13th International Conference on Fracture June 16–21, 2013, Beijing, China Is There a Threshold for PSB Formation in Iron Chong Wang1,*, Danièle Wagner, Claude Bathias 1 LEME, University Paris Ouest Nanterre La Défense 50, rue de Sèvres, 92410 VILLE D’AVRAY, France * Corresponding author: wangchongscu@163.com ABSTRACT Thin flat specimens (1mm thickness) have been tested on a piezoelectric fatigue machine in 20 KHz frequency on High cycle fatigue domain and beyond. The result shows that body centered cubic Armco iron (with 80ppm of carbon content) could fail after 109 cycles at a loading well below the yield stress. Observations under Scanning Electron Microscope on the specimen surface and the fracture surface indicated gigacycle fatigue failure originated in the specimen surface and related to the formation of Persistent Slip Bands (PSB). The microstructure evolution was observed by optical microscopy. It was found that PSB not only appear at the beginning of ultrasonic fatigue test but also increase with the number of cyclic loading. Keywords Persistent Slip Band, irreversible deformation, localized plasticity, very high cycle fatigue 1. Introduction For single phase materials without inclusions, testing in very high cycle fatigue regime, the first damage event of crack initiation is due to the occurrence of Slips Marks (SM) on the specimen surface [1, 2]. In f.c.c materials (which are the most studied materials), these SM are called Persistent Slips Bands (PSB), with a particular dislocation structure beneath the PSB [3]. When strain reached to cyclic stress-strain curve plateau, the deformation structure changes to the PSB instead of dislocation veins in matrix in order to accommodated the high value of plastic deformation [1]. Intrusions at PSB or PSB/matrix are interface preferential sites for the nucleation of microcracks[4]. In bcc materials, like Armco iron, the identification of these SM with PSB is matter of debate [3-9]. The reason lies in the very different temperature and strain rate dependent dislocation glide behavior in body centered cubic (b.c.c) metals, as compared to f.c.c metals. Moderate increase of temperature, low cyclic strain rates and alloying by substitution atoms (e.g. in Fe-Si) and interstitial atoms (e.g. C and N in -iron) make the dislocation glide modes of bcc metals more similar to those of f.c.c metals and then, “PSBs” may be observed. For a basic point of view, it is of interest to understand the initiation of a fatigue crack in the VHCF regime in pure alloys or metals without internal defects. Studies tested at different loading conditions given references for investigating fatigue features on iron. Dislocation arrangement in iron after fatigue bending test at low frequency are investigated by Mcgrath and Bratina[10]. They find that the average dislocation density for particular stress amplitude reaches a constant value after showing a rapid increase in the early portion of the fatigue life. When Wood et al [11] compare the fatigue mechanisms in bcc iron and fcc metals at low frequency in alternating
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