Fatigue tests under spectrum loading were conducted to evaluate hole cold-expansion in thick-section open-hole aluminium alloy specimens, some of which contained residual fatigue cracks before cold expansion. Cold expansion resulted in an increase in life by a factor of about 7. Small residual fatigue cracks did not inhibit the effectiveness of the cold-expansion process, indicating that it may not be essential to remove such cracks prior to hole cold-expansion. The increase in life is primarily associated with a reduced crack propagation rate compared with that for cracks from non-cold-expanded holes. Cold-expanded hole fractures displayed a marked disparity in crack depths adjacent to the two faces of the specimens. Considerable differences were evident in crack depths and fatigue crack areas at failure between cold-expanded and non-cold-expanded hole specimens. These findings have ramifications in the damage tolerance assessment of aircraft structures.
The cyclic behavior and endurance of austenitic stainless steels tested under NPP-relevant laboratory conditions has been studied. It had been earlier shown that long intervals between fatigue transients can affect the fatigue performance in stainless steels that are generally used in NPP primary piping. If this can be confirmed, then the transferability between laboratory results, design curves and the fatigue behavior of NPP components during plant operation shall be addressed. In addition to coolant water environmental effects, the material response during steady state normal operation should also be accounted for. Advanced Fatigue Methodologies (AdFaM), a joint project of European research laboratories, vendors and plant operators was focused on empirical and mechanistic investigations to confirm the claimed effects of hold times on fatigue life. Strain-controlled fatigue tests incorporating accelerated hold times at temperatures between 290°C and 420°C were performed on stabilized and non-stabilized stainless steel grades, which are used in Germany and the UK. Two material batches of both alloy types (304L and 347) were studied. The mechanisms responsible for the observed variations in stress response and fatigue life have been investigated using a range of microscopy techniques. The results confirmed the extension of fatigue life due to hold times in both stabilized and non-stabilized grades. This life extension appears to be linked to hold hardening observed in the cyclic behavior of both alloys. Tests incorporating hold times may be more representative of material behavior in NPPs, where temperature transients due to start-ups, shutdowns and major power changes may be separated by long periods of steady state operation. This gives reason to consider the transferability of standard laboratory test data to fatigue assessments of NPP components, and to propose a new factor ( Fhold ) as part of an advanced fatigue methodology and realistic transferability factor: Freal = Fen × Fhold.
This work focuses on the analysis of the data generated during the INCEFA+ project (INcreasing Safety in nuclear power plants by Covering gaps in Environmental Fatigue Assessment, a five year project supported by the European Commission HORIZON2020 programme). More specifically, this paper discusses how the outcome of this analysis can be used to evaluate existing fatigue assessment procedures that incorporate environmental effects in a similar way to NUREG/CR-6909. A key difference between these approaches and the NUREG/CR-6909 is the reduction of conservatisms resulting from the joint implementation of the adjustment sub-factor related to surface finish effect (as quantified in the design air curve derivation) and a Fen penalization factor for fatigue assessment of a location subjected to a PWR primary environment. The analysis presented in this paper indicates that the adjustment (sub-)factor on life associated with the effect of surface finish in air (as described in the derivation of the design air curve in NUREG/CR-6909) leads to substantial conservatisms when it is used to predict fatigue lifetimes in PWR environments for rough specimens. The corresponding margins can be explicitly quantified against the design air curve used for EAF assessment, but may also depend on the environmental correction Fen factor expression that is used to take environmental effects into account.
Future code development to adopt a risk-informed design methodology will require improved accuracy of fatigue initiation predictions. The ability to account for through wall strain gradients in plant components, particularly over the first 3 mm of wall thickness is one area where conservatism can be reduced. This is due to extant design fatigue curves being derived from strain controlled membrane loading tests where the 25% load drop definition for end of test equates to approximately a 3 mm crack. Being able to define initiation fatigue curves for much shorter crack depths would enable fatigue crack growth methods to then predict the additional cycles, taking into consideration the strain gradient, required before the defined end of life crack size is reached. The R5 procedure provides a method to adjust an existing Stress-Life (SN) curve representing an initiation crack depth, to a smaller depth. This method was developed for materials at higher temperature and for a CO2 cooling environment, thus its validity was uncertain for application to a Pressurised Water Reactor (PWR) plant. This paper details the development of best estimate and design basis SN curves and environmental fatigue enhancement factors (Fen) for crack initiation to a depth of 250 μm. It is concluded that the general methodology in R5 was found, through this work, to adequately describe fatigue initiation lives for stainless steel in a PWR environment when augmented with a crack size dependent Fen equation and with modified coefficients determined through regression. The resulting method is similar to R5 in that an SN curve can be derived for any crack size, however the current data set only provides validation down to a crack size of 250 μm, as striations at shorter depths were not visible with existing methods.
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