Issue 28

A. Brotzu et alii, Frattura ed Integrità Strutturale, 28 (2014) 19-31; DOI: 10.3221/IGF-ESIS.28.03 22 Cycles have been repeated up to the instrument break or up tomacroscopic defects appearance. Periodically the rotary instruments have been cleaned with Isopropyl alcohol and ultrasounds and then observed with SEM in order to evaluate presence of surface defects. After the instrument break, the fracture surfaces have been characterized by SEM. During the second cycle, when the rotary instruments reach the bottom of the simulator canal, sometimes the rotation has been interrupted and the simulator-rotary instruments system has been subjected to a traction test in order to evaluate the load required to extract the instruments from the Perspex simulator. These tests have been carried out employing an Instron 3367 tensile machine. Tensile tests have been carried out with a crosshead speed of 1mm/min. Load vs. displacements curves have beenobtained. The present investigation considers the mechanical behavior under torsion and extension to deepen in the study of the Ni-Ti instruments. Therefore a FEM Static analysis was carried out by applying an axial moment (3 Ncm), and an axial load (50 N) in order to test the structural status of endodontic Ni-Ti rotary instruments, using a representative loadings. This analysis does not take into account the forces applied to twomodels by any external structure like dentin or Perspex simulator. An accurate three-dimensional solidmodel ofNi-Ti rotary filewas created by reconstructing its 3D geometric shape using a computer aided design software (CATIA, Dassault System). Then finite element (FE) model of the instrument was obtained bymeshing the pre-established solidmodel. The geometricmodels are made by rotating the characteristic cross section, shown inFig. 3, through 360 degrees over thewhole length. Even if the performed tests do not allow us to confirm, in literature it is reported that continuous unidirectional rotation causes development of complex stresses in nickel titanium rotary instruments. The finite element analysis is aimed to study and compare stress distribution and behavior of two rotary instruments, of same shape, same taper but different root cross section diameter, as shown inFig. 3. Both instrumentswere tested under equal loads. Themodels are divided into discrete tetrahedral elements (a ten-nodes iso-parametric solid element). The total numbers of elements are 863972 (for 1302293 nodes) for the biggest file and 139863 (for 217475 nodes) for the smallest one. Figure 3 :Detail of the root cross sectionof the 3Dmodel and the tetrahedral meshes. Material properties of the instruments like Young'sModulus, Poisson's Ratio and elastic limit were incorporated, themain mechanical properties are resumed in Tab. 1. The surrounding conditions, force andmoments were fixed. In both cases, themodel was blocked at one end andwas loadedwith a concentrated torsional or bending all along the file. ElasticModulus 40GPa Density 6450 kg/m 3 Poisson ratio 0.34 Yield Strength 560MPa Linear thermal expansion coefficient 9.5 10 -6 /K Table 1 :Ni-Ti Alloymainproperties.