Creep-fatigue Conditions Research Articles

Application of an Advanced Creep–Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

The demand for steam turbine components is driven not only by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for life-time prediction. Fracture mechanics methods are usually applied when crack like defects are detected not only for new rotors but also for rotor components in service. Based on the findings, a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime. For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and long-term testing data, an advanced creep–fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth (CCG) behavior. Furthermore, recent studies show that the crack size for creep crack initiation (CCI) depends on material ductility and creep strain in the ligament. This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep–fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach—two-criteria-diagram (2CD). For the advanced evaluation concept, the CCI criterion is combined for analysis with a creep–fatigue crack growth (CFCG) procedure. The benefit of the method especially for ductile material will be highlighted.

Analysis of damage behaviour based on EBSD method under creep–fatigue conditions for polycrystalline nickel base superalloys

In order to detect creep–fatigue damage before the crack initiation stage, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method for a polycrystalline nickel base superalloy has been carried out. The average misorientation within grains increases linearly up to the initiation of cracks with the increase in creep strains. Even if stress distributions exist by stress concentration, assessment in all the damage areas allowed the evaluation of creep damage regardless of geometrical influence. Furthermore, the influence of strain rates, introducing fatigue and testing temperatures are hardly observed in the misorientation analysis. Misorientation almost corresponded to inelastic strain regardless of those influences. Quite a small misorientation caused by fatigue led to an equivalent result between creep conditions and creep–fatigue conditions. It is concluded that the misorientation analysis of damaged materials based on the EBSD method allows the quantitative estimation of creep strain and the assessment of remaining creep fracture life.

Creep-fatigue interactions in equiaxed and single crystal Ni-base superalloys

Ni-base superalloys are employed as structural materials for the most critical hot gas path components of gas turbines. The current market requirement is to cycle the machine every day, providing energy when it is most needed. It is therefore important to understand how creep and fatigue damages interact in these components. Starting from a significant knowledge base of mechanical and microstructural behaviour established from standard tests of the equiaxed and single crystal superalloys, creep-fatigue tests have been performed to evaluate how the two damage conditions develop together. The creep-fatigue testing conditions represent the maximum temperature and strain at the critical locations in real components, while the position of hold-time has been varied from tensile to compressive to understand the effect on reduction in crack initiation endurance with respect to standard LCF tests and on the microstructural mechanisms. The experimental test results have been explained in terms of microstructural evolution and they have been correlated to that observed at critical locations in real components.

Systematic Evaluation of Creep-Fatigue Life Prediction Methods for Various Alloys

Failure under creep-fatigue interaction is receiving an increasing interest due to an increased number of start-up and shut-downs in fossil power generation plants as well as development of newer nuclear power plants employing low-pressure coolant. Such situations have prompted the studies on creep-fatigue interaction and the developments of various approaches for evaluating its significance in design as well as remaining life evaluation, but most of them are fragmental and rather limited in terms of materials and test conditions covered. Therefore, applicability of the proposed approaches to different materials or even different temperatures is uncertain in many cases. The present work was conducted in order to comparably evaluate the representative approaches used in the prediction of failure life under creep-fatigue conditions as well as their modifications, by systematically applying them to available test data on a wide range of materials which have been used or are planned to be used in various types of power generation plants. The following observations have been made from this exercise: (i) The time fraction model has a tendency to be nonconservative in general, especially at low temperature and small strain ranges. Because of the large scatter of the total damage, this shortcoming would be difficult to cover by the consideration of creep-fatigue interaction in a simple manner. (ii) The classical ductility exhaustion model showed a general tendency to be overly conservative in many situations, especially at small strain ranges. (iii) The modified ductility exhaustion model based on the redefinition of creep damage showed improved predictability with a slightly nonconservative tendency. (iv) Energy-based ductility exhaustion model developed in this study seems to show the best predictability among the four procedures in an overall sense although some dependency on strain range and materials was observed.

On creep-fatigue endurance of TIG-dressed weldments using the linear matching method

This paper is devoted to parametric study on creep-fatigue endurance of the steel type 316N(L) weldments at 550°C identified as type 3 according to R5 Vol. 2/3 procedure classification. The study is implemented using a direct method known as the Linear Matching Method (LMM) and based upon the creep-fatigue evaluation procedure considering time fraction rule for creep-damage assessment. Seven configurations of the weldment, characterised by particular values of a geometrical parameter ρ, are proposed. Parameter ρ, which represents different grades of TIG dressing, is a ratio between the radius of the fillet of the remelted metal on a weld toe and the thickness of welded plates. For each configuration, the total number of cycles to failure N☆ in creep-fatigue conditions is assessed numerically for different loading cases defined by normalised bending moment M∼ and dwell period Δt. The obtained set of N☆ is extrapolated by the analytic function dependent on M∼,Δt and parameter ρ. Proposed function for N☆ shows good agreement with numerical results obtained by the LMM. Therefore, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) effected by creep, which are intended for design purposes, and dependent on Δt and ρ.

