Ring Creep Research Articles

Creep deformation and rupture behavior of 9Cr-ODS steel cladding tube at high temperatures from 700°C to 1000°C

ABSTRACT The Japan Atomic Energy Agency has been developing 9Cr-oxide dispersion strengthened (ODS) steel as a fuel cladding material for sodium-cooled fast reactors (SFRs). Previous studies have formulated the creep rupture equation for 650°C–850°C. However, little data have been obtained above 850°C, and no equation has been formulated. This study conducted creep tests to evaluate creep strength at 700°C–1000°C. Two creep test methods, the internal pressure and ring creep tests under development, were used, and the validation of the ring creep test method was conducted. The results showed that 9Cr-ODS steel undergoes almost no strength change due to the matrix’s phase transformation, and a single equation can express a creep rupture strength from 700°C to 1000°C. In validating the ring creep test method, analysis clarified the effect of stress concentration on the specimen. Plastic deformation occurs at high initial stress and may lead to early rupture. The results will be essential for future creep testing and evaluation of neutron-irradiated 9Cr-ODS steel.

Development of a Methodology for a WT Gearbox Robustness Test

The paper presents a novel test method to address non-calculative failure modes in wind turbine gearboxes through a metrological robustness evaluation. The method is exemplarily shown for the evaluation of the failure mode “fretting corrosion” which is caused by ring creep at roller bearings. The method was also verified for the failure modes smearing and rehardening. Through the proposed gearbox robustness test, it is for the first time possible to test gearboxes against relevant failure modes and to establish a baseline to compare the robustness of gearboxes with each other. This allows identifying design elements that increase the gearbox robustness and boosters a further increase in availability and reliability of wind turbines.

Simulation based prediction of ring creep on a planetary bearing of a 1MW wind turbine gearbox

Wind turbines (WT) must be further optimized concerning availability and reliability. One of the major reasons of WT downtime is the failure of gearbox bearings. Some of these failures occur due to the ring creep. Ring creep is a failure mechanism, which is mostly detected in the planetary bearings. Ring creep describes a relative movement of the outer ring to the planetary gear. In order to improve the understanding of ring creep, the finite element method (FEM) is used to simulate ring creep in a full-scale planetary bearing (NJ2330) of a 1MW WT planetary gear. The simulation results are validated with experimental data, obtained on a nacelle test bench. The study shows that the simulative prediction of ring creep is of the correct order of magnitude, but no accurate prediction of ring creep can be made without accounting for manufacturing tolerance classes.

Simulative investigation of ring creep on a\xa0planetary bearing of a\xa0wind turbine gearbox

Wind turbines (WT) must be further optimized concerning availability and reliability. One of the major reasons of WT downtime is the failure of gearbox bearings. Some of these failures occur, due to the ring creep phenomenon, which is mostly detected in the planetary bearings. The ring creep phenomenon describes a relative movement of the outer ring to the planetary gear. In order to improve the understanding of ring creep, the finite element method (FEM) is used to simulate ring creep in planetary gears. First, a sensitivity analysis is carried out on a small bearing size (NU205), to characterize relevant influence parameters for ring creep—considered parameters are teeth module, coefficient of friction, interference fit and normal tooth forces. Secondly, a full-scale planetary bearing (SL185030) of a 1MW WT is simulated and verified with experimental data.

Optimized cast components in the drive train of wind turbines and inner ring creep in the main bearing seat

Using large components made of nodular cast iron (GJS) in wind turbines enables the application of lightweight construction through the high degree of design freedom. Besides the sand-casting process, casting into a permanent metal mould, i.e. chill casting, leads to a finer microstructure and higher quasi-static mechanical properties as well as higher fatigue strength. Unfortunately, in present design methodologies specific fatigue data is only available for sand cast and not for chilled cast GJS. Thus, lightweight design strategies for large, chilled cast components are not achievable, which led to the publicly funded project “Gusswelle”. Based on material investigations of EN-GJS-400-18-LT chill cast, an optimized hollow rotor shaft is developed. The design process and the resulting shaft design are presented. The optimized hollow rotor shaft prototype will be tested on a full-scale test bench to validate the design methodology. The intended validation plan as well as the test bench setup is shown in this paper. Furthermore, the decreasing wall thickness influences the interference fit between main bearing and hollow rotor shaft. Thus, through the applied bending moment, inner ring creep is more probable to occur in the main bearing seat. The creeping behaviour is investigated with finite element simulations and a measuring method is presented.

