185 ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved. E-Journal of Advanced Maintenance Vol.5 (2013) 185-200 Japan Society of Maintenology Observation of Fatigue Crack Initiation and Growth in Stainless Steel to Quantify Low-Cycle Fatigue Damage for Plant Maintenance Masayuki KAMAYA 1,* 1 Institute of Nuclear Safety System, Inc., 64 Sata, Mihama-cho, Fukui 919-1205, Japan ABST RACT Quantifying the low-cycle fatigue damage accumulated in nuclear power plant components is one of the important issues for aged plants. In this study, detailed observations of crack initiation and growth were made using scanning electron microscopy in order to correlate the crack size and the magnitude of the fatigue damage. Type 316 stainless steel specimens were subjected to the strain-controlled axial fatigue test (strain range: 1.2%) in air at room temperature. The test was interrupted several times in order to observe the specimen surface. The spatial distribution of inhomogeneously accumulated damage by cyclic loading was identified by crystal orientation measurements using the electron backscatter diffraction technique. Cracks were initiated at grain boundaries and slip steps, where relatively large damage accumulated. The changes in the number of cracks and their length were quantified. The crack growth rates were well correlated with the strain intensity factor. The change in crack size during the fatigue test was predicted using the obtained growth rate and assumed initial crack size. The fatigue lives estimated by the crack growth prediction agreed well with those obtained experimentally. It was concluded that the fatigue damage could be estimated from the crack size measured in plant components. * Corresponding author, E-mail: kamaya@inss.co.jp KEYWORDS low-cycle fatigue, fatigue damage, crack growth, fatigue life, life prediction, stainless steel, strain intensity factor ARTICLE INFORMATION Article history: Received 17 June 2013 Accepted 2 September 2013 1. Introduction In the structural design of nuclear power plant components, fatigue damage has been assessed by quoting a usage factor (UF) [1], which is accumulated fatigue damage according to the linear damage accumulation rule. Nowadays, due to long time operation over forty years for some plants, UF may exceed the critical value for some components. Even if UF exceeds the critical value, however, the components may not be always damaged because sufficient margins were considered in the design. It is difficult to judge whether the components of interest are actually damaged or not from surface observations or inspections. It is important to correlate the fatigue damage to a material change which can be measured in order to quantify the fatigue damage accumulated in the components. In order to quantify the fatigue damage, it is important to understand what kind of change is brought about by cyclic loading, and to know the critical conditions for fatigue life. Using carbon steel (S45C) specimens, Murakami and Miller [2] showed that crack initiation and growth could be regarded as the damage caused by fatigue loading and that fatigue life was almost equivalent to the number of cycles for the initiated cracks growing to the critical size. Other researchers showed that the remaining fatigue life of damaged specimens was recovered by removing cracks initiated on the specimen surface for carbon steel [3][4] and stainless steel [5]. The fatigue loading causes various microstructural changes such as an increased number of dislocations, formation of cell structures, and alternation of deformation properties. However, these changes have minor influence on the fatigue life under the same strain range. Therefore, the fatigue damage can be correlated to the crack size. It is possible to measure (quantify) the fatigue damage by investigating the size of a crack initiated in components, or if no crack is found, it can be confirmed that damage has not been accumulated yet.