Issue 49

S. Pereira et alii, Frattura ed Integrità Strutturale, 49 (2019) 450-462; DOI: 10.3221/IGF-ESIS.49.43 461 an almost constant plateau concerning to the stress. This is in line with the known behaviour of the NiTi alloys, in which the life time will be smaller than the one observed in this plateau only when it is tested under strains on the fully martensitic phase. It can also be seen that, comparing the results of the two wires, the fatigue life of the thinner wire is always greater than the life time of the thicker one. This is in line with the literature, as for each different NiTi alloy, set of process parameters, and shape must be individually tested and analysed to provide accurate fatigue life data. The analyses of the fracture surfaces allow to confirm the surfaces resulting from low cycle fatigue, showing a small crack propagation zone and a rough surface in the rest of the fracture surface, as the crack propagates fast and through the grain boundaries, as expected. As we move into the high cycle fatigue, the crack propagation zone increases, while the rest of the surface, where the crack propagates fast and through the grain boundaries, showed the same rough appearance. Concerning the FEA, the results met the expectations and were validated from the theoretical model, showing an approximately constant strain zone between the first two pins. The specimens tend to present the highest value for the stress near the first pin, which statistically happened in the laboratory tests as that was the location where fracture occurred. Still concerning the FEA, the stress across the beam showed that, as predicted, specimens up to 1% of strain are fully austenitic, while specimens with greater strain levels have the central part still in austenitic phase, while the outer part is already in the Austenite to Martensite transformation phase (R-Phase). The comparison of FEA results with the theoretical model suggests that the models are suitable when the specimens are still in the isotropic elastic region. However, when the specimens start the Austenite to Martensite transformation, there is a slight loss in accuracy, although this is not significant to invalidate the FEA prediction of the fatigue life of the alloys tested. For future work, other NiTi alloys used in endodontic files should be tested, as well as other alloys used in dentistry or other applications. Also, a greater range of rotary speeds should be tested to study the influence of this parameter in the fatigue life of these alloys. In order to improve the quality of the results, in special to improve the accuracy of the FEA, the thermal expansion coefficient of the materials studied should be considered, as well as the thermal energy dissipated during the rotary fatigue tests. A CKNOWLEDGMENTS his work was supported by FCT, through IDMEC, under LAETA, project UID/EMS/50022/2019. R EFERENCES [1] Craig, R. G., McIlwain, E. D., and Peyton, F. A. (1968). Bending and torsion properties of endodontic instruments. Oral Surgery, Oral Medicine, Oral Pathology, 25(2), pp. 239–254. DOI: 10.1016 /0030-4220(68)90286-7 . [2] Thompson, S. A. (2000). An overview of nickel-titanium alloys used in dentistry. International Endodontic Journal, 33(4), pp. 297–310. DOI: 10.1046/j.1365-2591.2000.00339.x. [3] Civjan, S., Huget, E. F., and DeSimon, L. B. (1975). Potential Applications of Certain Nickel-Titanium (Nitinol) Alloys. Journal of Dental Research, 54(1), pp. 89–96. DOI:10.1177/00220345750540014301. [4] Schafer, E. (2002) Metallurgie und Eigenschaften von Nickel-Titan-Instrumenten zur maschinellen Wurzelkanalaufbereitung. In: Hulsmann, M., editor. Wurzelkanalaufbereitung mit Nickel-Titan-Instrumenten. Ein Handbuch. Berlin: Quintessenz, pp. 35–46 [5] Krupp, D., Brantley, J., and Gerstein, H. (1984). An investigation of the torsional and bending properties of seven brands of endodontic files. Journal of Endodontics, 10(8), pp. 372–380. DOI: 10.1016/s0099-2399(84)80157-0. [6] Montalvão, D., Alçada, F. S., Braz Fernandes, F. M., and de Vilaverde-Correia, S. (2014). Structural Characterisation and Mechanical FE Analysis of Conventional and M-Wire Ni-Ti Alloys Used in Endodontic Rotary Instruments. The Scientific World Journal, pp. 1–8. DOI: 10.1155/2014/976459. [7] Nagasawa, A., (1970). A new phase transformation in the NiTi alloy. Journal of the Physical Society of Japan, 29(5), pp.1386-1386. [8] Ling, H. C., and Kaplow, R. (1981). Macroscopic length changes during the B2 ⇆ R and M → B2 transitions in equiatomic Ni-Ti alloys. Materials Science and Engineering, 51(2), pp. 193–201. DOI: 10.1016/0025-5416(81)90195-6. T