Modeling of Hydrogen Diffusion Towards a NiTi Arch Wire Under Cyclic Loading

NiTi shape memory alloys belong to a class of metals with specific properties like corrosion resistance and biochemical compatibilities. These characteristics allow their use in a lot of medical fields, especially as arch wires in orthodontic applications. A significant alteration of the superelasticity of orthodontic arch wires can be notified after a few months of use in the oral cavity. This behavior is due to the presence of hydrogen. The objective of this work is to develop a constitutive model, which describes the effect of the degradations of the mechanical behavior of NiTi shape memory alloys caused by cyclic loading and hydrogen diffusion. To experimentally predict such effects, orthodontic specimens are charged by hydrogen in a 0.9% NaCl aqueous solution at room temperature with a current density of 10 A/m2 for 2 h, 3 h, 4 h and 6 h and are aged for 7 days in air. The obtained curves display an increase in the critical stress at the beginning and end of the martensite transformation and a decrease in the dissipated energy compared by the curve obtained by a non-charged wire. The developed model is implemented in ABAQUS software through the UMAT and UMATHT subroutines. Finally, the numerical simulation, based on the model proposed in our work, shows a good concordance with the experimental data.