Fatigue and Fracture of Small Cracks Induced by Focused Ion Beam in Superelastic Nitinol

Fatigue fracture in superelastic Nitinol commonly originates at near-surface intrinsic or extrinsic inclusions that are exposed to areas of elevated stress/strain. There has been a large driving force to reduce the size and number of these inclusions to increase the fatigue life in Nitinol, especially due to the material’s widespread use in biomedical applications. This study aims to more precisely analyze the critical flaw size to cause fatigue fractures in Nitinol by precisely milling in small crack like defects using Focused Ion Beam (FIB) in order to apply a more fracture mechanics-based approach to fatigue analysis. The initial findings for this study were first presented at SMST in 2019.

Small semi-elliptical crack-like flaws were milled into ELI superelastic Nitinol wire (0.635mm diameter, A_f =2.5°C) to a depth (a) of 5.6μm and four different widths (2c) ranging from 20 to 35μm. The four groups of flaw sizes were fatigue tested in tension-tension at 20Hz and at 6 different strain amplitudes (1% to 0.375%) until fracture or a 10 million cycle runout. The flaw geometry and loading modes allow for the application of fracture mechanics solutions for analysis. As such three different SN curves were plotted for investigation, strain, stress, and initial stress-intensity factor range (Δk_i). Additionally, size dependent fracture toughness was calculated for all wire fractures. Wires having a Δk_i of less than 0.45MPa√m survived to runout. The stress and strain amplitudes for runout were approximately 52MPa and 0.375%. Finally, the size-dependent fracture toughness of the wires was approximately 14.5 MPa√m.