Issue 28

A. Brotzu et alii, Frattura ed Integrità Strutturale, 28 (2014) 19-31 ; DOI: 10.3221/IGF-ESIS.28.03 28 Figure 21 :Micro crack on instrument no. 7 surface. Figure 22 : Fracture surface of instrument number 7. DSC ANALYSIS he Ni-Ti alloys can show both superelastic behaviour or shape memory effects. It depends on the manufacturing process. The materials employed for the realization of rotary instruments must show a superelastic behaviour. In order to verify if an eventual phase transition - which could modify the Ni-Ti alloy properties- happens in operation, aDSC analysis in the range of temperature - 30÷ 100 °C has been performed. Fig. 23 shows theDSCplot with the four typical phases transformation which characterize the SMA/superelastic alloys. In particular the higher transformation temperature, which usually is the Austenitic Finish Temperature (AF), is 11.55 °C. The material then operates in the superelastic field in which the high recoverable deformations are due to the austenite- detwinned martensite transformation. 11.55°C(I) 8.54°C 15.44°C 0.45 0.50 0.55 0.60 0.65 Heat Flow (W/g) -40 -20 0 20 40 60 80 100 Temperature (°C) ExoDown Figure 23 : DSCPlot. FEM ANALYSIS he FEM has been performed with the aim to estimate theoretically the deformations applied to the tools in the test conditions. A cylindrical hinge has been inserted at the section where the tool begins to bend, in order to simulate the real local constrain induced by the chanel design. In order to take the curvature of the cutter closer to T T