Crack growth behavior of 9Cr−1Mo (P91) steel under creep–fatigue conditions

Several components operated at high temperature are also subjected to cyclic loading making crack growth behavior under creep–fatigue conditions a significant concern during design and during service. This is especially the case for components designed for long term service. Creep–fatigue crack growth tests were conducted on a modified 9Cr–1Mo (P91) steel at 625°C under constant load amplitude conditions with various hold times using compact type specimens. The crack growth rates per cycle increased significantly with increase in hold time when crack growth data were plotted with the cyclic stress intensity factor, ΔK. It is shown that the creep–fatigue interactions during crack growth for various hold times are represented better by the (Ct)avg parameter implying that the P91 steel behaves in a creep-ductile manner. The test results are also used for assessing the newly developed American Society for Testing and Materials (ASTM) test standard, E2760-10 for creep–fatigue testing.

Comparison of fatigue crack propagation behaviour in two gas turbine disc alloys under creep–fatigue conditions: evaluating microstructure, environment and temperature effects

Gas turbine disc materials should possess excellent fatigue and creep performance due to the severe in service conditions experienced. In this study, a comparison of fatigue crack propagation behaviour in two turbine disc alloys, i.e. N18 and low solvus high refractory (LSHR) superalloy, has been made in terms of the propagation rate and fractography observed under equivalent testing conditions. Temperatures of 650 and 725°C are compared for a trapezoidal dwell fatigue cycle (1–20–1–1) in both air and vacuum at an R ratio of 0·1. It is found that coarse grained LSHR superalloy has better fatigue crack propagation resistance than fine grained N18 in vacuum, which is ascribed to its better creep performance. Oxidation causes significant degradation of fatigue performance of these two alloys, especially in the LSHR superalloy at higher temperature (725°C), resulting in its inferior fatigue performance compared with N18. In the LSHR superalloy, it seems that oxidation is the principal contributor to the deterioration of fatigue resistance. This is supported by observations of transgranular fracture in vacuum and intergranular fracture in air. In contrast, creep is a greater contributor to the deterioration in fatigue performance of N18 (as indicated by the intergranular failure modes observed in vacuum). An apparent activation energy analysis is able to provide further insight into the underlying mechanisms of fatigue crack propagation under creep–oxidation–fatigue conditions in these two alloys.

A parametric study on creep–fatigue strength of welded joints using the linear matching method

This paper presents a parametric study on creep–fatigue strength of the steel AISI type 316N(L) weldments of types 1 and 2 according to R5 Vol. 2/3 Procedure classification at 550°C. The study is implemented using the Linear Matching Method (LMM) and is based upon a latest developed creep–fatigue evaluation procedure considering time fraction rule for creep-damage assessment. Parametric models of geometry and FE-meshes for both types of weldments are developed in this way, which allows variation of parameters governing shape of the weld profile and loading conditions. Five configurations, characterised by individual sets of parameters, and presenting different fabrication cases, are proposed. For each configuration, the total number of cycles to failure N☆ in creep–fatigue conditions is assessed numerically for different loading cases including normalised bending moment M∼ and dwell period Δt. The obtained set of N☆ is extrapolated by the analytic function, which is dependent on M∼,Δt and geometrical parameters (α and γ). Proposed function for N☆ shows good agreement with numerical results obtained by the LMM. Thus, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) intended for design purposes and dependent on Δt, α, γ.

Developing Dislocation Subgrain Structures and Cyclic Softening During High-Temperature Creep–Fatigue of a Nickel Alloy

The complex cyclic deformation response of Alloy 617 under creep–fatigue conditions is of practical interest both in terms of the observed detriment in failure life and the considerable cyclic softening that occurs. At the low strain ranges investigated, the inelastic strain is the sole predictor of the failure life without taking into consideration a potentially significant environmental influence. The tensile-hold creep–fatigue peak stress response can be directly correlated to the evolving dislocation substructure, which consists of a relatively homogenous distribution of subgrains. Progressive high-temperature cycling with a static hold allows for the rearrangement of loose tangles of dislocations into well-ordered hexagonal dislocation networks. The cyclic softening during tensile-hold creep–fatigue deformation is attributable to two factors: the rearrangement of dislocation substructures into lower-energy configurations, which includes a decreasing dislocation density in subgrain interiors through integration into the subgrain boundaries, and the formation of surface grain boundary cracks and cavity formation or separation at interior grain boundaries, which occurs perpendicular to the stress axis. Effects attributable to the tensile character of the hold cycle are further analyzed through variations in the creep–fatigue waveform and illuminate the effects of the hold-time character on the overall creep–fatigue behavior and evolution of the dislocation substructure.

