Evaluation of NiTi under low-amplitude cyclic loading by means of thermographic harmonic analysis

Nitinol is a shape memory alloy exhibiting superelastic behaviour above a specific temperature. This property has allowed the design of a new breed of collapsible/expandable cardiovascular medical devices, which are generally characterised by high-risk classes. Therefore, it is crucial to gain a comprehensive understanding of the material behaviour under in-vivo operating conditions. These are typically characterised by large pre-straining and small strain amplitude cyclic loading (high-cycle fatigue). In the present study, nitinol strips are monitored by two full-field sensing techniques: digital image correlation (DIC) and infrared thermography (IRT). The latter in particular was used to sample temperature during small-strain amplitude sinusoidal loading at various mean strains. Results show that the analysis of the frequency domain content of the temperature signal can provide useful information to characterise the material under operating conditions. In particular, it is found that temperature modulation is mainly characterised by its first and second harmonics, i.e. the harmonics at the load frequency and at twice the load frequency. These are shown to be correlated to the local stress field, the actual material phase status and the phase transformation history. The proposed harmonic analysis can be performed in near-real-time, and has the potential to be a convenient and highly informative tool when monitoring nitinol devices under structural fatigue testing. The paper also discusses the nature of the load-induced temperature modulation, and the direct applicability of the thermoelastic effect theory to interpret the observed behaviour.