Issue 35

B. Lian et alii, Frattura ed Integrità Strutturale, 35 (2016) 389-395; DOI: 10.3221/IGF-ESIS.35.44 389 Focussed on Crack Paths Development of in-situ fatigue crack observing system for rotating bending fatigue testing machine B. Lian Yamamoto Metal Technos Co., Ltd. A. Ueno Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan T. Iwashita Graduate School Student of Ritsumeikan University A BSTRACT . To substitute for a traditional replication technique, an in-situ fatigue crack observing system for rotating bending testing machine has been newly developed. For verifying performance of this observing system, fatigue tests were carried out by using fatigue specimen having a small artificial defect. It is proved that this system can be detect a small fatigue crack and its propagation behavior. K EYWORDS . In-situ observation; Fatigue crack initiation; Fatigue crack propagation; Rotating bending testing machine. I NTRODUCTION n the field of the fatigue properties of metallic materials, the assessment of the S-N curve, which consists to the stress level and the number of cycles up to failure, does not give an entire viewpoint of the phenomenon. Indeed, a lot of information on the fatigue properties can be gathered by the observation of the fatigue crack initiation and propagation directly on the specimen surface. Unlike the case of pre-cracked specimens used for fracture mechanics experiments, the initiation and the propagation of the conventional fatigue specimens are often investigated by replica methods. Due to the performances of replica made of cellulose acetate film good enough to duplicate the specimen surface, it is possible to detect very small fatigue crack just after the initiation. However, it is necessary to have some experience on the use of this technique, in addition there are also cases where the utilization of replica is complex. It is particularly the case for fatigue specimens designed for rotating bending tests, with an hourglass shape at the minimum cross section spot. Nevertheless, performances of some optic devices have been improved in the latest years, in the field of: (i) affordable long working distance microscopy, (ii) high-definition video camera, and (iii) high brightness LED components. The combination of these new developments gives the possibility to design a system adapted for fatigue rotating bending tests, and thus to observe continuously the fatigue crack propagation. I