Environmentally Assisted Fatigue Research Articles

Increase of nuclear installations safety by better understanding of materials performance and new testing techniques development (MEACTOS, INCEFA-SCALE, and FRACTESUS H2020 projects)

Research to better understand the phenomena influencing materials and components’ performance is important for increasing the safety of Generation II and III nuclear plants. A crucial step for improving nuclear safety is the development of new experimental techniques that can provide the necessary data. The three H2020 projects presented in this paper, MEACTOS (2017–2022), INCEFA-SCALE (2020–2025), and FRACTESUS (2020–2024), cover the steps needed to realize those safety improvements. The goal of the MEACTOS project is to improve the resistance of critical locations, including welds, to environmentally-assisted cracking through optimizing surface machining and treatments. The project is currently in its final stage, and the complete analysis of the data is finished. The objective of INCEFA-SCALE is to improve predictions of component fatigue lifetime when subjected to Environmentally-Assisted Fatigue (EAF). The strategy consists of producing guidance on how to appropriately accommodate variable amplitude and plant-relevant loading in EAF assessments. Increasing the understanding of the EAF mechanism based on substantial testing, characterization, and analysis program will support the INCEFA-SCALE strategy. The FRACTESUS project will validate the use of miniaturized compact tension specimens by comparing the results of master curve-oriented fracture toughness tests performed with small and large specimens. The round-robin exercises will use irradiated and non-irradiated Reactor Pressure Vessel (RPV) materials. The material selection process is complete in time for the project to enter the testing phase. The output of the project will be beneficial from a long-term operation perspective and a saving in the material amount needed for RPV surveillance programs. Even though each project is devoted to different research areas, common aspects are clearly visible. All three projects investigate phenomena that are relevant to the performance and safe operation of the nuclear plant. Moreover, each project will provide valuable databases and analyses of test results for materials relevant to components in the nuclear plant. The output of these projects will be of great value to the nuclear industry. This paper presents the current progress for each project, emphasizing the common research domains between the projects.

Environmental Fatigue Analysis of nuclear components within the framework of INCEFA-SCALE project

INCENFA-SCALE is a five-year project funded by the EC Horizon2020 programme, successor of the INCEFA-PLUS project. INCEFA-SCALE started in October 2020. The objective of this project is to improve the capacity to predict the lifetime of Nuclear Power Plant (NPP) components subjected to environmental assisted fatigue. The project starts off by analysing the existing data and then provides new environmentally assisted fatigue data which allow the laboratory test outcomes to be applied to components with real geometries and loads. So far, the data mining of different finished projects (INCEFA-PLUS, USNRC, EPRI, MHI and AdFaM) has been carried out, and test conditions for filling the knowledge gaps have been established. Moreover, the test matrix for 2022 has been defined. In this first phase, tests are focused on producing reference data, analysing complex waveforms (variable amplitude) and the effect of the surface finish. The next testing phases will focus on particular conditions: multi-axial tests, notches, stress/strain gradient effect and size effect. Furthermore, the microstructural analysis of common materials and a guideline for fatigue striation measurement on the fracture surface have been developed. This article provides an update on the project status and the advances made in data analysis, mechanical understanding and testing conditions.

Differences of corrosion fatigue behaviors among 316LN base metal, 316LN heat-affected zone and 308L weld metal in a safe-end weld joint in borated and lithiated high-temperature water

Corrosion fatigue behaviors of 316LN base metal (BM), 316LN heat-affected zone (HAZ) and 308L weld metal (WM) in a safe-end weld joint were investigated in borated and lithiated high-temperature water. It was found that the 316LN HAZ showed the highest environmentally assisted fatigue (EAF) effect and the 308L WM had the lowest EAF effect. The corrosion fatigue cracking mechanisms are discussed in 316LN BM, 316LN HAZ and 308L WM by considering the effects of welding residual strain, corrosion resistance and δ-ferrite.

