PSI - Issue 42

Guilherme Saldanha et al. / Procedia Structural Integrity 42 (2022) 631–638 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction In case of infection of the living tissue in the root canals of teeth, its removal may be necessary, Caicedo and Clark (2012). The endodontic files to be studied are used for this mechanized removal process, and, given the highly variable geometries of the canals, Buican et al. (2017), they need to be as flexible and resistant as possible. In this sense, technological innovations in endodontic instrumentation have evolved. Initially, the instruments were manufactured from the twisting of carbon steel wires. However, with the discovery of new machining processes and materials, those materials were replaced by stainless steel alloys and, later, by Nickel-Titanium (NiTi) alloys, Bansode et al. (2016). These alloys proved to be much more flexible and resistant than the previous ones, adapting more easily to channels with sharp curvatures and difficult access. The main characteristic of NiTi alloys is their ability to reorganize their crystallographic microstructure in response to thermal and/or mechanical stimuli, a phenomenon that gives them shape memory (SME) and superelasticity (SE) effects, respectively, Jania et al. (2014). Thanks to these properties, the introduction of this type of materials in endodontic instrumentation has gained great attraction and popularity among odontologists. However, technological innovations did not stop with the search for new materials and new geometries. New methods of manipulating the transition temperatures of the crystalline structure of alloys through heat treatments started to be developed to take full advantage of their properties, Agarwal et al. (2018). In this research, the HyFlex CM files, manufactured from a NiTi wire by a conventional machining process, previously subjected to a series of heat treatments to maximize its flexibility at body temperature, were studied. Additionally, a recent generation of files introduced in the market was considered, namely the HyFlex EDM files, which, besides the heat treatments, are machined through an electrical discharge process. The purpose of all these advances is to make the instruments more resistant to mechanical fracture, either by fatigue or torsion. Still, although unlikely, the fracture is a reality in clinical treatment. In the case of the mechanical fatigue phenomenon, fracture occurs through the nucleation and propagation of a crack with the application of repeated stress cycles that are lower than the yield strength of the material. This type of phenomenon is highly problematic if it occurs during clinical treatment, given the difficulty in removing the fractured tip from the root canal, Parashos and Messer (2006). Therefore, the motivation underlying this work derives from the desire to find reliable methods that would allow timely detection of these cracks to avoid the endodontic files' breakage. In this sense, the possibility of detecting these flaws was evaluated using Non-Destructive Tests (NDT), namely, the eddy currents testing (ECT) and thermography. In the ECT method, the detection of discontinuities is possible by measuring the variations in the impedance of a probe, consisting of a coil, caused by the forced modification of the normal path of induced currents due to the presence of a defect. In the case of thermography, using a thermographic camera, it is possible to detect the thermal contrasts created by the defective regions, either by their different emissivities or even by the variations in thermal flow created by these sections. Thus, these methods were adapted and applied in a laboratory environment during this investigation to try to detect fatigue cracks in endodontic instruments. 2. Materials and methods This section, divided into three parts, presents a detailed description of the adopted methodology and the laboratory procedures used to achieve the predefined objectives. Firstly, the dimensional characterization of the endodontic files is presented, followed by the description of the experimental procedures used to determine the rotational flexural fatigue resistance of the instruments tested. In the second subsection, the procedures used to make it possible to carry out the eddy currents testing in endodontic instruments are identified, from the design of the probes and the parameters that were varied in them to the experimental setup used. The strategies developed to corroborate the experimental results through numerical simulations are also addressed. Finally, the third and last subsection describes the procedures and experimental setup used in the thermographic tests.