Experimental assessment of the functional fatigue in biocompatible Ti67Zr19Nb11.5Sn2.5 shape memory alloy in the vicinity of drilled holes

Abstract

Shape memory alloys (SMA) exhibit desirable and unique functionalities that are suitable for a plethora of applications in the automotive, aerospace, and biomedical industries. The TiNbZrSn SMA has been specifically developed for biomedical applications owing to its enhanced biocompatibility compared to the widely available and heavily investigated NiTi-based SMA. In this work, the effects of mechanical rolling, heat treatment, and grain size on the functional fatigue properties of TiNbZrSn are investigated under cyclic loading conditions. Superelasticity was observed for samples rolled to reduction levels exceeding 90 %. However, the recovery magnitudes improved significantly following 96.6 % rolling due to the optimization of the grain size. The reduction in grain size, from about 400 μm in the homogenized conditions to 50 μm for the 96.6 % rolled state, increases slip resistance which consequently promotes superelasticity. The functional superelastic properties were also evaluated for samples with geometric stress concentration (drilled hole). Using full-field measurement techniques, the applied, recoverable, irreversible strains, and their evolution under cyclic loading conditions were carefully measured. The use of virtual extensometers located in multiple locations around the geometric stress concentration provides means to not only evaluate the functional degradation of local superelastic properties, but also the transition to structural damage through the initiation and propagation of fatigue cracks.