PSI - Issue 47

Franco Maria Di Russo et al. / Procedia Structural Integrity 47 (2023) 765–781 Author name / Structural Integrity Procedia 00 (2023) 000–000

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the clinical relevance of these tests has been considered very low (“Mechanical Tests, Metallurgical Characterization, and Shaping Ability of Nickel-Titanium Rotary Instruments: A Multimethod Research,” 2020; Zanza et al., 2021a). Only recently have studies been conducted concerning the influence of some geometrical aspects, such as the relevance of mass distribution with respect to the axis of rotation or the shear efficiency of geometrically different cross sections. In this case, however, the constraint conditions were aimed at describing the case of torsion resistance (Seracchiani et al., 2022; Zanza et al., 2021b). One way to achieve a more reliable mechanical performance evaluation is to conduct dynamic analyses, which take into account the influence of geometric factors throughout the treatment: from insertion of the device into the canal and shaping. From a numerical point of view, phenomena of this kind can be analysed using finite element simulation tools. It is possible to reproduce the mechanical and structural behaviour of such devices in a scenario more in accordance with the clinical case, focusing on the influence relationships that exist between geometric factors and overall performance. The purpose of this study, then, is to evaluate the mechanical behaviour of some of the most popular commercial endodontic instruments under two different conditions: replicating static test conditions inherent to flexibility and, then, imposing sliding and rotational conditions within a simplified reproduction of the real root canal. Next, an analysis of the influence of geometric attributes on fatigue accumulation is to be conducted, and a Design Optimization procedure with the aim of increasing resistance to the fatigue phenomenon is carried out.

2. Material and method 2.1. Material

Material properties used for this analysis refer to a material model well known in the literature, the "Auricchio Model," which is widely used within this type of study for both SMA and superelastic materials (Auricchio, 2001; Prados-Privado et al., 2018). Assuming the adoption of a NiTi alloy with an austenitic crystal structure, relevant material properties are shown in Table 1.

Table 1. – NiTi alloy material properties (Auricchio, 2001) Parameter Magnitude Young’s modulus of austenite 42.53 GPa Austenite Poisson’s ratio 0,33 Young’s Modulus of Martensite Martensite Poisson’s ratio Uni-axial transformation strain Slope of the stress-temperature curve for loading 12.828 GPa 0.33 6% 6.7

492 MPa 630 MPa 22 °C 6.7 192 MPa 97 MPa 1200 MPa

Start of transformation loading End of transformation loading Reference temperature Slope of the stress-temperature curve for unloading

Start of transformation unloading End of transformation unloading End of martensitic elastic regime

This material model was then implemented in Ansys, in the "Engineering Data" section. Superelasticity properties were introduced through the "Superelasticity" tool.