Issue 35

B. Lian et alii, Frattura ed Integrità Strutturale, 35 (2016) 389-395; DOI: 10.3221/IGF-ESIS.35.44 393 E VALUATION OF THE PERFORMANCE OF THE INTRODUCED SYSTEM n order to assess the performances of this system, fatigue tests were carried out on valve spring Si-Cr oil tempered wire SWOSC-V[2]. Configuration of the fatigue specimen is presented in Fig. 7, where an artificial defect was added by 0.1 mm-diameter carbide drilling, with a depth of 0.1 mm, at the minimum cross section. Then, in order to minimize the effect of drilling, 250 °C × 30 min. vacuum heat treatment followed by mirror surface polishing condition of the minimum cross section area by OP-S were conducted. Fatigue tests are operated in air, at room temperature with a rotating speed of 3000 rpm. Figure 7 : Shape and dimensions of fatigue specimen. An experiment to assess the performance of the crack length has been undertaken. On a fatigue specimen, where the fatigue crack was already initiated, a comparison has been conduct between the crack length obtained by the system previously introduced and obtained by optical microscopy, where the fatigue test was stopped and bending weight removed (conditions equivalent to the replica method). Observation given by introduced system is presented in Fig. 8(a). Even though the photograph is dark and suffers from lack of clarity, as the bending load tends to open the crack, the crack tip can be easily indentified. In this case, this photograph points out a crack length of 2 s = 408  m, considering also the diameter of the artificial defect. On the other hand, after removing the bending weight and extracting the specimen from the testing machine, the photograph Fig. 8(b) has been taken, where the corresponding crack length found is 2 s = 419  m. Thus, even though the length obtained by introduced system is slightly lower, a relative error of approximately 3 % can be noted. The introduced system in this paper is thought to give relevant observation of the fatigue crack. Figure 8 : Comparison of fatigue crack images. (a) : Fatigue crack image obtained with observing system, (b) : Fatigue crack image obtained with optical microscope (No weight and no rotation). Change of the crack length at different cycle stages for a single specimen subjected to a fatigue test at  a = 800 MPa ( N f = 4.57 × 10 4 cycles) stress amplitude is shown in Fig. 9. As the crack propagates, if the observation field of the microscope does no more sufficient to see the entire crack length, it is necessary to switch to the configuration shown in Fig. 5(b). In I (a) (b)