Issue 35

B. Lian et alii, Frattura ed Integrità Strutturale, 35 (2016) 389-395; DOI: 10.3221/IGF-ESIS.35.44 390 This paper deals first with an explanation of the optical system designed here, then some fatigue crack observation results obtained are also introduced. S YSTEM OUTLINE Optic-System n the case of the rotating bending fatigue tests, since the fatigue specimen at the minimum cross section is incurvated, if the axis of microscope is the same as the axis enlightenment of the specimen, only the summit of the specimen will be observable, as shown in Fig. 1(a). Such a configuration is not suitable to obtain the full image of the entire crack on the specimen surface. Nevertheless, by adopting a configuration where several light sources are circularly placed around the fatigue specimen, it is possible to enlighten favorably a larger zone, as in Fig. 1(b). Figure 1 : Schematics of lighting. (a) : In case of point light source, (b) : In case of surround light source. In order to get observation in adequate synchronization with the rotation speed of the fatigue test, stroboscope device is usually set in order to make the observation of the highest position of the fatigue specimen, where the maximum tension stress level is reached. However, in the case of conventional stroboscope with xenon lamp components, the directive signal of flash, in other words the pulse frequency, is initially set, since the lamps’ flash and the pulse frequency is not totally in agreement. Thus, the timing of the observation will be shifted, resulting in a loss of luminosity compared to the optimum position for crack observation. To solve this problem, use of stroboscope with high luminosity LED components gives a more favorable observation condition than conventional stroboscope. In the rest of this paper, the introduced system gave acceptable observation conditions by setting the flash directive signal. Indeed, even though the flash period was extremely short, the luminosity given out by white color high luminosity LED, arranged on circuit boards illustrated in Fig. 2(a), was sufficient. More precisely, two circuit boards were installed around the specimen, as indicated in Fig. 2(b). Figure 2 : LED type stroboscope. (a) : LED array plan, (b) : Arrangement plan. I (a) (b) (a) (b)