[en] It is well established that corrosion is thermally activated: as for all thermally activated processes, the establishment of the process kinetics is of fundamental importance. It is the purpose of this presentation to develop the physical theory of the kinetics of corrosion fatigue and to define the corresponding experimental techniques and methodology for the evaluation of the rate controlling physical parameters. (author)

[en] The AREVA fatigue concept (AFC) provides for a multi-step and multi-disciplinary process (process engineering, fatigue monitoring, fatigue analyses, etc.) against fatigue in the design and construction phase and also during the entire operation cycle of nuclear power plants (NPPs). The ensuing fatigue analyses are based on the real operational loads measured continuously on site in the plant. The entire process of fatigue design is based on an installed fatigue monitoring system, the FAMOS. This way, realistic load data are available. This system enables a reliable and realistic fatigue assessment of stressed components. FAMOS, as the central module of the AFC, contributes critically to the management of the component ageing problem and enables the identification of operating modes unfavorable to fatigue. Moreover, code conforming fatigue usage factors can be determined for the lifetime of the plant. The direct processing of the measured temperatures is immediately used for a fast fatigue evaluation (FFE). By means of FFE, the inverse thermal field at the inner surface of the pipe is calculated and transferred from measuring location to the component of concern for stress calculation. Then, a real time fatigue evaluation of the cumulative usage factor (CUF) can be performed after every operational cycle. This procedure is highly automated and gives a first fatigue status of the power plant. In the framework of the periodic safety inspection (PSI), a detailed fatigue calculation conforming to the code rules is carried out in order to determine the current state of the plant. This detailed fatigue check is based on the real loads (specification of thermal transient loads) and finite element analyses (FEA) in connection with the local strain approach to design against fatigue. These FEA always include transient thermal determination of the temperature field and subsequent determination of stresses and strains. The latter analyses may consist of simplified elastic or of fully elastic-plastic analyses. As a conclusion, the AFC is a modern lifetime management concept for fatigue assessment based on realistic plant data retrieved by FAMOS. It enables the realistic estimation of service loads, follow up of fatigue tendencies and code compliant fatigue checks. The essential benefits for the customer to apply the AFC are readily recognized. Locations of potential fatigue failure are reliably identified and all efforts can be concentrated on these fatigue critical components. Thus, costs for inspection can be essentially streamlined with the prerequisite of implementing the temperature measurement system FAMOS in the power plant. (authors)

[en] Heat recovery steam generators (HRSGs) are responsible for more and more of the steam generating capacity of many utilities. Unfortunately, they are also increasingly responsible for the number of tube failures in the system. Corrosion that leads to failures often begins before commissioning and is exacerbated by cycling operation. This article reviews common waterside failure mechanisms in HRSGs, where they occur, and what can be done to prevent failures in the future. (orig.)

[en] The origin of fatigue in multiple sclerosis (MS) remains uncertain. However, the use of nonconventional magnetic resonance techniques has increased our understanding of this problem. We aimed to study the relationship between fatigue in MS and the presence of focal dysfunction in the basal ganglia and frontal white matter. Included in the study were 41 patients with relapsing-remitting MS with mild disability and 20 healthy controls. Fatigue was assessed by the Fatigue Severity Scale (FSS) and the Modified Fatigue Impact Scale (MFIS). Patients were classified as ''fatigued'' when they expressed a subjective feeling of fatigue, and the FSS score was ≥5.0 and/or the MFIS score was >38. Patients with no subjective fatigue were classified as ''nonfatigued'' when the FSS score was <4.0. Proton magnetic resonance spectra were obtained from two different regions: the frontal white matter and the lentiform nucleus. The relationships between fatigue and NAA/Cr, NAA/Cho and Cho/Cr ratios were analysed. A significant decrease in NAA/Cr in the lentiform nucleus region in patients with fatigue was observed. No differences between the groups were found in the frontal white matter. Although confirmatory studies are needed, our results would support the idea that a specific dysfunction or involvement of the basal ganglia might partly contribute to the development of MS-related fatigue. (orig.)