OS0917 改良9Cr鋼およびSUS316FR鋼におけるクリープ疲労特性とクリープ疲労が及ぼす微視的構造への影響

In this paper, creep-fatigue properties of modified 9Cr and SUS316FR steels are studied under several conditions of creep-fatigue tests at high temperature considering the use in energy equipment. Durations of stress holding, i.e., durations of creep were different among the experiments. The experiments were stopped when the specimens reached to the same strain for observation of the microstructures. The differences among macroscopic creep-fatigue properties under several creep-fatigue conditions were discussed in terms of the effects of the durations of stress holding. The electron backscatter diffraction (EBSD) was used to observe the microstructures such as grain size and deformation inside grains. Even if the macroscopic strains were same, the microstructures were different from each other especially in modified 9Cr steel. The microstructures were related to the effects of the creep-fatigue conditions.

128 クリープ・疲労条件下におけるAlloy617のき裂成長挙動(材料力学III,一般講演)

128 クリープ・疲労条件下におけるAlloy617のき裂成長挙動(材料力学III,一般講演)

Law of fracture life for directionally solidified and poly-crystal nickel-base super alloys (CM247LC and IN100) under creep–fatigue conditions based on non-equilibrium science

Nickel-base super-alloys strengthening by precipitation of γ' phase are used as a heat resistant material in gas turbines for electric power plant and jet engine. They were severely subjected to creep (time dependent mechanism, TD) and strain/stress controlled fatigue conditions (cyclic dependent condition, CD). In this paper, sophisticated investigation and analyses on the load frequency characteristics of fracture life under creep and fatigue conditions were conducted based on catastrophic function for nickel-based super alloys, CM247LC (directionally solidified nickel-base material, DS) and these results were compared with those of IN100 (polycrystalline nickel-base material). Keywords: Nickel-base superalloy, CM247LC, IN100, fracture life, creep-fatigue interaction, Q ∗ concept, non-equilibrium science

Procedures for Multiaxial Creep-Fatigue Verifications of Nuclear Components

Nuclear high heat flux components (HHFCs) experience large thermal gradients and high heat flux variations, which induce severe thermal cyclic loadings. The most critical design issue for these components is their endurance strength under the required number of thermal cycles.The aim of this work is to provide procedures to perform the multiaxial creep-fatigue life assessment of HHFCs. Since the existing design codes present limitations due to simplifying assumptions concerning procedures for the multiaxial fatigue verification considering interactive effects of creep-fatigue and local stress and temperature conditions, better accurate verification methods and rules are developed starting from the available scientific literature and experimental data. The new verification methods identify the shape of the most damaging hysteresis loop considering plasticity and creep strains in both tensile and compressive conditions.The developed procedures are used to post-process the thermomechanical results of finite element (FE) analyses. They foresee the calculation of the creep-fatigue damage in each node and for each cyclic loading of the analyzed FE model by using the fatigue curve corresponding to the shape of the local hysteresis loop. Furthermore, the most fatigued elements are bounded and the causes of damage are identified to improve the local design. The fatigue damage is evaluated considering the effects of local conditions: temperature, multiaxial stress-strain state, strain intensity range, effect of local mean stresses, material shakedown, accumulated damage for multiple cyclic loads, combined effect of creep-fatigue, hold periods, and neutron flux.The developed procedures are successfully verified by comparing the results with experimental data for different levels of mean stress.This paper presents a description of the procedures and design rules focusing on the innovative aspects. The new procedures have been developed in the framework of the activities for the design, manufacturing, and procurement of the ITER neutral beam injector, and they are applied for creep-fatigue verifications of the in-vessel HHFCs.