A new approach to evaluate material creep properties by C-shape ring specimen with fixed constraints

Small specimen test technique is widely used to evaluate material creep properties for those components operated at the elevated temperatures. However, the existing small specimen test techniques cannot be well applied for small miniature tubular components well, such as steam pipe and gas turbine recuperator in aviation, due to the equivalent gauge length, specimen volume, stress analysis method and conversion relationship. In this paper, combing the advantages of three-point bending specimen with fixed constraints and small ring creep specimen, a new C-shape ring specimen with fixed constraints (CRSFC) was proposed. Based on beam bending model, the conversion relationship between CRSFC and uniaxial creep test data was established by using complementary strain energy and reference stress method. Using finite element method, conversion coefficients and stress evolution process were obtained. Then the feasibility and accuracy of the above conversion relationship were verified by carrying out four sets of CRSFC creep tests of 1.25Cr0.5MoSi at 550 °C. It can be seen that creep parameters obtained by CRSFC agree well with those from uniaxial creep test with the maximum error of 22.6%. Finally, the effects of the straight edge length, indenter radius, eccentric load and indenter-specimen surface friction coefficient on the load-point steady-state displacement rate were investigated by finite element method. The results show that the effect of the straight edge length is negligible. In order to minimize the effects of other parameters, the indenter radius 0.6 mm ≤ r ≤ 1.2 mm, eccentric displacements e ≤ 0.2 mm and friction coefficient f ≤ 0.8 are recommended in CRSFC creep test.

Correlation of Planetary Bearing Outer Ring Creep and Gear Load Distribution in a Full-Size Wind Turbine

In this work, bearing outer ring creep in the planetary gear of a gear wind turbine gearbox is investigated. So far, gear creep has only been investigated experimentally on very small bearings under simplified load conditions. This paper presents experimental measurement results from a full-size 2.75 MW wind turbine. The test were performed on a system test bench capable of applying loads in all six degrees of freedom at the rotor hub. The objectives of this investigation are to provoke and measure bearing outer ring creep and to identify correlations to the gear load distributions acting on the planetary gear. After a motivation, a literature overview on bearing ring creep is given. Influences on and occurrences of ring creep are described and the two mechanisms roller-induced creep and gear creep explained. Then, the test setup and the evaluated experiments are presented. The influence of torque, rotational speed and additional non-torque loads on gear load distribution and bearing outer ring creep is shown, discussed and put in relation to findings from literature. Key findings are that outer ring creep speed increases with global rotational speed and torque. Also, gear creep – acting in the opposite direction of roller-induced creep – was found at the generator-sided bearing ring for loads below 40% nominal torque. The planet gear load distributions with sun and ring gear were highly uneven and opposed to each other at low torques while leveling out at high torques due to the load-dependent deformations of sun gear, planet carrier and ring gear. The average gear load distribution over all ring gear positions showed to be influenced only by torque and to be independent of non-torque loads. In accordance, even extreme non-torque loads were found to not influence the creep behavior of the two planetary bearing outer rings.

On the Determination of Material Creep Constants Using Miniature Creep Test Specimens

Full size creep test specimens, i.e., conventional uniaxial creep test specimen and Bridgman notch specimens are usually used to determine the full set of material constants for any creep model. However, in many situations, sufficient material is not available for theses specimens to be manufactured from it. Therefore, small creep test specimens have been introduced and used to determine (i) creep constants and (ii) the remaining life time for engineering components. Two commonly used small creep specimen types, i.e., the impression and the small ring creep tests, are used in this paper to determine the steady state creep constants. However, these specimen types are limited for use in determining the secondary creep properties, i.e., they are unable to determine the full set of material creep constants for creep damage models. In this paper the recently developed small two-bar creep test specimen and the newly developed small notched specimen test are described and used to determine a full set of material constants for Kachanov and Liu-Murakami creep damage models. The small notched specimen manufacturing, loading and testing procedures are described in this paper. P91 steel at 600 °C and (Bar-257) P91 steel at 650 °C have been used to compare the material constants obtained from the small two-bar and the small notched creep test specimens with those obtained from the conventional uniaxial creep test specimens and Bridgman notch specimens. The results show remarkably good agreement between the two sets of results.

