Fatigue modeling

Cardiovascular disease has become a major global health care problem in the present decade. To tackle this problem, the use of cardiovascular stents has been considered a promising and effective approach.
Several materials, as the 316L austenitic stainless steel, the cobalt-chromium alloys or the NiTi-based shape memory alloys, have been imposed as standard materials for cardiovascular stents. Although stent design is considered a relatively mature topic, recently, fatigue failure has emerged as one main cause of stent drawback. As an example, balloon-expandable stents have to withstand both their initial deployment within the artery and the long-term service loading induced by the pulsing blood pressure. In particular, such devices must withstand at least 10 years service without exhibiting failure (US Food and Drug Administration, 2010). Therefore, the design lies within the high- and very high-cycle fatigue.
Recently, the work [P1] has presented a fatigue life numerical method for the analysis of cardiovascular balloon-expandable stainless steel stents. The method is based on a two-scale continuum damage mechanics model. Numerical simulations predict a number of cycles up to microcrack initiation of 64 million for a PalmazShatz stent model and 53 million for a Cypher stent model. This is equivalent to approximately 22 and 18 months of the heart pumping in an average human adult, respectively.
Ongoing research is focusing on the introduction of a computational approach for the lifetime assessment of a classical coronary stent design. The main idea is to adapt and interpret classical notions, models, and techniques as well as to consider suitable experimental data for the calibration of stent fatigue criteria, in order to obtain a reliable fatigue prediction methodology for biomedical balloon-expandable stent. To this purpose, work first analyzes experimental data on 316L stainless steel from literature for smooth and notched mm-size components (see Figure 1) using a global computational approach (see Figure 2) in order to propose several fatigue criteria for finite and infinite lifetime (see Figure 3). Then, work applies the obtained results to lay down the basis for the prediction of fatigue-controlled service life of biomedical balloon-expandable stents.