PSI - Issue 2_B

Balázs Fekete et al. / Procedia Structural Integrity 2 (2016) 2164–2172 Author name / Structural Integrity Procedia 00 (2016) 000–000

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A new low cycle fatigue criterion is presented based on the stored energy, which accumulates in the material during fatigue loading. The new damage parameters are based on the assumption that only the stored part of the introduced energy causes the changes in the microstructure. The proposed model is physically consistent and its prediction accuracy is higher than by the classical strain amplitude and strain energy based approaches. The low cycle fatigue behaviour investigated with the developed engineering model can provide a reference for the remaining life assessment and possible operation life extension of nuclear power plant components. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ECF21. 1. Introduction In Hungary four Russian designed VVER-440 model 213 units are in operation in Paks Nuclear Power Plant. The owner’s intention of operating these reactors after the design life is under implementation. One of the possible damage mechanisms in pressurized water reactors (PWRs) is the low cycle thermomechanical fatigue (TMF) caused by simultaneous thermal and mechanical loading, during transient operating processes (such as start up and shut down) or accident conditions. The cyclic heat load (or simultaneous heat and mechanical loads) causes alternating plastic strain in the materials of the pressurized components. When considering the safety of nuclear power plant (NPP) operation and a possible lifetime extension, the key component is the reactor pressure vessel (RPV) [Slugeň et al (2013)]. It is essential that the RPV integrity is ensured during normal and off-normal operating conditions. The low-cycle fatigue (LCF) transients can often be responsible for reducing or even eliminating the safety margin of the RPV in terms of its usage factor (UF); they may therefore be life-limiting factors in long term operation. Consequently, knowledge of LCF degradation phenomena has become increasingly important to the operation of NPPs beyond their design life. Traditionally, isothermal (ISO) low cycle fatigue tests have been used to assess the performance of materials subjected to TMF [Nagesha et al. (2009)]. However, mechanical properties of polycrystalline materials are a function of temperature, and so basic crystalline deformation mechanics need not necessarily be identical during ISO and TMF testing. Under TMF conditions, the material is subjected to a more complex stress path compared to that with ISO testing. TMF tests will activate the non-isothermal damage mechanism, simulating service-like conditions more closely than the ISO experiments [Ramesh et al. (2011)]. Our motivation in carrying out this research was to develop a sophisticated test facility for the GLEEBLE-3800 thermomechanical physical simulator, which is able to perform TMF tests under the service temperature conditions of VVER-440 reactors. To study the microstructural evolution of the materials during the fatigue process transmission electron microscopy (TEM) and X-ray diffraction (XRD) investigations were performed using samples at different stages of fatigue life. For specimens fatigued to final failure for 15Ch2MFA samples, the crack positions, shapes and fracture surfaces were investigated with scanning electron microscopy (SEM). An important issue with respect to the LCF is the engineering description of the fatigue lives. In general, fatigue life assessments may be based on stress, strain or energy parameters. The energy parameters have several advantage: (a) both strain and stress parameters are included in these models (b) these variables are independent of the direction as they are scalar value, (c) cumulative damage is easy estimate, (d) the model results are more suitable for transferring to the real structure. To our point of view the weakness of the existing energy based LCF engineering models is that the damage parameter includes the dissipated heat generated during the tests, resulting in an inappropriately higher value for fatigue toughness. Due to the above mentioned reasons a new measure of LCF damage was developed: the absorbed strain energy density, which does not include the dissipated heat. Keywords: Low cycle fatigue; reactor steel; dislocation density