A Basis for Selecting the Most Appropriate Small Specimen Creep Test Type

Many components in conventional and nuclear power plant, aero-engines, chemical plant etc., operate at temperatures which are high enough for creep to occur. These include plain pipes, pipe bends, branched pipes etc., the manufacture of such components may also require welds to be inserted in them. In most cases, only nominal operating conditions (i.e., pressure, temperatures, system load, etc.) are known and hence precise life predictions are not possible. Also, the proportion of life consumed will vary from position to position within a component and the plant. Hence, nondestructive techniques are adopted to assist in making decisions on whether to repair, continue operating or scrap certain components. One such approach is to use scoop samples removed from the components to make small creep test specimens, i.e., sub-size uniaxial creep test specimens, impression creep test specimens, small punch creep test specimens, and small ring (circular or elliptical) creep test specimens. Each specimen type has its own unique advantages and disadvantages and it may not be obvious which one is the most appropriate test method to use. This paper gives a brief description of each specimen and associated test type and describes their practical limitations. The suitability of each of the methods for determining “bulk” material properties is described and it is shown that an appropriate test type can be chosen.

Interpretation of small ring creep test data

The small ring creep test method can be used to generate uniaxial secondary creep strain rate data from small samples of material. The technique used is based on the reference stress method; it enables the load-line displacement rates, obtained from testing the rings, to be converted to equivalent uniaxial creep strain rates. Accurate conversion relationships are required, in order to ensure that accurate creep properties are obtained. In this article, an analysis of the creep behaviour of small, ring-type specimens and the associated test procedures are described. This includes the determination of the conversion factors, which are used to convert the minimum creep displacement rate to the equivalent uniaxial minimum creep strain rate and to convert the nominal stress to the equivalent uniaxial stress, using the finite element method. In addition, an assessment of the effects of ring dimensions, for example, the radial thickness and mean radius, on the conversion factors, is made. Finally, the effects of the small geometry changes that occur during a test, on the conversion factors, are investigated.

113 モータ軸受のクリープ現象の診断(セッション4 振動利用・評価技術)

113 モータ軸受のクリープ現象の診断(セッション4 振動利用・評価技術)

링 시험편을 이용한 ZIRLO 피복관의 크리프 특성 평가

In this study, we suggested the ring creep test using the ring specimen of Arsene for estimating the burst creep properties of the cladding in stead of burst creep test. For this objective, we used the load-displacement conversion relationship of ring specimen called LCRR which had been determined on our previous study at high temperature by performing the ring tensile test and the numerical analysis. Then we carried out both the ring creep test and the burst creep test between 350°C and 600°C which were higher than the in-service temperature of the cladding in a reactor. The creep properties from the ring creep test with applying LCRR were compared with those from the burst creep test of closed-end specimens. From the results, it could be seen an very strong relationship between them, especially in Larson-Miller parameter. So, it is expected that we can easily predict the creep properties of not only claddings but also various small pressure pipes using the ring creep test.

The properties of the ring and burst creep of ZIRLO cladding

Cladding is used as a separator between fuel and coolant and a container of fuel in nuclear reactors. The cladding is the first part to break in accidents like LOCA or RIA in nuclear plant. Knowledge of the fracture characteristics of cladding is indispensable for the safe operation of nuclear plants. The objective of this study is to evaluate the creep properties of nuclear cladding using the ring tensile specimen. This study consists of two parts: ring and burst creep test. From the results of the ring tensile tests arid numerical analyses, the load–displacement conversion relationship for the ring specimen (LCRR) at high temperatures was determined. LCRR was applied to obtain creep properties from ring creep tests. The results from the creep tests of ring specimens were compared with those from the burst creep tests of closed-end specimens, and the results showed an applicability of the creep test using the ring specimen instead of burst creep test for evaluating the creep properties of the cladding.

Thermoactivated flux creep in high-temperature-superconducting rings in a low-frequency magnetic field

The effect of ac/dc magnetic fields and temperature on the trapped flux has been studied on superconducting rings, in which a random net of Josephson junctions has been established. Using the obtained experimental data, the dependences of the flux relaxation rate on temperature and magnetic field have been derived, and the flux-creep activation barrier has been evaluated. The electrodynamic properties of the samples have been calculated under three models of voltage-current characteristics under the assumption of a thin ring. When placed in an ac magnetic field and in the critical state, the ring displays a quasi-Meissner behavior, which has been verified by experiments. If ac/dc magnetic fields and temperature produce vortex density gradients throughout the ring, the flux locks up in the ring hole and stays invariable, while elsewhere in the ring creep continues. A peculiar graded decrease in the magnitude of the trapped flux has been observed under the action of ac field and has been supported by theoretical calculations; we suppose that this rapid dropoff can be accounted for by a change in effective time scale in the logarithmic creep law.