Damage Assessment of Heat Resistant Steels through Electron BackScatter Diffraction Strain Analysis under Creep and Creep-Fatigue Conditions

Damage Assessment of Heat Resistant Steels through Electron BackScatter Diffraction Strain Analysis under Creep and Creep-Fatigue Conditions

143 高温クリープ・疲労条件下におけるIN100合金のき裂成長寿命に及ぼす応力多軸効果と巨視力学と空孔拡散凝集挙動による損傷形成挙動の解明(材料力学II,一般講演)

143 高温クリープ・疲労条件下におけるIN100合金のき裂成長寿命に及ぼす応力多軸効果と巨視力学と空孔拡散凝集挙動による損傷形成挙動の解明(材料力学II,一般講演)

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.

Constitutive Laws for the Deformation Estimation of Extrusion Die in the Creep-Fatigue Regime

Tools are exposed to severe working conditions during the hot extrusion process. In particular, dies and mandrels can be subjected to an excessive amount of deformation as a result of the developed high cyclic loads and temperatures. In this scenario, a physical experiment reproducing the thermo-mechanical conditions of a mandrel in a porthole die was performed with the Gleeble machine on the AISI H11 tool steel with the aim to investigate the mechanisms that influence the die deformation. The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all the testing conditions, a comparable pattern of the mandrel displacement-time curve was found reproducing the 3 stages of softening typical of the strain evolution in a standard creep test but with a marked primary phase. Thus, with the aim to identify an easy-applicable equation to estimate the die deformation, the time hardening creep law was chosen. Coefficients of the time-hardening law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys taken from the literature. Results in terms of mandrel displacement were then compared to experimental data for the creep-fatigue condition at different stress and temperature levels. The values found were validated against additional experimental data performed with different specimen geometries. A good average agreement was found between experimental and numerical results. The developed procedure was then applied to an industrial die.

Risk based engineering for design, material selection and maintenance of power plants

Risk based engineering (RBE) provides a measure of decision making through risk assessment of design, manufacturing, operation and maintenance. The application study of RBE to fossil power plants is presented here especially focusing on steam turbines. Typical damage modes and mechanisms are presented, and failure scenarios are expressed in terms of event trees. The comprehensive risk assessment approach is expressed in terms of the conventional risk matrix method for prioritizing the important events and the probabilistic approach for more quantitative evaluation. To make more rational decision making, a ‘Bayesian’ network approach is introduced as an effective tool of casual structure determination in the probabilistic risk assessment framework. The Bayesian network approach for determining the casual structure of risk scenario is shown to be a potential tool for constructing the risk scenario automatically in the data-driven manner even in the case of insufficient available data. Finally a material selection and design map is presented for the creep-fatigue damage condition of turbine components using a two-parameter probability distribution function to make a reliable initial design of high temperature structures.

Creep-Fatigue at High Temperature of Notched Single Crystal Superalloys

Abstract The effect of hold time on the high temperature notched fatigue strength of the CMSX-4 single crystal superalloy has been experimentally investigated. Notched specimens with orientation 〈001〉 have been tested at 950°C in load control in the low-cycle fatigue (LCF) life regime both without and with dwell. For the same applied stress range (net-section stress), the effect of hold time results in an appreciable reduction of fatigue life. Several scanning electron microscope (SEM) observations of the failed specimens have been made in order to associate the actual failure (crack initiation) location to the correct crystallographic orientation and to the critical location in the notch as given by suitable multiaxial fatigue parameters. Moreover, SEM observations reveal the change of the failure mode from pure fatigue to fatigue/creep conditions. In order to model the material behavior in the notch, stress-strain analysis of the notched specimens was performed by an elastic-plastic finite-element method (FEM) procedure; the parameters describing the material behavior of the single crystal superalloy have been identified by comparison with experimentally determined stress-strain fatigue behavior and creep data of smooth specimens. A suitable model for the assessment of the reduction of fatigue life in creep-fatigue conditions is proposed.

Direct and indirect strain measurement during low cycle fatigue of metals at elevated temperature

Current testing practices in low cycle fatigue (LCF) determinations at elevated temperature recommend the use of side-contacting extensometers or other direct methods for the measurement of strain. In many cases (e.g., when an environmental chamber surrounds the test specimen) this is not possible and resort must be had to indirect methods, which require a correction factor depending on the cyclic stress– strain response of the material. By mounting direct and indirect extensometers on the specimen and grip assembly of the testing machine, direct comparisons were made on the output strain signals during LCF tests on alloys 316, Nimonic 90 and IN718 at temperatures in the range 600 – 900°C. Agreement for plastic strain range was in general good, the remote method however requiring an independent value of the elastic modulus for determination of total strain range. Typical uncertainties in these values are discussed. All comparisons were made under continuous cycling conditions and a correction factor for use under creep-fatigue conditions is suggested.