Fractographic analysis of K3 nickel-titanium rotary instruments submitted to different modes of mechanical loading.

The aim of the present study was to analyze the fracture surface of nickel-titanium K3 rotary files submitted to different modes of mechanical loading. Torsional and flexural fatigue tests were realized separately in 2 groups of files. Additional 2 groups of files were tested in mixed loading modes, torsional test after incomplete flexural fatigue test and flexural fatigue test after incomplete torsional test. Scanning electronic microscopy observation was used to make a fractographic analysis of the files under test. The files fractured during torsional resistance tests showed fractographies with a central area containing dimples, which are oval-shaped depressions in the microstructure in places of high local plastic deformation. The peripheral area of these files' fractographies presented a flattened aspect without dimples. The fractographies of the files fractured by flexural fatigue presented one area with radial striations and another area containing dimples. The fractographic patterns observed when mixed loading modes were imposed correspond to the pattern defined by the second stage test. The patterns established for the fractured files provide consistent bases to explain the fracture process in clinical conditions by fractographic analysis.