[en] The effect of loading variables and temperature on fatigue crack growth rate in SA508 C1.3 nuclear pressure vessel steel was investigated in air environment. Crack growth rate tests on compact tension specimen of thickness 12 mm were conducted by using sinusoidal waveform. The crack length was monitored by compliance method. Test conditions were at 0.1 and 0.5 of load ratio, at 1 and 10 Hz of loading frequency, and at room temperature to 400.deg.C. At the lower temperatures, the fatigue crack propagation was not affected by the frequency and temperature, while at the higher temperatures above 120.deg.C, fatigue crack growth rate increased with decreasing loading frequency and increasing temperature. This accelerated fatigue crack propagation was associated with the increase of oxidation rate at the ahead of crack tip. Fatigue crack growth rate increased with increasing the load ratio. The effect of load ratio was more significant at the lower temperature, while the dependence on load ratio decreased with increasing temperature. The sensitivity of load ratio to temperature can be explained by crack closure with the oxidation process. 8 figs., 25 refs. (Author)

[en] During maintenance visit on 93 D type primary pumps of French 900 MWe nuclear units, cracking has been evidenced on the thermal barrier, first on the flange, on the face of connection of the cooling, water coils, and then on the weld between the housing and the flange. Laboratory examinations have exhibited that this cracking is due to a fatigue phenomenon which is initiated on locations where high residual stresses are present. One pump, in service in a plant, has received an instrumentation in order to determine stress cycling. Measurements of temperature on the surface of the metal have shown the presence of thermal cycling due to the thermohydraulic conditions inside the thermal barrier. A non destructive testing method using ultrasounds has been developed in order to asses the magnitude cracking. Corrective and preventive actions have been implemented for repairing and improving thermal barrier when cracking is detected. (authors)

[en] The cyclic life of IN738LC, a widely used nickel base superalloy for blades in stationary gas turbines, was investigated under thermo-mechanical fatigue loading using a temperature variation range of 1023 to 1223 K, with temperature variation rate in the range of 6 to 15 K/min. Simple thermo-mechanical cycles with linear sequences corresponding to in-phase (IP) and out-of-phase (OP) tests were performed. Both the IP and OP tests were carried out at different constant mechanical strain ranges varied between 0.8 to 2.0% and at a constant mechanical strain rate of 10^-5 s^-1. Thermo-mechanical fatigue lives under both test conditions were compared with each other and with those of isothermal LCF tests at a temperature of 1223 K. The results show that the life under thermo-mechanical fatigue is strongly dependent on the nature of the test, i.e. stress controlled or strain controlled. (orig.)

[en] For one stage realistic long term alternating strain tests on two forged steels with the duration of tests up to an order of magnitude of 45,000 hours, the generalised damage accumulation rule, using an optimised evaluation process dealing with pre-stress effects leads to a relative creep fatigue service life of one. A replacement description by the modified service life share rule is indicated for the long term area. First results from realistic three step tests are classified in the scatter band of single stage stress, where there are only slight differences from different cycle counting processes. (orig.)

[en] The existing limited information which is relevant to thermal striping is considered and design proposals to avoid damage are made. Additional tests are identified which should reduce the uncertainties associated with materials behaviour and permit a strengthening of the basis on which to make design recommendations. (author)

[en] Fractographic examination has been performed on a number of corrosion fatigue specimens, which had been exposed to a 290 C aqueous environment simulating the primary coolant of an operating Pressurised Water Reactor (PWR). The aim of the examination was to help provide an explanation for the significant variations in fatigue crack growth behaviour observed between the various specimens. Characteristic differences were observed in the appearance of the fracture surface between specimens that had shown environmentally enhanced crack growth and those that had not. Second phase sulphide inclusions were observed to have a major influence on the onset of environmentally assisted cracking. A localised influence of the test environment on the fracture surface morphology could frequently be detected even when macroscopic crack propagation rates were not enhanced. The highest crack growth rates were measured in environments which were contaminated with sulphur-bearing anions or oxidising agents. (author)