Abstract
The objective of this study was to assess the resistance to fatigue fracture of conventional nickel–titanium files after undergoing shot peening. Forty NITIFLEX endodontic files, number 30, were divided into two groups; one was submitted to shot peening treatment and the other was not. All instruments were tested for fatigue fracture in simulated canals with a TRI-AUTO ZX endodontic motor. One file of each group was subjected to a residual stress analysis by XRD. Finally, the fractured surface was observed and elemental analysis performed by means of SEM and EDX. Roughness analysis was made by focal variation microscope. The shot peening group showed greater resistance to fatigue fracture; there was no difference in the length of the fractured fragments. XRD results showed the presence of residual compression stresses in the file submitted to shot peening, a decrease in the interplanar spacing, and an increase in the full-width-at-half-maximum and the microstrains. SEM and EDX showed a ductile fracture with zones of fatigue and an equiatomic ratio between the nickel and titanium. Surface roughness increased after the file was subjected to the shot peening procedure. In conclusion, shot peening increases the resistance to fatigue fracture due to the presence of residual compression stresses in files manufactured from a conventional nickel–titanium alloy.
Highlights
The objective of this study was to assess the resistance to fatigue fracture of conventional nickel– titanium files after undergoing shot peening
With regard to time to fracture and number of cycles to fracture (NCF), statistically significant differences were observed between the two groups, with the highest fatigue resistance being found in the group that was treated with Shot peening (SP) (p ≤ 0.001)
The parameters used in this article allow us to calculate that the endodontic file makes 4.7 revolutions in one second, with alternation of tension and compression stresses directly related to the angle of curvature of 86°
Summary
The objective of this study was to assess the resistance to fatigue fracture of conventional nickel– titanium files after undergoing shot peening. Shot peening increases the resistance to fatigue fracture due to the presence of residual compression stresses in files manufactured from a conventional nickel–titanium alloy. Fatigue fracture has been prevented with measures such as a decreased number of uses[5,9] and conventional NiTi alloy heat treatments[10], which change the original characteristics of the alloy, making it more ductile and reducing its superelasticity Surface treatments such as electropolishing have been proposed[11] to try to prevent the origin or growth of cracks by making their surface smooth; their effectiveness in increasing resistance to fatigue fracture has been q uestioned[12,13].
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