Issue 49

S. Pereira et alii, Frattura ed Integrità Strutturale, 49 (2019) 450-462; DOI: 10.3221/IGF-ESIS.49.43 451 K EYWORDS . Fatigue; Superelastic Alloys; NiTi wires; Life Evaluation I NTRODUCTION ndodontic files have been used in dentistry since the middle ages and, as so, the shape, material and operation mode have changed since those days. These files are used in root canal procedures to remove the inflamed pulp and nerve endings in a tooth. In the past, endodontic files used in this procedure were made from highly flexible steel alloys. However, steel alloy files, while being flexible, are still too rigid to avoid damaging the walls of the root canals. [1-4] In order to minimise these adverse effects, Nickel-Titanium (NiTi) alloys are used in the design of endodontic files. This change in the material allowed the use of rotary systems to be used to prepare root canals more safely and predictably. [5] NiTi alloys are superelastic metal alloys able to fully recover from large deformations (up to strains of 10% [6]). The Niti alloys, as others superelastic alloys, that are included in Shape Memory alloys family, present a particular behaviour for the stress vs strain plot, both on loading and unloading. On loading, the alloys start to present a linear phase, where the alloy presents a stable crystalline phase, known as austenite. After this first part, a second region of the plot shows a phase, also linear but with a much smaller slope and where occurs the transition from the austenitic phase to the martensitic phase. This transition phase is often referred as R-phase [7, 8], and a large strain is produced with a small variation of the stress. The third phase, in which the material presents a fully martensitic phase, is again linear and presents a greater slope than in the second region. During the unloading, the plot also presents three different parts, yet the stress plateau of the transition phase is much lower comparing to the loading. [9] These alloys, however, have a drawback: when compared to steel files their fatigue life is relatively shorter than steel and, seen in commercial endodontic files, they break without a previous mechanical warning (such as visible plastic deformation), increasing the risk of the file failing inside the teeth. [10] To prevent these accidents the NiTi files are used only once, which leads to an increase of cost, making this material not that viable. In order to increase the viability of the use of NiTi alloys in dentistry, some studies were made to determine the fatigue life of these alloys, usually through traditional uniaxial fatigue tests and rotary bending fatigue tests [11]. Although rotary bending tests are the tests that most accurately replicate the kind of loads and deformation a file is subjected to when inside a root canal, the great majority of the existing machines in the literature only perform the fatigue test with a predetermined set of shapes. However, most of the imposed deformations are far from the complex shapes of the root canals [12]. To work- around this problem, Cheung and Darvell in 2007 [12] and Carvalho et al. in 2015 [10] developed machines with a variable set shape, being able to evaluate the fatigue life of files and specimens in a more versatile way [13-16]. However, even with the efforts to characterise this material, the mechanical properties of NiTi change with the manufacturing process and, consequently, the mechanical behaviour during the endodontic procedure is highly variable. This variability creates the need to study NiTi alloys from different suppliers. In this work, the mechanical properties of Endodontic-grade NiTi wires, from Alfa Aesar®, will be studied both by experimental tests and numerical simulations. This paper is part of a research project that aimed to study the fatigue life of endodontic files. Since there is a plethora of different geometries and alloys (of which the manufactures do not disclose de composition), there is a need to compartmentalize the study. Studying a specimen with simple geometry, the results of the fatigue life can be directly correlated with the fatigue properties of the material itself. From this point forward, the study can be taken to the actual files, where the differences in fatigue life can then be corelated with the complex geometry of the files(the fatigue properties of the material are known). For this, it is of special interest the rotary fatigue machine designed by Carvalho et al. (2015) [10], as this is the machine used to perform the fatigue test presented herewith. This machine consists of three pins that can be positioned precisely through numerical control to deform the test specimen with a degree of bending that can range from simple point bending to more complex multi-point bending, depending on the need of the user. The file/wire is then put into rotation using a brushless DC motor with variable speed, until failure is detected. M ATERIAL AND METHODOLOGY Material o perform the rotating fatigue four-point bending tests two NiTi alloy wires (Alfa Aesar® Nitinol wire) are used, with a composition of 55.75% Nickel and 44.25% Titanium. One with a diameter of 0.58mm and the other with a diameter of 0.25 mm, both straight annealed and with an oxide surface. [17] E T