Creep-fatigue Crack Initiation Research Articles

Neural network-assisted probabilistic creep-fatigue assessment of hydrogenation reactor with physics-based surrogate model

In order to obtain better hydrogenation reaction efficiency, the prospective hydrogenation reactor faces harsh working conditions, including the high-stress level, high-temperature environment, long creep dwell period and degradation of material resistance, leading to significant creep-fatigue interaction at the structural critical locations. The uncertainty widely existing in such design conditions further aggravates the creep-fatigue failure risk of the hydrogenation reactor structure, and makes the damage evolution and lifetime difficult to capture and predict. According to the latest appendix A15 of the R5 Procedure, a series of probabilistic analysis practices are delivered based on Volume 2/3 regarding creep-fatigue crack initiation. In this study, the probabilistic analysis for the hydrogenation reactor is implemented by the neural network-assisted creep-fatigue surrogate model under the probabilistic Linear Matching Method Framework (pLMM), with the superior balance between the computational efficiency and prediction accuracy fully achieved. The statistical distribution of structural creep-fatigue lifetime is investigated with the physics-based modelling technology, and the reliability-centred evaluation diagram and safety factors of the hydrogenation reactor are proposed for different reliability levels, which is dedicated to facilitating the risk management of crucial components in the hydrogen industry.

Investigation of creep-fatigue crack initiation by using an optimal dual-scale modelling approach

In this work, three dual-scale modelling approaches containing sub-modelling approach, coupled modelling approach and full-field crystal plasticity finite element (CPFE) approach are implemented for notched structures under creep-fatigue loading conditions. Based on the comprehensive comparisons, the sub-modelling approach is determined to be an optimal simulation strategy among them. Furthermore, the combined effects of grain orientation and stress concentration on crack initiation are investigated by adopting the sub-modelling approach. Under low stress concentration factor, the most potential positions of crack initiation randomly locate at the root of geometric discontinuities owing to the influence of grain orientation distribution. With the increase in stress concentration factor and decrease in the number of grains at hotspots, the cooperative relation between grain orientation and stress concentration factor for creep-fatigue crack initiation is revealed through a sequence of simulation results, followed by the competitive relation. On this basis, a feature region map is constructed to distinguish the factor-dominated crack initiation. Finally, a case study of turbine disk is provided to illustrate the engineering meanings of the dual-scale modelling approach. The link from scientific problem to engineering application for the crack initiation prediction is bridged by the damage mechanics approach, which is also expected to be promoted in many high-temperature components.

Data‐driven prediction of the probability of creep–fatigue crack initiation in 316H stainless steel

AbstractStainless steel components in advanced gas‐cooled reactors (AGRs) are susceptible to creep–fatigue cracking at high temperatures. Quantifying the probability of creep–fatigue crack initiation requires probabilistic numerical simulations; these are complex and computationally intensive. Here, we present a data‐driven approach to develop fast probabilistic surrogate models of creep–fatigue crack initiation in 316H stainless steel. We perform a set of Monte Carlo simulations based on the R5V2/3 high temperature assessment procedure and determine the sensitivity of the probability of crack initiation to loads and operating conditions. The data are used to train different supervised machine learning models considering Bayesian hyperparameter optimization. We discuss the relative performance of such models and show that a gradient tree boosting algorithm results in surrogate models with the highest accuracy.

Creep-Fatigue Crack Initiation Simulation of a Modified 12% Cr Steel Based on Grain Boundary Cavitation and Plastic Slip Accumulation.

High-temperature components in power plants may fail due to creep and fatigue. Creep damage is usually accompanied by the nucleation, growth, and coalescence of grain boundary cavities, while fatigue damage is caused by excessive accumulated plastic deformation due to the local stress concentration. This paper proposes a multiscale numerical framework combining the crystal plastic frame with the meso-damage mechanisms. Not only can it better describe the deformation mechanism dominated by creep from a microscopic viewpoint, but also reflects the local damage of materials caused by irreversible microstructure changes in the process of creep-fatigue deformation to some extent. In this paper, the creep-fatigue crack initiation analysis of a modified 12%Cr steel (X12CrMoWvNBN10-1-1) is carried out for a given notch specimen. It is found that creep cracks usually initiate at the triple grain boundary junctions or at the grain boundaries approximately perpendicular to the loading direction, while fatigue cracks always initiate from the notch surface where stress is concentrated. In addition to this, the crack initiation life can be quantitatively described, which is affected by the average grain size, initial notch size, stress range and holding time.