Characterization of Austenitic Stainless Steels with Regard to Environmentally Assisted Fatigue in Simulated Light Water Reactor Conditions

A substantial amount of research effort has been applied to the field of environmentally assisted fatigue (EAF) due to the requirement to account for the EAF behaviour of metals for existing and new build nuclear power plants. We present the results of the European project INcreasing Safety in NPPs by Covering Gaps in Environmental Fatigue Assessment (INCEFA-PLUS), during which the sensitivities of strain range, environment, surface roughness, mean strain and hold times, as well as their interactions on the fatigue life of austenitic steels has been characterized. The project included a test campaign, during which more than 250 fatigue tests were performed. The tests did not reveal a significant effect of mean strain or hold time on fatigue life. An empirical model describing the fatigue life as a function of strain rate, environment and surface roughness is developed. There is evidence for statistically significant interaction effects between surface roughness and the environment, as well as between surface roughness and strain range. However, their impact on fatigue life is so small that they are not practically relevant and can in most cases be neglected. Reducing the environmental impact on fatigue life by modifying the temperature or strain rate leads to an increase of the fatigue life in agreement with predictions based on NUREG/CR-6909. A limited sub-programme on the sensitivity of hold times at elevated temperature at zero force conditions and at elevated temperature did not show the beneficial effect on fatigue life found in another study.

Incefa-Plus Project: Lessons Learned From the Project Data and Impact on Existing Fatigue Assessment Procedures

Abstract This work focuses on the analysis of the data generated during the INCEFA+ project (INcreasing safety in nuclear power plants (NPPs) by Covering gaps in Environmental Fatigue Assessment, a five-year project supported by the European Commission Horizon 2020 program). 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 subfactor 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 pressurized water reactor (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 environmentally assisted fatigue (EAF) assessment, but may also depend on the environmental correction Fen factor expression that is used to take environmental effects into account.

Environmentally assisted fatigue behavior of 308L weld metal in borated and lithiated high-temperature water

Environmentally assisted fatigue (EAF) tests in borated and lithiated high-temperature water were conducted to investigate the effect of dissolved oxygen (DO), strain rate and strain amplitude on fatigue life of 308L weld metal. It was found that fatigue lives were comparable in 0.005 ppm DO and 0.1 ppm DO water, and decreased slightly with decreasing the strain rate from 0.04%/s to 0.004%/s. The EAF effects were more pronounced at low strain amplitudes. The EAF cracking mechanisms involved with dendrite boundary orientation relative to cyclic stress axis and δ-ferrite in 308L weld metal are discussed.

High temperature fatigue of carbon/polyimide 8-harness satin woven composites. Part I: Digital Image Correlation and Micro-Computed Tomography damage characterization

This paper focuses on the experimental high temperature fatigue of Carbon/polyimide 8-harness satin woven composite materials. Traction-traction (R = 0.1) fatigue tests at 250 °C are carried out on Carbon/polyimide [45]6 8-harness satin woven composites under three different environments (air, 2 bar N2, 2 bar O2). Part I is specifically devoted to image based damage characterization by employment of Digital Image Correlation (DIC) and Micro-Computed Tomography (µ-CT) techniques. For the used samples, intra tow matrix cracks along fibres on the external plies are the first damage form at the ply-scale. Damage evolution on the sample surface can be followed during the test (in situ) by using 2D DIC. Moreover, fatigue tests are periodically interrupted to perform µ-CT scans to characterize and quantify the damage in the volume of the samples. The obtained results prove that the damage assessment methods presented in this work are suitable for environmentally-assisted fatigue tests on composite materials.

Predicting fatigue crack initiation from coupled microstructure and corrosion morphology effects

The onset of fatigue crack initiation is driven by the microstructure and the corrosion morphology; however, these mechanisms are rarely studied synergistically. In this work, characterizations of the microstructural features and the corrosion morphology resulting from two different environmental exposures are instantiated into crystal plasticity models. Fatigue indicator parameters (FIPs) are calculated from the micromechanical fields to quantify failure. The FIPs compare favorably to predict the experimentally observed location of fatigue crack initiation. These results show the potential behind analyzing environmentally-assisted fatigue crack initiation from a multivariable perspective.

