Issue 35

B. Lian et alii, Frattura ed Integrità Strutturale, 35 (2016) 389-395; DOI: 10.3221/IGF-ESIS.35.44 392 tooth A is detected, the LED stroboscope flashes for a lapse time the position indicated by plain star. If the position of the magnetic sensor is changed to position 2, the LED stroboscope will flash for a lapse time the position indicated by hollow star. Thus, it is possible to adjust the flash timing over these 4 possible angular positions of the magnetic detector. As depicted in Fig. 5, as the gear used here consists of 9 teeth, in the case where there are 4 possible angular positions of the magnetic sensor, we can flash at 36 different angular positions, which means an angular interval between two positions of 10°. One can note that we can reduce such an angular interval by introducing more possible angular positions of the magnetic sensor, or installing a gear with a higher number of teeth. In the case where an artificial defect has been added at the surface of the fatigue specimen, since the crack initiation site is already known, we are in the case depicted in the Fig. 5(a), where the same angular position of the magnetic sensor will be used for the duration of the fatigue test. Nevertheless, if the crack initiation site is unknown, after initiation of the crack, it is possible to switch over the possible angular positions of the magnetic sensor in order to have an appropriate flash timing. However, between fatigue test’s start and detection of the crack initiation, the interval meter of the video camera have to record the time interval in order to obtain the number of fatigue cycle data, which is mandatory to get appropriate results. Figure 5 : How to set up LED flushing timing. (a) : Case 1 for fixed point observation, (b) : Case 2 for circumference observation. Figure 6 : Newly developed high-speed rotating bending fatigue testing machine. (a) : Side view photograph, (b) : 3D-CAD image near specimen end, (c) : LED type stroboscope and attachment cover. Rotating Bending Testing Machine The fatigue crack observation system previously introduced can be adapted to all types of rotating bending testing machine. In this study, the system was installed on a single axis spin motor high speed rotating bending machine as one can see in Fig. 6. The overview of the testing machine, details on the environment close to the hanging weight and a photo of the LED stroboscope and its plastic cover are given in Fig 6(a), (b) and (c), respectively. For this machine, motor engine is designed for machine tool usage. Thus, it is possible to reach loading frequency up to 500 Hz (=30,000 rpm), however since the chuck of the fatigue specimen is directly integrated in the principal axis of the motor, the fatigue specimen must have a very low eccentricity characteristic. (a) (b) (a) (b) (c)