A validated approach for probabilistic structural integrity design code implementation

This paper provides a possible approach for incorporating probabilistic structural integrity assessment which is sufficiently simple to encourage its adoption by design engineers. The approach, called the “Three-Term Reference Damage Model (3T-RDM)”, is developed in the context of a particular benchmark problem and a proposed Design Chart where the probability of crack initiation by creep-fatigue in a widely used material in nuclear plants, namely 316H austenitic stainless steel, is estimated based on the R5V2/3 high temperature procedure. In principle, the 3T-RDM is not restricted to a particular failure mechanism, i.e., fracture, creep rupture, creep-fatigue crack initiation and growth are all within its scope, and non-metallic materials may also be addressed. To further validate the accuracy and generality of the proposed 3T-RDM and Design Chart, deterministic and Monte Carlo probabilistic structural integrity assessments are conducted for three EDF nuclear plant case-studies and the results are superimposed on the Design Chart obtained from the benchmark problem. The analysis indicates an excellent consistency between the results, justifying the adoption of the 3T-RDM for an intermediate level of probabilistic assessment in the nuclear sector.

A dual-scale modelling approach for creep-fatigue crack initiation life prediction of holed structure in a nickel-based superalloy

In this paper, a dual-scale modelling approach is developed to investigate creep-fatigue behavior and predict crack initiation life for holed structures under multi-axial stress state. The macro-scale simulation supplies local deformation histories to the dual-scale simulation as boundary conditions. In the dual-scale simulation process, the micro-mechanical behavior and damage evolution are described by using crystal plasticity. In order to validate the dual-scale simulation procedures, a series of creep-fatigue tests as well as the post-test characterizations were carried out for nickel-based Inconel 718 at 650 ℃. The detailed results of macro- and micro-scale simulations are presented in terms of stress–strain behavior, damage evolution and life prediction. Regarding the macro-scale simulations as the benchmark, it may provide an assistant support and precognition for the micro-scale damage calculation at higher cycles. The predicted cycle numbers to crack initiation are in agreement with the experimental ones. More advantages are manifested in the potential scientific and engineering significance for the dual-scale modelling approach.

Semi-quantitative creep-fatigue damage analysis based on diffraction-based misorientation mapping and the correlation to macroscopic damage evolutions

A large number of strain-controlled creep-fatigue tests under wide loading waveforms are conducted at 650 ℃ in nickel-based forged GH4169 superalloy. Comprehensive characterizations, including scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), are observed from the post-test examinations. Particular focus is brought to the physical understanding of damage mechanisms under wide creep-fatigue loading conditions using EBSD analysis. The representative misorientation parameters are calculated for constructing diffraction-based misorientation mapping. Semi-quantitative analysis of longitudinal EBSD observations is conducted to prove that strain ratio has little influence on creep-fatigue damage degrees, while dwell time causes noticeable changes to damage progressions. In particular for geometrically necessary dislocation (GND) map explored in this work, more fundamental information based on failure physics is obtained to analyze the creep-fatigue crack initiation mechanism.

A review of probabilistic creep assessment reporting relating to volume 2/3 of the R5 procedure

Probabilistic assessments allow the uncertainties present in reactor design and operation to be understood and reflected in the quantification of reliability. Increased awareness of the limitations of deterministic structural integrity assessments have highlighted that the unquantified margins do not provide an adequate measure of component risk.The R5 procedure is a “UK nuclear power industry standard, frequently used in safety cases for structural integrity assessments of Advanced Gas-Cooled Reactor components operating in the creep range” [Assessment Procedure R5, EDF Energy Nuclear Generation Ltd.]. Within this procedure, Volume 2/3 covers creep-fatigue crack initiation in defect-free structures and a drafted appendix provides guidance on the application of probabilistic approaches.A review of probabilistic approaches using the R5 procedure has been conducted, including a wider review of probabilistic assessments to ascertain general good practice. The review identifies a lack of consistency in the reporting of probabilistic assessments, particularly for correlations and convergence of outputs. Providing a structured method for reporting probabilistic assessments would be valuable to ensure consistent application and enable reproducibility of results.

