Issue 28

A. Brotzu et alii, Frattura ed Integrità Strutturale, 28 (2014) 19-31; DOI: 10.3221/IGF-ESIS.28.03 26 during the test without lubricant. The gap between detachment and maximum loads and the displacement necessary to reach themaximum load are reduced. Figure 13 : Fracture surface – dimples. Figure 14 : Fracture surface – striations. R ED INSTRUMENTTESTRESULTS ab. 4 reports the results of the test performed on the red instruments. Fig. 15 and 16 show the load vs. displacements curves of the extraction test carried out with andwithout the lubricant use. Instrument n° Lubricant gel Break/damage revolutions Break/damage cycles Simulators employed Damage type 7 Yes 1820 26 7 Break at the revolution 4 8 No 350 5 2 Unwinding at the revolution 2-4 9 No 840 12 3 Unwinding at the revolution 2-4 (cycle 6), Break at revolution 3 10 Yes 1120 16 4 Break at revolution 2 11 No 1260 19 5 Unwinding at revolution 7-9 12 No 980 14 4 Unwinding at revolution 8-10 Table 4 : Red instrument test results. The behaviour of the larger instruments is similar to those observed for the yellow instruments. Particularly, the failure mechanism for the red instruments workingwithout lubricant is the unwinding (Fig. 17). It can be observed the formation of:  Severalmicro cracks on the instrument surface (Fig. 18).  Deep cracks on the instrument file (Fig. 19). Instead, for test performed with lubricant, the instrument fails suddenly without any macroscopic previous sign. On the surface andon the file of the instruments appear the usual micro crack (Fig. 20 and 21). Also in this case the fracture surface are characterized by a dimples networkwithout the presence of any fatigue sign (Fig. 22). Concerning the load vs. displacements extraction curves, the consideration taken for the yellow instruments can be extended to the red one. The differences observed concern both the detachment load and the maximum load, which in this case are quite higher. T