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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stabilizing in the last third of the instrument. Moreover, in the last third of the instrument (faulty region), the probe no longer detects additional impedance variations, and it was found that the decay rate of the currents increases with the excitation frequency. The explanation lies in the conicity of the endodontic file; if the amount of material decreases, so does the intensity of the induced currents. Additionally, Eddy Currents describe a circular path, while the rectangular cross section of the instruments means that the currents ignore the cutting edges, where the accumulation of stresses is most significant. Similar results and conclusions were drawn for the differential probe type: the defects are detected in the cylindrical wire but not in the conical endodontic file. Finally, the experimental results were confirmed in the numerical electromagnetic analyses carried out.

Fig. 9. Electrical impedance variation recorded on probe SA30 for f = 3, 4 and 5 MHz in the cylindrical wire and in the endodontic file.

4. Conclusion This section presents the relevant conclusions drawn from this work, namely:

• Concerning the application of thermography, it was possible to detect the defect, especially in the thermogram corresponding to the thermal equilibrium phase; • Fatigue tests have shown that the latest generation of the Hyflex EDM endodontic files are more resistant than their predecessors, mainly due to the electrodischarge machining process used to produce them; • The Eddy Currents were not effective in the detection of a large artificial defect introduced in the endodontic file due to the dimensional and geometric constraints of the instrument, namely its taper. Additionally, Eddy Currents describe a circular path, while the rectangular cross-section of the instruments means that the currents tend to ignore the cutting edges where the accumulation of stresses is most significant and where the artificial defect was located. Acknowledgements The authors acknowledge Fundação para a Ciência e a Tecnologia (FCT.IP) for its financial support through the grant UIDB/00667/2020 (UNIDEMI).