A crystal plasticity-based approach for creep-fatigue life prediction and damage evaluation in a nickel-based superalloy

A numerical process based on crystal plasticity finite element (CPFE) was implemented to predict creep-fatigue crack initiation life. CPFE-based model can describe the macroscopic cyclic deformation and reveal grain-level damage mechanism. A new life prediction approach was then constructed by introducing fatigue and creep indicator parameters. Furthermore, a series of strain-controlled creep-fatigue tests in GH4169 superalloy at 650℃ were used to validate predicted accuracy of this model, where most of the data points lied within ±1.5 error band. Finally, a potential methodology for conservative creep-fatigue life evaluation was provided by flexible creep-fatigue damage summation rule in engineering applications.

Plant loading uncertainties and their incorporation in probabilistic creep damage assessments

This work presents methodologies for the examination of uncertainties in plant loading conditions for incorporation in probabilistic creep-fatigue (CF) crack initiation assessments. These approaches are based on the use of plant data for transient (TR) as well as steady-operating (SO) conditions, with both potentially having large contributions towards creep-fatigue damage. Conventionally, the stress-states in a boiler plant component are found using thermal and mechanical (elastic) finite element (FE) models. The main inputs to these models are the boiler steam temperatures, while the outputs are the six stress components (SCs) and the metal temperature (MT) at the assessment location(s) of interest. The proposed methodologies were developed based on experience gained from examining a tube-plate (TP) plant component, for which historical data was available. In a probabilistic lifetime assessment, the aim of these approaches is to replace time intensive FE runs with probabilistic alternatives which incorporate the load variabilities of interest. The value of these approaches lies in the avoidance of running FE models for every probabilistic trial (e.g. a Monte-Carlo simulation may require more then 105 trials), which may be computationally prohibitive.

Probabilistic structural integrity: methodology and case-study in the creep regime

ABSTRACTThis paper presents a methodology for conducting probabilistic assessments for structural integrity applications based on the Monte Carlo method. This starts with the definition of the underlying procedure for assessing the failure mechanisms of interest, followed by the statistical modelling of the key input parameters, leading to the estimation of desired probabilities. A case-study assessing a plant component (the tubeplate) for creep-fatigue crack initiation using the R5 Volume 2/3 procedure was conducted to provide context and demonstrate the utilities of implementing a probabilistic framework. Expanding on previous work, this paper highlights the main areas of focus for the proposed probabilistic methodology: probabilistic representation of input parameters, correlations, treatment of loading uncertainties, conducting post-assessment sensitivity analyses, the extrapolation of assessment location probabilities to component-level and, thereafter, population-level estimates. Through presenting a case-study implementing the full probabilistic methodology, the aim is to promote wider application and acceptance within the international structural integrity community, and further development of the methodology and constituent methods.

Creep-fatigue and cyclically enhanced creep mechanisms in aluminium based metal matrix composites

An aluminium (Al, 2024T3) matrix composite reinforced with continuous alumina (Al2O3) fibres is investigated under tensile off-axis constant macro stress and thermal cyclic loading. The micromechanical approach to modelling and three different fibre cross-section geometries have been employed. The effect of creep is included by considering three dwell times at the peak temperature of the thermal loading history. The presence of the hold time gives rise to different sources of failure such as cyclic enhanced creep and creep ratchetting. These failure mechanisms are carefully discussed and assessed. The linear matching method framework has been used for the direct evaluation of the crucial parameters for creep-fatigue crack initiation assessment at the steady cycle. A detailed representation of the steady-state hysteresis loops is provided by using the strain range partitioning and a method for dealing with multiaxiality is reported with regard to the algebraic sign of the Mises-Hencky equivalent stress and strain. All the results obtained have been benchmarked by fully inelastic step-by-step (SBS) analyses. The design of a long fibre metal matrix composite should consider not only the detrimental effect of their dissimilar coefficient of thermal expansion, but also the state of stress at the interface between the matrix and fibre.

