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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3. Results and discussion Concerning the fatigue tests results (Fig. 7a), one can see that Group B (OneFile EDM files) showed a higher mechanical fatigue resistance than the endodontic files of the other two groups. On the contrary, in the opposite direction, group A (EDM Glidepath files), which are used in the intermediate region of the tooth and not in the apical region, showed a low fatigue resistance due to its lower flexibility and higher dimensions of its cross section in the bending region, resulting in higher induced stresses and lower fatigue lives. Moreover, in Fig. 7b, the rectangular cross-section of the OneFile type and the regions where nucleation, propagation and final fracture occurred can be seen. When comparing the higher fatigue lives obtained for the OneFile EDM files with the lower fatigue lives obtained for the conventional machined Hyflex CM endodontic files, the main reason is the better surface finish obtained with the electro discharge machining process that lowers the local stress concentration factors. a b

Fig. 7. (a) Experimental fatigue parameters and results, (b) Fracture surface of endodontic file EDM OneFile_2 . Regarding the images obtained using the thermographic camera (Fig. 8), it is possible to see that the thermographic method allows for identifying the location of the defect introduced in the endodontic fi les from the heating phase to the cooling phase.

Fig. 8. Thermograms obtained during the thermal cycle applied

Additionally, when looking at the results obtained with the Eddy Currents method (Fig. 9), considering an absolute probe type for the different exciting frequencies, it is possible to observe two different results for the cylindrical wire and the conical endodontic file. For the cylindrical wire, the eddy currents method allows to detect all the defects introduced; however, when looking at the results obtained for the conical endodontic file, one can see that the probe impedance decreases with the inspected distance in an approximately exponential way, almost always