Zooming in on inclusions: in-situ SEM-DIC observations of damage and crack formation at particle/void assemblies

The durability of NiTi implants is limited by nonmetallic inclusions and surrounding voids ("particle/void assemblies," or PVAs). PVAs are often the primary sites for crack initiation, increasing the risk of premature/unpredicted failure in key NiTi applications (stents, heart-valve cages, etc.). Unfortunately, the micromechanical understanding of PVAs is limited, largely due to their small size (less than 10 μm). To provide more confident fatigue predictions, this work is zooming in on PVAs and their role in NiTi fatigue, especially with scanning electron microscope digital image correlation (SEM-DIC) to quantify strain around PVAs before and after crack initiation. This work utilizes in-SEM fatigue testing to capture in-situ strain maps around inclusions of high-purity Nitinol dogbones. Fatigue cycles are applied to the dogbone samples via uniaxial, constant amplitude testing (R = 0.1) for millions of cycles. After a sample has failed, it is analyzed via fractography to determine the characteristics of the fatal defect and connect these to the SEM-DIC strain fields gathered before failure. Our results include details of reliable SEM-DIC speckle patterning for NiTi, along with the initiation and development of damage around inclusions and voids. In measuring displacements and strains around inclusions, this work provides a new pathway to gather micromechanical insights for defect and damage modeling, such as parameters for traction-separation laws for cohesive zone modeling and finite element analysis.