Advances on creep–fatigue damage assessment in notched components

AbstractIn this paper, the extended Direct Steady Cyclic Analysis method (eDSCA) within the Linear Matching Method Framework (LMMF) is combined with the Stress Modified Ductility Exhaustion method and the modified Cavity Growth Factor (CGF) for the first time. This new procedure is used to systematically investigate the effect of several load parameters including load level, load type and creep dwell duration on the creep–fatigue crack initiation process in a notched specimen. The results obtained are verified through a direct comparison with experimental results available in the literature demonstrating great accuracy in predicting the crack initiation life and the driving mechanisms. Furthermore, this extensive numerical study highlighted the possible detrimental effect of the creep–ratchetting mechanism on the crack growth process. This work has a significant impact on structural integrity assessments of complex industrial components and for the better understanding of creep–fatigue lab scale tests.

Creep Fatigue Crack Initiation Assessment for Weldment Joint Using the Latest R5 Assessment Procedure

Boiler tube to tubeplate welds are often seen in the power generation industry. In the nuclear power industry, especially in Advanced Gas-Cooled Reactors, the welds are exposed to long term creep temperature and are subjected to fatigue cycles due to start-up and shutdown. The creep-fatigue crack initiation life assessment for the weld is required for the purposes of ensuring of safe operation and life extension. This paper presents the detailed assessment methodologies to perform a creep-fatigue crack initiation assessment using the latest R5 volume 2/3 (Issue 3 Revision 002, 2014) for the boiler superheater header tubeplate to tube Type 316 welds. The key changes of the revised R5 procedure are also outlined in the paper.

A Recent Development in Creep-Fatigue Testing

Abstract Procedures for the assessment of components subject to cyclic loading at high temperatures require material property input data that characterize the creep-fatigue deformation response and resistance to cracking. For many years, there was no testing standard or code of practice to ensure that such information was generated in a uniform way. This was mainly because the creep-fatigue test data requirements for organizations in various industrial sectors appeared to be so different that the need for standardization was questioned. In the mid-2000s, it was recognized that even though it would make no sense to be prescriptive about such details as cycle shape, there were many aspects of creep-fatigue testing for which guidance would be beneficial to ensure acceptable uniformity in deformation and endurance data generation. In response to this realization, the state of the art relating to creep-fatigue interaction was extensively reviewed by an international group of specialists, and the generated knowledge base was used to underpin a new ASTM testing standard, ASTM E2714-09. The gathered knowledge is reviewed. There is a requirement for all ASTM standards to include a precision and bias statement, and an international interlaboratory creep-fatigue test comparison activity was facilitated to form the basis of this section of ASTM E2714-09. An integral part of the guidance given in the new standard is the recommendation for post-test metallurgical inspection and the way in which this information can be used to give added value to creep-fatigue crack initiation endurance results. The evidence gathered from this study is also examined.

Comparison and Assessment of the Creep-Fatigue Evaluation Methods With Notched Specimen Made of Mod.9Cr-1Mo Steel

In components design at elevated temperature, creep-fatigue is one of the most important failure modes, and assessment of creep-fatigue life in structural discontinuities is an important issue in evaluating the integrity of components. Therefore, a lot of creep-fatigue life evaluation methods were proposed until now. To compare and assess the evaluation methods, a series of creep-fatigue test was carried out with notched specimens. All the specimens were made of Mod.9Cr-1Mo steel, which is a candidate material for primary and secondary heat transport system components of the Japan sodium-cooled fast reactor (JSFR). Mechanical creep-fatigue tests and thermal creep-fatigue test were performed by using a conventional uni-axial push–pull fatigue test machine and a thermal gradient generating system with an induction heating. The stress concentration levels were adjusted by varying the notch radius in the each test. The creep-fatigue lives, crack initiation, and propagation processes were monitored by a digital microscope and the replica method. A series of finite element analysis (FEA) was carried out to predict the number of cycles to failure by the several creep-fatigue life evaluation methods. Then, these predictions were compared with the test results. Several types of evaluation methods such are stress redistribution locus (SRL) method, simple elastic follow-up method and the methods described in the design and constriction code for fast reactor (FR) published by the Japan Society of Mechanical Engineers (JSME FRs code) were applied. Through the comparisons, it was appeared that SRL method gave rational conservative prediction of the creep-fatigue life when the factor of κ = 1.6 was applied for all conditions tested in this study. A comparison of SRL method and simple elastic follow-up method indicated that SRL method applied factor of κ = 1.6 gave the smallest creep-fatigue life in practicable stress range level. The JSME FRs code gave an evaluation 70–100 times conservative lives comparing with the test results.

