13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Study of fatigue crack mechanism on an armco iron in the gigacycle fatigue by temperature recording and microstructural observations Chong WANG1, Danièle WAGNER1*, Claude BATHIAS1 1 University Paris Ouest Nanterre La Défense – LEME Laboratory – 50, rue de Sèvres – 92410 VILLE D’AVRAY – France *Corresponding author : daniele.wagner@u-paris10.fr , Abstract: Whatever the fatigue domain, the fatigue crack mechanism consists of an initiation crack stage (stage I) and a propagation stage (stage II). For materials without inclusions with a single phase, the first damage events in the stage I are due to the occurrence of Slips Marks (SM) on the specimen surface. In this study, the fatigue crack mechanism was studied in the VHCF domain on a body centered cubic Armco iron (with 80ppm of carbon content).The tests were performed on a piezoelectric fatigue machine on plate specimens. During the tests, the microstructure evolution was observed by optical microscope, and the temperature recording on the specimen surface was achieved by an infrared camera. After the failure specimen, fractographic observations were performed under a Scanning Electron Microscope; From the temperature recording on the specimen surface near the section where the fracture occurs, the spatio temporal localization of the point where the temperature was the highest was achieved. The highest temperature corresponds to the crack tip where the plastic zone takes place. This allows to follow the evolution of the crack tip during the test. The results are compared with the fractographic examinations on the fracture surface, and the number of cycles in each stage can be calculated. Keywords: Very High Cycle Fatigue, fatigue crack mechanism, Armco iron, piezoelectric device, Slips Marks, infrared thermography 1. Introduction Whatever the fatigue domain, the fatigue crack mechanism consists of an initiation crack stage (stage I) and a propagation stage (stage II). For materials without inclusions with a single phase, the first damage events in the stage I are due to the occurrence of Slips Marks (SM) on the specimen surface [1,2]. At the beginning of the long crack propagation stage, striations occur (stage II). Previously, the temperature recording was performed on the surface of cylindrical specimens [4,5] on materials with inclusions which leads to subsurface crack initiations with “fish eyes” formations . In this paper, flat specimens were used in order to improve the temperature recording on the surface specimen. The studied material is an Armco iron without inclusions which leads to a surface crack
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