of shorter cycle, higher force/deformation tests. In addition, by “testing to fracture,” one can validate the fatigue analysis capability, learn the consequences of in vitro fractures, and provide information that can either be used for product improvement or for clinicians to make life-saving decisions.

Advances in fatigue lifetime predictive techniques: Second volume

Based on the success of the first symposium on this topic in 1990 (published as ASTM STP 1122), this follow-up symposium was again intended to review recent progress in the understanding of fatigue phenomena and in the development and application of methods for predicting the fatigue performance of materials and structures in service environments. Topical content was purposely kept broad in an effort to represent the efforts and viewpoints of a range of researchers and practitioners who often participate in more specialized forums. This strategy, it is felt, provides an excellent opportunity for cross-fertilization and establishment of common ground between the various disciplines and interest groups. A cursory scan of the contents clearly reveals the breadth of coverage. The papers included in the volume cover: fundamental issues in damage development and crack growth; behavior in both low- and high-cycle regimes, under constant amplitude and spectrum loading conditions; the performance of advanced materials in hostile environments, including creep-fatigue and thermomechanical fatigue; and predictive techniques for the real-world environments. Separate abstracts were prepared for 14 papers of this conference.