Multiscale Modeling of fracture in Nitinol using the Quasicontinuum method: Bridging Atomistic Mechanisms to Component-Level Fracture

Nitinol stents are subjected to cyclic loading from pulsatile blood flow, making them susceptible to fatigue crack initiation and growth. Since fracture is fundamentally an atomistic process—governed by the breaking of atomic bonds and the competition between cleavage, dislocation emission, and phase transformation at the crack tip—a nanoscale understanding is essential for predicting device lifetime. However, modeling a crack with purely atomistic methods is computationally challenging.

We overcome this by using a multiscale quasicontinuum (QC) method. This method efficiently embeds an atomistic region at the crack tip, where the key mechanisms occur, while the far-field elastic response is captured by a finite element region described by the Cauchy-Born model. This approach allows us to apply realistic K-field boundary conditions, simulating a clinically relevant loading environment. Our study systematically investigates the effect of crystallographic orientation, including high-index planes, on the active fracture mechanism.

The QC method utilizes modern interatomic potentials capable of capturing phase transformations, with ongoing work to employ machine learning potentials for greater accuracy. Results are validated against experimental fracture studies from the literature, linking observed atomistic mechanisms to macroscopic material behavior. This work establishes a powerful framework for predicting fracture in Nitinol devices, providing a direct pathway from atomistic insights to improved component-level durability.