Development of the Z Specimen for Tensile-Tensile, Tensile-Compression, Compression-Compression Wire Testing

Abstract

A new wire test specimen was developed as part of the Safe Technology consortium project to produce a nitinol algorithm for the fe-safe Ô fatigue postprocessor. The requirements for the design were permit tensile and compressive loading, accommodate data collection of strains from ±0.1 to 6% at high-frequency cycling. The requirements for the wire were inexpensive, stable, consistent, and commercially available. The requirements for the processing were incorporate deformation and multiple anneal cycles to simulate manufacturing methods associated with vascular stent production. The final design was 0.508 mm (0.020 in.) diameter wire formed in a two-stage process to produce a Z-shaped specimen. The final part met all requirements for testing. Subsequently, a second specimen, Z3, and a straight wire specimen were produced to supply additional data to the consortium.

A statistical approach to understand the role of inclusions on the fatigue resistance of superelastic Nitinol wire and tubing

“…The displacements required to reach the desired pre-strain of 6% and mean strain of 3% were computed at cycle 1, whereas the displacement inputs to achieve the various strain amplitudes were computed at cycle 3 of the FEA analysis. This technique of utilizing computational modeling to establish experimental displacement inputs is common in the evaluation of Nitinol stent-like surrogates (Tolomeo et al, 2000;Pelton, 2008;Robertson and Ritchie, 2008;Gall et al, 2008;Lin et al, 2011Lin et al, , 2012Pike et al, 2011;Robertson et al, 2012;Pelton et al, 2008). Fatigue testing was performed using a 12-station Instron Model E3000 (each with isolated load cells for dynamically monitoring fracture) in 37 1C distilled water, at a frequency of 20 Hz, and under displacement-control conditions.…”

“…Similarly, in recent years, the effects of impurities, primarily C and O, that may form non-metallic inclusions (NMIs), in biomedical-grade NiTi (Nitinol) have been studied. In particular, investigations have demonstrated fatigue crack nucleation in Nitinol from NMIs (and/or voids) in the simple raw material form (Wick et al, 2004;Schaffer and Plumley, 2009;Rahim et al, 2013), surrogate medical device implant fatigue specimens (Tolomeo et al, 2000;Pelton, 2008;Robertson and Ritchie, 2008;Gall et al, 2008;Lin et al, 2011Lin et al, , 2012Pike et al, 2011;Robertson et al, 2012;Pelton et al, 2008), and even finished commercial devices (Pelton et al, 2008;Hull and Robertson, 2009). On a commercial scale, Nitinol is melted by vacuum arc remelting (VAR), vacuum induction melting (VIM) or a combination of the two.…”