Tarek Hassine

In situ stress relaxation mechanism of a superelastic NiTi shape memory alloy under hydrogen charging

Abstract On account of its good biocompatibility, superelastic Ni–Ti arc wire alloys have been successfully used in orthodontic clinics. Nevertheless, delayed fracture in the oral cavity caused by hydrogen diffusion can be observed. The in situ stress relaxation susceptibility of a Ni–Ti shape memory alloy towards hydrogen embrittlement has been examined with respect to the current densities and imposed deformations. Orthodontic wires have been relaxed at different martensite volume fractions using current densities of 5, 10 and 20 A/m2 at 20 °C. The in situ relaxation stress shows that, for an imposed strain at the middle of the austenite–martensite transformation, the specimen fractures at the martensite–austenite reverse transformation. However, for an imposed strain at the beginning of the austenite–martensite plateau, the stress decreases in a similar way to the full austenite structure. Moreover, the stress plateau has been recorded at the reverse transformation for a short period. For the fully martensite structure, embrittlement occurs at a higher stress value. This behaviour is attributed to the interaction between the in situ austenite phase expansion and the diffusion of hydrogen in the different volume fractions of the martensite phase, produced at an imposed strain.

Tensile behaviour of superelastic NiTi alloys charged with hydrogen under applied strain

ABSTRACT The tensile behaviour of NiTi alloys is investigated after hydrogen charging during the austenite, half-transformation and martensite phases. The specimens are charged with different current densities and charging durations. During the tensile tests, the strain of the plateau transformation decreases due to hydrogen-induced residual martensite variants. This decrease becomes important when the charging happens during the martensite phase. Accordingly, the hydrogen ensures the stability of the phase in which the charging process occurs. Moreover, a heightening of transformation stress is noticed during the plateau. The transformation stress increases when the current density grows and the charging duration rises. This occurrence is caused by the interaction between the hydrogen and NiTi structures, where hydrogen delays the NiTi martensite transformation. This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Finite element analysis of hydrogen effects on superelastic NiTi shape memory alloys: Orthodontic application:

Thanks to its greater flexibility and biocompatibility with human tissue, superelastic NiTi alloys have taken an important part in the market of orthodontic wires. However, wire fractures and superelasticity losses are notified after a few months from being fixed in the teeth. This behavior is due to the hydrogen presence in the oral cavity, which brittles the NiTi arch wire. In this article, a diffusion-mechanical coupled model is presented while considering the hydrogen influences on the NiTi superelasticity. The model is integrated in ABAQUS finite element software via a UMAT subroutine. Additionally, a finite element model of a deflected orthodontic NiTi wire within three teeth brackets is simulated in the presence of hydrogen. The numerical results demonstrate that the force applied to the tooth drops with respect to the increase in the hydrogen amount. This behavior is attributed to the expansion of the NiTi structure after absorbing hydrogen. In addition, it is shown that hydrogen induces a loss of superelasticity. Hence, it attenuates the role of the orthodontic wire on the correction tooth malposition.

Finite Element Investigations of the Shrink-Fit Assembly with Radial Cyclic Load

Fretting is a widely used process to assemble mechanical parts and its knowledge is based on simple calculation models applied for simple and symmetric geometries. In this paper we model this process for cylindrical parts in order to study the behavior of a modified assembly under a cyclic radial load through a rigid drive shaft. A tangential movement of the shrink relative to the hub is raised. When these movement are localized and do not present a global rotation of the shrink, the behavior of the assembly is ensured. However, under certain conditions, these tangential displacements accumulate from one cycle to another up to initiate a global relative movement of the shrink, which in some cases lead to the failure of the assembly. In this work, we highlight this phenomenon and other studies will follow to improve the knowledge of this behavior.