Towards understanding the initiation and growth of fatigue failures at defects and inclusions in NiTi

This talk will provide a status update for work that has commenced as part of the SMST Founders’ grant titled “Zooming in on inclusions and their role in the fatigue of NiTi devices.”

It is known that failure in cardiovascular implants is often due to inclusions in NiTi, rather than the NiTi itself. To understand these defects better, this work is probing the initiation and growth of fatigue cracks in and around inclusions and particle-void assemblies. Our approach is utilizing a combination of fatigue testing, scanning electron microscopy (SEM), both optical and SEM digital image correlation (DIC), and focused ion beam milling (FIB).

Samples in tensile dogbone and Z/diamond strut shapes are being examined. For the tensile dogbones, the samples include ones produced from NiTi tube with stringer inclusions. The dogbones were laser cut in two geometries: stringers along the tensile axis, and stringers perpendicular to the tensile axis. Furthermore, the investigation includes FIB milling of two types: first, to study the 3D cross-sectional view under and through particle-void assemblies; and second, to use FIB to prescribe surface defects of particular geometries and study their effects in device-like sample dimensions.

The results will include SEM-DIC strain maps presented at different stages in the fatigue life of the samples, to investigate crack initiation and propagation. Analyses will include defect populations statistics with information from the SEM-DIC in the early phases of damage accumulation, with the goal of seeing damage forming and identifying critical inclusions even before cracks are visible.