L. Saint-Sulpice, S. Calloch, ENSIETA, Brest Cedex 9, France; S. Arbab Chirani, ENIB, Brest Cedex 3, France; V. Chevalier, R. Arbab Chirani, UBO-UFR d'odontologie, Brest Cedex 3, France
NiTi superelastic endodontic instruments use the great deformability of these alloys to realize the root canals preparation. Generally they work under torsion and flexion nonproportional loadings. Because of the cyclic mechanical loadings produced during their use, they present fatigue effect. To ensure the reliability of the global structure, we developed a multiaxial nonproportional model for cyclic behaviour of superelastic shape memory alloys. A particular attention is paid to the evolution of residual strain with number of cycles. To study the phenomenology of the cyclic behaviour and to identify the origin of the developed residual strain a series of cyclic uniaxial tensile tests on SMA wires has been realized. The originalities of the model are, on the one hand, the definition of a particular elasticity domain when the material is in a two phased state and, on the other hand, an ad hoc kinetic of transformation strain taking into account a residual strain evolution. The proposed model has been identified using our experimental data base and has been used to simulate various cyclic multiaxial loadings.This model has been integrated in the CASTEM finite elements code in order to study the endodontic instruments. They are used in real conditions under the flexion-torsion nonproportional cyclic loading. This path has so been simulated integrating the exact form of the root canal as boundary conditions. This study permits to know the more loaded zones of the existent instruments and also to optimize the shape of the future instruments.
Summary: To ensure the reliability of the NiTi endodontic instruments, we developed a multiaxial nonproportional model for cyclic behaviour of superelastic shape memory alloys. The originalities of the model are, on the one hand, the definition of a particular elasticity domain when the material is in a two phased state and, on the other hand, an ad hoc kinetic of transformation strain taking into account a residual strain evolution under cyclic loadings. This model has been integrated in the CASTEM finite elements code in order to study the mechanical behaviour of endodontic instruments.
