A workbench applying combined loading torsion-bending tests for shape memory alloy endodontic files

In endodontics, Nickel-Titanium (NiTi) files are used for their superelasticity in order to obtain more reproducible and safer treament with a better respect of the root canal anatomy than stainless steel files. Loadings applied on them are mainly bending, due to root canal curvature and torsion due to their use in continuous rotation. These two solicitations are combined during treatment. Standard ISO 3630-1 allows to qualify and characterize endodontic files under bending or torsion loading. Usually, these tests are carried out separately although both loads are occurring in a combined way during the root canal treatment. This presentation describes a prototype workbench allowing to apply separately or simultaneously a bending/torsion loading on a Shape Memory Alloy endodontic file. It allows to apply monotonic, cyclic or any complex loading paths. Two engines provide bending and torsion with controlled rotation and two sensors measure corresponding moments. The main design idea consists on gather the torsion and the bending loadings and avoid any unexpected solicitation. Bending and torsion tests on NiTi wires were carried out in order to calibrate this device. Further, this device was adopted to analyse the response of various NiTi files under bending and/or torsion loadings. Corresponding results are presented and discussed. In particular, it shown that combining bending and torsion induces a stiffness decreasing compared to simple solicitations. In fact, more martensitic transformation appears in the case of combined loading. This workbench is suitable for qualification of new developed SMA endodontic files.