Application of probabilistic modelling to the lifetime management of nuclear boilers in the creep regime: Part 2

Monte Carlo probabilistic simulation has been applied to a large population of nominally identical components in an AGR boiler operating in the creep regime. Some of the components have a history of partial steam flow restrictions which can cause an elevation of their operating temperature, potentially raising the rate of creep life usage. Metal losses due to steam-side and gas-side oxidation and chemical cleaning operations can also exacerbate the rate of creep life usage. The R5 procedure has been used within a probabilistic program to calculate the expected frequency of both creep rupture and creep-fatigue crack initiation. The probabilistic approach is shown to provide a better quantitative guide to the commercial threat than traditional deterministic methodologies based on bounding data. In particular, probabilistic assessments identify the parameters which most significantly influence plant life.

Characterizations of Creep-fatigue Crack Initiation and Growth Life for P92 using Circular Notched Round Bar Specimen

Abstract A testing method under creep-fatigue interaction conditions has been proposed as the ASTM standard E2714-09. In accordance with the ASTM standard, round bar smooth specimens have been recommended as a testing specimen. Additionally, creep-fatigue crack initiation and growth life are usually defined as the attainment of a specific rate of decrease in the maximum tensile stress or the modulus of elasticity ratios. However, these criterions of crack initiation and growth life on ASTM standard are not characterized in terms of physical crack size. In this study, the creep-fatigue crack initiation and growth tests for P92 were conducted using circular notched round bar specimens. An attempt is made to measure the crack length during the tests by the direct current potential drop method, which results in the establishment of a high accuracy crack length measurement. Additionally, the life of creep-fatigue crack growth was characterized by linear cumulative damage law combined with the Q*concept which has been proposed as fracture parameter to describe the life of creep crack growth.

Creep-fatigue interaction in power plant steels

The consideration of a steel’s resistance to creep-fatigue crack initiation and development is an important part of the integrity assessment of high temperature power plant components. Defect-free or defect assessment procedures may be employed depending on the product form from which the component is manufactured, and the criteria adopted for design, inspection-management and remaining-life evaluations. Such assessment procedures require a comprehensive knowledge of the creep-fatigue properties of the component materials, and these are reviewed.

Recent developments in the R5 procedures for assessing the high temperature response of structures

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and for assessing components containing defects (Volume 4/5).The current R5 Volume 2/3 procedure for assessing creep-fatigue initiation in weldments involves analysis assuming homogeneous parent material properties with a single Fatigue Strength Reduction Factor (FSRF) used to account for both the reduction in fatigue endurance and enhancements in strain due to material mismatch and local geometry effects. A new development of this approach involves splitting the FSRF into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhancement Factor (WSEF), which accounts for material mismatch and local geometry effects. The new approach results in less conservative predictions of creep damage than are obtained using the existing approach as it is only the WSEF that affects the stress at the start of the creep dwell rather than the larger FSRF in the existing approach. However, the new approach has little effect on the calculated fatigue damage as the combined effects of the WER and the WSEF are essentially equivalent to the previous FSRF.In addition to methods for predicting creep and creep-fatigue crack growth for load-controlled situations, R5 Volume 4/5 gives advice on the prediction of crack growth for situations involving combined primary and secondary loading. This advice has recently been extended to cover cases involving significant amounts of crack growth, C(t) estimation methods for elastic-plastic-creep behaviour and explicit advice on the treatment of combined loading for situations involving significant welding residual stresses. This extended advice can be used to predict the growth of defects in non-stress relieved austenitic weldments operating at elevated temperatures.This paper briefly outlines the current R5 Volume 2/3 and Volume 4/5 procedures and then focuses on these two significant recent developments in R5, both of which are highly relevant to real instances of creep and creep-fatigue cracking that have been observed in high temperature austenitic plant components.