Fatigue properties by “self-heating” method: Application to orthodontic Ni-Ti wires after hydrogen charging

Ni-Ti superelastic alloys have been successfully used in orthodontic clinics thanks to their good biomechanical and biochemical behavior. However, during treatment, some orthodontic wires may break in the oral cavity. The susceptibility of these alloys to cyclic loadings and to hydrogen embrittlement is supposed to be main causes of these unexpected failures. This study presents a contribution to studying the effect of hydrogen, obtained after cathodically charging in 0.9% NaCl solution, on the fatigue behavior of Ni-Ti commercial orthodontic wires subjected to high-cycle fatigue. Fatigue tests were analyzed using self-heating method based on observing thermal effects under mechanical cyclic loading. The results obtained with self-heating approach imply that the increase in hydrogen charging time is connected with an increase in the mean stabilized temperature and a decrease in the fatigue life. Self-heating method allows a rapid prediction of the endurance limit with a good reproducibility of fatigue tests at high number of cycles. Furthermore, cyclic stress–induced transformations and conventional fatigue tests under strain control are considered in this work to investigate the effect of hydrogen on cyclic loading type and to acquire for a better understanding of the interaction between hydrogen and thermo-mechanical mechanisms in Ni-Ti alloys.