Effect of zinc on the environmentally-assisted fatigue behavior of 316 stainless steels in simulated PWR primary environment

In a simulated pressurized water reactor (PWR) environment with 30 ppb of Zn and peak holding, the low-cycle fatigue (LCF) life of 316 SS increased about 3 times compared to that without them. For the specimens tested with Zn and strain holding, the crack tip oxide layer was enriched with Zn. Electrochemical analysis showed the oxide layer had higher impedance and less defect density, indicating less corrosion compared to oxide layers from non-Zn-injection conditions. Thus, the improvement in LCF life of 316 SS was attributed to crack tip oxide modification by the combined effect of Zn-addition and peak strain holding.

Research on the Application of Fen for Environmentally Assisted Fatigue Evaluation of the Austenitic SS Pipe Under Combined Transient Loads

Accumulative test data indicate that the effects of the light water reactor (LWR) environment could cause the fatigue resistance of primary pressure boundary components materials to be significantly reduced. Environmentally assisted fatigue (EAF) is the abbreviation of the environmentally assisted fatigue. In 2007, Nuclear Regulatory Commission (NRC) issued RG. 1.207. It was updated in 2014. And, it requires that the effects of LWR environment on the fatigue life reduction of metal components should be considered for new design plants. And it suggests to use environmental correction factor, Fen, to account for EAF. NRC regulation (NUREG), NUREG/CR-6909 (NRC, 2013, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” U.S. Nuclear Regulatory Commission, Argonne, IL, Standard no. NUREG/CR-6909), presents the detail Fen calculation formula. Fen is a function of temperature, strain rate, dissolved oxygen level in water, and sulfur content of the steel. Accordingly, Fen calculation will present a comparatively conservative result. Depending on the experience of the primary pressure boundary piping transient operation, Fen varies during each transient. More uncertainty and confusion are raised during the application of the Fen method. The research work in this paper includes: first, the typical character of piping thermal transient is derived based on the existing experience. Second, small specimen EAF tests are conducted depending on the above derived combined loading characters. Then effort is taken to improve the application of the Fen method for the combined multitransient loading conditions. And the results are compared with those of the lowest instantaneous Fen method and equalization of the weighted Fen method. Finally, a designed test plan is presented.

Structural Integrity for the RPV of French NPP during the LRA

Most of the French Nuclear Power Plants (NPPs) are currently embarking upon efforts to renew their operating license, while the pressurized thermal shock (PTS) events and environmentally assisted fatigue (EAF) pose potentially significant challenges to the structural integrity of the reactor pressure vessel (RPV) which has the potential to be NPP life-limiting conditions. In the assessment of the PTS events, the deterministic fracture mechanics (DFM) is still used as the basic mechanics in most countries except for the USA. While the maximum nil-ductility-transition temperature (RTNDT) is about 80°C for 54 French RPVs after 40 years operation, the maximum allowable RTNDT is only about 70 oC and 80 oC for the typical PTS events in the IAEA and NEA reports, respectively. On the other hand, the effects of light water reactor (LWR) environmental (other than moderate environment in the code) were not considered in the original design, while the effects of LWR environmental are needed to be considered in the LRA according to the USA regulations. In this paper, the challenges of the PTS and EAF are discussed, and some suggestions are also given for the LRA

An Approach for a Statistical Evaluation of Uncertainty in Assessing Fatigue Usage Including Environmental Effects

