Transcatheter heart valve replacement is a key advancement in the cardiovascular device industry and provides an alternative to open surgical procedures for patients that suffer from severe symptomatic stenosis and/or regurgitation. The functions and boundary conditions of the four heart valves are unique and must be considered separately. It is essential that the structural durability of these high-risk valve replacement implants is thoroughly assessed through testing and analysis. As such, ISO 5840 outlines a comprehensive device durability approach that incorporates worst-case boundary conditions, computational stress/strain analyses, and benchtop fatigue testing. The present study is focussed on 100,000,000-cycle fatigue testing of custom-designed “diamond-shaped” coupons of process-optimized high purity VAR/EBR Nitinol. Benchtop testing was coupled with finite element analysis (FEA) and microstructural characterization to provide an in-depth understanding of durability.

The stress-strain-temperature thermomechanical responses of NiTi shape memory alloys due to B2-B19′ martensitic transformation (MT) should ideally be phase and strain reversible in closed-loop thermomechanical load cycles, where the austenite and martensite phases do not undergo plastic deformation. However, this ideal behavior is only observed when MT occurs under zero or very low externally applied stresses. When MT occurs under higher externally applied stresses, it generates small plastic strains. These strains accumulate whenever MT proceeds under external stress, leading to the accumulation of residual plastic strains, internal stress, and lattice defects during cyclic thermomechanical loads. This accumulation results in the instability of cyclic thermomechanical responses, a phenomenon known as “functional fatigue.”

Damage-tolerant approaches, relying on fracture mechanics, are powerful tools adopted in many engineering sectors to predict the initiation and propagation of fracture. In the last two decades, there has been a growing interest in extending these methods to shape memory alloy (SMA) components despite their greater complexities related to phase transformation and the thermomechanical coupling characterizing material response. SMA devices for actuation, damping, and energy absorption are the most appropriate applications of fracture mechanics, being large enough to sustain crack growth before fracture. Nevertheless, also the biomedical field is moving toward these methods for a more accurate prediction of device failure. This work investigates a damage-tolerant approach for predicting the fatigue fracture of Nickel-Titanium (Ni-Ti) stent-like devices. In the case of stenotic lower limb arteries, Ni-Ti SMAs are the top choice for designing self-expanding stents for mini-invasive deployment. The stents throughout their lifespan are subjected to cyclic loads mainly due to gait (10 6 cycles/year at 1 Hz) causing fatigue failures related to dramatic drawbacks. Since fatigue fracture is typically related to crack growth from manufacturing-related defects, this research aims to identify the basic ingredients needed to apply fracture mechanics, highlighting the critical aspects that should be considered given the particular features of the material and its application in the cardiovascular field.

Implanted cardiovascular devices must undergo a range of applied cyclic loads after implantation. Peripheral devices, for example, are subjected to both radial pulsatile loading and non-radial loading such as cyclic bending from limb motion. Coronary stents may also see a range of loads due to increases in heart exertion, such as during exercise. As highlighted by these examples, such variable loading commonly involves a smaller number of high amplitude cycles combined with a larger number of low amplitude cycles.

Inclusions and surface defects are the primary drivers of fatigue fractures for Nitinol. Surface defects can take the form of die marks, surface roughness, or scratches, and when present, these will typically be the sites for fatigue fractures. When such surface defects are not present, however, surface-exposed inclusions become the primary sites of fatigue fractures. Surface-exposed inclusions are either fully embedded (no surrounding voids) or particle-void assemblies (PVAs). PVAs are up to 600 times more likely to be the sites of crack nucleation compared to fully embedded particles, so they are of primary interest. PVAs typically take the form of a single, small, and unbroken inclusion with one or two associated voids (coined a "teardrop"), or a longer, drawn-out, and broken-up inclusion with many voids (coined a "stringer").