José Mendez

Fatigue Crack Initiation of 304L Stainless Steel in Simulated PWR Primary Environment: Relative Effect of Strain Rate

The French Regulatory Commission insisted on a survey justifying the assumed mechanical behavior of components exposed to Pressurized Water Reactor (PWR) water under cyclic loading without taking into account its effect. In the US and Japan, the fatigue life correlation factors, so called Fen, are formulated and standardized on the basis of laboratory data to take into account the effect on fatigue life evaluation.However, the current fatigue codification, suffers from a lack of understanding of environmental effects on the fatigue lives of stainless steels in simulated hydrogenated PWR environments. Samples tested in a recent study were analyzed to highlight the strain rate effect (within a range 0.4%/s to 0.004%/s) at the early stage of fatigue life in PWR primary environment for a 304L stainless steel. The deleterious effect of PWR primary environment on fatigue crack initiation was observed with a quantitative microscopic approach. Multi scale observations of oxide morphology and microstructure were carried out from common optical microscopy using recent technologies such as 3D oxide reconstruction, and DualBeam observations.Copyright

Influence of the Strain Rate on the Low Cycle Fatigue Life of an Austenitic Stainless Steel with a Ground Surface Finish in Different Environments

This paper focuses on the influence of strain rate in Low Cycle Fatigue (LCF) of a 304L austenitic stainless steel at 300 °C in different environments (secondary vacuum, air and Pressurized Water Reactor (PWR) water environment). Moreover test samples are ground to obtain a surface finish rougher than all that could be found in nuclear power plants. Different strain rates (4x10-3, 1x10-4 and 1x10-5 s-1) are studied, with a triangular waveform at a total strain amplitude of ±0.6%. The influence of strain rate on cyclic stress-strain behavior and fatigue life is firstly analyzed in secondary vacuum, considered as a non-active environment. Then, interactions between stain rate and environmental effects in Air and in PWR environment are presented. In all environments, a decrease in strain rate leads to a negative strain rate dependence of the stress response and a reduction in fatigue life. Finally, SEM observations of fatigue striations in PWR environment indicate a crack propagation rate enhancement when the strain rate is decreased.

3D Imaging Using X-Ray Tomography and SEM Combined FIB to Study Non Isothermal Creep Damage of (111) Oriented Samples of γ / γ ' Nickel Base Single Crystal Superalloy MC2

An unprecedented investigation consisting of the association of X-Ray tomography and Scanning Electron Microscopy combined with Focus Ion Beam (SEM-FIB) is conducted to perform a 3D reconstruction imaging. These techniques are applied to study the non-isothermal creep behavior of close (111) oriented samples of MC2 nickel base superalloys single crystal. The issue here is to develop a strategy to come out with the 3D rafting of γ’ particles and its interaction whether with dislocation structures or/and with the preexisting voids. This characterization is uncommonly performed away from the conventional studied orientation [001] in order to feed the viscoplastic modeling leading to its improvement by taking into account the crystal anisotropy. The creep tests were performed at two different conditions: classical isothermal tests at 1050°C under 140 MPa and a non isothermal creep test consisting of one overheating at 1200°C and 30 seconds dwell time during the isothermal creep life. The X-Ray tomography shows a great deformation heterogeneity that is pronounced for the non-isothermal tested samples. This deformation localization seems to be linked to the preexisting voids. Nevertheless, for both tested samples, the voids coalescence is the precursor of the observed damage leading to failure. SEM-FIB investigation by means of slice and view technique gives 3D views of the rafted γ’ particles and shows that γ corridors evolution seems to be the main creep rate controlling parameter.

Influence of PWR Primary Water on LCF Behavior of Type 304L Austenitic Stainless Steel at 300°C: Comparison With Results Obtained in Vacuum or in Air

