Study Of The Ultrasonic Fatigue Behavior And Mechanism For Nickel-Titanium Endodontic Files

The nickel-titanium endodontic files are selected as the studying samples, the ultrasonic fatigue behavior of which under the unconstrained and flexural conditions has been studied by using an self-designed ultrasonic fatigue testing equipment and simulated root canals, and the numbers of ultrasonic vibrations before the ultrasonic fatigue fracture of nickel-titanium endodontic files are recorded, as well as the fracture positions. The features of vibration and harmonious response of nickel-titanium endodontic files are analyzed by the methods of the finite element analysis (FEA), and the characteristics of ultrasonic fatigue fractures under the unconstrained and flexural conditions has been studied, the reasons and mechanism of which have been also discussed.The experimental results show that the average ultrasonic fatigue lifetime of nickel-titanium endodontic files under unconstrained and flexural conditions are all more than 10~8 , which are in the range of ultra-high fatigue.Under the unconstrained condition, the numbers of ultrasonic fatigue vibrations before fracture vary with the diameter of nickel-titanium endodontic files, which indicates that the ultrasonic fatigue lifetime is proportional to the diameter of nickel-titanium endodontic files. In addition, the fracture of nickel-titanium endodontic files under unconstrained conditions usually occurs at the position which is 16.78 to 17.73mm away from the clamped tip, namely at the 2-3mm near the working tip of the files, which is irrelative to the diameter of the files. According to the results of FEA, the 11- and 12-order natural frequency is close to the testing frequency, which indicates that the nickel-titanium endodontic files are in the resonance vibration state and subject to 6-order bending mode during the experiments. The maximum stress locates at 2-3mm near the working tip of the nickel-titanium endodontic files, where is tend to generate fatigue cracks, the results of which is consistent with the experimental results. The analysis of ultrasonic fatigue fracture by SEM shows that the ultrasonic fatigue cracks always initiate from surfaces of the nickel-titanium files under unconstrained conditions, where the secondary phases or impurities probably exists that creates fatigue crack under the repeated tension-compression stress, while no abnormality exists at the center of the cross section of the files.The results of the ultrasonic fatigue experiments of the nickel-titanium endodontic files under the flexural condition show that under the same flexural condition, the average ultrasonic fatigue lifetime of the nickel-titanium endodontic files increases with the increase of diameters of the files; while for the same-sized endodontic files, the average ultrasonic fatigue lifetime increase with the decrease of angle of curvature and the increase of radius of curvature. Besides, the fracture of nickel-titanium endodontic files frequently occur at the positions near the maximum curvature of resinous simulated curved root canals, where is also the maximum deformation of the endodontic files in the resinous simulated curved root canals. Because at these positions, the outer surfaces of nickel-titanium endodontic files are in tension stress, while the inner surfaces are in compression stress. Under this state of stress, the outer surfaces in tension-stress state play an important role in the generation of fatigue cracks, where easily originate cracks, propagate under the continuous vibration, and finally give rise to the fracture. SEM analysis results also indicate that the ultrasonic fatigue cracks of the nickel-titanium endodontic files under the flexural conditions originate from the outer surface of curved potion of the files. There are clear fatigue striations and dimples existing in the propagation region of the fatigue cracks, which indicate the excellent ductility and the resistivity to the propagation of fatigue cracks of nickel-titanium endodontic files. To sum up, the nickel-titanium endodontic files possess the excellent resistivity to the ultrasonic fatigue fracture, which provides an experimental basis to the application of nickel-titanium endodontic files in the fields of the ultrasonic root canal therapy.