High Compressive Prestrain Reduces the Fatigue Life of Electropolished Nitinol Wire

Nitinol-based transcatheter devices undergo a complex mechanical history before implantation.  This history, which includes a large bending prestrain due to constraining the device onto a delivery catheter and subsequent deployment, may affect the fatigue behavior of these devices.  The objective of this study is to investigate the effect of prestrain on the fatigue life of Nitinol wires. 

 Electropolished Nitinol wires were prestrained to 8% or 10% by wrapping around a mandrel at 0°C, followed by immersion in 60°C and 23°C baths prior to release.  Fully-reversed rotary bend fatigue testing was conducted on 0.5mm diameter Nitinol wire (A_{f ~} 15°C, typical of implantable devices) in a 37°C water bath. The strain life of as-received (0% pre-strain) and prestrained wires was measured for strain amplitudes (e_a) between 0.3%-1.5%.  Samples were cycled until fracture or runout at 100M cycles.  Fracture surfaces were characterized using SEM.

 No difference in fatigue life was observed between prestrain groups above 0.8% e_a.  Below 0.8% e_a, fatigue life diverged: 0% and 8% prestrained samples reached runout at 0.6% and 0.4% e_a, respectively, while 10% prestrained wires fractured around 75,000 cycles at 0.3% e_a.  The reduction in fatigue life was attributed to the nonuniform prestrain-induced plasticity and deformed martensite retention.  SEM revealed that all wires had comparable morphology, with a region of fatigue crack initiation and growth followed by ductile tensile failure.   No differences in the fatigue crack growth depth or area were noted.   Cracks were found to initiate preferentially on the compressive prestrain surface.  Since Nitinol accommodates less transformational strain in compression, fibers undergoing compressive prestrain likely experience greater plastic deformation, making them more susceptible to fatigue damage.

 This study demonstrates that large compressive prestrains are detrimental to the fatigue life of Nitinol, and underscores the importance of accounting for mechanical history in the design of endovascular implants.