The assessment for adequacy in managing the effects of fatigue in the ASME Code Class-1 (pressure boundary) components is based on a calculated measure of the projected fatigue damage. This measure is the highest cumulative usage factor (CUF) in a given component under a specified set of cyclic loadings and their expected number of repetitions. The Code based calculation of CUF and its adjustments for potential environmentally assisted fatigue (EAF) damage accumulation utilize a multitude of inputs, and conservative assumptions and applied margins. To support the extended service life beyond the original design, or longer life of new designs, changes in inputs and/or conservative assumptions used in these deterministically calculated CUFs are often made to meet a deterministic performance criterion. This makes the impact of uncertainty in the inputs and/or changes in the conservative adjustments difficult to assess. This paper presents a generic, engineering approach for estimation of the uncertainty distribution of CUF based on the expected statistical characteristics of input variables used in the calculation of EAF-based CUF. The approach does not involve Monte Carlo sampling. The proposed statistical approach analytically combines variances of the inputs leading to an acceptable estimation of the total variance of the CUF. The approach does not require specification of full probability distribution(s) for the input variables, nor is the dependence between variables a critical issue from the analytical point of view. Feasibility and limitations of the approach are discussed in relation to the NB-3200 and NB-3600 procedures of the ASME Code and the current Fen-based augmentation for environmental effects. This approach is further examined in the framework of stress–strength interference methodology to account for the uncertainty in the fatigue performance criterion which can lead to a rational deterministic safety factor interpretation and its relation to a quantifiable measure of the probability of exceeding the fatigue performance criterion.

Study on Environment Correction Factor of Design Fatigue Curve Sample Problem and Strain Rate Calculation

Shanghai Nuclear Engineering Research and Design Institute (SNERDI) had been involved in ASME Section III environment assistant fatigue (EAF) task group (TG) and undertaken the calculation and verification work of first round sample problem, which was intended to test the engineering application of environmentally assisted fatigue (EAF) using the Fen correction factor method of Code Case N-792 Rev.0 and the proposed Code Case for evaluation of strain rate in EAF evaluations. This paper will present the sample problem of a light water reactor (LWR) primary loop stainless steel nozzle under some thermal transients and the calculation procedure of detailed integral methods according to Code Case N-792. Moreover, the method of strain rate calculation was particularly studied and discussed which is essential to the Fen value. The Fen correction factor results and the comparison with other participants is presented in this paper.

Environmentally-assisted fatigue crack growth in prestressing steel wires

We present the evolution of the surface crack front in prestressing reinforced-concrete steel wires subjected to fatigue in air and to corrosion fatigue in Ca(OH)2 + NaCl. To this end, a numerical modeling was performed on the basis of discretization of the crack front (with elliptic shape) by considering that the crack advance at each point is perpendicular to the analyzed front according to the Paris–Erdogan law and using a three-parameter solution for the stress intensity factor (SIF). Each analyzed case (a particular geometry of the initial crack) was characterized by the evolution of the aspect ratio of the semielliptic crack (ratio of the semiaxes of the ellipse) with the relative crack depth and by the variations of the maximum dimensionless SIF at the crack front.

Environmentally-assisted fatigue crack growth mechanisms in advanced materials for aerospace applications

This paper addresses the issue of environmentally-assisted fatigue crack propagation in metallic and intermetallic alloys, which is essential to predict the durability of aerospace components. A review of the current understanding of the mechanisms accounting for the detrimental effect of air on fatigue crack growth resistance of conventional alloys is first presented. The adequacy of the two sequential processes identified, namely water vapour adsorption on crack tip surfaces and hydrogen embrittlement of cyclically deformed material within the plastic zone, to account for environmentally-assisted fatigue crack propagation in alloys based on intermetallic compounds is then examined. The last section is devoted to the analysis of environmental effects during creep–fatigue crack growth in an age-hardened aluminium alloy for supersonic aircraft. In particular the influence of environment on the intergranular cavitation damage process is analysed.

Environmentally-assisted fatigue crack propagation in aluminides at room temperature

This paper reports on a study on the effect of upscaling on the fatigue crack growth resistance of a FeAl alloy prepared by mechanical alloying and forging. The alloy selected is a FeAl 40 grade 3 alloy reinforced by oxide dispersion. A batch of material prepared on a large scale mechanical alloying facility and subsequently forged is considered and compared to a reference material prepared at a laboratory scale and hipped or forged. Microstuctural analysis indicates that the grain size is coarser and the grain size distribution wider in the material obtained by upscaling. It is shown that, despite of these differences, no significant effect on the fatigue crack growth behaviour is noticed. The dadN curve presents two characteristic regimes in the ΔK range explored, regardless of microstructure. The effect of periodic overloads is examined in order to get further insights into the damage tolerance of the material. It is shown that the propagation is significantly retarded as compared to constant amplitude loading, regardless of the processing route. In addition this behaviour can be correctly accounted for by a Wheeler model. This point is supported by fractographic observations of the post-overload crack advance. This result constitutes, to the authors’ knowledge, the first report of a retardation effect on crack growth in intermetallics.

