Pseudoelastic shape memory alloy model with stent deployment simulation
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In recent years, Shape Memory alloys characterized by the peculiar properties of shape memory effect and pseudoelastic effect have found use over broad range of applications from dampers for seismic control of structural systems to endovascular stents for treating coronary artery diseases. Although extensive work has been done to characterize the behavior of Shape Memory Alloys (SMAs), there still remains a need to develop stress-strain models and finite element formulations that are valid under arbitrary loading. The first portion of the thesis focuses on investigating a newly developed cyclic constitutive model that describes the pseudoelastic effect in SMA. This incremental model provides a phenomenological description of the behavior by including an internal state variable associated with the austenite-martensite phase transition. In order to perform numerical simulations of the stress-strain response, the model was implemented as a User MATerial subroutine in ABAQUS software. In the second portion of the thesis, one of the widely used applications of SMA as a cardiovascular stent is studied. Finite Element analyses based on patient specific data to treat Coronary Artery Disease is becoming an important tool for designers while designing stents and also as an requirement for Food and Drug Administration submissions. Due to the dearth of literature available about the Finite Element simulation of Nitinol stent deployment, the implemented UMAT subroutine is used effectively to do a structural interaction study as the stent is deployed into arteries infected with plaque. Overall a finite element analysis similar to the one proposed herewith could help in designing new stents or analyzing actual stents to ensure ideal expansion, structural integrity and to identify the stresses that may lead to restenosis. Thus, ultimately a Finite Element Analysis could perhaps substitute for in vitro experiments, which are often difficult and impractical.