Improving the reliability of NiTi endodontic files using eddy currents, thermography and fatigue tests: A multimethod approach

Non-Destructive Testing (NDT) by Eddy Currents (EC) allows for determining material properties and detecting surface and subsurface defects that can harm structural integrity of electrically conductive components. How-ever, despite being a standard technique and with many years of accumulated experience, there are some conditions where difficulties persist, such as detecting small defects in complex components with small dimensions and conicity (e.g. endodontic files used in a clinical environment).This work aimed to design, numerically simulate, produce, and experimentally validate different configurations of EC probes for defect detection in endodontic files. For the application of NDTs, an artificial defect was introduced in the apical region of a commercial endodontic file with 4% taper and a tip's diameter equal to 0.20 mm. The experimental results showed that most of the methodologies developed for the inspection of files through NDTs proved unable to detect the defective region, primarily due to its taper and the special Pauli paramagnetic properties of the NiTi alloy. Nevertheless, the customized EC toroidal probe operating in differential bridge mode could detect the artificial defect in the endodontic file. In parallel, a thermographic inspection of the defective instrument was made. As a result, it was possible to detect the defect by thermography, although with low Signal-to-Noise Ratio (SNR) due to its small dimensions.Finally, commercial endodontic files manufactured by Electrical Discharge Machining (EDM) were tested and compared with the results obtained for conventional manufactured endodontic files. The endodontic instruments were placed in rotation in an experimental setup designed to simulate a root canal with a radius of curvature equal to 5 mm applied along a 45 degrees angle. The experimental results showed that the latest generation of endodontic files, manufactured using EDM process, are significantly more resistant to fatigue fracture when compared to the Controlled Memory (CM) machined files.