The application of microstructural fracture mechanics to various metal surface states

The fatigue crack initiation period, previously thought to be a necessary precursor to fatigue crack propagation and eventual failure, is considered here to be a negligible phase in the fatigue failure of polycrystalline metals. Rather this period is considered to be the propagation of a defect of microstructural dimensions by a variety of processes. The significance of this alternative view is examined in relation to corrosion fatigue, models of short crack growth, different loading modes, and the enhancement of fatigue resistance by surface shot-peening treatments. In both inert and aggressive environments, the fatigue lifetime of plain steel specimens of various strengths and treatments is predominantly determined by the early propagation of short cracks of microstructural dimensions. Microstructural fracture mechanics, rather than continuum mechanics, can quantify both pit growth and Stage I shear crack growth behavior before the defect reaches the dominant microstructural barrier which controls the fatigue behavior of the material. The important processes that determine lifetime are those that are strongly dependent on the synergism between the aggressive environment and cyclic stresses; these are the pitting, Stage I and the Stage I-to-Stage II crack propagation processes. A model has been produced to quantify these three important stages of lifetime named above. Under torsion loading, where Stage I cracks prefer to propagate along the surface, an intermittent series of deceleration/acceleration events of crack growth occur across the first few grain boundaries until the defect is blocked in its further development by a major microstructural barrier. When this barrier is breached, the environmentally-assisted Stage I crack rapidly becomes a Stage II crack. Under push-pull loading, the Stage I environmentally-assisted crack can propagate faster into the bulk material and, as a consequence, the transition to a Stage II environmentally-assisted crack is rapid thereby eliminating the need for the intermittent process observed under torsion loading. With no environmentally-assisted fatigue processes (i.e., testing in air) reversed torsion and push-pull loading test data can best be correlated by a von Mises criterion. Corrosion fatigue lifetimes can best be correlated by a Rankine (tensile stress) criterion. Shot-peening enhances the corrosion fatigue resistance of polycrystalline metals by inducing residual compressive stresses in the surface and creating numerous and more rapidly formed microcracks. This is probably caused by the presence of variously oriented plastically deformed bands within the surface microstructure and “short crack-short crack” interactions both of which delay the progress of the dominant crack toward its final Stage II phase.

On fatigue crack propagation enhancement by a gaseous atmosphere: Experimental and theoretical aspects

This paper is concerned with environmentally-assisted stage II fatigue crack propagation and specifically the interaction between the fatigue damage mechanisms and the exposure of the material to an active environment like ambient air. A brief state-of-the-art in the understanding of these well-knit connected processes is first presented. On the basis of original experimental results obtained on a high-strength low-alloy steel exposed to an atmosphere of very low water vapour pressure, a new light is shed on the physical description and on the modelling of environmentally-assisted fatigue damage. The salient feature is that physical adsorption of water vapour on freshly cracked surfaces can by itself enhance the damage process at the crack tip and therefore promote the stage II crack propagation in a wide range of alloys.

Effect of stress ratio on the mechanism of environmentally-assisted fatigue crack growth in high strength steels

The environmentally-assisted fatigue crack growth (FCG) behavior of high strength steels, the effect of cathodic potential on the FCG rates, and fracture surfaces were examined in some aqueous solutions. The effect of stress ratio on the critical values of maximum stress intensity, as well as cyclic stress intensity range and da/dN, which define quantitatively the role of hydrogen-induced cracking and local anodic dissolution in crack propagation, were also examined.