Nowadays, it is well known that the low cycle fatigue (LCF) life of austenitic stainless steels can be affected in specific conditions of temperature, strain rate, strain amplitude or dissolved oxygen concentration by the effect of Pressurized Water Reactor (PWR) primary coolant environment. Nevertheless, questions remain about the best methodology that must be used to consider environmental effects for nuclear power plant licensing and for operating lifetime extensions.These environmental effects are most commonly evaluated from a mean fatigue curve based on tests conducted in air at room temperature. However, it is well established that air is not a neutral environment for metallic alloys and its effect can be highly dependent on the temperature level.Thus, in order to evaluate the intrinsic fatigue resistance of a 304L austenitic stainless steel at 300°C and the importance of complex fatigue – environment interactions in air or in PWR water, LCF tests were performed in both environments and specifically designed ones were conducted in secondary vacuum. Tests were performed on 304L cylindrical specimens at 20 or 300°C in vacuum or in air and only at 300°C in PWR water, under total axial strain control using a triangular waveform at strain amplitudes of ±0.3 or ±0.6% and strain rates of 4 × 10−3, 1 × 10−4 or 1 × 10−5 s−1.It was found that compared with vacuum, air is responsible for a strong decrease in fatigue lifetime in this steel, especially at 300°C and low strain amplitude. The PWR water coolant environment is still more active than air and leads mainly to increased damage kinetics, with slight effects on initiation sites or propagation modes. More precisely, the decreased fatigue life in PWR water is essentially attributed to an enhancement of both crack initiation and “short crack” micropropagation stages. Furthermore, a detrimental influence of low strain rates on the fatigue lifetime at 300°C was observed in PWR water environment or in air, but also in vacuum without environmental effects, and was in the last case exclusively attributed to the occurrence of the dynamic strain aging (DSA) phenomenon.So, the use of data obtained in a neutral environment as a reference allows the evaluation of the intrinsic effect of each environmental or loading condition. Moreover, in an active environment such as air or PWR primary water, damage evolutions as well as fatigue lives cannot be predicted by a simple multiplication of each parameter effect taken separately because they are the result of numerous interactions.The last conclusion is supported by complementary results showing that the PWR water environment effect as well as the ground surface finish effect can be attenuated when LCF tests are performed with a more representative loading signal shape.Copyright

A Strain Rate Sensitive Formulation to Account for the Effect of \gamma ^{\prime } Rafting on the High Temperature Mechanical Properties of Ni-Based Single Crystal Superalloys

The **Polystar** model was recently developed to fulfill the effects of possible fast microstructure evolutions occurring upon high temperature non-isothermal loadings. New internal variables were introduced in a crystal plasticity framework to take into account microstructure evolutions such as \(\gamma ^{\prime }\) dissolution/precipitation and dislocation recovery processes, and their effects on the creep behavior and creep life. Nevertheless, this model does not take into account one of the main microstructural evolutions occurring specifically at high temperature, the \(\gamma ^{\prime }\) directional coarsening. Fedelich and Tinga have already proposed models respectively based on a modification of the kinematic hardening and on the level of the von Mises stress. Nevertheless, if the Fedelich’s model is implicitly strain rate sensitive, improvements have to be performed for strain controlled tests under fast conditions for which such a model may overestimates the \(\gamma \) channel width evolutions. A new formulation has been proposed to explicitly account for such a strain rate sensitivity and was successfully implemented in the **Polystar** model. The effect of \(\gamma ^{\prime }\) rafting on the mechanical behavior is well reproduced for both cyclic and monotonic tension tests.

Fatigue Life of the Strain Hardened Austenitic Stainless Steel in Simulated PWR Primary Water

Over the last 20 years or so, many studies have revealed the deleterious effect of the environment on fatigue life of austenitic stainless steels in pressurized water reactor (PWR) primary water. The fatigue life correlation factor, so-called Fen, has been standardized to consider the effect on fatigue life evaluation. The formulations are function of strain rate and temperature due to their noticeable negative effect compared with other factors [1,2]. However, mechanism causing fatigue life reduction remains to be cleared.As one of possible approaches to examine underlying mechanism of environmental effect, the authors focused on the effect of plastic strain, because it could lead microstructural evolution on the material. In addition, in the case of stress corrosion cracking (SCC), it is well known that the strain-hardening prior to exposure to the primary water can lead to remarkable increase of the susceptibility to cracking [3,4]. However, its effect on fatigue life has not explicitly been investigated yet.The main effort in this study addressed the effect of the prior strain-hardening on low cycle fatigue life of 304L stainless steel (SS) exposed to the PWR primary water. A plate of 304LSS was strain hardened by cold rolling or tension prior to fatigue testing. The tests were performed under axial strain-controlled at 300 °C in primary water including B/Li and dissolved hydrogen, and in air. The effect on environmental fatigue life was investigated through a comparison of the Fen in experiments and in regulations, and also the effect on the fatigue limit defined at 106 cycles was discussed.Copyright